gardening in the global greenhouse - the impacts of climate change on gardens in the united kingdom

Gardening in the Global Greenhouse: The Impacts of Climate Change on Gardens in the UK

Technical ReportNovember 2002

Richard Bisgrove and Paul HadleyCentre for Horticulture and Landscape, School of Plant Sciences, The University of Reading

First published in 2002 byThe UK Climate Impacts Programme

This report should be referenced as:

Bisgrove, R. and Hadley, P. (2002) Gardening in the Global Greenhouse: The Impacts of Climate Changeon Gardens in the UK. Technical Report. UKCIP, Oxford.

Further copies of this report and the accompanying summary report are available from:

The UK Climate Impacts ProgrammeUnion House12-16 St. Michael’s StreetOxford, OX1 2DU

Tel: 01865 432076Fax: 01865 432077

Email: [email protected]: www.ukcip.org.uk

Gardening in the Global Greenhouse Technical Report

iiiPreface

PrefaceThere are three interrelated phenomena whichneed to be identified in reviewing the potentialimpacts of climate change on gardens.

The first is climate change itself. The climaticchanges expected in the UK are described in thereport Climate Change Scenarios for the UnitedKingdom: The UKCIP02 Scientific Report (Hulmeet al., 2002). This report examines the potentialimpacts of the expected climate changes on gar-dens in the UK.

The second phenomenon is the occurrence ofextreme weather events such as floods anddroughts. Climate change is expected to increasethe frequency of some extreme weather events, butpredictions of such events are less certain thanthose for average changes in climate. Predictions ofgale frequency in future are particularly uncertain.

The third phenomenon is development. The Earth’ssurface has changed dramatically as a result ofhuman activity. Forests have been cleared (in theUK as elsewhere), grasslands have been ploughedand fields covered with houses, factories, motor-ways and airports. People travel much more wide-ly and much more frequently than was the caseeven twenty years ago. Some of these changes areroot causes of climate change. Others serve tointensify the impacts of climate change or to bringthem to wider notice. Covering previouslyabsorbent land surfaces with concrete alters thehydrological balance and exacerbates the severityof floods and droughts caused by extreme weatherevents. Building houses in floodplains increasesthe risk and cost of flood damage by orders ofmagnitude. Moving around the globe results in thespread of pests and diseases, of plants and ofhumans, to new areas so it is often impossible tosay if changes in disease incidence are the resultsof climate change or of human activity.

Much of the information relating to climate changeand gardens is anecdotal. In order to draw on wellfounded scientific research it has been necessary tomove outside the garden and use data from

research on agricultural and horticultural crops,and in forestry and nature conservation. This islogical because the plants grown on farms and inforests, or which are managed in nature reserves,also play an important part in gardens. There areimportant differences though between monocultur-al stands of a crop in a field or forest and the mix-ture of many plants in a garden. There are alsoimportant differences between the behaviour ofplants in a natural community and in a highly man-aged garden. These differences have been dis-cussed where appropriate.

Our review is unusual in covering an exceptionallywide range of subject matter, from photosyntheticpathways and evapo-transpiration, to garden histo-ry and concepts of garden conservation. It has beennecessary at times to deviate from the central topicto present short summaries of specific issues, suchas the historical evolution of gardens and plantgrowth and development, to set the impacts of cli-mate change in context. It has been difficult,always, to steer a path between scientific jargonand naïve generalisation. Those with expertise inparticular aspects of the review will find some pas-sages highly simplified but we hope readers willappreciate that this report is intended for an audi-ence with wide ranging interests.


vContents

ContentsPreface iii

Executive summary vii

CHAPTER 1: INTRODUCTION 11

1.1 The UK’s garden heritage 11

1.2 Climate change and gardens 13

1.3 Background to the study 14

1.3.1 Aims and objectives 14

1.3.2 Methodology 14

CHAPTER 2: CHANGES IN THE UK CLIMATE 17

2.1 Observed trends 17

2.1.1 Temperature 17

2.1.2 Precipitation 20

2.2 Causes of climate change 21

2.3 Climate change in the 21st century: the UKCIP02 scenarios 22

2.4 Climate change in the UK 23

2.4.1 Temperature 23

2.4.2 Precipitation 26

2.4.3 Cloud cover, relative humidity &

soil moisture deficits 27

2.4.4 Wind 30

2.4.5 Weather extremes 30

2.4.6 Sea level rise 31

CHAPTER 3: THE PHYSIOLOGICAL BASIS OF

PLANT RESPONSES TO CLIMATE CHANGE 35

3.1 Plant growth and development 35

3.2 Plant responses to carbon dioxide 36

3.2.1 Carbon dioxide and growth 36

3.2.2 Partitioning of assimilates 37

3.2.3 Carbon dioxide and development 38

3.2.4 Interaction of responses to carbon

dioxide and water 38

3.2.5 Interaction of responses to

carbon dioxide and nitrogen 39

3.3 Plant responses to temperature 40

3.3.1 Temperature and growth 40

3.3.2 Temperature and development 40

3.3.3 Plant phenology 41

3.3.4 Dormancy 43

3.3.5 Frost susceptibility 44

3.3.6 Interaction of responses to

temperature & carbon dioxide 45

3.3.7 Plant adaptability and plant breeding 47

3.4 Plant responses to water 47

3.4.1 Water supply 47

3.4.2 Water availability 47

3.4.3 Impacts of water deficits 48

3.4.4 Impacts of water surfeits 49

3.5 Plant responses to changes in pest, disease and weed incidence 50

3.5.1 Climate change and pests 50

3.5.2 Climate change and diseases 52

3.5.3 Climate change and weeds 55

3.6 Climate change and symbiotic organisms 57

CHAPTER 4: PLANTS IN NATURAL

AND MANAGED COMMUNITIES 59

4.1 Plants in the natural environment 59

4.2 Plants in the garden environment 60

4.2.1 Hardiness of garden plants 63

4.2.2 Water availability and garden plants 65

4.2.3 Climate averages versus weather

extremes 66

CHAPTER 5: GARDEN CHARACTER AND

RESPONSES TO CLIMATE CHANGE 69

5.1 The domestic garden 69

5.2 The heritage garden 70

5.2.1 Conserving the heritage garden 70

5.2.2 Climate change and heritage gardens 71

5.2.3 Potential effects of longer term

extreme climate change 72

5.2.4 Botanic gardens 73

5.2.5 Managing historic parks and

gardens through climate change 73

CHAPTER 6: GARDEN MANAGEMENT IN A

CHANGING CLIMATE 75

6.1 Climate change impacts on soil 75

6.1.1 Climate change impacts on

particular soils 77


vi Contents

6.2 Climate change impacts on water 78

6.2.1 Water supplies 78

6.2.2 Water features 84

6.2.3 Water management 86

6.3 Climate change impacts on trees 87

6.3.1 Tree selection 89

6.3.2 Fruit trees and bushes 91

6.3.3 Tree management 91

6.4 Shrubs and sub-shrubs and climate change 92

6.4.1 Shrubs 92

6.4.2 Sub-shrubs 92

6.4.3 Shrub and sub-shrub management 92

6.5 Herbaceous perennials 93

6.5.1 Herbaceous perennial management 93

6.6 Bulbs 94

6.7 Annuals and tender perennials 94

6.8 Lawns and other grass areas 95

6.8.1 The diversity of grass areas 96

6.8.2 Climate change impacts on the

growth of grass 96

6.8.3 Grass management 97

6.9 Paths, walls and garden structures 99

6.9.1 Paths and walls 99

6.9.2 Garden buildings and structures 99

6.10 Garden staff 99

6.10.1 Perceptions of climate change

by garden managers 100

6.10.2 Responses of garden staff 101

CHAPTER 7: CLIMATE CHANGE AND

GARDEN VISITORS 103

7.1 Impacts of climate change on garden visitors 103

7.2 Impacts of visitors on gardens in a changing climate 104

CHAPTER 8: CLIMATE CHANGE

AND GARDEN-RELATED INDUSTRIES 107

8.1 Impacts on operations 107

8.2 Impacts on marketing opportunities 108

CHAPTER 9: RESEARCH AND FURTHER ACTIONS 111

9.1 Climatological research 111

9.2 Horticultural research 111

9.3 Research on soils and water 112

9.4 Economic research 112

9.5 Networks 113

9.6 Policy development 114

CHAPTER 10: CONCLUSIONS 115

REFERENCES 117

APPENDICES

A1 Questionnaire 129

A2 Glossary 133

A3 Examples of climate change scenarios for three case study gardens 135

A3.1 Ardtalnaig, Loch Tay, Central Scotland 135

A3.2 Cambridge, Eastern England 136

A3.3 Torquay, Devon 137

A3.4 Summary 139


viiExecutive Summary

Executive summary1. In the past forty years, the importance of UKgardens in our culture and as a significant contribu-tor to the tourist industry has been increasinglyrecognised. Over that same period the effects ofextreme weather events on gardens have beenincreasingly apparent.

2. This report is the outcome of a desktop studyto review the potential impacts of climate changeon gardens and to identify future research needs. Itwas undertaken under the UK Climate ImpactsProgramme (UKCIP) and funded by AnglianWater, the Department for Environment, Food andRural Affairs (Defra), English Heritage, theForestry Commission, the National Trust, NotcuttsNurseries, the Royal Botanic Garden, Kew and TheRoyal Horticultural Society. The study was under-taken by Richard Bisgrove and Professor PaulHadley from the School of Plant Sciences at theUniversity of Reading.

The aims of the study were:

(i) to provide an overview of the best currentinformation on the potential impacts that cli-mate change may have on UK gardens, gar-den plants and the garden industry;

(ii) to identify information gaps in assessingthese impacts on gardening, heritage gardensand the garden industry and, from these gaps,to define a future research agenda.

3. Analysis of long term weather records indi-cates that annual mean temperatures haveincreased by 1.7°C since the beginning of theIndustrial Revolution in 1750. 1°C of this increasehas occurred in the 20th century, with the lastdecade being the warmest on record. The numberof cold days has decreased and frost incidence hasdecreased very substantially. These changes havebeen reflected in changes in dates of leaf emer-gence, flowering, appearance of many species ofbutterfly and other phenological events.

4. Four climate change scenarios have beendeveloped for UKCIP (the low, medium low, medi-um high and high emissions scenarios) reflectinguncertainties about future emissions of greenhousegases. The UKCIP02 scenarios suggest that meanannual temperatures in the UK will increase by 2-3.5°C by the 2080s, depending on scenario andregion, with increases in the south east greater thanthose in the north west. These increases will beassociated with more hot and very hot days andless frost and snow.

The scenarios also suggest that annual precipita-tion will decrease slightly as a net result of higherwinter rainfall (10-30% higher under the highemissions scenario by the 2080s) and decreases insummer rainfall (a 20-50% reduction by the 2080sunder the high emissions scenario). The year toyear variation in precipitation will also increaseleading to an increased frequency of very drysummers and very wet winters.

Mean sea levels may rise by 60cm (south eastScotland) to 85cm (London) around the coast bythe 2080s, increasing the risk of damaging stormsurges along some coasts. Storms may becomemore frequent, especially in winter, when moredepressions could cross the UK.

5. Climate change will affect plant growth inseveral ways. For example:

(i) increased carbon dioxide levels will increaserates of plant growth and perhaps develop-ment (bud burst, flowering and leaf fall);

(ii) temperature will have more complicatedeffects but an earlier onset of growth inspring and a longer growing season are antic-ipated. Spring has advanced by 2-6 days perdecade and autumn has been delayed by twodays per decade. With the higher emissionscenarios, the chilling requirement for bud-break in fruit trees may not be met in mildwinters, leading to reduced yields;


viii Executive Summary

(iii) the various components of climate changewill interact, often in quite complex ways.For example, while carbon dioxide increasesgrowth in most plants, increased tempera-tures may hasten maturity of some plants andtherefore reduce or negate the impact ofincreased carbon dioxide.

6. Increasing temperatures are expected toaccelerate loss of organic matter in soils, releasingnitrogen which may increase plant growth or, ifleached from the soil, increase pollution of water-courses.

The annual moisture content of soils is likely todecrease by 10-20% across the UK by the 2080s,with substantial reductions (of 20-50%) in soilmoisture possible in the summer by the 2080s.

7. Climate change will affect garden plantsindirectly by affecting the range and virulence ofpests, diseases and weeds. The severity of pest anddisease attacks in general is likely to increase, aswill the geographical spread of many organismscurrently on the edge of their climate range.

8. The impacts of climate change on plants ingardens will be less than on those in the naturalenvironment because of the attention they receivein cultivation.

Increased temperatures in themselves will rarely bedirectly damaging to plant growth but will enable amuch wider range of plants from warmer parts ofthe world to be grown. Higher temperatures com-bined with decreased summer rainfall, though, willcause stress, especially in plants with extensive,shallow, fibrous root systems.

Extreme weather events such as gales, floods anddroughts will be much more damaging than willlong term and subtle changes in average climaticconditions.

9. The impacts of climate change on gardenswill depend in large measure on the regional andlocal setting of the garden. In Scotland and northwest England, change will be less marked than inthe south east where summer heat and drought are

likely to pose serious problems. Throughout theUK, hilltop gardens will be particularly prone todrying and to gales while low-lying gardens will besusceptible to flooding, as at present.

The significance of climate change impacts willdepend on whether the garden is a domestic gar-den, in which case a warmer climate and the oppor-tunity to grow new plants may be welcomed, or agarden of significant historic interest, where con-servation is important. In the latter case the cost ofadapting to climate change while conserving as faras possible the form and content of the garden willoften be considerable.

10. Climate change will have impacts on themany components of the garden. In particular, thisreport addresses the potential impacts of climatechange on:

• soils, water supplies and water bodies;• trees, shrubs, sub-shrubs, herbaceous

perennials, bulbs and annuals;• lawns;• paths, buildings and other structures;• garden staff.

11. Climate change will have impacts on thenumbers of people visiting gardens and on theeffects of those visitors in compacting wet lawns,for example. However these impacts will be rela-tively minor in relation to other social and culturalchanges affecting visitor numbers. Each major gar-den will therefore have its own set of parametersdetermining its catchment area and anticipatedthreats and opportunities arising from climatechange. The most important influence on a gar-den’s attractiveness to visitors and on visitor num-bers will be marketing in its broadest sense.

12. Climate change will also have impacts ongarden-related industries. In terms of risks ofgrowing and potential damage to property theimpacts may be negative. In selling, the overalleffects should be neutral or positive as the ability tocultivate many new plants and an increasingly out-door lifestyle should stimulate demand. Caution inguarding against major losses from extreme weath-er events, and flexibility in adapting to the hazards


ixExecutive Summary

and benefits and problems of climate change willbe the keys to commercial survival.

13. By regarding the garden as a microcosm ofthe wider environment and using it to develop anddemonstrate practices which will alleviate and mit-igate the adverse effects of climate change, the gar-dening community has the potential to set anexample of good practice which will furtherincrease public appreciation of and support for gar-dens and which could ultimately alter the course ofclimate change.

14. In order to achieve this goal, a programme ofresearch is proposed in the report. In particular, theestablishment of a ‘garden network’ to exchangeand coordinate observations, ideas and actions, andto combine research activity on the natural and thecultural environment to the benefit of both, is rec-ommended.


11Chapter 1

Francis Bacon (1625) commented that “when Agesgrow to Civility and Elegancie, Men come to BuildStately, sooner than to Garden Finely: As ifGardening were the Greater Perfection”. In this herecognised the position of the garden as an art formand as an essential component of a civilised soci-ety. He also pointed out that the climate in Englandwas more conducive to being outside than in anyother country but that it was seldom warm enoughto sit still out of doors. To this meteorological situ-ation he attributed the love of gardens and espe-cially of gardening in the UK. It is this involvementof the populace with plants and the ability to growa very wide range of plants in a mild and equableclimate that gives the UK a unique character andsense of place.

The relationship between gardens and climate hastaken on a new significance in recent years with thegradual awareness of the existence of climatechange and its potential impacts on gardens. Thisreport is a desktop study to review the potentialimpacts of climate change on gardens, and identifyareas for further research.

It is necessary to start by looking briefly at thehistorical development of gardens in order toappreciate something of the diversity and signifi-cance of gardens in the UK before considering thesignificance of climate change on gardens of dif-ferent types.

1.1 The UK’s garden heritage

The evolution of garden design has been driven bymany influences but particularly by the inspirationof new ideas, a desire to react against existingestablished styles, by response to the cultural andphysical environment of the time, by diffusion ofideas from pioneers to the wider population and byrediscovery and reappraisal of earlier styles(Bisgrove, 2000).

As a result of these various influences the UK haswitnessed the development of small, enclosedmedieval gardens full of herbs and other useful

plants, the larger 17th century formal gardensinspired by France and Italy, often extending intothe countryside with great avenues and the 18thcentury landscape garden with pleasure groundsmerging into broad expanses of parkland, woodsand lakes. The combination of beauty and utilitywhich the landscape garden represented helped toreshape much of the UK’s lowland landscape anddid much to create the ‘green and pleasant land’which UK citizens take for granted as the ‘natural’landscape and which tourists find so attractive.

In the 19th century gardens were greatly enrichedby exotic plants from all parts of the globe and byequally exotic buildings in Chinese, Japanese,Gothic, Indian, Egyptian and other styles. The rich-ness and diversity of plant introductions in the 19thcentury stimulated the creation of gardens in areasparticularly suited to cultivation of these newplants: the wooded hills on acid soils near toLondon and other major cities, the south west ofEngland with its exceptionally mild, moist climateand outliers such as Tresco (Scilly Isles), Bodnant(North Wales) and Inverewe (Ross and Cromartyon the west coast of Scotland).

The 19th century, especially, saw the developmentof the smaller suburban garden, sometimes mim-icking its aristocratic counterpart but also intensi-fying the interest in the cultivation of plants asobjects in their own right in addition to their use tocreate garden scenery. The introduction of exoticplants came to a climax in the second half of the19th century as a result of growing internationaltrade, increased wealth, improved technology forthe cultivation of plants and a burgeoning nurseryindustry. The excitement of growing unusual or dif-ficult plants remained a feature of 20th centurygardening, to the extent that many gardeners were(and still are) more interested in their plants than inthe garden as an artistic entity. The 19th centuryalso saw the development of public parks in mostmajor cities and towns, extensive landscapes whichrivalled the large private gardens of the day and inwhich their superintendents vied with each other toproduce the most elaborate floral displays.

Introduction


12 Chapter 1

The 20th century was marked by a move awayfrom the most flamboyant excesses of the 19th cen-tury in favour of Gertrude Jekyll and WilliamRobinson inspired wild gardens and flower gardenswith carefully graded colour schemes. In the sec-ond half of the 20th century, large increases inhome (and garden) ownership, increases in themedia attention paid to gardens, increased travelabroad and interest in the garden as an extension ofthe house combined to stimulate, and be stimulat-ed by, a flourishing garden centre industry.

At the beginning of the 21st century one can dis-cern in the UK both the rediscovery of minimalismin gardens by the avant garde (Tunnard, 1938) andits rejection by the mainstream gardener (Taylor,1936), combined with continued diffusion of theideas attributed to Gertrude Jekyll (1908) andWilliam Robinson (1870, 1879). These variousstrands were disseminated through Scandinavia andGermany in ecologically inspired but sophisticatedplant communities of Karl Foerster, Friedrich Stahland others (Hansen and Stahl, 1993) and returnedto the UK in the late 20th century to inspire a newgeneration of gardeners at the end of the century.Interest in ecological or wildlife gardens in the UKparalleled enthusiasm for prairie restoration in theUSA, both developments being partly in responseto perceived damage to the environment by chemi-cally based farming and gardening.

The fusion of European and American ideas in the‘bold romantic gardens’ of Oehme and van Sweden(1990), the grass gardens of Piet Oudolf, BethChatto’s ‘dry garden’ (Chatto, 1994) and the morerecent revival of interest in earlier Victorian styles offlamboyant bedding all contribute to a lively currentinterest in more or less spectacular planting design.Christopher Lloyd’s replacement of the Edwardianrose garden at Great Dixter (Sussex), a strikingendorsement of Robinson’s ‘subtropical garden’ isone of the most recent and most influential examplesof the fusion of these many trends (Lloyd, 2000).

In the smaller domestic garden increased levels ofdisposable income, increased home ownership, thecolour magazine and garden makeover programmehave all widened demand for exciting and usableoutdoor spaces. As new gardens shrink in size there

has also been a revived interest in allotment gardensfor the cultivation of vegetables, fruit and cut flow-ers, allowing the small patch of land around thehouse to be used entirely for recreational purposes.

Nurseries have responded to the demand for novel-ty by offering a wider range of architectural andincreasingly ‘exotic’ plants: phormiums, bamboos,cannas and now palms, tree ferns, bananas andolives. Climate change is beginning to make possi-ble the cultivation of many succulent and otherwisespectacular plants which were once the exclusiveprovince of the gardens of the extreme south west.Tresco (Scilly Isles) has spread to Tunbridge Wellsand is on its way to Teeside

On a technical level the availability of sophisticat-ed, often computer controlled, irrigation systems,pumps and other equipment is beginning to make iteasier to cultivate water demanding plants or plantsin containers and to create fountains, waterfallsand other such features (see section 6.2.1). At thesame time the more environmentally aware garden-er is turning away from such gadgetry to adopt amore ecological approach using composting,mulches, water butts and drought tolerant plants.

One important feature of the late 20th century wasa dramatic increase in interest in historic or heritagegardens. The Garden History Society was formed in1965. In the 1970s the National Trust embarked ona series of ambitious restoration and conservationprojects in such gardens as Claremont (Surrey),Erddig (Wrexham) and Westbury Court(Gloucestershire) (Bisgrove, 1990). Membership ofthe National Trust, which now manages the largestassembly of historic gardens in the world, grewfrom 278,000 in 1971 to 2.6 million in 2000 and arecent survey indicates that 57% of people joiningthe National Trust do so because of its gardens. In1981 English Heritage began its Register of Parksand Gardens of Special Historic Interest in England,which now includes more than 1500 parks and gar-dens, while the National Council for Conservationof Plants and Gardens (NCCPG) was established in1978 to conserve the gene pool which the immense-ly rich garden flora of the UK represents. Mostrecently the Heritage Lottery Fund has grant-aidedthe restoration of many formerly neglected public


13Chapter 1

parks. There are now estimated to be 2,500 publicparks, gardens and other designed landscapes ofnational, regional or local historic importance in theUK, and some 25,000 recreational open spaces(DTLR et al., 2001).

The result of these centuries of development is thatthe number and diversity of gardens and gardenowners in the 21st century is such that one wouldneed to use the sophisticated cluster analysis tech-niques of the plant taxonomist to begin to classifyUK gardens. A garden may be a small oasis ofcalm in the city, a source of productive pride grow-ing quantities of fruit and vegetables, a nationalcollection of saxifrages or apples, a blue-deckedproduct of last week’s television garden makeoverprogramme, a perfect example of the work ofCapability Brown or an important repository ofplants collected by one of the renowned planthunters such as David Douglas.

An important characteristic of UK gardens is thatvery few have been designed then made and left tomature. The majority evolved year by year and per-haps generation by generation. Most gardens areessentially private places. An increasing number ofthe larger private gardens rely on income from vis-itors to support the maintenance costs of the gar-den, while many of the finest gardens in the UKare now owned and managed by public bodies andby organisations such as English Heritage and theNational Trust. Large gardens frequently extendinto even larger parks with tree-scattered pastures,lakes and woodlands which combine visual beautywith economic value and substantial nature conser-vation importance.

At the beginning of the 21st century gardening isestablished as the leading hobby in Britain with anestimated 27 million people owning or havingaccess to a garden. Gardens form the basis of amulti-billion pound industry (Calnan, 2002).

Over the past forty years, especially, the importanceof UK gardens as part of our cultural heritage hasbeen increasingly recognised. With increased inter-est matched by rapidly growing personal mobility,leisure opportunities and living standards, gardentourism by UK citizens and by overseas visitors has

also become a significant contributor to the nation-al economy. Visitors to the Royal HorticulturalSociety’s garden at Wisley, for example, increasedfrom 181,000 in 1970 to 614,000 in 2000 (Prior,pers. comm.). The National Gardens Scheme, whichincludes 3,500 gardens in 2002, raised £1.2 millionin 2001 and has raised over £20 million in its 75-year existence. There are 24 million visitors eachyear to gardens in the UK and garden tourism is esti-mated to be worth £300 million per year. Perhapsmore important than this direct financial contribu-tion to the economy, the quality and quantity of ourgardens and parks also contribute to the image of theUK as a green and pleasant land and to the healthand happiness of its inhabitants.

1.2 Climate change and gardens

In the second half of the 20th century there was anincreasing awareness that the climate was changing,initially because of observed changes in the weath-er and subsequently through the development of cli-mate modelling techniques. Recognising the poten-tial problems inherent in global climate change theWorld Meteorological Organisation (WMO) andthe United Nations Environment Programme(UNEP) set up an Intergovernmental Panel onClimate Change (IPCC) in 1988 to assess thenature and scale of these problems.

Since the 1960s the effects of extreme weather eventson gardens has also been increasingly apparent. Thesevere winter of 1962/3 killed many supposedly

The UK has a remarkable history of gardens and

gardening spanning a thousand years. This results in

a rich heritage of gardens – formal and natural,

planned and planted – and a lively tradition of

making and cultivating gardens. There are estimated

to be 27 million active gardeners and approximately

27,500 parks, gardens and other designed

landscapes of national, regional or local importance.

Gardens make a direct contribution to the tourist

industry of about £300 million per year, but they are

much more important in underpinning the essential

character of the UK as a green and pleasant land, for

the benefit of its citizens and visitors alike.


14 Chapter 1

hardy plants (Anon, 1964; Booth, 1964; Salisbury,1963). The drought of 1976 weakened large trees andcaused lakes to dry out (Bisgrove, 1978). The stormsof 1987 and 1990 felled an estimated 15 million treesin southern Britain alone (Rich, 1988) and the effectsof devastating flooding in many parts of the countryin 2000/01 are still being felt at the end of 2002 asplants whose root systems were inundated slowly die.

The increasing appreciation of gardens and theexpanding catalogue of damage imposed on themby extreme weather events, combined with mount-ing scientific evidence of the existence and scale ofclimate change, has led to growing concern that thechanging climate must inevitably have significantimplications for the future of gardens and theirmanagement.

1.3 Background to the study

Awareness of the implications of climate changefor gardens and the gardening industry led to theorganisation of a workshop on Climate Change andGardens, coordinated by the National Trust, TheRoyal Horticultural Society and the UK ClimateImpacts Programme in April 2000.

The purpose of the workshop was to develop anunderstanding of the potential implications of cli-mate change on gardens and garden-related indus-tries, to begin to identify research needs and to testsupport for a study into the issue. The workshopwas attended by sixty participants including repre-sentatives from botanic gardens and gardens organ-isations, commercial horticulturists, landscapeconsultants, the horticultural press, local andnational government representatives, universitiesand other research organisations.

The primary outcome of the workshop was theexpression of need for a desktop study to reviewthe potential impacts of climate change on gardensand to identify future research needs.

This report is the outcome of that expression of need.It has been undertaken within the framework of theUK Climate Impacts Programme with funding fromAnglian Water, the Department for Environment,Food and Rural Affairs (Defra), English Heritage, the

Forestry Commission, the National Trust, NotcuttsNurseries, the Royal Botanic Garden, Kew and TheRoyal Horticultural Society.

1.3.1 AIMS AND OBJECTIVES

The primary aims of the study were:

(i) To provide an overview of the best currentinformation on the potential impacts that cli-mate change may have on UK gardens, gar-den plants and the garden industry.

(ii) To identify key information gaps in ourknowledge and understanding of theseimpacts on gardening, heritage gardens andthe garden industry.

(iii) To define a future research agenda.

The objectives were:

(i) To identify the aspects of gardens and gar-dening most at risk from climate change, andthe general patterns of distribution and typeof garden most vulnerable to these changes.

(ii) To identify those aspects of gardens and gar-dening which might benefit from climatechange.

(iii) To suggest techniques and practices whichmight be used to reduce the negative impactsand derive maximum benefit from the posi-tive aspects of climate change in gardens.

(iv) To present findings in a report to increase thegardening public’s awareness of climatechange and to communicate the wider bene-fits of environmentally sound practices inadapting to and perhaps mitigating the effectsof climate change.

1.3.2 METHODOLOGY

The main focus of the study was a review of the liter-ature, including searches of bibliographic databases.This review was facilitated and supplemented by theauthors’ expertise in horticultural plant physiologyand landscape management. The authors also derivedmuch support and information from the study’sSteering Committee. Colleagues in or associated withthe School of Plant Sciences at the University ofReading also provided valuable information.


15Chapter 1

The literature review was supplemented by consulta-tion with key experts in the garden and landscapesector. A questionnaire (see Appendix 1) was sent tothe director, curator or head gardener of fourteenmajor gardens strategically distributed throughoutEngland, Scotland and Wales to obtain informationon how past weather had affected their gardens and toexplore expectations in relation to climate change.Three leading nurserymen were interviewed aboutthe commercial impacts of climate change. Theircontribution to this study is gratefully acknowledged.

This report first summarises observed changes inthe UK climate and the changes expected over thecoming century from the UKCIP02 climate changescenarios. It then considers potential implicationsof anticipated future changes for gardens, workingfrom the small scale of the plant cell to the largescale of the landscape as follows:

• the effects on plant growth and development• the effects on plants as individual organisms• the effects on plant communities• the effects on gardens, garden types and gar-

den components such as borders, shrubberies,woodlands, water features and architecturalstructures

• the effects on human use of gardens and theconsequent impacts

• the resultant economic/financial implicationsof climate change in gardens

• adaptive responses to climate change

It concludes with recommendations for researchand action.

A long series of extreme weather events – frosts,

extreme heat, floods and droughts – have caused

severe damage to many gardens in the past forty

years. A growing awareness of the reality of long

term climate change has led to concern for the

future of UK gardens.

Following a workshop to discuss the implications

of climate change on gardens, a decision was made

to commission this desktop study within the

UK Climate Impacts Programme to consider the

issues in more detail.


17Chapter 2

Changes in the UK climate2.1 Observed trends

This section describes changes that have alreadyoccurred to the climate of the UK by examininglong-term measurements of temperature and pre-cipitation.

2.1.1 TEMPERATURE

One of the longest series of temperature measure-ments in the world is the Central EnglandTemperature (CET) record developed initially byProfessor Gordon Manley between 1941 and 1974(Wheeler and Mayes, 1997). This data set collatesrecords from several sites in central England, somedating back to 1659. When the data are smoothedto remove year to year variability, or noise, fromthe record (Figure 1), they show that the averagetemperature in central England increased by 0.7°C

between 1750 (the start of the IndustrialRevolution) and 1900 and almost 1°C during the20th century, with two-thirds of the 20th centurywarming occurring since the 1970s. The recordalso shows that the 1990s was the warmest decadein central England since records began. Five of thesix warmest years since 1659 were 1989, 1990,1995, 1997 and 1999. The year 2000 had thelongest thermal growing season (see Glossary inAppendix 2) in a 230-year history. On a globalscale, nine of the ten warmest years in the 142-year history of the Global Instrument Record haveoccurred since 1990, including the four consecu-tive years from 1998-2001 (WMO, 2001).Analysis of the data indicates conclusively that,although some warming can be attributed to natu-ral fluctuations, the temperature rise since 1970can only be explained if human activity is takeninto account.

Figure 1: Observed temperature trend for Central England from 1659 to 2000. Source: Hadley Centre for Climate Prediction

and Research

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Figure 2 shows the frequency of ‘hot’ days (averagetemperature above 20°C) and ‘cold’ days (averagetemperature below 0°C) since 1772. Since the 18thcentury, the frequency of hot days has increased,with eight hot days per year over the last decade,about twice the long term average. In contrast, thenumber of cold days has fallen from 15-20 days peryear, to approximately 10 days per year.

The number of days with frost in central Englandhas also declined steadily, from about 55 per yearin 1880 to 35 per year in 1980 (Figure 3).

Temperature changes have extended the growingseason. The record-breaking thermal growing sea-son of 2000 lasted for 328 days, from 29 Januaryto 21 December. Such temperature changes haveobvious impacts on plant growth.

Long-term studies in plant phenology (the study ofthe timing of developmental changes in plants,such as expansion of first leaves, opening of flow-ers, leaf fall) have played a useful part in demon-strating plant responses to climate change.

Figure 2: The annual number of ‘hot’ and ‘cold’ days extracted from the Central England Temperature series for the period1772 to 1997. ‘Hot’ days are those with mean daily temperature above 20°C, ‘cold’ days are those with mean daily temperature below

freezing. The smoothed line emphasises variations on a 30-year time scale. Source: Hulme and Jenkins, 1998

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Phenological records for the past 30 years in Europeindicate that spring events such as leaf unfoldinghave advanced by about six days, whilst autumn leafcolouring is delayed by nearly five days (Menzel etal., 1999). Conditions in Britain are changing evenmore rapidly, with spring arriving six days earliereach decade, and autumn being delayed by two days

each decade, an extension of the growing season of24 days since records began 30 years ago. The ther-mal growing season extended on average by 0.7days per year between 1920 and 1960 and by 1.7days per year between 1980 and 2000 (Figure 4).Implications of these changes for plants are dis-cussed in more detail in section 3.3.3.

Figure 3: The number of frost days recorded in central England from 1880-2000. The smoothed line emphasises variations on a

30-year time scale. Source: Hadley Centre for Climate Prediction and Research

Figure 4: The length of the thermal growing season in central England. The bars emphasise deviations in duration from the 1961-

1990 average (242 days). The smoothed curve emphasises variations on time scales of at least 30 years. Source: Hulme et al., 2002


20 Chapter 2

2.1.2 PRECIPITATION

Precipitation records show that winter rainfall hasincreased in Scotland in recent years, whilst sum-mer rainfall has been falling over the same periodin England and Wales. Moreover, a larger propor-tion of winter rainfall now falls as heavy rainfallthan it did 50 years ago (Figure 5).

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Figure 5: The trend (1961-2000) in the fraction of the total seasonal precipitation contributed by the ‘most intense’precipitation events in the winter (left hand bars) and in summer (right hand bars) for a number of UK regions.Positive numbers indicate an increasing trend in the proportion of the total precipitation that comes from the ‘most intense’ events, i.e.

‘most intense’ events are increasing either in frequency or intensity. The lower bound to the class of ‘most intense’ events is defined

(separately to each season and region) by an amount (mm) calculated from the 1961-1990 period, namely the daily precipitation

exceeded on a minimally sufficient number of days necessary to account for precisely 10 per cent of the seasonal precipitation.

Source: Osborn, TJ (2000) in Hulme et al. 2002

Analysis of the Central England Temperature

record shows a rise in average annual temperature of

0.7°C from 1750-1900 and nearly 1°C from 1900-2000.

Nine of the ten warmest years globally since 1860

have occurred since 1990. In the past 30 years the

incidence of frost has declined steadily


21Chapter 2

2.2 Causes of climate change

There is now indisputable evidence that some ofthe changes in the global climate are occurring asa direct result of human activity on the planet(IPCC, 2001). One of the main causes for this isthe worldwide increase in the atmospheric concen-tration of carbon dioxide and other greenhousegases. Carbon dioxide concentration has beenmonitored at a number of locations around theworld since measurements first began at MaunaLoa in Hawaii in 1955 (Keeling et al., 1955). Theyshow that carbon dioxide concentrations areincreasing by approximately 1% per year (Figure6). Thus, since pre-industrial times (i.e., since

1750), the atmospheric concentration of carbondioxide has increased by about 30%.

In the same period, atmospheric methane concen-trations have increased by 145%, nitrous oxide hasincreased by 15% and tropospheric ozone hasincreased by 100% (Scarascia-Mugnozza et al.,2001). These changes can be ascribed directly tothe use of fossil fuel for industrial use and trans-port, and to rapid changes in land use.

These gases have the characteristics of absorbinginfra-red radiation and are commonly called ‘green-house gases’, because of their ability to trap energywithin the lower atmosphere in a manner analogousto the trapping of energy within a greenhouse.Increases in these gases result in more energy beingtrapped, and therefore an increase in global tempera-tures and consequent changes in our climate.Approximately 60% of global warming can beattributed to increases in atmospheric carbon dioxideconcentration, 16% to the increase in methane and14% to increases in ozone (Shine and Forster, 1999).

Figure 6: The steady upward trend of carbon dioxide in the Earth’s atmosphere; the saw-toothed pattern reflects seasonal biospheric changes. Source: Dave Keeling and Tim Whorf (Scripps Institution of Oceanograpny)

and the growing season has extended by 24 days.

Winter rainfall has increased in Scotland, with more

of this rain falling in heavy downpours. Summer

rainfall has decreased in England and Wales.


22 Chapter 2

The extent to which our climate changes in future,will depend largely on future rates of emissions ofgreenhouse gases.

2.3 Climate change in the 21st Century: theUKCIP02 scenarios

Scenarios of greenhouse gas emissions have beenproduced by the IPCC to investigate probablefuture climate changes. The emissions scenariosare based on different views of how the world maydevelop in the decades to come, in terms of eco-nomic growth, population increase, the balancebetween unrestrained market mechanisms anddevelopment of global approaches to sustainabilityand other sociological and economic factors.

Four of the most recent emissions scenarios devel-oped by the IPCC (the SRES scenarios) have beenused by the Meteorological Office’s Hadley Centrefor Climate Prediction and Research and theTyndall Centre for Climate Change Research, togenerate four climate change scenarios for theUnited Kingdom. The low emissions, medium lowemissions, medium high emissions and high emis-sions scenarios describe future UK climate changesfor three 30-year periods centred on the decades ofthe 2020s, 2050s and 2080s. The standard baselineclimate period is used, namely 1961-1990. The sce-narios are described in detail in the UKCIP02 sce-narios report by Hulme et al. (2002) and sum-marised in section 2.4. They show that a certaindegree of climate change is inevitable and thatmany components of our climate will be affected.

The emissions scenarios on which they are basedsuggest that, by the 2080s, the atmospheric carbondioxide concentration may be between 525 partsper million (ppm) (low emissions scenario) and810 ppm (high emissions scenario). This repre-

sents a 45-25% increase over current carbon diox-ide concentrations, or 2-3 times the pre-industrialconcentration. The UKCIP02 scenarios representthe UK climate for about 124 grid squares, each50 x 50 km.

The climate change scenarios are developed on theassumption of more or less smooth evolution of cli-mate change within the parameters used in themodel. Possible discontinuities could arise if thereare chaotic changes to tropical rainforest, for exam-ple, or to the Gulf Stream.

An uncertain element in global climate modellingis the point at which the temperature in tropicalrain forests exceeds the optimum for growth.Temperatures above this optimum would lead todecline in productivity, and ultimately to thedeath of the forest, its decomposition and a mas-sive release of additional carbon dioxide andother greenhouse gases into the atmosphere(Betts, 2000).

The Gulf Stream (and its north eastern extension,the North Atlantic Drift) has a major influence onUK temperatures especially along the west coast,currently conferring on the west coast of Scotland,at the latitude of Labrador and Moscow, a climatein which palms and tree ferns flourish. Completecollapse of the Gulf Stream is not thought to belikely, but some weakening of the Gulf Stream isanticipated in the high emissions scenario for the2080s.

There are many lesser unknowns and uncertaintiesimpinging on climate change modelling whichmight result in more severe or more benign scenar-ios. However, the Hadley Centre’s climate modelsare powerful and robust tools that validate againstcurrent climate very well. The scenarios generatedby them represent convincing synopses of climatechange for assessing the potential impacts on gar-dens. It is clear from the scenarios that a certainamount of climate change is inevitable. Steps willbe needed to prepare for, and where possible tocapitalise on, any benefits of these changes and toreduce the rate and the extent of any negativeimpacts. Key aspects of UKCIP02 scenarios arenow presented.

Analyses of meteorological data indicate

conclusively that temperature rise since 1970 can

only be explained if human activity, especially

increasing emissions of carbon dioxide and other

greenhouse gases, is included in the climate change

models.


23Chapter 2

2.4 Climate change in the UK

2.4.1 TEMPERATURE

According to the UKCIP02 scenarios, averageannual temperature in the UK is expected to rise by

0.1-0.3°C per decade (low) to 0.3-0.5°C (high).This is similar to the general pattern of globalwarming (Figure 7) and compares to a currentobserved rate of global warming of approximately0.14°C per decade.

All four UKCIP02 scenarios suggest that warmingwill be greater in the summer (June-August) andautumn (September-November) than in the winter(December-February) and spring (March-May),and greater in the south east than in the north west.By the 2080s a large part of southern England andSouth Wales will be 4°C warmer in summer and 3-3.5°C warmer in winter, while in north westScotland summers will be 3°C warmer and winters2-2.5°C warmer.

Increases in temperature will lengthen the growingseason for plants such that, for each 1°C increase,the growing season can be expected to increase by

International recognition of the existence of climate

change led to the establishment of the International

Panel on Climate Change (IPCC) in 1988. The Panel

has produced a series of scenarios of future

greenhouse gas emissions based on different views

of how the world might develop.

Four of the most recent IPCC scenarios have been

used in the UKCIP02 report to generate four climate

change scenarios, taking the 1961-1990 climate as a

baseline and forecasting changes for the three 30-

year periods centred on the 2020s, 2050s and 2080s.

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(thinner lines) conducted using the same A2 (medium high) emissions scenario. The upper and lower dotted curves represent the full

IPCC range of global temperature change when both emissions uncertainties and model uncertainties are considered.

Source: Hulme et al., 2002


24 Chapter 2

Figure 8: Relative changes in the inter-annual variability of temperature for the 2080s for the four scenarios.Source: Hulme et al., 2002

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25Chapter 2

approximately three weeks in the south east and byabout ten days in northern areas. The average ther-mal growing season in the Scottish Highlands, cur-rently approximately 150 days, might thereforeextend by 20-60 days in the various scenarios. Insouth west England, where the current averagethermal growing season is approximately 250 days,the extension might be 40-100 days, giving theprospect of year round thermal growing conditionsin some years well before the 2080s.

In winter, minimum temperatures will rise morerapidly than the maximum temperatures, leading towarmer winters with a reduced diurnal temperaturerange. In summer, the opposite will occur with max-imum temperatures rising faster than minimum tem-peratures, leading to more frequent hot summers.

Analysis of inter-annual variability suggests thatwinter and spring temperatures will be less variablefrom one year to another while summer andautumn temperatures will be more variable, espe-cially in the west (Figure 8).

Temperature extremes are also expected to increase.The 1961-1990 average temperatures of ‘extremelywarm’ days (the temperature exceeded on only 10%of days in a given period) were 11°C in winter and23°C in summer in England, 7°C and 17°C respec-tively in Scotland. By the 2080s the temperature ofan extremely warm day in winter could increase by3°C (to 14°C) in south east England and by 2°C (to9°C) in Scotland under the high emissions scenario.In summer, the gradient is from south west to northeast with increases of between 7°C (to 30°C) and4°C (to 21°C) respectively.

The effects of these increases can be expressed in arange of outcomes. For example, under the mediumhigh emissions scenario, a hot August such as in1995 (which was 3.4°C higher than average), mightbe expected to occur one year in five by the 2050sand more than one year in every two by the 2080s(Table 1). Currently, the UK might expect to expe-rience a once in a decade daytime temperatureexceeding 35°C. Under the medium high emissionsscenario, such a once in a decade event may exceed42°C in lowland England by the 2080s.

At the other end of the scale, very low temperaturesare expected to become less common. A minimumtemperature of less than -5°C currently occurs on15% (1 in 7) of winter days around Inverness (east-ern Scotland). In the medium high emissions sce-nario, this frequency decreases by the 2080s to 4%(1 in 25). Frost in many parts of the UK, particular-ly in the south west, will largely become a thing ofthe past, with frosts on the western fringe ofCornwall occurring possibly once every 10 years bythe 2080s (see Figure 3 in section 2.1.1).

Table 1: The percentage of years experiencing various seasonal anomalies across the southern UK (England and Wales) for the medium high emissions scenario. Simulated by HadCM3. Source: Hulme et al., 2002

2020s 2050s 2080s

Mean temperature

A hot ‘1995-type’ August (+3.4° C) 1 20 63

A warm ‘1989-type’ year (+1.2° C) 28 73 100

Precipitation

A dry ‘1995-type summer (37% drier than average) 10 29 50

A wet ‘1994/95-type winter (66% wetter than average) 1 3 7

Although there are complex regional variations,

the general picture for the high emissions scenario

by the 2080s, is for a temperature increase of 2-3°C

in winter and 2.5-5°C in summer, increases in the

number of ‘hot’ (mean temperature above 20°C)

and ‘very hot’ (mean temperature above 27°C) days,

and a once in a decade chance of temperatures

as high as 42°C, with a marked decline in the

number of frosts.


26 Chapter 2

2.4.2 PRECIPITATION

Average annual precipitation is predicted todecrease in all scenarios by up to 10%, as a resultof shifts in the pattern of precipitation with sub-stantial decreases in summer rainfall outweighingsmaller increases in winter. As with temperature,precipitation patterns will vary across the country,with the greatest changes and greatest extremesoccurring in the south east.

Summer precipitation is expected to decrease byabout 20-40% across the UK by the 2080s under thehigh emissions scenario, with a reduction of 50% ormore occurring in the south east, already the driestpart of the country. Winter precipitation will increaseby 5-30% depending on the scenario. Spring precip-itation will decrease in inland areas. Autumns maybecome some 5-20% drier in the south east, butslightly wetter in the north west. Autumn and winterrains will become more intense than at present.

The inter-annual variability in seasonal precipita-tion is also expected to change, with winter rainfallbecoming more variable from one year to the nextacross the UK and summer precipitation becomingless variable, especially in the south and west. Anincrease in the frequency of very dry summers andvery wet winters is likely by the 2080s.

Snowfall: As winter temperatures increase, a greaterproportion of the winter precipitation will fall as rainrather than snow. Indeed, snow has already become arare phenomenon in southern Britain. All scenariosindicate that there will be less snow over the wholeUK, with the largest percentage reductions – perhaps90% or more by the 2080s for the high emissions sce-nario – around the eastern, southern and south west-ern coasts and in the English lowlands (Figure 9).

In relative terms, the Scottish Highlands andNorthern Ireland experience the smallest reduc-tions, but even in Scotland the total snowfall by the2080s might decrease by 60-80% relative to pre-sent day totals. Some areas of the UK are increas-ingly likely to experience a long succession ofsnowless winters.

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Figure 9: Changes in average winter snowfall by the 2080s (per cent) for the four scenarios. Source: Hulme et al., 2002

Under the UKCIP02 high emissions scenario, average

annual precipitation is likely to decrease by 10-20%

by the 2080s, the net result of a 20-50% decrease in

summer precipitation and a 10-30% increase in

winter precipitation. Autumn and winter rainfall will be

more intense. Snowfall could decline by 90% in the

south and by 60-80% in the north. Changes to

temperature and precipitation will be most marked

in the south and east.


27Chapter 2

2.4.3 CLOUD COVER, RELATIVE HUMIDITY AND SOIL

MOISTURE DEFICITS

The effects of changing precipitation patterns ongardens will be exacerbated by the inverse rela-tionship between precipitation and solar radiation.Annual cloud cover and relative humidity areexpected to decrease by 3-9% by the 2080s (Figure10a, b) so evaporation will increase.

The combination of reduced precipitation andincreased evaporation will have marked effectson soil moisture deficits (Figure 11). On anational scale, long term average precipitationcurrently varies approximately four-fold, from600mm each year in south east England to2500mm in west Wales and the west of Scotland.Potential evaporation varies from 350-800mm,with the higher rate of evaporation in the sunnier

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Figure 10a: Changes in annual and seasonal cloud cover (relative to 1961-1990) for the four scenarios for the 2080s.Source: Hulme et al., 2002


28 Chapter 2

and drier south and the lower rate in the northwest. The potential loss of water is thereforegreatest where water is least available. In futuresummers, average soil moisture will decrease

across the UK with the largest reductions (20-50% by the 2080s) occurring in south eastEngland compared with a 0-20% reduction in thenorth west by the 2080s.

Figure 10b: Changes in relative humidity (relative to 1961-1990) for the four scenarios for the 2080s. Source: Hulme et al., 2002

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29Chapter 2

In winter, there will probably be modest increasesin soil moisture availability in Scotland and little, ifany, change in Wales or Northern Ireland, butEngland will see a 10% decrease by the 2080s.

Foggy days are expected to decrease by 20%under the medium high emissions scenario by the2080s because of the generally drier and warmerconditions.

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30 Chapter 2

2.4.4 WIND

Average wind speeds over the UK are expected tochange little as a result of climate change, althoughthis is uncertain. In winter, there may be somewhathigher average wind speeds in south and centralEngland with the largest increase (4-10% by the2080s) expected along the south coast. No changeis expected in the north or west.

In summer, changes along the south coast are expect-ed to be even smaller, with probable small reductionsin average wind speed in other coastal areas. Inspring, changes will be small, if any, and in autumn a5% decrease in average wind speed in England islikely in the high emissions scenario by the 2080s.

2.4.5 WEATHER EXTREMES

One very important consideration in many climateimpact studies, not least in relation to gardens, is thepossible increase in extreme weather events. Theseare potentially much more damaging than steadylong term climatic changes, as plants do not have theopportunity to adapt to the stresses imposed by them.

There is much anecdotal evidence on a global scalethat increasing energy in the climate system as awhole is leading to more erratic weather patterns:torrential rains, very strong winds or short periodsof exceptionally high or low temperatures. Extreme

weather conditions associated with El Niño haveincreased in frequency in recent decades from oneyear in ten, to one in four (Pain, 2002).

Climate change is expected to increase the frequen-cy of some extreme events, such as droughts andhigh temperature events (see Table 1); but trends infuture extremes of wind speed cannot be predictedwith a great deal of confidence, because of the smallnumber of gales in the historical record. The periodof increased gale frequency between 1960-1990, forexample, is matched by similar periods at the end ofthe 19th and beginning of the 20th centuries.However, although the UKCIP02 report states that“the evidence for the recent increase in gale fre-quencies over the British Isles being related tohuman-induced warming remains unconvincing”,the graph plotting deviation of gale frequency fromthe 1961-1990 average (with data smoothed over 30-year averages), does show that frequencies werebelow the 1961-1990 average of 12.5 gales per yearfor much of the period 1881-1980, then climbedsteadily to 17 gales per year in 2000. Only one yearsince 1987 has had fewer gales than the 1961-1990average (Figure 12).

Winter depressions across the UK are expected toincrease in number, from an average of five eachyear currently to eight by the 2080s in the mediumhigh emissions scenario. The track of depressionsis also expected to move further south, resulting inmore strong winds in the winter over the south.Depressions in the summer months may fall fromfive to four in an average season.

Although extreme event predictions, particularly forstorms, are less certain than those for averagechanges in climate, the investment that a gardenrepresents, and the long time scales over whichmost historic gardens are managed, make it wise toconsider the potential implications of increased galedamage in any long term management strategy.

Cloud cover and relative humidity are anticipated to

decrease by 5-25% by the 2080s. Soil moisture

content may decrease in summer by 20-50% in the

south and east under the high emissions scenario by

the 2080s. In winter, soil moisture content will

increase slightly in the north and west. The

combined effects of lower rainfall and higher

evaporation rates may lead to increasingly severe

water shortages in future in some parts of the

country by the 2080s.

Little change is expected in mean wind speed,

although there is a possibility of more strong

winds in the winter in the south, especially along

the south coast.

Scenarios of the frequency or severity of storms and

gales remain uncertain, but the severity of damage

that these events can cause is such that their

potential impacts should be considered when

planning for climate change.


31Chapter 2

2.4.6 SEA LEVEL RISE

The average global increases in sea level from theUKCIP02 scenarios range from 14-18cm by the2050s, and from 23-36cm by the 2080s, dependingon the scenario.

The change in sea level will vary across the UK,mainly because of the readjustment of the land-mass since the last ice age, with much of southernBritain sinking at between 1-1.5mm per year, andmuch of northern Britain rising at between 0.5-1.0mm per year relative to the sea. Net sea levelchanges by the 2080s range from 0-60cm forScotland and 15-85cm for much of England.

Even modest sea level rises will have impacts onlow-lying coastal gardens by raising the water tablebeneath, thereby increasing the risk of flooding, andpossibly resulting in salt intrusion into aquifers.

Storm surges will potentially be much more dam-aging than steady rises in sea level but are much

more difficult to model. Surges occur when hightides are combined with low atmospheric pressureand strong on-shore winds. They can be exacerbat-ed by tidal conditions and the shape of the coast-line. Surges are expected to increase most signifi-cantly on the south east coast, by 80-140cm in the2080s for the high emissions scenario. A decreaseis implied by the model for the Bristol Channel ,butthis does not take into account the shape of thechannel (Figure 13).

Even modest rises in sea level can have dispropor-tionate impacts by allowing increased transmissionof wave energy to the shore. The combined effectsof increased wave energy, wave height and smallincreases in wind speed could result in an order ofmagnitude increase in frequency of surges. Thenorth west coast, for example, experienced a 10cmrise in mean sea level in the last century. An extra15cm rise in levels, and 2mm increase in waveheight likely to be achieved by the 2050s coulddouble or treble dangerous storm surges in the IrishSea (Shackley and Wood, 1998).

Figure 12: The annual frequency of severe gales over the UK for a July-June year (1881/82 to 2000). The bars emphasise

deviations from the 1961-1990 average (12.5 gales), and the lower curve emphasises variations over time scales of at least 30 years.

The upper curve is the normalised July-June index of the North Atlantic Oscillation (1850/51 to 2000/01), smoothed to emphasise vari-

ations over time scales of at least 30 years. Source: Osborn, TJ (2000) in Hulme et al., 2002.


32 Chapter 2

Figure 13: Change in 50-year return period surge height (metres) for the 2080s for three different scenarios. The combined

effect of global average sea level rise, storminess changes and vertical land movements (from Shennan, 1989) are considered. (Top)

Low emissions scenario (low sea level rise estimate; 9cm); (Middle) medium high emissions scenario (central estimate; 30cm);

(Bottom) high emissions scenarios (high estimate; 69cm). Source: Hulme et al., 2002.


33Chapter 2

Higher sea levels combined with increases in windspeed may also result in salt spray being carried inlarger volumes and over longer distances, causingdamage to plants and soil structure further inlandthan hitherto.

These various components of climate change willhave significant influences on plant life and there-fore on gardens. In the next section we look at howplants, the main components of gardens, will beaffected by climate change.

Sea levels will increase as sea water warms and

expands. Changes are likely to be small in the low

and medium low emissions scenarios but might

reach 60-85cm in the high emissions scenario by the

2080s. Even small increases in mean sea level,

combined with a modest increase in wind speed and

wave height, could result in a significant increase in

damaging storm surges. Higher seas and stronger

winds could also result in salt spray being carried

farther inland.


35Chapter 3

The physiological basis of plantresponses to climate changePlants are fundamental to life, utilising the sun’sradiant energy to combine carbon dioxide andwater to produce sugars and oxygen by the processof photosynthesis. The sugars are combined withnutrients from the soil to produce proteins andother complex compounds which enable the plantto grow and to function. The pattern of growth anddevelopment from seed to flowering plant produc-ing more seeds is governed by many factors suchas carbon dioxide concentration, the quantity, qual-ity and duration of light and availability of waterand nutrients. Each species of plant has evolved tosuit a particular ecological niche, using signalsfrom the environment - especially temperature andday length - to synchronise their growth with sea-sonal changes. This chapter provides a basic out-line of how plants function and explores howchanges in climate might affect their growth.

3.1 Plant growth and development

Plants grow, by accumulating material to becomelarger, and they develop, in response to internal andexternal stimuli, from juvenile to mature state,from vegetative to flowering, and from active todormant for example.

The fundamental process driving plant growth isphotosynthesis, by which the green tissues of theplant, especially its leaves, combine carbon dioxidefrom the air with water taken up from the soil byroots to produce carbohydrates (sugars, starch andcellulose), the basic building materials of the plant.Oxygen is also produced in the process. The pho-tosynthesis process may be summarised as;

6 CO2 (carbon dioxide) + 6 H2O (water)

➜ with energy from sunlight ➜

C6H12O6 (glucose) + 6 O2 (oxygen) (a carbohydrate)

The carbon dioxide needed for photosynthesis dif-fuses into the leaves through small pores (stomata)mainly in the undersides of the leaf. In order to sur-vive the plant has to balance its need to take in car-bon dioxide with the need to reduce loss of watervapour through the stomata. It does this by openingand closing the stomata through very sensitivefeedback mechanisms which respond to carbondioxide and water availability. The energy requiredfor photosynthesis comes from sunlight.

The plant then combines the products of photosyn-thesis with nitrogen (usually as nitrate), phospho-rous, potassium and other nutrients taken up fromthe soil in solution, to produce proteins and othercomplex materials. The factors which most affectthe growth of a plant are those required for photo-synthesis (light, water and carbon dioxide) and thelevels of available nutrients (nitrogen, phosphorus,potassium and minor nutrients).

Water plays a key role in plant growth by makingnutrients in the soil available to the plant, but it isalso vital in keeping the plant turgid. If water sup-plies in the soil are inadequate, or if evaporationfrom the plant’s leaves exceeds the ability of theplant roots to replenish this loss, the plant will suf-fer water stress.

The immediate response to water stress is closureof the stomata, thereby preventing further waterloss but at the cost of cutting off the carbon diox-ide supply needed for growth. If water stress con-tinues the plant will usually react by wilting, thenshedding leaves to reduce its evaporative surface.Continued water stress will lead to damage to celltissues and ultimately to the death of the plant.

Plants not only grow, by accumulating carbohy-drates and proteins, but they develop by germinat-ing from seed, producing new shoots and leaves, byflowering, and setting seed. In a temperate climatesuch as that experienced in the UK, perennial


36 Chapter 3

plants (those which live for several, often many,years) have yearly cycles in which they produceleaves, flowers and seed then prepare for winter,for example by developing underground restingorgans (bulbs, tubers) or by losing their leaves andproducing resting buds tolerant of low tempera-tures. This winter resting state often develops intotrue dormancy which can only be broken by a peri-od of low temperatures or short day lengths orboth. When dormancy is broken the plant is readyto begin growth again as soon as the temperatureincreases sufficiently.

The growth of a plant is controlled mainly by lightlevels, the availability of carbon dioxide, water andnutrients, and temperature. Its development, fromvegetative to flowering to resting for example, isoften controlled by more complex mechanismssuch as changes in day length or changes in tem-perature. The onset of dormancy and release fromit are particularly complex processes. In someplants temperature change is the main stimulus. Inothers the process is entirely or predominantly con-trolled by day length while in other plants daylength and low temperature will substitute for eachother in varying degrees.

From this brief and very much simplified descrip-tion of plant growth and development it is clearthat some aspects of growth and development willbe significantly affected by climate change (whencarbon dioxide concentration, temperature or wateravailability are important for example) while otheraspects, especially those controlled by day length,will be largely unaffected.

3.2 Plant responses to carbon dioxide

In most climate change impact studies carbondioxide concentration is important only insofar asit is the principal driver of climate change. Instudying the impact of climate change on gardens,as with agriculture, forestry and nature conserva-tion, carbon dioxide itself has a significant impactby its involvement in photosynthesis.

3.2.1 CARBON DIOXIDE AND GROWTH

If other factors remain favourable, increased carbondioxide concentrations will lead to greater rates ofphotosynthesis in plants. Current carbon dioxideconcentrations limit plant photosynthesis. Growersof protected horticultural crops have known forsome many years that artificially raising the concen-tration of carbon dioxide in greenhouses can sub-stantially increase crop growth and yield. It is gen-erally accepted (Kimball et al., 1983; Poorter, 1993)that a doubling of carbon dioxide concentrationswill lead to approximately a 40-50% increase in thegrowth of plants. However, there are strong interac-tions between increased temperature and carbondioxide such that increases in carbon dioxide con-centration will not always lead to increases in theyield of food crops for example (see Section 3.3.6).

Response to elevated carbon dioxide concentra-tions varies between different species. A review byPoorter (1993) indicates that herbaceous cropplants responded more than herbaceous wildspecies (58% vs 35%), and potentially fast growingwild species increased more than slow growingspecies (54% vs 23%). Leguminous species capa-ble of symbiosis with nitrogen fixing organismshad larger responses to carbon dioxide comparedto other species. There is also a tendency for herba-ceous dicotyledons (broadleaved plants) to show alarger response than monocotyledons like grasses.Poorter (1993) suggests that the more responsiveplants are those with a greater sink strength, that isin those plants with active regions such as devel-oping fruits or rapidly expanding shoots capable ofutilising the products of photosynthesis.

There have also been comprehensive reviews of theeffects of elevated carbon dioxide levels on woody

Plants grow and develop in response

to a range of stimuli but especially to the

availability of carbon dioxide, water and

mineral nutrients and to the quality and

quantity of light. Most of these stimuli will be

affected directly or indirectly by climate change,

except that light quality and the natural

rhythm of variation in day length will

remain unaltered.


37Chapter 3

plants, notably Curtis and Wang (1998). The rate ofgrowth accumulation in trees will be significantlyhigher as a result of elevated carbon dioxide con-centrations (Jach and Ceulemans, 1999). Recentfindings have shown that forests are currently grow-ing at an accelerated rate, particularly in the north-ern hemisphere. Annual increase of wood volume inconiferous and hardwood forests in Sweden,Germany, France and other European countries hasincreased by up to 50% as a result of rising carbondioxide levels, a longer growing season and increas-ing nitrogen deposition (Spiecker et al., 1996;Scarascia-Mugnozza et al., 2001). Broadmeadow(2002b) suggests that timber yields in the UK maybe 20-40% higher over the course of the 21st centu-ry as a result of higher carbon dioxide levels.

The benefits of elevated carbon dioxide levels may,however, be relatively short term. A process ofacclimation (becoming adapted to) is often seen inplant responses to carbon dioxide. Here, the shortterm photosynthetic response to instantaneouschanges in carbon dioxide is much larger than thelong term response. Long term exposure to elevat-ed carbon dioxide leads to the accumulation of car-bohydrates in the photosynthetic tissues of the plantand this accumulation leads to a reduction in pho-tosynthetic rates (Clough et al., 1981). Medlyn etal. (2000) noted an initial 20% increase in net pri-mary productivity of coniferous forests in responseto a doubling of carbon dioxide concentration butthe increase was not persistent, whereas a 2°Cincrease in temperature caused a 10-15% increasein long term productivity in both cool (Swedish)and warm (Australian) climates. (Medlyn attributedthis temperature effect to increased soil nitrogenavailability at the higher temperatures.)

In the long term, leaves developing under elevatedcarbon dioxide concentrations appear, in manyspecies, to have fewer stomata than under lowercarbon dioxide levels (Woodward, 1993). In fact,studies of herbarium specimens indicate that stom-atal numbers in leaves collected from tree speciesat early stages of the industrial revolution werehigher than present day numbers (Woodward,1987, Penuelas and Matmala, 1990). Since the late18th century the mean atmospheric carbon dioxideconcentration has increased from about 277 parts

per million by volume (ppmv) to current levels ofover 350 ppmv and thus there appears to be a closerelationship between the historical trends in stom-atal number and carbon dioxide concentration.

Nevertheless, although photosynthetic responses tocarbon dioxide are less marked in the long termthan those anticipated from short term measure-ments, these responses are significant and con-tribute substantially to the increase in growth anddry matter accumulation under climate changeconditions. The increase in carbohydrate concen-tration of tissues leads to higher dry matter content(cellulose, starch etc) of plant tissues which couldhave implications for the quality of some horticul-tural products. This may have less importance ingardens than in commercial horticulture but itcould, for example, affect the storage life of allot-ment and kitchen garden produce. It might alsoaffect pest and disease incidence (Ciesla, 1995)(see section 3.5).

3.2.2 PARTITIONING OF ASSIMILATES

About half of all assimilates (carbohydrates andproteins) are exported from the shoot to belowground parts of the plant where they are used forroot respiration, nutrient uptake and transport pro-cesses in the roots, and as an energy source fornitrogen fixing bacteria and mycorrhizal fungiassociated with the plant (Lambers, 1987). Underoptimal levels of water and nutrient supply, thispartitioning of assimilates to the roots does notappear to be changed by elevated carbon dioxideconcentrations (Stulen and Hertog, 1993). Whennutrients are in limited supply, varying responseshave been noted but some experiments (egOberhauer et al., 1986) have demonstrated a high-er allocation to roots in elevated carbon dioxideconditions where naturally occurring species aregrowing under nutrient limited conditions. Thissuggests that, in soils with low nutrient status, thehigher carbon dioxide concentrations associatedwith climate change may enable plants to foragemore effectively for their nutrients.

Experiments in which the effects of elevated car-bon dioxide on the responses of plants to waterstress have been examined show a variable


38 Chapter 3

response. In some species, partitioning to the rootis not influenced by elevated carbon dioxide con-centration whereas in other species there is anincreased proportion of photosynthetic productsmoving to the root system implying a greaterinvestment in roots to increase their access to avail-able soil moisture (Tolley and Strain, 1985). Inthese species, higher carbon dioxide concentrationsin the air will enable the plant to tap limited soilwater resources more effectively.

3.2.3 CARBON DIOXIDE AND DEVELOPMENT

In addition to their impact on photosynthesis andtherefore plant size, carbon dioxide levels can alsoaffect other aspects of plant development. Cannell(1990) noted an effect on timing of bud burst andthe cessation of growth: altered concentrations ofcarbohydrates and plant hormones in turn alteredthe dormancy status of trees thereby changing thetiming of bud burst and the length of the activegrowing period. Jach et al. (2001) compared thir-teen different studies of the effects of doubling ofcarbon dioxide concentrations on bud burst in ninedifferent tree species. They showed that, whilsttime of bud burst of five species was unaffected byelevated carbon dioxide concentrations, onespecies (Scots Pine [Pinus sylvestris]) wasadvanced and three were delayed (Sitka spruce(Picea sitchensis), Sweet chestnut (Castanea sati-va) and the hybrid poplar (Populus trichocarpa xPopulus deltoides). There did not appear to be anydifferences in the responses between coniferousand broad leaved species. In addition, elevated car-bon dioxide resulted in a shortening of the growingseason in three species (Castanea sativa, Piceasitchensis and Populus sp.). In one of the reportedstudies on Pinus sylvestris (Jach and Ceulemans,1999), elevated carbon dioxide also stimulated thebuds to develop more rapidly than under ambientconditions. However, there appears to be stronginteraction between these responses and nutrientavailability. Increased nutrient availability increas-es the growing season in many tree species (Bigraset al., 1996) and may mask the effects of elevatedcarbon dioxide concentration.

Flowering and fruiting of trees are likely to be has-tened under conditions of elevated carbon dioxide.

For example, flowering of roses is hastened and thenumber of flower buds is increased (Andersson,1991). The yields of Valencia orange (Citrus sinen-sis) (Downton et al., 1987) and orange trees (Citrusaurantium) (Idso and Kimball, 1997) wereincreased when they were grown in elevated carbondioxide levels. These increases in yield resultedfrom increases in both fruit numbers and fruit size.

The evidence for an effect of carbon dioxide con-centration on leaf senescence and leaf fall is rathercontradictory and may be species dependant. Somestudies (eg, McConnaughay et al., 1996) suggestthat leaf fall could be accelerated by elevated car-bon dioxide. However, others (eg, Gunderson et al.,1993) suggest that leaf fall in some species is unaf-fected whilst yet other studies suggest that leaf fallcan be delayed (Norby et al., 1986, McConnaughayet al., 1996). Clearly, the effect of carbon dioxideconcentration on leaf senescence is still poorlyunderstood and requires further study.

Most predictions of the direct effects of carbondioxide suggest that average yields will increase byabout 40-50% with a doubling of carbon dioxideconcentrations. However, this does not address howplant growth responses to carbon dioxide areaffected by changes in other climatic variables suchas water and soil nutrient availability or tempera-ture conditions. Interactions of carbon dioxide andtemperature are particularly important and aredescribed in section 3.3.6 below.

3.2.4 INTERACTION OF RESPONSES TO CARBON

DIOXIDE AND WATER

The plant manages its intake of carbon dioxide andits control of water loss by the same mechanism,the opening and closing of its stomata. As men-tioned briefly in section 3.1 and discussed morefully in section 3.4 the plant responds to waterstress by closing its stomata. Conversely, if the sup-ply of carbon dioxide is greater than the plant canutilise, it will react by closing its stomata and itwill, in so doing, reduce its water use.

Leaves are able to detect and respond rapidly tocarbon dioxide concentration. Stomatal openingdecreases in response to increased carbon dioxide


39Chapter 3

concentrations (Woodward et al., 1991). Decreasedstomatal aperture under conditions of elevated car-bon dioxide also leads to an increased resistance towater loss from leaves. Thus, as carbon dioxideconcentration increases, the water use efficiency(carbon dioxide gained in relation to water lost)also increases. This suggests that the rate of evapo-transpiration decreases under conditions of elevat-ed carbon dioxide. Crop simulations used to predictthe irrigation requirements of potatoes under cli-mate change conditions suggest that there will bevery little change in irrigation requirements undermost climate change scenarios, as reduced precipi-tation is balanced by increased water use efficien-cy (Wolf, 2000). Indeed, use of an earlier crop vari-ety and an earlier planting date, made possible byincreased temperatures, could considerably reduceirrigation requirements.

Taking into account long term reductions in stom-atal numbers and short term closure of stomata inresponse to increased carbon dioxide concentra-tion, Woodward (1993) estimates that leaf wateruse efficiency has increased by about 28% over thelast century. Kimball et al. (1983,1984) measuredseasonal water use (essentially evapo-transpiration)for well watered, field grown cotton in open topcarbon dioxide chambers. Although not very con-sistent the data overall showed a slight decrease inwater use at elevated carbon dioxide concentra-tions. This, coupled with the large increase ofyields, suggest that these beneficial effects of ele-vated carbon dioxide may, in some instances atleast, compensate for increased evaporation fromplants in the drier conditions anticipated by climatechange scenarios.

Response to elevated carbon dioxide may be influ-enced by water stress. Kimball et al. (1993) againshowed that seed cotton yields were increased moreby a doubling of carbon dioxide concentrationsunder drier than under wetter conditions (74%compared to 54%).

It is important to remember, though, that while theimpact of the environment (in terms of higher car-bon dioxide levels) on the plant may be to increasethe efficiency of water use, the impact of the planton the environment will be to reduce humidity

(Ciesla, 1995) and, by not using energy for evapo-ration, to increase the temperature of both the plantand its surroundings. The valuable air-conditioningeffect of plants will be reduced during periods ofwater stress.

3.2.5 INTERACTION OF RESPONSES TO CARBON

DIOXIDE AND NITROGEN

The increase in carbohydrate content of tissuesunder elevated carbon dioxide is not necessarilyaccompanied by increases in nitrogen uptake andso a likely response to climate change is a decreasein the nitrogen concentration in plant tissue and anincrease in the nitrogen efficiency of plants.Overall, therefore, nitrogen use by plants may stayessentially the same and fertiliser requirements willbe unaltered by increasing carbon dioxide levels.

One of the many uncertainties surrounding climatechange impacts on gardens is whether nitrogenavailability in the soil will increase (as a result ofhigher nitrous oxide levels in the atmosphere andhigher rates of mineralisation in soils) (Medlyn etal., 2000) or will decrease as a result of increasedleaching (Jeffery, 2001). This would be a fruitfularea for research.

The response to carbon dioxide may also changeunder conditions of low soil fertility. Data fromKimball et al. (1993) show that, even under nitro-gen limited conditions, the response to a near dou-bling of carbon dioxide concentration led to a 53%increase in seed cotton yields under both irrigatedand dry conditions.

Carbon dioxide is important because carbon

atoms form the structural skeleton of the plant.

A doubling of carbon dioxide levels may increase

plant growth by 40-50% though continuous high

levels saturate the plant’s ability to use

carbon dioxide and the benefits decrease with

time. Higher carbon dioxide levels also allow the

plant to use water more efficiently and may make

the plant sturdier, more fruitful and more resistant

(or less appetising) to pests.


40 Chapter 3

3.3 Plant responses to temperature

There are two main categories of temperature effectson plant growth and development. The first is theeffect of temperature and temperature fluctuationson general growth and development; the second isthe effect of temperature extremes on survival.

3.3.1 TEMPERATURE AND GROWTH

Each plant species has its own characteristicresponse to temperature. Most biological activityslows almost to zero below 5°C. At still lower tem-peratures cell functions may be impaired and theplant damaged. At some point below 0°C ice mayform between and within plant cells, causing dam-age or death of the plant, although many plants havestrategies for surviving temperatures far below 0°C.

Above 5°C growth increases exponentially towardsan optimum which varies widely from plant toplant, usually reflecting the natural climate withinwhich a particular species has evolved.

Higher summer temperatures, like higher carbondioxide concentrations, will favour plant growth ifother factors are not limiting. As temperaturesexceed the optimum for any particular plant itsgrowth rate then falls, often sharply, to the point atwhich damage to tissues leads to complete cessationof growth and ultimately to the death of the plant.Temperatures in the UK are unlikely to reach levelsin the next 50-100 years at which they cause directdamage to plants (essentially ‘cooking’ the plant)rather than causing indirect damage by increasingwater stress, although the possibility can not be ruledout on very hot days, especially in greenhouses.

3.3.2 TEMPERATURE AND DEVELOPMENT

Plant developmental responses are somewhat dif-ferent to the growth response to temperature, inthat developmental rates increase approximatelylinearly with temperature above a threshold tem-perature which is often referred to as the ‘base tem-perature’ for plant development (Ellis et al., 1990).As for growth (see section 3.3.1 above) this basetemperature is about 4.5-5°C for many species butcan be lower for some species such as some

Brassicas (Hadley and Pearson, 1999) and higherfor species from a tropical or subtropical origin(Hadley et al., 1984). This linear increase in therate of plant development reaches an optimum typ-ically at between 20°C and 25°C but again thisvaries between species, varieties and even differentdevelopmental processes in the same plant. Abovethis optimum temperature, developmental ratesoften decline at approximately the same rates atwhich they increase at sub-optimal temperatures.

Because rates of plant development increase linear-ly with temperatures above a threshold, events suchas germination, leaf appearance and flowering of daylength insensitive species and varieties often occurafter a fixed accumulation of heat above this basetemperature, often called ‘thermal time’ and mea-sured in day degrees (number of days multiplied bydegrees above the base temperature). This can bevery useful in predicting the effects of climatechange. For example, if time from germination toflowering for a particular species occurs after theaccumulation of 900 day degrees above 5°C, thenflowering time will be 90 days at 15°C (90 days x 10degrees above 5°C). The effects of an averageincrease in season temperature of 2°C above 15°Cover the period of flower development would thenlead to a date of flowering 15 days earlier (75 days at12 degrees above 5°C). Thus, providing that the ther-mal time requirements for particular events areknown, the effects of increased temperature on theseevents can be estimated relatively easily. However,the resources needed to determine thermal timerequirements are such that the information is likelyto be available only for important crop plants. Thismay yield useful information for the management ofkitchen gardens and allotments but for most orna-mental plants, phenological studies of floweringdates and other developmental responses to temper-ature change are likely to be more informative.

As already stated, higher temperature generallyincreases the rate of growth and of development ofplants, particularly at the lower end of the range oftemperatures suitable for growth. One effect of anextended growth period which has thus far been lit-tle studied is that plants will be growing at sub-stantially lower light levels and in shorter days atany given temperature than at present (van de Geijn


41Chapter 3

et al., 1998). The effect of this on quality of growthand, for example, on susceptibility to pest attack,needs further investigation.

One of the most important effects of climate warm-ing is likely to be changes to the onset and cessationof growth (i.e. the beginning and the end of the grow-ing season). Current estimates suggest that spring isadvancing by 2-6 days per decade and autumn isdelayed by about two days per decade (see section2.1.1), and it is anticipated that a year round thermalgrowing season may be experienced in the south ofEngland before the 2080s in the high emissions sce-nario (Hulme et al., 2002). This will affect the rate ofdevelopment (the ‘phenology’) of the plant.

3.3.3 PLANT PHENOLOGY

As mentioned in section 2.1.1, examples of longdelayed autumn leaf fall, flowering extending into

the winter months, ‘unseasonal’ flowering ofspring bulbs, and other indicators of climaticchange are widespread in the horticultural andnational press (see, eg, Anderton, 2000; Fletcher,1999; Greenwood, 2000) to the extent that it isbecoming necessary to redefine what is meant by‘unseasonal’. Observations show already that inyears with mild winter temperatures and warmersprings, bud burst is advanced and the onset ofgrowth occurs earlier (Last, 2001).

The most conspicuous manifestation of climatewarming in a garden situation will be earlier flower-ing times of many plants. Historical data on flower-ing times of many of our garden species provide auseful guide to how our gardens many change underconditions of climate change. Many long termrecords exist and already show substantial changesin flowering time as a result of recent changes in ourclimate. Analysis of long term records of floweringof a number of garden plants (eg Last, 2001) sug-gests that some species, including Mexican orange(Choisya ternata) and Rhododendron ‘Praecox’ willbe very responsive to climate change, whilst others,such as Honesty (Lunaria annua) and bleeding heart(Dicentra formosa) will flower at approximately thesame time as they do now.

Table 2 shows trends in flowering times of somecommon garden plants recorded by Mary Manningin her garden since the mid 1960s (Sparks andManning, 2000) with flowering times for manyspecies advancing by one to two weeks per decadeover the last twenty years.

Table 2: Flowering time of some common garden plants recorded in East Anglia over the last forty years expressed as a mean date of flowering averaged over the periods 1965-1980, 1981-1990 and 1991-2000Source: Sparks and Manning (2000)

1965-1980 1981-1990 1991-2000 Days earlier per decade

Primrose (Primula vulgaris) Feb 8 Jan 6 Nov 23 23.1

Aconite (Eranthis hyemalis) Jan11 Jan12 Dec 14 10.7

Hazel (Corylus avellana) Feb 3 Jan 14 Dec 14 10.7

Daffodil (Narcissus cv.) Mar 10 Mar 7 Feb 25 7.9

Crocus (Crocus sp.) Feb 8 Jan 22 Jan 24 7.1

Snowdrop (Galanthus nivalis) Jan 19 Jan 10 Jan 6 5.5

Willow (Salix sp.) Feb 16 Feb 19 Feb 8 –

Temperature has very complicated effects on

plant growth. Higher temperatures increase growth

and speed up the rate of plant development so plants

will flower earlier, though the scale of the response

is different in different plants. In recent decades

spring has been advancing by 2-6 days per decade

and autumn has been delayed by two days per

decade. With temperature increases anticipated in

the high emissions scenario, a year round growing

season in the south of England will be likely in some

years before the 2080s.


42 Chapter 3

A more detailed analysis of 23 British native andgarden species is given by Sparks et al. (2000).This shows that 22 out of 23 species showed a sig-nificant advancement in flowering time with a 1°Cincrease in temperature (Table 3).

The results suggest a 2-10 day earlier flowering foreach degree Centigrade of temperature rise.Interestingly, autumn crocus produced the onlypositive response, an indication that in plantsresponding to declining temperatures as a stimulusfor autumn flowering, flowering may be delayed asmean temperatures increase.

Butterfield et al. (2000) simulated the effect of cli-mate change on grape vine production in the UKand showed that under climate change conditionspredicted by the HadCM2 climate model, date ofbud burst occurs 10 to 25 days earlier, while dateof maturity occurs 20-50 days earlier. Currentlythe area suitable for grape production (wheregrapes are able to reach maturity by 15 November)covers the southern and central counties ofEngland, with Lancashire in the west and

Humberside in the east marking the northern lim-its. Maturity in the UK varies from late Septemberto mid November with maximum achievableyields varying from 175-200 grams per squaremetre (gm-2). Although temperature has a nega-tive effect on grape yields this is more than offsetby the substantial positive effect of increased car-bon dioxide. The net effect will be increases inyield ranging from 10-25% by 2050. It is estimat-ed that mean yield, quality and quantity of grapesused for wine making in the UK will increaseunder conditions of climate change (Bindi andFibbi, 2000). Grape production, at present on thenorthern limit for economic production in the UK,could extend into Scotland as the Iberian peninsu-la becomes less and less suitable (Schulz, 2000).In gardens, the grape might eventually replacesuch fruits as raspberry and blackcurrant whichwill not respond well to increasing temperatures.

One result of warming at the lower end of the tem-perature range is already, and will increasingly be,the near continuous growth of lawns through thewinter months. In recent research at the Cambridge

Table 3: Results from stepwise regression of flowering time on Central England monthly temperatures (fromthe preceding October through to the month of mean flowering of that species). Source: Sparks et al., 2000

The value in the second column gives the pooled effect of a 1°C rise across all months on the date of flowering (expressed indays). A negative sign indicates an advancement in flowering time, a positive sign indicates a delay in flowering time. Six species for which 58 years of data exist are given first followed by species with 20 years of data.

Species Net effect (days) Species Net effect (days)

Greater bindweed (Calystegia silvatica) -9.9 Ox-eye daisy (Leucanthemum vulgaris) -4.9

Bird cherry (Prunus padus) -9.1 Redcurrant (Ribes rubrum) -4.9

Almond (Prunus dulcis) -8.9 Horse-chestnut (Aesculus hippocastanum) – leafing -4.9

Purple lilac (Syringa vulgaris) -8.8 Winter aconite (Eranthis hyemalis) -4.7

Hawthorn (Crataegus monogyna) -8.6 Coltsfoot (Tusilago farfara) -4.2

Dog rose (Rosa canina) -8.2 Hazel (Corylus avellana) -4.1

Laburnum (Laburnum anagyroides) -7.9 Garlic mustard (Alliaria petiolata) -4.1

Horse Chestnut (Aesculus hippocastanum) – flowering -7.7 Wood anemone (Anemone nemorosa) -3.6

Ivy (Hedera helix) -7.3 Snowdrop (Galanthus nivalis) -3.4

Lesser celandine (Ranunculus ficaria) -6.7 Harebell (Campanula rotundifolia) -2.6

Elder (Sambucus nigra) -6.5 Christmas rose (Helleborus niger) -1.9

Madonna lily (Lilium candidum) -6.4 Autumn crocus (Colchicum speciosum) +3.8

Yellow crocus (Crocus aureus) -5.8


43Chapter 3

Botanic Garden, Jeffery (2001) found that lawngrowth in plots heated to 3°C above ambient washigher in March (a small but significant increase)and 18% higher in April.

Another effect of a general increase in tempera-ture, especially if combined with wetter winters,might be the increased incidence of mosses andalgae, many of which have lower threshold temper-atures for growth than those of most floweringplants. Hotter and drier summers may limit orcounter this increase, or at least result in the moss-es and algae adopting their dry resting state for alarger proportion of the summer. However, diffi-culties could well arise as more gardens open ear-lier in the year in response to progressively earlierflowering seasons: algae and mosses will exacer-bate the slipperiness of wet paths.

3.3.4 DORMANCY

Much of the UK garden flora consists of speciesthat have a perennial habit, for example, trees,shrubs and herbaceous perennials. These have dis-tinct annual growth cycles which can be dividedinto three phases; a rest period, a period of quies-cence and an active growth period (Leinonen,1996; Battey, 2000). The rest period and the quies-cent period together constitute the dormant period.The rest and quiescent period are often describedas periods of innate and induced dormancy respec-tively and have evolved to ensure that plants haveno soft, young growing tissues that could be dam-aged by the unfavourable conditions that prevailduring the winter period.

During the autumn and winter, active growth ceas-es and plants then have very limited ability to groweven if placed in conditions that allow activegrowth. Although the plant appears to be inactiveduring dormancy, this is often a period of highinternal activity, with the plant producing leaf andflower initials in readiness for rapid spring growth.Exposure to a period of low ‘chilling’ temperaturesis required before a plant can resume activegrowth. This chilling requirement is often mea-sured as the accumulation of temperature below aparticular threshold temperature. For example,sweet cherry requires the accumulation of 1000

chill units at 3.8°C in order to complete or ‘break’dormancy (Mahmood et al., 2000). If chilling isinadequate, the development and/or the laterexpansion of leaf and flower buds may beimpaired. Problems have already been experiencedwith poor cropping of blackcurrant after mild win-ters (Carew, pers. comm.) and the same might hap-pen with raspberry, apple and other fruits as wintertemperatures continue to increase.

After the completion of the rest period, plants entera quiescent period in which they have the potentialto grow but are limited by the prevailing conditionsin late winter and early spring. Once temperaturesattain a certain threshold, plants then begin theiractive growth period. During this growth phase,providing the temperature is warm enough, theamount of growth is a function of the amount oflight intercepted by the plant canopy and the effi-ciency of photosynthesis. Finally, growth isbrought to a halt again by a combination of short-ening day lengths, lower light levels and coolertemperatures during the autumn.

Cannell (1989) assumes a continuously changingresponse to temperature for woody perennials fromautumn through to spring flowering, so that the needfor chilling temperatures is related to the thermaltime requirement for flowering. Thus, as autumnprogresses and the tree accumulates exposure tochilling temperatures, the thermal time required forflowering decreases progressively. Using Cannell’sapproach, Battey (2000) points out that beech(Fagus sylvatica), which has a large chilling require-ment and thermal time for bud burst (beech is one ofthe last trees to leaf out in the spring), will accumu-late less chilling under conditions of climate change.This will increase the thermal time for bud burst andmake bud burst even later. However, species such ashawthorn (Crataegus monogyna) have a small chill-ing requirement which is easily met by the Britishclimate. Here climate warming would cause earlierbud burst.

Although dormancy has been studied widely inwoody plants, it has received less attention inherbaceous perennials. However, commonly theunderground resting organs of species from tem-perate regions require a period of chilling before


44 Chapter 3

growth can recommence (Heide, 2001). Manyherbaceous perennials possess prominent winterbuds, whilst the shoot dies down in the autumn.Studies by Heide (2001) on Sedum telephium, andpreliminary studies on three other herbaceousperennials with prominent winter buds, Rhodiolarosea, Epilobium adencaulum and Oxyria digyna,suggest that dormancy was controlled by daylength rather than by temperature, with the plantsbecoming dormant under short days and beingreleased from dormancy under long days. Clearly,this suggests that, for those herbaceous perennialswhich are under strict photoperiodic control fordormancy release, this mechanism will preventearly growth initiation in mild winters and suggestsa much greater stability of emergence under condi-tions of climate change.

3.3.5 FROST SUSCEPTIBILITY

A widely expressed concern among horticulturistsand gardeners is that climate change will lead toearlier growth and therefore to greater susceptibili-ty to, and damage from, late spring frosts. Increases

in winter temperatures, anticipated in all scenarios,will result in a very substantial increase in the num-ber of days with temperatures above freezing, andabove 5°C, thus extending and advancing the grow-ing season. The concern expressed is that suchearly onset of growth as a result of climate changemay increase the risk of frost damage to plants(Hanninen, 1991).

However, the earlier onset of spring growth inperennial species has to be seen in the context of adecline in the number of damaging spring frosts.For tree species, modelling exercises suggest aprobable decline of spring frost damage in treeswith climate warming, at least in the Netherlandsand Germany (Kramer, 1994). Other predictions(eg, Hanninen, 1997) range from no change to amoderate increase in the incidence of frost damage.

Although this aspect of climate change merits fur-ther study, it is logical that damage to precociousyoung growth from late frosts is unlikely toincrease in response to an increase in average tem-perature. At worst, damage may occur earlier in theyear but at the same erratic frequency and with thesame unpredictability as at present. What is morelikely is that the reduced frequency and severity offrosts as average temperatures increase will resultin less frost damage, including less damage to pre-cocious growth. This is not to say that growers andgardeners can ignore the possibility of a severefrost. In terms of mathematical probability it isunlikely that another winter of 1962/3 severity willoccur but, taking an indefinitely long term view,another winter of 1962/3 severity is almostinevitable at some time in the future.

Although the incidence of spring frost damage toprecocious growth is not expected to increase withclimate change, there is some indication thatautumn frosts may become more damaging.Reduced or delayed hardening of plants in theautumn combined with reduced cloud cover and anincreased diurnal temperature range could lead toincreased damage (Broadmeadow, 2002a).

Frost damage can also occur during the dormantperiod, so the ability of plants to withstand winterfrosts may also be affected by climatic warming.

Many perennials found in cool temperate climates

adapt to low winter temperatures by becoming

dormant, in which state they are resistant to low

temperature damage. Many trees and shrubs in

particular have periods of dormancy which can only

be broken by more or less prolonged periods of

chilling, which is most effective at 0-5°C. Although

externally dormant, many plants undergo active

internal development, producing leaves and flowers

which will emerge in the following spring.

Higher mean winter temperatures will have a variety

of effects. Some plants, such as hawthorn, have a

small chilling requirement so higher temperatures

will accelerate growth. Others, notably beech, have

a longer chilling requirement. If this is not met,

growth in spring will be delayed, so increasing

winter temperatures will result in later leafing.

In blackcurrant, raspberry, apple and other fruits

the plant needs a cold period to form flower buds.

Insufficient chilling will result in delay, abnormality

or failure of flowers.


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Higher rates of tree activity under elevated carbondioxide concentrations may result in increasedmetabolic activity during the dormant period, par-ticularly at elevated temperatures. Plants maybecome less deeply dormant, leading to anincreased probability of frost damage (Repo et al.,1996; Ogren et al., 1997). However, equally,increased soluble carbohydrate concentrationsunder future elevated carbon dioxide conditions(effectively increasing the concentration ofantifreeze in dormant plants) may improve frosthardiness in some species (Ogren et al., 1997). In aUK context, Murray et al. (1994) concluded thatelevated carbon dioxide concentrations and climat-ic warming would reduce the risk of frost damageto Sitka spruce (Picea sitchensis) in Scotland.Nevertheless, mild winters could also lead to high-er rates of respiratory activity, resulting in adecrease in soluble sugars and thus a loss of coldtolerance (Ogren, et al., 1997). Certainly, the rangeof possible outcomes to the effects of climatechange on the internal tissue conditions during thedormant phase reflects the spectrum of responsesthat have been recorded on frost hardiness in dor-mant tree species (Jach et al., 2001) which rangefrom increased frost hardiness in Scots pine (Pinussylvestris) to increased frost injury in Black spruce(Picea mariana).

In colder climates than that of the UK, snow canplay an important role in gardens in protectingplants against winter injury, by providing a protec-tive blanket against freezing in very low wintertemperatures and desiccation in cold winter winds.However, the climate of the UK is not usually suchthat the protection from snow cover is vital. Inmost of the UK, lack of snow will signify moregenial growing conditions and reduced winterinjury rather than an increased risk of low temper-ature damage.

3.3.6 INTERACTIONS OF RESPONSES TO

TEMPERATURE AND CARBON DIOXIDE

There is widespread evidence of a positive interac-tion between carbon dioxide concentration and tem-perature. Response to higher carbon dioxide concen-trations is greater at higher temperatures (Idso et al.,1987) and the optimum temperature for photosynthe-sis increases with increasing carbon dioxide concen-tration (Allen et al., 1990). The combination ofincreased temperature and increased carbon dioxidepredicted in all climate change scenarios suggeststhat for some species the growth stimulation may begreater than the 40-50% suggested above (section3.2.1). Kimball (1993) predicts from an extensivedata set, that a doubling of carbon dioxide concentra-tion combined with a 3°C increase in temperaturecould lead to a 56% stimulation in growth. This issimilar to values obtained for carrots under increasedtemperature and carbon dioxide concentrations pre-sented below (Wheeler et al., 1994). ConverselyKimball et al. (1993) suggest that the response tocarbon dioxide may be very variable or even negativeat cool temperatures, suggesting that photosynthesismay be stimulated less, or even be reduced, at cooltemperatures by increasing carbon dioxide concen-tration. This implies that an increase in temperatureshould be even more effective in stimulating the ben-efits of increased carbon dioxide levels at the low endof the temperature range than at the higher end.

An important aspect of temperature on plant

growth is the effect of very low temperatures

which may freeze plant tissues and kill the plant.

Plants vary enormously in their tolerance of low

temperatures. Some people fear that climate change

will encourage earlier growth of soft new shoots

and that this will increase risk of frost damage.

It is more likely in most cases that precocious

growth will be paralleled by reduced incidence

of frost. The timing of frost damage to precocious

growth may change but its frequency will not

increase. As frost becomes increasingly rare,

especially in the south, then frost damage will also

be reduced.

There is some risk that clearer skies in autumn

and delayed dormancy in plants may lead

to increased frost damage in autumn, and possibly

in winter.

Reduced snow cover will not usually result in

increased winter damage to plants.


46 Chapter 3

Although very little information exists for orna-mental garden plants, studies of the effects of cli-mate change, including the interaction of carbondioxide and temperature, on growth, developmentand yield of several important crop plants havebeen conducted over the last decade. Most of theseare also widely grown in gardens.

In general, all crops show a positive effect of car-bon dioxide on yield. However, it appears that onlycrops that are harvested at an early stage of theirphysiological development (eg, carrot) show a pos-itive effect of increased temperature. Crops that areharvested later in their physiological development(eg, onion and cauliflower) show a negative effectof increasing temperature. The net effect ofincreased carbon dioxide levels and increased tem-peratures therefore varies from plant to plant.

Carrot yields are likely to increase substantiallywith predicted changes in climate in the UK.Studies by Wheeler et al. (1994) showed that car-rot growth is stimulated by increases in tempera-ture, although temperatures greater than 18°C leadto progressively more foliage growth than rootgrowth. A 1°C increase in soil temperatureincreased yield by 34%. Responses to increasedcarbon dioxide are also large: an increase in carbondioxide concentration from 325 to 530 ppm alsoincreased yield by 34%. This reflects the responsesto climate change of most root crops which areconsidered to be larger and more positive thanmost other crops (Kimball, 1983).

In onion, warmer temperatures shorten the dura-tion of growth whilst elevated carbon dioxidestimulates growth with no overall effect on cropduration (Daymond et al., 1997). However, thenegative effect of temperature on crop durationappears to predominate, so that the overall effectof climate change is likely to reduce yield, becausethe stimulation in growth brought about by carbondioxide is more than offset by a shorter period ofbulb growth brought about by elevated tempera-tures. In general, it appears that a 1°C increase inaverage temperature decreases bulb yield by 3.5-15% whereas an increase in carbon dioxide levels,from current ambient levels to 530 ppm, lead to anincrease in bulb onion yield of 30-50%.

It has long been known that reproductive growthin cauliflower and broccoli (the initiation andgrowth of the cauliflower curd or broccoli spears)is very sensitive to temperature (Salter, 1969).Thus, the date of curd or spear initiation isadvanced by increased temperature but unaffectedby light and carbon dioxide (Wheeler et al., 1995).Curd or spear growth for both species is increasedby temperature up to a maximum of 14-15°C, buttemperatures above this lead to a decrease ingrowth (Hadley and Pearson, 1999). Althoughmean curd dry matter yield is increased by 34%when carbon dioxide concentrations are increasedfrom ambient levels to 530 ppm, a 1°C rise intemperature reduces yield by approximately 6%(Wheeler et al., 1995).

The optimum temperature conditions for produc-tion of potato, the main tuber crop grown in theUK and an important plant in many gardens, aretypically those that are currently experienced(Wolf, 2000). Increases in temperature alone, as aresult of climate change, will accelerate the senes-cence and death of foliage and therefore precipi-tate the end of growth. There are also more dayswith reduced growth as a result of higher temper-atures, resulting in lower tuber production.However, this is compensated for by a positiveresponse to increased carbon dioxide concentra-tions so that, for most climate change scenarios,small yield increases are expected. Warmer sea-sons offer the possibility of earlier planting datesthan are presently possible, although the risk offrost during the early spring will limit the earliestdate of planting. Earlier planting dates will givehigher potato yields and this yield increase willbecome larger if carbon dioxide levels continue toincrease.

There are varietal effects on potato yield. In gener-al, early maturing varieties have a lower optimaltemperature range than late maturing varieties,suggesting that earlier varieties may be more nega-tively affected by increases in temperature. Thevariation will inevitably be exploited to producenew potato varieties suited to changing climaticconditions, so potato yields are not likely to changesignificantly in response to climate change.


47Chapter 3

3.3.7 PLANT ADAPTABILITY AND PLANT BREEDING

The facts, cited above, that the optimum tempera-tures for tuber production in potato are typicallythose currently experienced, and that early varietieshave a lower optimum temperature range than latermaturing varieties, indicate a very important aspectof the responses and adaptations of cultivatedplants to climate change. There is variability with-in the population of any one species and this willlead to some adaptation to climate change by natu-ral selection. The pace of change anticipated by cli-mate change scenarios is such that plants in thenatural environment may be unable to adapt suffi-ciently quickly and may face elimination, but in ahorticultural context the plant breeder will have amarked influence in accelerating the selection pro-cess and in shaping plant responses to the environ-ment. This will be the case especially in annualplants, where the life-cycle is very short, and inplants of widespread use and value, such as turfgrasses. For trees and for plants which have verylimited commercial importance, intervention byplant breeders is unlikely.

As an example of past success, old varieties ofbeetroot were subject to bolting (accelerated flow-ering instead of root formation) in cold springs and

this restricted sowing dates. ‘Boltardy’ beet wasselected for reduced susceptibility to bolting inresponse to low spring temperatures, allowing ear-lier production. Similarly the production of sweetcorn varieties able to mature in shorter and coolerseasons has greatly extended the range of UK gar-dens in which sweet corn can be cultivated. Theseselection and breeding practices will undoubtedlymake a significant contribution in the adaptation ofannual garden plants, especially, to climate change.

3.4 Plant responses to water

Water plays a vital role in plant growth and sur-vival in three ways: as one of the ‘raw materials’ ofphotosynthesis, in transport of dissolved nutrientsfrom the soil into and through the plant, and inmaintaining plant turgor. Before one can considerthe impacts of climate change resulting from plantresponses to water, it is necessary to have a basicunderstanding of the distinction between watersupply and water availability.

3.4.1 WATER SUPPLY

Water supply to the plant is derived from precipita-tion and in some situations from the net result ofinflow to and outflow from the catchment area instreams or rivers. In all the UKCIP02 scenarios,water supply to the plant is expected to decrease inspring, summer and autumn. Although precipita-tion is expected to increase throughout the UK inthe winter, evaporation will also increase in thesouth, so winter water supply will be increased inthe north but reduced in the south. The wider envi-ronmental implications of water supply in affectingrun-off, erosion and flooding, for example, areconsidered in section 6.2.

3.4.2 WATER AVAILABILITY

Water availability to the plant depends in part on thepresence of an adequate water supply, but it is alsoaffected by evaporation from the plant which, in turn,depends on solar radiation. Water evaporates fromthe cells inside the leaf and is lost to the atmospherethrough the stomata. This process of transpirationuses energy supplied by solar radiation and dependson the humidity of the air and on windspeed.

Temperature and carbon dioxide concentrations

interact. Higher temperatures and higher carbon

dioxide levels combine to stimulate more rapid

growth and development but the end result is not

always a higher yield. Increased speed of

development may mean that the plant is unable

to use the full length of the growing season before

it dies. Plants like carrot, which are harvested early in

their development, will increase in yield.

Plants harvested at the end of their natural growing

season, like broccoli, cauliflower and onion, may

produce lower yields as the accelerating effect of

temperature exceeds the increase in growth caused

by carbon dioxide.

There has been little work on ornamental plants but it

is likely that they will respond in the same way.

Hardy annuals, in particular, may go to seed earlier

so their flowering season will be curtailed.


48 Chapter 3

As the temperature increases, the capacity of theair to carry water vapour increases very steeply.The humidity of the air decreases or, put anotherway, its drying capacity (and therefore its capacityto ‘pull’ water from a plant) increases. If water isavailable it will evaporate and, by absorbing verylarge amounts of energy in so doing, will cool theplant and its surroundings (or prevent them heatingup). If the water supply is limited, evaporation isnecessarily reduced and more energy will be usedin heating the plant and its surroundings. The valu-able air-conditioning role of plants will decreasewhen they are subjected to water stress.

The plant is not a passive tube conducting water fromthe soil to the air. Water movement through the plantresults partly from active uptake by the roots (requir-ing food reserves and oxygen) and partly from evap-oration by the leaves. If root uptake cannot keep pacewith evaporative loss, as is often the case in hot, dryand windy weather, the plant will close its stomata.This prevents further water loss, but also preventscarbon dioxide uptake for photosynthesis.

The impacts of climate change on water availabili-ty and the resulting impacts on plant growth willarise in part from changes in the water supplyitself, but also from increased temperatures anddecrease in atmospheric humidity. Changes in sup-ply to the plant will result from increased precipi-tation in the winter (when water is least useful forgrowth) and decreased precipitation in the summerand autumn, when high light levels and increasing-ly high temperatures combine to maximise poten-tial evapo-transpiration. The change in temperatureis expected to be upwards in all seasons in all sce-narios, leading to increased evaporation of water.

3.4.3 IMPACTS OF WATER DEFICITS

As discussed briefly in section 3.1, water deficits inthe plant will lead in the short term (seconds, min-utes, hours), to closure of stomata, loss of ability totake up carbon dioxide for photosynthesis and wilt-ing of soft plant tissues. Even short periods of wilt-ing can cause substantial reductions in plant growth.

In the longer term (days, weeks, months), the plantwill respond to water stress by shedding its older

leaves and by becoming more compact with small-er, thicker leaves. It may also divert more resourcesto root development in order to exploit waterresources in a greater soil volume. Annual plants,especially, will often flower more rapidly in condi-tions of water stress in order to set seed before theydie, so the flowering season will be curtailed.

In the very long term (centuries, millennia), plantssubject to the continuous selective pressure of waterstress evolve adaptive mechanisms such as hairi-ness (to reflect light and thus reduce evaporation) orwaxiness (to reflect light and reduce water lossthrough the surface), resulting in grey or silverfoliage. In continuously dry environments, evolu-tion results in very compact habits, succulent waterstorage tissues (as in cacti and succulents), and spe-cialised metabolism to allow photosynthesis to takeplace while stomata are closed. These long termadaptations, which often have an ornamental valuein themselves, are also subject to short term varia-tion. In low light conditions or with plentiful watersupplies, leaf hairiness will be reduced and normal-ly grey or silver leaved plants will become moregreen. Plants grown in gardens for their grey leavesusually look less attractive in the winter months butin the wetter and duller winters anticipated in cli-mate change scenarios they will look worse.

The impact of decreased precipitation in other sea-sons will depend on the interaction of many factors,especially on regional variation and on soil type. Inthe north and west, reduced summer precipitationand higher light levels (from reduced cloudiness)may result in a more favourable climate for plantgrowth. Those plants attuned to a cool, moist cli-mate, such as Meconopsis, primulas and ferns, maysuffer. In the south and east, already the driest partsof the UK, temperature increases and precipitationdecreases are anticipated to be greater than else-where. Water deficits and their impacts on plantsurvival and growth may be severe, especially forlong established plants attuned to a softer climate.Particular concern has been expressed for the futureof beech trees (Harrison et al., 2001; Wade et al.,1999), an important and characteristic componentof the landscape in the south east of England and animportant feature of many larger gardens (see sec-tion 6.3.1).


49Chapter 3

Paradoxically, though, adaptation to increasingwater deficits may be easier in some respects inthe south east than in other regions. There aremany highly attractive plants which will toleratevery dry conditions, but they will only flourish ifthey can escape winter wet. Although winter pre-cipitation is anticipated to become higher in allparts of the UK, higher temperatures in winter areexpected to result in reductions of soil moisturecontent in the south east, rather than increases. Inareas in which the summer becomes drier but thewinters distinctly wetter the potential of adaptingby using ‘xeriscape’ planting schemes (i.e. usingxerophytic, or drought tolerant, plants) will beseverely limited.

Soil type will also have a major influence. The dry-ing of heavy and poorly drained soils in reducedprecipitation scenarios will increase the range ofplants which can be grown, and will reduce the riskof waterlogging, though it will, of course, reducethe opportunity to grow plants specifically adaptedto wet conditions (and may have serious implica-tions for factors other than plant growth, such asthe oxidation of soils, as discussed in section 6.1).

3.4.4 IMPACTS OF WATER SURFEITS

Increase in winter precipitation is a feature of allUKCIP02 scenarios, but more rain does not neces-sarily mean more waterlogging. On light soils,increasing winter precipitation will be a great ben-efit in improving the health of trees and in recharg-ing the water table, thus extending the period forwhich water will be available for growth in springand summer.

Water surplus will only be damaging to the plantif it causes waterlogging of the soil. The impact ofwaterlogging is to deprive the plant roots of oxy-gen so that they cease to function. In the shortterm, the symptoms of waterlogging of leafyplants are similar to those of drought: roots areunable to pump water into the plant, so the upperpart of the plant wilts. More seriously, in theanaerobic conditions of waterlogged soils, plantmetabolism is altered and toxic compounds accu-mulate. If conditions do not improve, the roots andthen the whole plant will die.

Some plants are highly adapted to waterloggedconditions in the soil, with air channels in theleaves, stems and roots - or the pneumatophores(‘knees’) of swamp cypress (Taxodium distichum)- conducting atmospheric oxygen down to theroots. Many other plants will tolerate short periods(hours, days, even weeks) of waterlogging espe-cially if the plant is inactive in winter so that oxy-gen demand is minimal. Others are very intolerantof waterlogging, or even of wet soils, and willquickly be killed either as a direct result of reducedoxygen supply or indirectly as a result of succumb-ing to root pathogens. In general, those plantswhich are most highly adapted to dry conditions(xerophytes) are the least tolerant of wet and espe-cially waterlogged soils.

One important implication of climate change is thatincreasing winter temperatures will increase theactivity (and therefore the oxygen demand) of plantsin the winter, so roots may become less tolerant ofwaterlogging. This will especially be so in the southwest, where both temperature increases and winterprecipitation increases are expected to be high.

Water is vital to plant growth and to all life.

Water availability to the plant depends on the

relative rates at which water is taken up from the

soil by the roots and lost from the leaves.

If water is in short supply in the soil, or water loss

from the leaves is too high because of increasing

temperatures and increasing light levels, the plant

will suffer water stress. It will react by closing its

stomata (leaf pores) to conserve water and will

therefore shut off its carbon dioxide supply.

Growth will suffer. Prolonged stress will cause loss

of leaves and hardening of the plant. Extreme stress

will kill it.

Adaptability to water stress varies greatly. Plants

adapted to growing in cool, shady positions, ferns

for example, will wilt after a few minutes in full sun

even if the roots are freely supplied with water.

Succulents, on the other hand, have reduced or

fleshy leaves and very thick, waxy or hairy

light-reflecting leaf surfaces, so can tolerate very

severe drought for months or years.


50 Chapter 3

3.5 Plant responses to changes in pest,disease and weed incidence

The general increase in summer and winter temper-atures, combined with wetter winters and driersummers, are likely to have considerable impact onthe severity of pest and disease attack on horticul-tural plants. These effects are among the most diffi-cult to extrapolate from climate change scenariosbecause of the complexity of interactions, particu-larly in relation to specialised feeding relationshipsof pests. For example Visser and Holleman (2001)found that increasing spring temperatures advancedthe hatching of oak moth (Operophtera brumata)more than they accelerated the unfolding of the oakleaves on which it feeds. Such disruption of the syn-chrony could have disastrous consequences for themoth but would result in reduced pest damage tooaks unless and until the moth, is able to evolve suf-ficiently rapidly to adapt to the changing situation.Where the interactions extend to the synchronousdevelopment of the host plant, an insect pest and adisease transmitted by the insect, each with its ownparticular responses to climate change, attempts toextrapolate from climate change scenarios to dis-ease incidence become increasingly difficult.

This is clearly an area in which further research isneeded in relation to garden plants.

3.5.1 CLIMATE CHANGE AND PESTS

The focus in this section is on those insects andother arthropods which cause damage to plants butconsideration is also given to other species associ-ated with gardens, such as butterflies and bees.

For example, an increase in the mean annual tem-perature of 2°C (as anticipated for south eastEngland by the 2050s under the medium highemissions scenario), will mean that butterflies willappear 2-3 weeks earlier (Sparks and Yates, 1997)and the range and distribution of butterflies willshift dramatically (Fox et al., 2001). Many nativebutterflies will extend their territories northwardswhile new species, or those with only a localisedexistence in southern England, will migrate fromthe European mainland. The same will be true forpest species of insects. On the other hand, warmerwinter temperatures are already leading to greaterwinter activity in bee populations and are resultingin poorer winter survival (Fletcher, 2000).

Many insect pests cause damage by eating leavesand other plant parts, so the nutrient status of theplant has a very significant effect on its suscepti-bility to attack. Increasing carbon dioxide levelswill result in ‘harder’, less succulent plants whichshould be more resistant to attack (Ciesla, 1995). Awell documented effect of increased carbon diox-ide is a decrease in the nitrogen concentration ofplant tissues, leading to a reduced nutritive value toinsects (Nicolas and Sillans, 1989). Leaf nitrogenis a limiting nutrient in many herbivorous insectdiets (Lincoln et al., 1986). When feeding onleaves of low protein content, insects respond in anumber of ways including increasing consumption,choosing more nutritious leaves or species,reduced fecundity (production of young) and/orsurvival, reduced population density or more effi-cient food use (Scriber and Slansky, 1981). Suchresponses have also been observed when insectsfeed on plants that have been grown in elevatedcarbon dioxide concentrations. Consumption canbe increased by 20-40%, compared to feeding onplants grown under ambient carbon dioxide condi-tions (Lincoln et al., 1986). This has been attribut-ed directly to the reduction of nitrogen concentra-tion in the leaf (Lincoln, 1993).

On the other hand, reduced water supplies will leadto increases in cell sap concentration. Suckinginsects and mites will have a more concentratedfood supply and may increase more rapidly. It iswell known that low humidity in greenhousesencourages the build up of red spider mite. In the

With too much water, plant roots will be

deprived of oxygen and will die. Again, plants differ

widely in their tolerance of waterlogging, but those

most adapted to very dry conditions will be

least tolerant of wet soils and vice versa. Where

drier summers are combined with wetter winters,

plant choice will present considerable challenges,

but it will often be possible to circumvent the worst

effects of waterlogging by improving drainage.


51Chapter 3

very dry summer of 1995, red spider mite andaphids were major problems on outdoor crops,such as lettuce, apple and raspberry (Orson, 1999).In experiments with sap feeding insects on plantsgrown in elevated carbon dioxide concentrations,responses varied from no significant effect(Docherty et al., 1997; Salt et al., 1996; Butler etal., 1996) to increased settling times (Smith, 1996)and increased fecundities (Awmack et al., 1996).

There is little direct evidence that chemical deter-rents in the leaf, which many plants have evolvedto resist pest or grazing attack, will be altered inplants growing under elevated carbon dioxide con-centration.

Arthropod species that are likely to be mostresponsive to the effects of climate change arethose that produce many generations in a singleseason (‘multivoltine’ arthropods). These include,for example, thrips, aphids and spider mites andrepresent the opportunistic insects and miteswhich, given a suitable food supply, will multiplyrapidly on plants. Many of these cannot survivecold winters as adults and so the first insects toappear in the spring hatch from over wintered eggs.However, the speed of development and thus thenumber of generations that will be produced, is adirect function of temperature. Two components ofclimate change will have a direct impact on thistype of pest.

First, milder winters will enable some species toover winter as adults rather than as eggs as at pre-sent. This will allow the pest to have a significanthead start given the appropriate food supply, sincethey will not need to go though an initial genera-tion prior to reproducing. Zhou et al. (1995)showed that warmer winter and spring tempera-tures will increase the over wintering survival ofaphid species in the UK and, in some cases,advance the appearance of winged adults by asmuch as a month. An increase in spring tempera-tures of 2°C will mean that cabbage root fly willalso become active a month earlier than at present(Collier et al., 1991). Mild winters will also favoursuch insects as green spruce aphid (Elatobium abi-etinum) which can feed and multiply throughoutthe year (AAIS, 1999).

Second, higher average temperatures will meanshorter intervals between generations.

Both factors will increase the potential for earlierpest attacks than at present, and for more seriousattack as a result of rapid population growth. Thiseffect will be exacerbated in the case of manysucking pests, as higher temperatures will beaccompanied by increased water stress, leading toincreased uptake of increasingly concentrated plantsap, as referred to above (Orson, 1999) in relationto red spider mite.

Insect population growth increases by 10-14 foldbetween generations, so that just one more genera-tion over a growing season can have a profoundeffect on population numbers. A good example ofan insect pest in this group is the cabbage aphid(Brevicoryne brassicae) which in a mild winterwill over winter in rural areas on oil seed rape. Thispest can then attack a wide range of garden brassi-cas (cabbage, cauliflower, Brussels sprout) in thespring and, since the population is initiated byadults, generations of invasive winged aphidsoccur much earlier than after a cold winter, whenpopulations are initiated from eggs. Work at theInstitute of Arable Research, Rothamsted hasshown that typically, aphid attacks occur approxi-mately two weeks earlier for every 1°C increase inaverage temperature. This has been confirmed byobservations made over the last twenty years, dur-ing which spring temperatures have increased.

Many aphid species in particular are of concern togardeners, not only as plant pests but as vectors(transmitters) for a number of serious virus dis-eases. Many of the impacts of climate change onpopulation dynamics of aphids will, therefore,have consequential effects on virus disease attack.Zhou et al. (1996) conclude that the severity ofaphid outbreaks in the UK will be increased underclimate change conditions and will lead to anincrease in the period of virus infestation furtherinto the growing season (see section 3.5.2 below)

Warmer autumns may lead also to greater numbersof aphid vectors later in the season and may givegreater problems to over wintering plants. Climatechange, whilst increasing the opportunities for over


52 Chapter 3

wintering of vegetables and bedding plants, forexample, may also be responsible for direct effectsof later surviving insect pests and their indirecteffect through the spread of virus diseases.

Water stress in plants results in greater nutrient con-centration in the sap, and thus in greater growthrates and fecundity in aphids feeding on the plants.However, predator levels would also be expected tobuild up in response to increased food supply. Oneof the most important natural controls of aphid pop-ulations is rainfall, as the impact of raindrops dis-lodges the aphids or damages their feeding parts(Doggett, 1863; van Emden, pers. comm.). Whetherlower summer rainfall will reduce the operation ofthis control mechanism, or less frequent but heavierrainfalls will increase it, is uncertain.

Higher temperatures may allow some pests that arelargely confined to the sheltered environment of theglasshouse to move out into the open. This is alreadyhappening with red spider mite (Tetranychusurticae). However, while extending the period ofpest activity, warmer conditions may also extend theperiod during which biological control agents canoperate. Currently the nematode used for biologicalcontrol of vine weevil is not as effective outside as itis under glass, because soil temperatures are too low.Consistently higher outdoor temperatures may resultin better control, and perhaps even control by natu-rally accumulating nematode populations.

There is considerable evidence of changes in thenatural distribution of birds and butterflies with ageneral northward movement of species as temper-atures rise (Fox et al., 2001). This leads to loss ofspecies favoured by low temperatures, but inwardmigration of species from mainland Europe. Thissame phenomenon can be expected for insect andother pests, especially if the spread of exoticspecies is exacerbated by international transport ofcrops or individual plants.

Termites have already been found in Cornwall.Japanese beetle, spruce moth, and stem-borers ofbirch, Sorbus and other woody species make thecultivation of many garden plants difficult in theUSA where summer temperatures are substantiallyhigher than in Britain. Cameraria ohridella, the

moth which has devastated horse chestnuts in Spain(Bedoya, pers. comm.) and has recently spread intonorthern Italy (Garibaldi, pers. comm.), is now to befound in Belgium and, as of July 2002, inWimbledon (Prior, pers. comm.). A small colony ofthe voracious Asian gipsy moth already present inEpping Forest could spread if a series of dry springsallows it to gain a firm foothold (Gruner, 2000). Acomprehensive review of the effects of climatechange on insect pests is given by Canon (1998).

Among larger organisms, roe deer and grey squir-rel are both favoured by warm winters, and areboth likely to survive in increasing numbers as aresult of climate change. The grey squirrel has aparticular predilection for the bark of beech trees.The combined effects of increasing squirrel dam-age and direct impacts of climate change onattempts to retain or regenerate plantings of beechin gardens, parks and woodlands will pose majorchallenges in the future. Attempts to regeneratebeech woodland on Box Hill, Surrey in the pastfifty years have failed because of the total destruc-tion of the young trees by bark stripping (Piggott,1988). Grey squirrels have also caused widespreaddamage to sycamore and Japanese maples inNational Trust and other gardens, including theUniversity campus at Reading.

3.5.2 CLIMATE CHANGE AND DISEASES

As with pests, with diseases it is very difficult todetermine the exact impact of climate change onthe development of a particular disease because ofthe complexity of the relationship between pest,host plant and environment. It is also very difficultto differentiate between the effects of climatechange and the effects of increased internationaltravel as causes of increased attack. The generalimpact of climate change on diseases, however, canbe summarised as follows:

• wetter, warmer winters will favour diseasessuch as phytophthora that need water tospread;

• drier, warmer summers will favour diseasesuch as powdery mildew that can spread indry conditions;


53Chapter 3

• warmer conditions will allow diseases thatcannot establish under current climatic condi-tions in the UK to survive and establish, butwill cause the decline of existing diseasesunable to adapt to higher temperatures.

For most diseases, the important epidemiologicalimplications of climate change stem from warmerwinters, which will result in greater availability ofsurviving host material, greater survival of overwin-tering inoculum (spores or growing fungi), and thusa more rapid onset and spread of disease as thegrowing season proper begins. However, this couldbe offset by greater dryness in the summer.Perennial plants growing on the margins of their cli-matic tolerance are likely to be subjected to longterm, chronic stresses, resulting in a decline in planthealth and increased susceptibility to diseases.

Faster rates of reproductive development underhigher temperature conditions may well alsoincrease the rate of disease spread, as will the morerapid increase of insect vectors of disease. Forexample, higher aphid vector populations generallymean that virus diseases will increase (Zhou et al.,1995). Moreover, earlier attacks by aphid vectorswill lead to virus disease infection occurring earli-er in the development of plants. Generally, virusdiseases are always more serious when plants areinfected at an early stage of development. As aplant becomes older it has a greater inherent toler-ance of attack by virus diseases. Thus, the effects ofclimate change on virus diseases may well be moreserious plant infection.

This may have implications for the Scottish seedpotato industry. Some of the important potato virus-es are aphid vectored. By raising the seed in Scotland,growers are currently able to produce virus free stockbecause the low temperatures inhibit the develop-ment sufficiently to allow the potatoes to producetubers before the virus gets into them. Warmer, earli-er seasons in Scotland could eliminate that advan-tage. The same problems arise with raspberry andstrawberry, threatening the supply of virus free plantsfor gardens as well as for commercial growers.

Milder winters will also favour winter activity of awide range of bark and wood invading fungi which

are able to overcome the defences of trees duringdormancy, and are likely to result in increasingseverity of fungi which are currently limited bylow temperatures (such as Phytophthora cinnamo-mi), while summer drought will favour diseasessuch as sooty bark disease of sycamore(Cryptostroma corticale) which attack droughtstressed trees (AAIS, 1999). In a survey of horti-cultural crop responses to the hot, dry summer of1995, Orson (1999) found reduced disease levelsoverall, but higher incidence of powdery mildews,rusts and Fusarium diseases.

The establishment of exotic diseases in the UKunder conditions of climate change will providegreat challenges for quarantine research in the UK.A number of diseases which have a wide host rangeand which are currently ubiquitous in the tropicsand subtropics could prove major threats. Forexample, Athelia (Corticium) rolfsii is a widespread disease in warmer climates where it infectsa number of species including cotton, tomatoes andgroundnuts. It is a soil borne disease, which isequivalent to Sclerotinia sclerotiorum and, giventhe opportunity, will attack a wide range of gardenplants. A. rofsii is known to have been accidentallyintroduced into the UK once (where it was a prob-lem during the season of its introduction), but diedout. Under climate change conditions, the likeli-hood is that, if reintroduced, it would be able to sur-vive the winter. Quite clearly, there is a threat wait-ing to happen.

Climate change conditions will offer an opportuni-ty to introduce new plants into the UK, but eachwill have its own suite of pathogens. This will putgreat pressure on plant health authorities to ensurethat future quarantine measures are effective. Thereare already instances where lack of plant quaran-tine measures have lead to the introduction of dis-eases, because quarantine is usually imposed onlywhen plants are of economic importance. For thisreason, no quarantine measures were approvedwhen mature olive trees were imported into theUK, with the result that olive scab (Spilocaeaoleagina), a major disease of olive inMediterranean areas, was introduced. The problemis already here, therefore, if olives become a morecommon feature in the UK in the future.


54 Chapter 3

Other problems are likely to occur if plant healthauthorities concentrate their quarantine efforts onlyon plants of economic importance and not on plantsof garden interest. Camellia petal blight (Ciboriniacamelliae), a disease of the petals at flowering time,originated in Japan but has spread around theworld. It poses a threat to UK gardens as it isspreading up through Europe. The plant healthauthorities did not think it necessary to imposestrict quarantine measures on imported Camelliasbut posters were distributed alerting the nurseryindustry to the chances of this disease. Withinmonths of the issue of the posters, the disease wasfound in Cornwall and Devon, where it had proba-bly already been present for several years. Undercurrent conditions the disease is not devastating,presumably because the fungus is at its northernlimit. However, under conditions of climate changeits effects could be more severe and the potential forits spread to the rest of the UK much greater. In2002 there has in fact been a significant spreadeastwards to the south and east of England.

Many diseases are spread by insect vectors andthe effect of climate change on the biology of vec-tors may also effect the spread of disease. Forexample, Oak Wilt (Ceratocystis fagacearum) is aNorth American disease spread by insect vectorswhich are not found in Britain. Their distributionand activity and the number of generations eachyear are limited by climate in North America.Under conditions of climate change, the environ-ment of the UK could become more favourablefor these insects, increasing the potential threatfrom the disease.

Aphids and thrips are vectors for a number of dis-eases. The potential impact of climate change onvirus transmission in Scottish seed potatoes, forexample, has already been discussed. Westernflower thrips, a pest that was introduced a fewyears ago from Holland, is a major vector for toma-to spotted wilt virus and impatiens necrotic spotvirus. Currently this insect is mainly a problemunder greenhouse conditions, but it could moveoutside under climate change conditions and couldthen attack a range of vegetables. This could beaccompanied by a large increase in other virusesspread by thrips.

Popular garden species such as yew (Taxus bacca-ta) and box (Buxus sempervirens) have recentlybeen affected by serious diseases. Box, a majorfeature of many formal gardens and shrubberies, isunder increasing threat by box blight(Cylindrocladium buxicola) (Henricot et al., 2000).This is a disease which is dependent on watersplash for its spread and will therefore be favouredby wetter, warmer conditions. Phytophthora rootrot of Yew is a very destructive disease of hedgescaused by Phytophthora cinnamomi. This is alsolikely to be more severe under wetter warmer win-ters. Brasier and Scott (1994) suggest that, underclimate change conditions, the fungus will causemore severe damage in the regions where it is cur-rently present and will tend to spread northwardsand eastwards. They also predict that the host rangeof the fungus will increase if it spreads into areaswhere it is not currently present. Phytophthora isan important disease associated with poordrainage, as its spores are spread by water move-ment through the soil and host plant resistance isdecreased in anaerobic conditions. Increased rain-fall and higher winter temperatures, allowing thefungus to develop during periods of waterlogging,are likely to lead to increased incidence of the dis-ease and symptoms will be exacerbated whenplants suffering root loss caused by Phytophthoraare further stressed by summer drought.

Holly is affected by holly leaf blight caused byPhytophthora ilicis, a disease that was first describedin the USA but came to Britain in the 1980s.Recently, in the late 1990s, there have been upsurgesof the disease for reasons that are unclear, but may beassociated with wetter, warmer winters. It has causedserious damage to hollies at the National Trust’s gar-den at Nymans, Sussex, in 2001/02.

Lawsons Cypress is attacked by Phytophthora lat-eralis, a disease which is native to the northernUnited States but has spread to other countries,and, for example, is present in France. It is notthought to exist in Britain but, as with allPhytophthoras, it is favoured by warmer wetterwinters, so that the threat from other disease willbe greater under conditions of climate change andexacerbated by the free movement of plantsthroughout the European nursery industry. As with


55Chapter 3

yew, a combination of warmer and wetter winters,predisposing to infection, and of hotter, drier sum-mers increasing root stress will greatly increasepotential damage from these root pathogens.

Brasier (2000) has studied a new Phytophthora dis-ease affecting alder. This is an interesting potentialeffect of climate change since it is believed thatthis new disease has arisen as a result of thehybridisation between two Phytophthora specieswhich may have come into contact as a result offlooding. Although the individual parent species donot attack alder, the hybrid does. This clearly intro-duces new possibilities for the threat byPhytophthora diseases.

On lawns one might expect an increase in redthread (Laetisaria (formerly Corticium) fuci-formis) which has a high optimum temperatureand perhaps a reduction in the incidence of snowmould (Monographella nivalis), which thrives atlower temperatures (Dawson, 1977). However, ifclimate change results in longer periods when thetemperature is at 5°C rather than zero, for exam-ple, snow mould could increase rather thandecrease in severity.

Reasons for a widespread decline in tree health inBritain are not well understood, but stress causedby water shortage in the summer (as a result ofdecreasing precipitation, increased evaporationand higher rates of extraction), or flooding in thewinter, is probably already contributing to acceler-ated losses of trees and shrubs to Armillaria,Phytophthora and other pathogens.

3.5.3 CLIMATE CHANGE AND WEEDS

As weeds are simply plants in the wrong place, theeffect of climate change on weeds will be as forplants in general. Higher carbon dioxide levels,higher winter temperatures and perhaps greaterwater availability in early spring will favour earliergermination and growth of weeds. This will espe-cially be the case for highly competitive annualweeds which demonstrate the high sink strengthidentified by Poorter (1993) (see section 3.2.1).Many annuals will be able to germinate and growthrough the winter and to set seed before the onsetof summer drought. The result will be a need forincreased garden maintenance.

Drier summer conditions may reduce weed growth,but will also reduce the effectiveness of glyphosateand hormone weedkillers such as 2-4D and MCPAwhich work best when the treated plants are inactive growth. It will be more necessary to ensurethat herbicide spraying is carried out earlier in theyear, with a narrower window between the weedachieving sufficient leaf cover for chemical uptakeand the onset of dry conditions, and the prospect ofhaving to spray at a time when maintenancedemands are at a peak.

Bracken (Pteridium aquilinum) is expected tobenefit significantly from climate change (Farrarand Vaze, 2000; Pakeman and Marrs, 1996).Intolerant of exposure or shade, it will be able to

Plants will be affected by climate changes

indirectly, by the effects of these changes

on the virulence of pest and disease attack,

as well as directly. Pests and diseases are likely to

be more troublesome as a result of climate change,

because higher temperatures will allow

increased survival and activity in winter and more

rapid increase in spring. Some pests (mites, aphids)

and diseases (powdery mildews, rusts) will be

favoured by hot, dry summers. Leaf eating pests

may be slightly disadvantaged by the higher

carbohydrate status (and therefore reduced

protein content) of host plants growing in the higher

concentrations of carbon dioxide, and higher light

levels associated with climate change. Warmer but

wetter winters will favour root rots of various kinds,

especially phytophthora.

Hotter summers will encourage the spread of new

pests from warmer parts of Europe as well as the

northward migration of pests found commonly only

in the south of England. With increasing human

mobility from other parts of the world, more care

will be needed to monitor pests and to develop

quarantine procedures to prevent the import of new

pests and diseases.


56 Chapter 3

colonise at higher altitudes as the temperatureincreases and to penetrate further into thin wood-land as light levels increase.

Garden escapes which come from warmer and/ordrier climates will also be favoured by climatechange. The classic example is Oxford Ragwort(Senecio squalidus), a native of Sicily and southernItaly which is thought to have escaped from theOxford Botanic Garden (Clapham et al., 1962). Itwas first noticed on walls in Oxford and subse-quently spread through much of lowland Britain,its spread accelerated in the 19th century by thedevelopment of the railway system. The ballast onwhich the rails were laid provided the very dryconditions needed by the plant and the air currentscaused by the trains helped to distribute the windborne seeds along the track. In the late 20th andearly 21st centuries the road system has had thesame effect: dry embankments and expanses ofgravel or coarse stone along hard shoulders and ontraffic islands - and the wind currents created byspeeding juggernauts - have created enormouslyexpanded areas suited to the weed. Hotter and driersummers have also undoubtedly assisted in theRagwort’s success.

Rhododendron ponticum, Himalayan balsam (or‘policeman’s helmet’, Impatiens glandulifera)and Japanese knotweed (Fallopia japonica) havealso invaded large areas of the UK, the north andwest in the first two cases (Farrar and Vaze,2000), the south in the case of Japaneseknotweed. In this last case, human activity inspreading the plant, initially deliberately as a gar-den ornamental and more recently accidentally intransporting knotweed-contaminated soil, hasbeen a major factor in its invasion, but all threehave been, and will increasingly be, favoured byclimate change. The spread of these plants erodesthe biodiversity of habitats and the quality ofparks and gardens.

Vines (2002), refers to the “tender” and very orna-mental perennial Hedychium gardnerianum, asnow growing out of doors in Durham in a shelteredgarden. Hedychium is becoming a major pest inNew Zealand in a climate not dissimilar to that ofsouthern England.

There are very well documented cases of damagingspread of introduced plants in the hotter parts ofthe world: water hyacinth blocking African rivers,prickly pear in the Australian desert and Kudzuvine engulfing areas of the southern United Statesfor example. Phormium tenax has naturalised on StHelena to the exclusion of large areas of naturalvegetation (though mainly because of its toleranceof the heavy grazing pressures imposed by feralgoats), and Gunnera manicata is becoming natu-ralised in southern Ireland. As the UK climate con-tinues to heat up, it will be necessary to monitorcarefully the potential and observed threats ofinvading exotic plants.

It is important, though, to maintain a sense of pro-portion when dealing with this potential problem.To speak in terms of ‘alien invasions’ raises thespectre of a triffid conquest. The first 150 years ofincreasing temperatures in the UK have not result-ed in a dramatic change in its vegetation cover,other than that attributable directly to habitatdestruction due to urbanisation and changes infarming practice (Bailey, 2000; Milne and Hartley,2001). Rhododendron ponticum (introduced in1763), Oxford ragwort (1794) and Japaneseknotweed (1886) have become serious weeds insome locations, but only in recent years (in part, atleast, because of changes in land use and manage-ment) and have not yet colonised to the exclusionof other species.

Clearly, the rate of temperature change, in partic-ular, is accelerating and it will be necessary tomonitor carefully the distribution and activity ofany potentially invasive species. Hossell et al.(2001) suggest that attention should be given toupdating the list of invasive species in Schedule 9of the Wildlife and Countryside Act to includespecies with the potential to become invasive.However, given a history of exotic plant introduc-tions to Britain extending over at least a thousandyears and an immensely rich and widely distribut-ed garden flora, there is as yet a conspicuousabsence of invasive species.

A review of the characteristics of potentially inva-sive plants and the potential causes and impacts in aUK context would be very helpful, not only in iden-


57Chapter 3

tifying the nature of any potential risk but equally inreducing the likelihood of any panic reaction whichmight pose an even greater risk to UK gardens, bylimiting the movement and use of exotic plants.

3.6 Climate change and symbiotic organisms

Many plants have symbiotic (mutually beneficial)relationships with other organisms. This is mostcommonly seen in the relationships between legu-minous plants (peas and beans for example) andnitrogen fixing bacteria, and between many conif-erous trees and mycorrhizal fungi. The microor-ganisms benefit from the photosynthates producedin the leaves of the trees and provide in return soilmineral nutrients which the tree roots are them-selves unable to acquire.

Although there has been limited research in thearea of climate change impacts on these symbioticrelationships, it appears that the exudation of solu-ble compounds from roots tends to be greater in

carbon dioxide enriched plants so that more carbonis available for mycorrhizal associations (Norby etal., 1986, 1987). Similarly, the greater allocation ofcarbon to the roots of carbon enriched legumesleads to increased nitrogen fixation as a result ofgreater nodule mass, although there is little evi-dence for any effect on specific nodule activity.This ability of mycorrhizae to extend theirexploitation of soil nutrients and to supply theirhost plant with additional nitrates could help to off-set some of the disadvantages associated withreduced summer precipitation and drier soils.

More research is needed to investigate the impactsof increasing temperatures and increasing waterstress on the mycorrhizae themselves and on thepotential uses of mycorrhizae to aid tree establish-ment and growth.

The impacts of climate change on weeds will be

the same as for plants in general. Increased carbon

dioxide levels will favour growth of competitive

annual weeds more than it will favour plants in

general. Higher winter temperatures and increased

water availability, where the latter does not result in

waterlogging, will allow overwinter growth of many

annuals, more rapid growth in spring and earlier

seeding as summers become hotter and drier.

Perennial weeds will grow more quickly and most will

flower earlier, if not controlled. Chemical weed

control, with glyphosate in particular, will be less

effective in hot, dry conditions.

A small number of introduced plants have become

serious nuisances in recent decades. The reasons for

the recent territorial expansion of plants present in

the UK for a century or more are not clear, but a

combination of climate change and changes in land

management are probable contributors to the spread.

Other garden species have the potential to become

weeds if climate change accelerates, so careful

monitoring will be needed.


59Chapter 4

Plants in natural and managed communitiesThe behaviour of individual plants to climatechange can be anticipated with some degree ofconfidence, because plants function in accordancewith well established principles of plant physiology(see Chapter 3). However, when plants grow inclose association with other plants, the outcome ofany change in the environment will be subject tothe complex science of ecology. As a very simpleexample, use of fertiliser in 1970s public plantingschemes resulted in poor tree growth. The grassaround the tree was stimulated by the fertiliser andcompeted more strongly with the tree roots forwater, so tree growth suffered.

In examining the effects of climate change on plantsin gardens, it is therefore necessary to consider thecharacter of the garden and the extent to which theplants in it exist as individuals or as parts of a com-munity. Gardens vary greatly in this respect. Some‘wild’ gardens or ‘wild’ parts of gardens, and manyof the18th century parklands and woodlands, wereplanned and planted to be gardened and managed asnatural landscapes. Indeed, many landscape parks arenow valued and managed as much for their natureconservation value as for their aesthetic appeal.

At the other extreme, a garden may be a collectionof individual specimens, whether prizewinning del-phiniums, North American conifers, or old applevarieties. In such cases, management interventionis more intense with pruning, staking, spraying andother maintenance practices aimed at fostering thegrowth of the individual and protecting it fromexternal threats.

There are important differences between plants inthe natural environment and plants in the highlycultivated garden in relation to the impacts of cli-mate change. It is, therefore, necessary to consider,in each garden, the extent to which the particulargarden resembles a natural community or, in J CLoudon’s words, “a work of art and a scene of cul-tivation” (Loudon, 1838).

4.1 Plants in the natural environment

The survival of a plant in nature depends on its fineadjustment to many components of the habitat, andthe ability to exploit a niche more effectively thanany potential competitor. It is not a matter of how itwill respond to higher carbon dioxide levels, butwhether it will respond to higher carbon dioxidelevels more or less successfully than its neighbour.Very subtle changes in soil moisture levels, lightlevels (as nearby plants grow, for example), ornutrient levels can make the difference between aplant dominating its surroundings, or becomingeliminated. Increased susceptibility to, or preva-lence of, pests or diseases will also have long termimpacts on the success or failure of a species, andhence affect the composition of the plant commu-nity as a whole.

The static nature of a plant, being literally rooted tothe spot, is a challenge to its survival in a changingenvironment. In natural, as in managed forests,there is some evidence that trees are capable of sur-viving (Myking, 2000) and even benefiting from(Saxe et al., 2000) the higher mean temperaturesexperienced to date. However, the current rate ofchange in annual mean temperature is unprecedent-ed. As is already apparent in the Chiltern beech-woods, for example, some trees have adapted to 150years of moderate climate change, but continuingincreases in temperature and particularly the moretaxing extremes of temperature and drought, areresulting in widespread decline (see section 6.3).

In nature, plant species have responded to climatechange by physiological changes in the individualplant (fewer stomata for example), and by changesin distribution of the species as seeds are dispersedto more or less favourable sites and flourish or per-ish accordingly.

Experience in nature conservation suggests that dis-persal of many species will be too slow to respond


60 Chapter 4

to the changes anticipated in the climate changescenarios. Van de Geijn et al. (1998) equate a 1°Crise with a geographical shift of 150-200km, andCiesla (1995) with a 100km shift, or a 170m rise inaltitude. Given the 3-5°C rise in temperature antici-pated by the high emissions scenario by the 2080sfor the UK, a plant species would need to migrate 4-7km each year to stay in the ‘same’ climate, assum-ing of course that the terrain permitted this.

Harrison et al. (2001) have simulated the responseof 34 terrestrial plant species to climate changeusing the UKCIP98 climate scenarios (a previousgeneration of climate change scenarios that havesince been updated and replaced by the UKCIP02scenarios). Their results show that the distributionof some species such as Beech (Fagus sylvatica)and bog rosemary (Andromeda polifolia) declines(Figure 14), but the distributions of others, such ascross-leaved heath (Erica tetralix), remainsunchanged.

Another factor in the potential risk to natural plantcommunities to climate change, is the very limitedmobility of many plant species, and thus theirinability to migrate from one area to a morefavourable one if there are discontinuities in thelandscape because of habitat fragmentation. Aspecies may be able to spread northwards to escapeor exploit higher temperatures, but it will rarely beable to move from one mountain top to a higher oneacross a valley, or even from one damp hollow to anearby wetter one across dry ground, so it may beeliminated as the local or microclimate changes. Inmountainous areas of Europe, plant communitiesare moving to higher altitudes as temperaturesincrease (Ciesla (1995) estimates 1-4m higher eachyear in Austria), but local extinctions are anticipat-ed as plants are unable to relocate from a previous-ly favourable area to a new area, because of habitatdiscontinuity (Gottfried et al., 1999). Managementand selection – human intervention – will be need-ed if substantial components of natural ecosystemsare to be conserved.

4.2 PLANTS IN THE GARDEN ENVIRONMENT

In a garden, the ‘habitat’ is often extensively mod-ified prior to planting, by soil cultivation and fer-tiliser addition, for example. This modificationcontinues with mulching, feeding, irrigation, pestand disease control and, most importantly, removalof potentially competing plants, often throughoutthe life of a plant. Even in supposedly ‘wild’ gar-dens, grass is cut and bracken, brambles, sycamoreand other potentially dominant plants are con-trolled periodically. Competition seldom plays amajor part in determining the fate of individualgarden plants.

In gardens, plants are not necessarily required toflower (especially in the case of vegetables) or toset seed. Plants do not require pollinating insectsbecause, if an increase in numbers is required, seedcan be obtained from suppliers in warmer climatesor plants can be propagated vegetatively. Undernatural conditions, plants are especially vulnerableduring seed germination and early growth, whichis why a plant may produce tens of thousands of

When plants grow together in communities it is much

more difficult to anticipate the impacts of climate

change than when they grow as individuals or as a

crop of one plant type. Survival of a plant in a natural

community will depend on how much more or less

able it is to respond to climate change than its

competitors, rather than on its innate response.

Because plants are static, those in natural

communities, in particular, need to be closely adapt-

ed to their environment. Changes in that environment

are likely to threaten their survival. In natural

communities, plants will respond to higher

temperatures and reduced water availability by

migrating to cooler, wetter areas (north and west in

the UK) and/or to higher altitudes – by about 1-

200km, or 170m in altitude per degree Celsius. This

migration can only occur if there is a continuum of

suitable environment across which migration can

take place. Plants with patchy, localised distribution,

such as those in heaths, bogs or mountain tops,

could be threatened with extinction.


61Chapter 4

Figure 14a: Current and future distribution of beech (Fagus sylvatica) using the SPECIESv1 model and the UKCIP98 climatechange scenarios: (a) simulated current distribution (1961-90); (b) 2020s low scenario; (c) 2020s high scenario; (d) 2050s low sce-

nario; and (e) 2050s high scenario. Source: Harrison et al. (2001)

(a)

(b) (c)

(d) (e)


62 Chapter 4

Figure 14b: Current and future distribution of bog rosemary (Andromeda polifolia) using the SPECIESv1 model and theUKCIP98 climate change scenarios: (a) simulated current distribution (1961-90); (b) 2020s low scenario; (c) 2020s high scenario;

2050s low scenario; and (e) 2050s high scenario. Source: Harrison et al., 2001.

(a)

(b) (c)

(d) (e)


63Chapter 4

seeds in order to continue the survival of thespecies by establishing one successor. In the gar-den or nursery, seedlings and young plants aregrown in carefully controlled conditions, usingsterilised compost for example, and are onlyplanted in the open ground at a later, more robuststage of the life-cycle, into carefully preparedsites. It is unusual to see natural seeding of thegreat majority of our garden plants in gardens.

The effectiveness of all these cultivation practicesis clearly evident when one compares the verypoor native flora of the UK, estimated at 1500species, with its immensely rich garden flora of10000-15000 species (Royal HorticulturalSociety, 1992) or the 70000 taxa (species, sub-species, varieties and cultivars) listed in The RHSPlantfinder (Lord, 2002). The very wide geo-graphical distribution of many garden plantsthroughout England, Scotland and Wales, and theability of plants produced in Italian, German orDutch nurseries to acclimatise more or less suc-cessfully to British gardens, all testify to the abil-ity of plants to tolerate a much wider range of con-ditions in cultivation than they would normallyexperience in the natural environment.

Because of these advantages, garden plants arevery elastic in their response to environmentalconditions, and are likely to display a similarlyelastic response to climate change. However,some changes in the garden flora will undoubted-ly occur as a result of increasing temperaturesand, especially, changing patterns of precipita-tion. Where conservation of the existing plantpopulation of a garden is a high priority, forexample in historically important designed land-scapes or in gene banks of particular species,careful management will be required to accom-modate and adapt to changing conditions.

Two key factors influencing the tolerance of gardenplants to climate change will be their hardiness andtheir water requirements.

4.2.1 HARDINESS OF GARDEN PLANTS

Low temperature tolerance varies greatly amongstgarden plants. Some plants, for example dahliasand pelargoniums, will not survive even short peri-ods below freezing, while others will tolerate tem-peratures of -40°C or lower.

The United States Department of Agriculture(USDA) developed a system for defining planthardiness by dividing the United States into tenhardiness zones based on 6.25°C bands of averageannual extreme minimum temperatures (the lowesttemperature recorded in each year averaged over anumber of years). These bands extend from zone 1(eg, central Alaska) where the average annualextreme minimum temperature is below -46°C tozone 10 (eg, southern Florida) where the annualextreme minimum temperature is between -1°C to4°C. In the context of the large land mass of theUnited States, the zones run in more or less paral-lel east-west bands across the country, curvingsouthwards across the major mountain ranges andnorthwards as the moderating influence of theAtlantic and Pacific Oceans on winter tempera-tures is felt along the coasts.

The USDA system has also been adopted inEurope (Krussmann, 1984; Royal HorticulturalSociety, 1992; Schacht and Fessler, 1990) but inEurope as a whole, and especially in the UK, thezones are much less neatly defined (Figure 15).The patchy influence of changing altitude on tem-perature and the moderating influence of the seaare greater than the smooth influence of latitude isin Europe, compared with the United States.

In gardens, plants grow in very favourable

conditions. They are usually propagated in controlled

conditions, planted into carefully prepared ground

and protected to a greater or lesser degree from

pests and diseases and especially from competing

plants. In such conditions, the elasticity of response

to climate change is very much greater than in

nature. Two factors are particularly important in

determining climate change impacts: hardiness and

water availability.


64 Chapter 4

The relationship between hardiness zones and plantsurvival is also less clear, because milder wintertemperatures in the UK are not currently accompa-nied by high summer temperatures. In particular,the uncertain progression from autumn to winter tospring in the UK frequently leads to winter damageof young twigs which were not sufficiently maturedor ‘ripened’ in the previous summer, and to frostingof premature growth in spring. Plants such as sugarmaple (Acer saccharum), redbud (Cercis canaden-sis) and flowering dogwood (Cornus florida) toler-ate very low temperatures in the northern UnitedStates but are much more temperamental in the UK.

As a broad generalisation, however, much of theUK equates to zone 8 (-12°C to -7°C), the Scottishhighlands to zone 7 (-17°C to -12°C), and thesouthern and western coastal fringe to zone 9 (-7°Cto -1°C) (Figure 15). Significantly, zone 9 plants

include Abutilon vitifolium, Callistemon,Carpenteria californica and Phlomis fruticosa,which can now be found thriving in many gardensin the south west and in sheltered locations muchfarther north.

The 1-4.5°C rise in mean temperature anticipatedby the 2080s by the UKCIP02 scenarios equates toabout half a zone in the USDA scheme. Althoughmean annual temperature and mean annualextreme minimum temperature are very different,this comparison does serve to put the temperatureeffect of climate change in the UK into some kindof context. Most plants currently growing in UKgardens would be expected to survive a tempera-ture lift of this magnitude, especially as it has beenestimated that 85% of plants grown in UK gardensoriginate from areas with warmer climates(Thoday, pers. comm.).

Figure 15: The USDA hardiness zones applied to Europe. Source: Schacht and Fessler, 1990.

HARDINESS ZONETEMPERATURE RANGES

°F Zone °C

Below -50 1 below -45-50 to -40 2 -45 to -40-40 to -30 3 -40 to -34-30 to -20 4 -34 to -29-20 to -10 5 -29 to -23-10 to 0 6 -23 to -17

0 to 10 7 -17 to -1210 to 20 8 -12 to -720 to 30 9 -7 to -130 to 40 10 -1 to 5


65Chapter 4

Sukopp and Wurzel (2000) suggest that the heatisland effect of major cities also lends support tothe conclusion that many plants will adapt readilyto the increasing temperatures predicted by the cli-mate change scenarios. Native and exotic plantssurvive at least as well in cities, where the meantemperature is often 2-3°C warmer, than they do insurrounding rural areas.

The response to temperature is further complicatedbecause the variations in microclimate within asingle large garden are probably equivalent to thevariation occurring on the macro scale across atleast half of England. If there is a great desire togrow a particular plant, much of the change antici-pated in plant tolerance as a result of climatechange could, for several decades at least, be com-pensated for by moving plants to more sheltered ormore shaded situations for example, though ofcourse only if the plants are small enough and tol-erant enough to be moved. This might also apply inspecial circumstances to heritage gardens where aparticular plant needs to be conserved, but it wouldnot apply, of course, if the particular plant had anhistorically determined particular place.

4.2.2 WATER AVAILABILITY AND GARDEN PLANTS

Although higher temperatures anticipated by climatechange scenarios are, in themselves, unlikely to havea major impact on the garden flora, the relationshipbetween temperature and water availability will be ofcritical importance to survival of many plants. If theexpected climate changes were limited to highertemperatures and drier summers, it would not be atall difficult to find plants tolerant of the climate con-ditions suggested by the UKCIP02 scenarios for the21st century. A rule of thumb of looking for parallelsabout 150km further south for each °C could be used(see Ciesla, 1995; van de Geijn, 1998).

However, most of the plants used in ‘dry’ or ‘water-wise’ or ‘xeriscape’ gardening are intolerant of thewinter wet, and increased winter rainfall is a com-ponent of all the UKCIP02 scenarios. By the 2080swinter precipitation is likely to increase by 10-30%across the UK with regional variations. Butincreased winter precipitation will not necessarilylead to wetter soils. This is because the warmer,drier summers and autumns expected could sub-stantially reduce soil moisture content before theonset of winter, particularly in the south and eastwhere soils may become some 20-50% drier by the2080s. By the 2080s, winter soil moisture contentsare expected to increase by around 4% in parts ofWales and south west England, and by 4-10% inScotland, while parts of south east and north eastEngland could see a 10% reduction in winter soilmoisture. Annual soil moisture content is expectedto decrease by 10-20% across the UK by the 2080swith regional variations. The impacts of increasingwinter rainfall may, therefore, not be as serious asthe figures of precipitation change imply.

The local setting of the garden will be important indetermining the impact of hydrological changes.Clearly, only coastal gardens or those in low-lyingareas near the coast will be affected by increases insea level, although gardens farther inland may suf-fer from increasing salt spray damage. Low-lyingareas inland, such as the Bedfordshire Plain, maysuffer from increasing flood risks in periods ofheavy rain. Most larger gardens from the early 18thcentury onwards were consciously sited on emi-nences for the prospect which that afforded

A scale of hardiness developed initially in the

United States, from zone 1 (Alaska) to zone 10

(Florida), has also been adopted in Europe.

On this scale, most of the UK falls into zone 8,

with the Scottish highlands falling into zone 7,

and the southern and western coastal fringe to zone

9. The average annual temperature change

anticipated by the UKCIP02 low and high emissions

scenarios by the 2080s is equivalent to about

half a zone.

Hardiness and satisfactory growth and flowering are

more complex than the ability to withstand a

particular minimum temperature. Some North

American plants, in particular, are tolerant of low

temperatures, but will not grow well in Britain

because of cool summers and an erratic transition

from autumn to winter to spring. Climate change

will favour the growth of some of these plants and

will encourage good autumn colour in these and

many other plants.


66 Chapter 4

(Bisgrove 1978), and so should be at lower risk ofinland flooding as winter rainfall increases, thoughthey will be at risk from summer drought.

Soil type will also be important. On light, sandysoils, increased winter precipitation is likely to beadvantageous, replenishing the water resourcesdepleted in hotter and drier summers. There will,though, be some increased leaching of nutrientsfrom the soil. On heavier soils and in low-lyingareas (the two situations are often linked, becauseclay deposits occur on flood plains and old lakebeds), higher winter rainfall could lead to anincreased risk of waterlogging. Improved soildrainage and careful positioning of plants (perhapsin raised beds) may assist in countering wetter win-ters but the effects of long periods of higher humid-ity, surface wetness and low light conditions willnot be so easily remedied. Plants in raised beds willalso be more susceptible to summer drought.

Another important factor in relation to increasedwinter rainfall is its intensity. A greater proportionof winter rain is expected to fall in more intensedownpours. If the soil is capable of absorbing largeamounts of water quickly, the garden will benefitfully from the higher rainfalls anticipated by cli-mate change scenarios. If it is not, water will runoff across the surface, causing erosion and possi-bly flooding, and it will be lost as a resource to thegarden. Dawson et al. (2001) (using the UKCIP98scenarios) suggest that by the 2050s run-off couldincrease by up to 20% in the winter and decreaseby as much as 20% in the summer, with the largestchanges in the south and east. Good soil hus-bandry will play a very important role in allowingplants and gardens to adapt to this aspect of cli-mate change.

Given the continued survival of traditional garden-ing skills, it will be possible for the garden plantsto survive fairly substantial changes in climate.Indeed, the lessons offered by long established gar-den maintenance techniques are being recognisedincreasingly as analogous to techniques whichcould be required in the wider aspects of landscapeand nature conservation, if fragile habitats such asrelict alpine meadows, lowland heaths and fens areto survive and adapt to climate change.

4.2.3 CLIMATE AVERAGES VERSUS WEATHER

EXTREMES

Although plants may survive significant changes inaverage temperature and average precipitation rates,it is important to realise that climate change will notcome as a smooth gradient of change, but as theaccumulation of a very large number of fluctuationsabout a mean. Weather will remain variable infuture, and extreme conditions will occur. Thesewould have more immediate impact on the gardenthan average changes in climate would. A week-long heat wave, a night of severe frost, drought,floods and very high winds, for example, will stressmost garden plants and kill the more sensitive ones.

As part of this study, fourteen garden managerswere asked to cite any examples of extreme weath-er events in the past five years that had affected theirgardens. All ten respondents listed events rangingfrom heavy or prolonged rain (7 responses), throughfrosts and prolonged cold spells (5), high winds (5),floods (2), drought (2), variable rainfall (1), mildwinters(1), very warm springs (1), unusuallyhigh/low temperatures (1) and deep snow (1).

Eight respondents thought that extreme weatherevents had become more frequent during theircareers (two of these adding the qualification,“without doubt”). Another thought that extremeweather events were now so common that they nolonger justified the term ‘unusual’. These responsesdo not constitute hard evidence for the existence ofclimate change: not all of the phenomena listed are

Winter precipitation is expected to increase across

the UK. However, increased precipitation will not

always lead to increased soil moisture content. By

the 2080s, slight increases in winter soil moisture

content are expected in south west England, Wales

and Scotland, but annual soil moisture content may

decrease by 10%-20%, with regional variations.

Rainfall intensity is likely to increase in winter,

increasing the risk of flooding. It may be difficult to

find plants that will tolerate hotter drier summers but

also survive wetter winter conditions, particularly in

gardens with heavy soils.


67Chapter 4

Climate change – long term changes in temperature

and precipitation, for example – is the result of

averaging widely variable daily, monthly and

annual weather conditions. Short term weather

events will have more immediate impact on the

garden than will long term changes in climate.

Plants in gardens will be less vulnerable to climate

change than plants in natural habitats, because of

the management they receive. However, climate

change will have significant impacts on garden

plants and additional management inputs will be

required to reduce adverse impacts.

necessarily associated with climate change and therecollections might well be biased by the freshnessof recent events. However, they do offer some indi-cation of recent climatic impacts on gardens and, byanalogy, illustrate the effects and reactions whichmany of the components of climate change antici-pated by the UKCIP scenarios are likely to create.


69Chapter 5

Garden character and responses to climate changeThe magnitude of climatic changes to which a gar-den is likely to be subject, will depend on itsregional and local setting. Gardens in the north andwest of the UK may be more vulnerable to flood-ing than those in the south and east. Gardens onhilltops are likely to suffer more from drought,while those in valleys or on flood plains will besusceptible to flooding. Only gardens on the coastor near sea level will feel the impacts of increasesin mean sea level.

The significance, or impact, of those climaticchanges will depend very much on the particularcharacteristics of the garden, on the ‘genius of theplace’. The enormous diversity of gardens, and thereasons for this, has been outlined briefly in section1.1. Within this great diversity though, and accept-ing that every categorisation has its misfits, it ispossible to distinguish two main types of gardens:the small, private or domestic garden and the her-itage garden. There are, of course, also many largeprivate gardens. These have some of the character-istics of domestic gardens (a considerable degreeof freedom to determine and modify the form andcontents of the garden) and some of heritage gar-dens (availability of machinery, and usually trainedstaff to carry out large scale operations). Theimpacts of climate change on domestic and her-itage gardens are examined in this chapter.

5.1 The domestic garden

In the domestic garden, the design and content ofthe garden result from a complex interplay betweenwhat is available and what is considered desirable.The garden layout often evolves and may undergoradical changes from time to time. Change – evencomplete change on occasions – is usually accept-able and will often be considered desirable.

Supply and demand interact in determining theform and contents of the domestic garden, but incomplex ways. Such is the contrariness of the

archetypal keen British gardener, that many willseek to grow some plants because they are difficultor not widely available and will reject other plantsbecause they are easy and to be found in every gar-den centre. In cases where the garden is seen as aroom outside rather than as a plot for cultivation,the gardener may be more inclined to follow fash-ion and to accept what is portrayed on televisionand readily available at the garden centre.

In both of these types of gardens, climate changewill influence the ease of cultivation of some plantsand the perception of what gardens are for, but itwill be only one of many social, cultural, econom-ic and environmental influences determining theprogress of garden fashions. Cultivation of the cur-rently popular cannas, tree ferns and other tropicallooking plants may be made easier by climatewarming, but it is very unlikely that climate changeis the primary reason for their popularity.

For keen gardeners and for advocates of an outdoorlifestyle garden, climate change offers excitingopportunities and few threats. The main strategyfor dealing with problems in the private garden hasalways been avoidance of the problem. If one plantfails for any reason, it will cease to be used by allbut the most determined enthusiast, and morerobust alternatives will be adopted; the range ofplants available is so wide that it will not usually bedifficult to find alternatives. Difficulties in thegrowing of particular plants, as a result of climatechange or for any other reason, will be more or lessindefinitely circumvented by changing to a differ-ent plant palette.

Keen gardeners have always enjoyed the challengeof growing marginally hardy plants, seeking shel-tered corners of the garden and using a variety ofprotective covers in winter to increase the chancesof success. Their gardens represent the limit ofwhat is possible in that location. Climate changeshould allow such gardeners to succeed more fre-


70 Chapter 5

quently in growing plants which previously couldonly be grown in warmer areas. As successesincrease, so will the demand. Sought after plantsmay cease to be the province of specialist nurseries,and may become available in garden centres andadopted by the wider gardening fraternity.

Challenges will arise for the more conservativegardener as a result of climate change. A luxuriantherbaceous border and an immaculate green lawnwill be much harder to achieve in a hotter and drierclimate. However, for those who insist on retainingsuch features, the expenditure in time and money towater plants, for example, and to mulch and feed,could be small in absolute terms because of thesmall size of the garden.

5.2 The heritage garden

There are more than 3000 gardens in Britain regu-larly open to the public. Of these, over 1530 areregistered for their special historic importance(English Heritage, 1998). The gardens range fromsmall sites, like Barbara Hepworth’s home in StIves, to great landscaped gardens, like Stourhead,and from privately owned gardens to public parksand properties held in trust. The register includesmany other types of designed landscapes and gar-dens, like allotments, town squares and cemeteries.

A number of gardens and parks, like the magnifi-cent water gardens at Studley Royal along withFountains Abbey, are of such international impor-tance they have been designated World HeritageSites. Together, the UK’s garden heritage reflectsthe evolution of garden design spanning the last500 years, and illustrates the diversity of designedlandscapes and gardens and plant collections, andtheir historic interest.

Gardens may be historically significant becauseof their design, their planting or their associations,or a combination of these. The garden at AudleyEnd (Essex) for example, has a Victorian parterre,set in a Capability Brown landscape. The gardenat Killerton (Devon) contains many plants intro-duced from Japan and elsewhere in the 19th cen-tury by James Veitch. The gardens at Down Houseand Chartwell (both in Kent) were developed byCharles Darwin and Winston Churchill respec-tively. Neither of these Kent gardens is outstand-ing in its design or planting, but each sheds lighton the life of an historically important person.Gardens important for their plant collectionsalone are increasingly being acknowledged fortheir aesthetic, cultural, botanical and historicalsignificance.

5.2.1 CONSERVING THE HERITAGE GARDEN

Heritage is a fragile and non-renewable resource(Farrar and Vaze, 2000), and the cultural heritage– especially heritage gardens – is disproportion-ately sensitive to change (Shackley and Wood,1998) because it necessarily involves long timespans, during which extreme climatic events arelikely to occur.

The fragility and uniqueness of the historic envi-ronment underpins the Government’s own policy(DCMS and DTLR, 2001) on protecting and sus-taining historic buildings, monuments, gardens andlandscapes. In A Force for Our Future (DCMS andDTLR, 2001) the Government says: “If we fail toprotect and sustain it [the heritage environment] werisk losing permanently not just the fabric itself,but the history of which it is the visible expression.It is therefore essential that decisions taken at alllevels – national, regional and local – have regard

The domestic garden results from personal whim.

It often changes from year to year as new ideas

are tried. In many gardens the challenge of growing

difficult plants is part of the excitement of

gardening and the effort required to meet the

challenge is focused on a small area.

For the domestic gardener climate change poses

few problems and offers several opportunities.

Lawns will be more difficult to maintain,

but irrigation will often be practical, given their small

scale. Plants needing cool, moist conditions

could be moved to deeper shade. With warmer

weather, climate change offers opportunities of

using the garden more often and of growing a whole

new range of plants.


71Chapter 5

to any potential impact on the physical remains ofthe past”. The aim of conserving historic parks andgardens is to protect and maintain these land-scapes, and the investment of resources and theskills that went into their creation over the cen-turies. The conservation approach is as diverse asthe range of gardens, and individual to each one.

As well as sustaining the authenticity of the his-toric design, garden conservation seeks to sustainthe character, qualities, traditions and the plantmaterial of individual gardens. Many gardens havea long history of development and evolution,sometimes with different phases overlaying eachother. The historic interest may relate to the design-er, design ideas or the progression of designs, itsperiod or rarity, associations with notable events orpeople, or the botanical interest of plants or plantcollections, or a combination of all these factors.

English Heritage’s criteria for gardens of specialhistoric interest are:

• sites with a main phase of developmentbefore 1750 where at least a proportion of thelayout of this date is still evident, even per-haps only as an earthwork;

• sites with a main phase of development laidout between 1750 and 1820 where enough ofthis landscaping survives to reflect the origi-nal design;

• sites with a main phase of developmentbetween 1820 and 1880 which is of impor-tance and survives intact or relatively intact;

• sites with a main phase of developmentbetween 1880 and 1939 where this of highimportance and survives intact;

• sites with a main phase of development laidout postwar, but more than 30 years ago,where the work is of exceptional importance;

• sites which were influential in the develop-ment of taste whether through reputation orreferences in literature;

• sites which are early or representative exam-ples of a style of layout, or a type of site, orthe work of a designer (amateur or profes-sional) of national importance;

• sites having an association with significantpersons or historical events;

• sites with strong group value (EnglishHeritage, 1998).

In garden conservation, significance is determinedby establishing the past history of a garden and itsdesign influences, what survives today and howthe garden has developed since first begun.Garden designs, whether formal or informal, relyon a precise relationship between structural fea-tures like avenues, hedges and groups of trees, andopen spaces like lakes and lawns. Flower displaysand plant collections add a rich decorative layer tothe landscape designs. The original planting of thegarden, the choice of trees, shrubs and flowerswould have been governed by species availability,the garden’s location and geology, and the skillsand interests of the designer, owners and theirhead gardeners. In all cases, a garden’s signifi-cance, historical precedent and traditions shapethe policies for its conservation and the plantsgrown, and where they should be grown. Gardensmay also be of interest for their artistry or horti-cultural styles, plant collections and scientific col-lections. The great tree collections of the 19th cen-tury, for example at Killerton (Devon), Westonbirt(Gloucestershire) and Sheffield Park (Sussex)were not simply collections but were arranged foraesthetic effect. Features may be of architectural,archaeological or wildlife importance too, andgreat educational value. These interests, and othernew developments such as opening the gardens tovisitors, also need to be embraced in planning thegarden’s future management.

5.2.2 CLIMATE CHANGE AND HERITAGE GARDENS

Climate change potentially poses an escalatingrange of threats for heritage gardens, from theimpact on choice of plants grown, through to thelong term sustainability of historic designs due tochanging environmental conditions. The long term


72 Chapter 5

cumulative impact of repeated storm damage,drought, pest and diseases, flooding, lake siltation,sea level rises and so forth is likely to be of great-est significance. Within the next 50 years, somegarden features may become vulnerable, and a fewgardens may be at risk of complete destruction as aresult of climate change, despite the best efforts tocounteract its effects on a local level. This will beparticularly the case where plants and plantschemes are close to their ecological and physio-logical limits, for example, moisture loving herba-ceous borders grown on the thin gravely soils ofthe Thames and Chilterns, or ferneries in locationswhere drought and exposure might be a regularfeature in future. However, the majority of plantspecies that are currently cultivated in historic gar-dens could be maintained by the use of suitable soilmoisture conservation techniques and irrigation insummer, albeit at increasing cost.

Climate change is anticipated to be most markedin the south, and especially the south east, of theUK so clearly it will be most difficult to adapt gar-den management in these regions. In the northwest temperature increases are expected to be lessmarked and several of the parameters of seasonalprecipitation and cloudiness, for example, are notexpected to change beyond the limits of presentvariability. The climate change impacts will beless dramatic, but this is not to say that there willbe no concerns. Small changes in temperature andwater availability may make the differencebetween survival and loss of a rare plant or a dis-tinctive plant community, which may be an impor-tant factor in the uniqueness and significance of aparticular garden.

5.2.3 POTENTIAL EFFECTS OF LONGER TERM

EXTREME CLIMATE CHANGE

In the most extreme case, maintaining original, his-torically authentic trees species and varieties maynot be possible. Recent storms in the north haveresulted in the loss of many fine specimens andchampions of Douglas fir (Pseudotsuga menziesii)at Cragside, Northumberland. This damage mayhave been due as much to the fact that the treeshave grown to the limit determined by current galeincidence, as to increases in gales themselves.

Perversely, such events can have beneficial conse-quences. The 1987 and 1990 storms devastatedmany landscape gardens, by toppling the maturetrees and overgrown features, but it also createdopportunities to rejuvenate the gardens throughcarefully planned repair and restoration pro-grammes. A body of expertise and skills on historicpark and garden restoration has developed from thestorm damage work. However, the cost implica-tions of the storms for owners and the Governmentwere significant. English Heritage, alone, released£4 million of grants for storm damage relatedrepair programmes to historic parks and gardens(English Heritage, 1997).

Repeated and prolonged summer droughts, as areprojected especially for the south, could turn largeareas of parkland brown, threatening their aestheticappeal, grassland flora and fauna, grazing regimes,and agricultural income. Longer-lived tree speciesmay also be threatened, especially the beech (Fagussylvatica), a characteristic and important compo-nent of many southern gardens and parks, such asAshridge (Hertfordshire), on the thin chalky soils ofthe Chilterns. The serene verdant English parklandwe have become used to could disappear.

In the longer term, there may well be situations inwhich, because of climate change, a whole gardenis at risk of being destroyed by inundation as sealevels rise or by increasing incidents of flooding, ifthe frequency of heavy rainstorms increases. Insuch circumstances, ‘managed retreat’ will be theonly viable option, and it will only be possible to‘preserve’ the garden as an archaeological site,unless the garden is of such importance and theresources so freely available that defensive mea-sures, such as a wall or earth bund are possible. Thefinancial ramifications of such protection, especial-ly if a lengthy stretch of river bank is involved, as isthe case at Westbury Court on the River Severn(Gloucestershire), will be considerable; and thecontext of the historic garden will be altered. Socialand economic considerations will also have to beconsidered, with some gardens in flood plains pos-sibly being ‘sacrificed’ to ensure the protection ofupstream urban communities. Conversely, therewill be garden management issues like flood waterswashing soil and nutrients into ornamental lakes,


73Chapter 5

and creating algal blooms which can be damagingto human health or animal life, as well as beingunpleasant, difficult and expensive to resolve.

Historically important plant collections, with orig-inal introductions from known collectors for exam-ple, pose a particular challenge if climate changethreatens the survival of unique trees. Exact repeti-tion of a planting scheme may not be possible, andanyway can be argued to be seldom desirable for,for example, plant hygiene reasons. Where accu-rate replanting is desirable, such as with a formalavenue, and where the tree species or varieties canno longer be grown easily, it will be necessary torethink the conservation policies for these designelements of the garden.

Where a particular species is considered of thegreatest importance, it may be propagated andreplanted, if necessary in a more favourable posi-tion in the garden or even elsewhere in the UK(requiring coordination between owners and organ-isations). This is a sensible insurance against acci-dental loss, regardless of climate change. Wherethe particular specimen is important because of itsage, position or historical associations, good culti-vation (aeration of the soil, mulching, removal ofcompeting grass, irrigation) may extend its life.

In very general terms, architectural gardens thatrely on terraces, steps, balustrades and fountainsfor much of their drama will be less obviouslyaffected by climate change, than will plantsman’sgardens. Architectural garden features may sufferfrom settlement problems and cracking of wallsand steps on some soils in hotter and drier condi-tions. Such damage will usually be much moreexpensive to repair, but the costs could be coun-tered by reduced frost damage in winter. EnglishHeritage has commissioned UCL’s Centre forSustainable Heritage to develop a method forunderstanding and assessing climate change riskfor the historic environment, and to identify fur-ther areas of research.

5.2.4 BOTANIC GARDENS

Botanic gardens constitute a particularly interest-ing group, as most of the major botanic gardens in

the UK are simultaneously heritage landscapes andoutdoor laboratories, expanding our knowledge ofplants – as is the case with the Royal BotanicGardens, Kew. Many of the most notable plants areof great age and some have important historicalassociations, so climate change may have impactson the heritage aspects of the garden. Historically,however, botanic gardens have had to be intensive-ly managed in order to grow the widest possiblerange of plants in the living collections. The ethosand skills to deal with management challenges(including the potential challenges associated withclimate change) already exists within these gar-dens, although there may be additional costs suchas irrigation.

There will also be the advantage, for many botanicgardens, in being able to grow a wider range ofplant than hitherto. Several gardens use the mostsheltered corners of the garden (eg, narrow south-facing borders against the glasshouse range) – togrow the least hardy plants. With a general warm-ing of the climate, these plants may be able to moveinto the open garden, releasing their locations foreven more tender plants.

The advancement and dissemination of knowledgeis another important aspect of the botanic garden,so development of the collections is important tothe vitality of the garden. Climate change mayoffer new opportunities for the collections. Afterthe 1987 storm the Royal Botanic Gardens, Kewextended their tree collection to include morespecies better suited to increasing temperatures.The botanic garden at the University of Cambridgehas developed a dry garden as an example of‘waterwise’ gardening.

5.2.5 MANAGING HISTORIC PARKS AND GARDENS

THROUGH CLIMATE CHANGE

Garden conservation management plans are alreadyextensively used as practical tools to develop con-servation policies and to monitor ongoing manage-ment and maintenance. These plans could readily beused to appraise climate change impact risks, adjustconservation policies accordingly, and to measurecumulative effects. Managing the effects of climatechange could also impact on the use of surrounding


74 Chapter 5

land, its character and landscape setting for individ-ual gardens. Management agreements could be usedto develop cooperative approaches to larger scaleland management, to ameliorate climate changeimpact such as flood prevention schemes which, inturn, could bring benefits for individual gardensthrough the control of soil and nutrients beingwashed into lakes and streams. Specific measures,like silt traps for mirror lakes, will be an essentialand incur additional costs to build and maintain.

Britain’s historic gardens and parks were mostly

developed during a climate that itself is becoming

historic, therefore adaptation in future will be

unavoidable. If anything of the original effects and

layouts are to be conserved in perpetuity, changing

and/or more intensive maintenance regimes will have

to be introduced, accepting the cost implications

arising. Greater coordination will be required

between organisations and owners, to ensure the

conservation of the country’s valuable plant

collections for future generations. In all cases,

managing gardens is about managing natural

processes, including the human desire for change.

Extreme climate change will make such changes

common place and so challenge the way we think

about our historic gardens, what we expect of them

and what we mean by conservation.


75Chapter 6

Garden management in a changing climateIn this chapter, the components of a garden aretaken individually and the potential impacts of cli-mate change on each are considered in relation tothe domestic and the heritage garden. Some of thecosts associated with managing the impacts of cli-mate change in gardens are identified in Box 6.1 atthe end of this chapter.

6.1 Climate change impacts on soil

While much attention has been paid to changes inthe aerial environment in response to climatechange, our review suggests there has been muchless work on soil. Soil changes brought about byclimate change will have a very profound influenceon plant growth, and garden management and use.These issues are considered briefly here.

The total carbon content of world soils is nearlythree times that of above ground biomass, andtwice that of the atmosphere (Piccolo and Teshale,1998) (Figure 16).

Soils under arable cultivation have only 15-20% ofthe humus content of forest or grassland soils. Ona world scale, the conversion of forests and grass-lands to cultivated arable has had a major impact inreducing soil carbon content, contributing signifi-cantly to the increased carbon dioxide levels in theatmosphere, and hence to climate change. Theimportance of the soil as a sink for atmosphericcarbon (Lal, 2000; Pearce, 1998), the effect of cli-mate change on the soil itself (Norby and Jackson,2000) and the vulnerability of organic soils (Braggand Tallis, 2001) and wetlands (Winter, 2000), haveall been considered to some extent at the agricul-tural level and in natural ecosystems, but these top-ics apply equally in gardens.

Changes in atmospheric levels of carbon dioxidewill, in itself, not have a significant impact on soils,because carbon dioxide diffuses from the soil intothe atmosphere, rather than from atmosphere to soil.

The higher temperatures resulting from climatechange will be more important. As soil temperatureincreases, so does the rate of biological activity inthe soil. Higher temperatures typically result inincreased breakdown of soil organic matter, releas-ing available nitrogen in the process and thusincreasing plant growth (Medlyn et al., 2000).

However, higher air temperatures will substantiallyincrease evapo-transpiration by plants, therebyreducing soil moisture content. In experiments atCambridge, a 3°C increase in soil temperaturecaused a 30% increase in evapo-transpiration(Jeffery, 2001) and a 25% decrease in soil moisture(Harte et al., 1995). The UKCIP02 scenarios allpoint to higher soil moisture deficits over increas-ingly longer periods across the UK in the future. Inconditions of extreme drought, this will result incessation of organic matter breakdown. However,for most of the time, soils will retain some mois-ture, and the combination of increased aeration (airreplacing water in the larger soil pores) andincreased temperature will accelerate loss of soilcarbon by oxidation to carbon dioxide.

Loss of soil carbon, as organic material is brokendown, also results in release and mobilisation ofsoil nitrogen. Intermittent wetting and drying inearly autumn causes accumulation of soil nitrateand, when the soil is once again at field capacity,leaching of this nitrate is increased. Jeffery (2001)measured a 47% reduction in the volume ofdrainage from soils heated to 3°C above ambient,but a doubling of nitrate loss in drainage water.Imposition of a two month drought during theexperiments increased nitrogen loss from the soil,when the drought ended.

The principal effects of climate change on soilswill be to accelerate loss of soil organic matter andto release nutrients in increasing amounts. Theseincreases in oxidation of soil organic matter andmobilisation of soil nitrogen cannot continue indef-


76 Chapter 6

initely. If not replaced by natural processes or dur-ing cultivation, carbon content and nutrient statusof the soil will be diminished, causing loss of fer-tility. Broadmeadow (2002a) suggests that micro-bial breakdown of leaf litter with increased car-bon:nitrogen ratios, as plants fix atmospheric car-bon dioxide in a scenario of reduced availability ofsoil nitrogen, will result in further reductions ofavailable soil nitrogen to the point that plant growthwill be adversely affected.

Loss of organic matter will also result in loss of soilstructure. The soil therefore becomes more suscep-

tible to wind erosion in the drier summers antici-pated by the climate change scenarios, and less per-meable to water, leading to increased water erosionand run-off in the heavier downpours of wetter win-ters (Piccolo, 1998).

When mobilisation of soil nitrogen coincides with aslowing of plant growth in the autumn, much of thenitrate will be lost from the soil in drainage water,possibly causing problems elsewhere in pollution ofstreams, ponds and lakes and adding to the prob-lems being caused by increasing levels of nitrousoxide present in the atmosphere (see section 6.2.2).

Figure 16: Carbon in the soil as compared to carbon stored in land cover. Source: Piccolo, 1996.

TROPICAL FOREST TEMPERATE GRASSLAND AGRIC. URBAN DESERT,FOREST LAND AREA TUNDRA

LEAF

WOOD

LITTER

ROOT

HUMUS

RESISTANTCARBON

1000 mhs

1 kg C

m2

SOIL SURFACE


77Chapter 6

6.1.1 Climate change impacts on particular soils

The local effects of these changes on gardens willdepend on soil type. In light, freely drained soilsthe increased biological activity in warmer andwetter winter months will lead to increasinglyrapid loss of organic matter, and thus to increasedsusceptibility to erosion. Organic matter loss willalso reduce waterholding capacity of the soil andfurther accelerate and exacerbate effects of sum-mer drought as summer rainfall decreases.

Highly organic soils are at risk of rapid oxidation asthe combined effects of increased evapo-transpira-tion and reduced summer rainfall lead to aeration ofpreviously waterlogged profiles. The most dramat-ic example of this is in the East Anglian fens, wheresoil levels have dropped by tens of metres since thearea was drained for cultivation in the 17th century.The loss, both by oxidation and wind erosion, iscontinuing to the extent that large areas will soon beunsuitable for commercial crop production.

A similar, though less visible, problem exists inupland areas where increasing oxidation of peatsoils will not only affect the natural vegetation inSSSI and other designated areas, but may exposearchaeological remains which are important cultur-al components of many parkland and garden land-scapes (Farrar and Vaze, 2000).

Heavy soils may retain sufficient water and nutri-ents to sustain plant growth through the summermonths, despite the drier regime, though on veryretentive clay soils water available to plants will bemuch less than that held by the soil. However, theseheavy soils may suffer from waterlogging in winter.

One obvious sensible response to climate changewill be to work within constraints imposed by soilconditions. The need, also, to maintain or improvesoil organic matter content, and hence its water-holding capacity, and to ensure adequate drainage,can not be over-emphasised.

i) Domestic gardensThe soil in the smaller, domestic garden is likelyto have been much modified from the natural state

during construction of the house and other builtstructures, such as drains, paths and driveways.There is potential for soil improvements to bemade to minimise the effects of climate change.For example, the structure and nutritional status ofsoils can be improved by incorporating organicmaterial and by mulching to reduce loss of organ-ic matter as temperatures increase. Incorporationof grit or the construction of raised beds cancounter the risk of waterlogging from heavy win-ter rains.

Such treatments will also ameliorate the impact ofincreased drought and increase the infiltration ofheavy rainfall. The cost per square metre of thesetreatments, especially if materials are bought insmall units at retail prices, will be very high, butthe area to be treated will generally be small.

ii) Heritage gardensThere may be some scope for soil amendment andimprovement in the intensively managed parts ofheritage gardens, as in the domestic garden. Theavailability of leaf litter and other organic waste,and the availability of machinery for shredding,composting, transporting and incorporating organ-ic matter will make handling relatively easier thanin the domestic garden, though at considerablecapital cost.

In less intensively managed areas, the preventionor reversal of soil deterioration will depend onmaintenance of plant cover, retention of fallenleaves (perhaps using strategically placed plantgroups to minimise wind-blow of leaves onto pathsor lawns) and, where acceptable, return of lawnclippings to maintain organic matter levels and anopen soil surface.

On slopes, it may be necessary to use interceptorgullies and soakaways to reduce soil erosion risks.Regular maintenance, perhaps upgrading andrenewal of drainage systems will also be necessary.

Traditional gardening techniques have always beendirected at maintaining fertile, water retentive, butwell-aerated soil. The challenge in the 21st centurywill be to continue that tradition in the face of lim-ited and often dwindling resources.


78 Chapter 6

6.2 Climate change impacts on water

Water exists in two forms in gardens: water suppliesare used for irrigation; water features provide aes-thetic, and often ecological and productive, bene-fits. In practice the two components are oftenlinked. At the simplest level, a hosepipe (water sup-ply) can be used to top up a garden pond, while thepond (a water body) can be used to fill a wateringcan. On a larger scale, a borehole might be used tofill a lake, while a lake or stream might provide awater supply for irrigation or domestic use.

The availability of water for use and its presence inwater features is determined by the hydrologicalcycle. A very brief and much simplified descrip-tion of the cycle is provided in Box 6.2, to set theimportant topic of climate change impacts on waterinto context.

6.2.1 WATER SUPPLIES

Water supply for human use may be obtained bytaking water directly from rivers or by pumping itfrom underground aquifers. As river flows areuneven and demand is more or less even throughoutthe year, it becomes necessary to impound and storewater in reservoirs, so that large flows in winter canbe used to meet large demands through the summer.This is particularly the case in the more hilly androcky parts of the country, where most water runsoff the surface into streams rather than soaking intothe soil, so stream flow is most variable.

Much of the water supply in the UK is providedfrom private water companies on a regional basis,but some users have their own private water supply,either directly from a river or, more commonly, bypumping from a borehole (Table 4). Such extrac-tions require a licence.

Total water abstractions declined between 1971 and1991 as a result of the closure of old, watercooledpower stations and the decline of heavy industry(Table 4). Indeed the water table is rising undermajor cities in the UK as a result of this decline,threatening an increased risk of flooding and a dan-ger of pollution of water supplies, as the water tablerises through industrially polluted soils (Shackleyand Wood, 1998). In the same period, public watersupply increased from 14-18 billion litres per day, anincrease from 34%-51% of total water abstractions,partly as a response to some increase in population,but mainly as a result of higher living standards.

The direct impact of climate change on overalldemand for water is expected to be rather small. Themain uses of water – for cooling of power stations inindustry, and domestically for flushing toilets andwashing clothes – will be little affected by climatechange. The likelihood, though, of reduced supply asa result of climate change will present challenges in

The reduction of soil carbon by clearing forests and

ploughing grassland has been a major contributor to

climate change. Soils are, in turn, significantly

affected by climate change. Many factors interact

in determining the soil’s susceptibility to climate

change.

Increase in temperature will increase the rate of

loss of soil carbon by oxidation. This will lead to loss

of soil structure and loss of permeability, so intense

rainfalls may cause run-off (and therefore erosion

and flooding) rather than the recharge of soil

moisture reserves. Oxidation of soil organic

matter also releases nitrates, which may increase

plant growth, or leach out of the soil to pollute rivers

and lakes.

Decreased rainfall will slow conversion of soil

carbon to carbon dioxide, but the relationship is

complicated and effect will differ from day to day.

Usually, plants will cease to take up water (and

therefore nitrates) from the soil before the soil has

dried to the extent that organic matter breakdown

stops. Nitrates will therefore accumulate in the soil

and be leached out in heavy rains.

Highly organic soils, in fen and moorland areas, will

be most susceptible to climate change and may lose

their ability to support their characteristic vegetation.

Any action which increases soil organic matter will

help to reduce all these problems and thereby will

reduce the cause, as well as the impact, of climate

change. Caring for and covering the soil will play a

significant role in countering the adverse impacts of

climate change.


79Chapter 6

Box 6.2 The Hydrological Cycle

The primary source of water supply is from precipitation. Water reaching the soil surface will either be absorbed into the surface

or will run off. Water filtering down through the soil and underlying rocks will eventually reach an impermeable layer, above which

it will accumulate to form an underground zone in which pores in the rock are more or less saturated with water. This natural

reservoir is called an aquifer, and the level of water in the aquifer is the water table.

Because of inequalities in the soil and in the underlying rocks, this water table will sometimes reach the soil surface and the

water will bubble out as a spring. For example, where a thick stratum of chalk (a very porous rock) overlies impervious clay,

water will accumulate on top of the clay layer and emerge wherever the chalk/clay interface is exposed at the ground surface, to

form a line of springs along a hillside. Water from the springs will flow down hill in streams. These streams meet each other and

build into rivers, which make their way down to the sea.

It usually takes months or years for a drop of water arriving at the soil surface to filter down through the soil to the water table

and then to emerge at the surface again, so short term fluctuations in precipitation are evened out and the output from springs is

much less variable than is the input of rainfall. There will be a gradual increase of stream levels in winter, and a gradual decline

in summer but day to day fluctuations are eliminated.

If water does not soak into the soil, but instead runs across the surface, it will flow down hill carrying with it soil particles.

Trickles of water coalesce to form runnels, then streamlets and streams. Erosion of the land, as soil particles are carried down

by the water, creates channels which determine the future course of the streams. As these channels become more clearly

defined, the concentration of water increases and, with it, the ability to carry more sediment, including larger stones and rocks.

Surface run-off feeds into streams quickly, so the level and force of the stream will increase rapidly during periods of rainfall and

decrease equally quickly when the rain stops. In heavy rains, the stream channel will be inadequate to carry the extra volume of

water; the water will flood onto surrounding land, depositing most of the silt it carries and, in time, building up a flood plain.

Some of the flood water will drain back into the stream as its level drops. Some will soak into the surface to replenish the water

table. By eroding and depositing soil, the stream will gradually reshape the landscape.

In a natural ecosystem, rivers and streams will have a more or less steady base flow of water emerging from springs throughout

the year, supplemented by short term increases of water from run-off after periods of rain or snow-melt. If the land surface is

made less pervious, by compacting the surface or by covering it with concrete or tarmac, for example, infiltration and the steady

base flow will be reduced, and the flash flows after rainfall will increase. The long term effects of this are to increase the risk of

flooding, and to deplete groundwater reserves which feed the steady flow of streams and rivers.

Loss of groundwater is exacerbated by extraction for domestic, industrial and agricultural use. Decrease in surface permeability

as a result of development also increases the sensitivity of the hydrological system to short term heavy downpours, and so

greatly increases the risk of sudden floods in heavy rainstorms. Flooding is further exacerbated by channelling water into drains

and river channels, which pass the problem more rapidly downstream. Every step in the process, from absorbent forest soil, to

grassland, to bare arable soil, to tarmac accelerates and exacerbates the change from steady year-round flow of rivers and

streams, to the sudden oscillations from spates after rainfall, to low base flows, and intensifies the problems of insufficient water

supply to dilute pollutants. In the most extreme cases, streams will dry up completely in dry periods then turn into raging torrents

for a few hours or days after heavy rainstorms.

Climate change is likely to intensify the hydrological cycle. Rates of evaporation will increase due to higher temperatures,

variability of precipitation will increase (with increases in winter and decreases in spring, summer and autumn), as will variability

of run-off owing to more intense rainfall. Water and soil management will be inextricably combined when protecting gardens and

the wider landscape from the adverse effects of climate change. Increasing the humus content of the soil will help to reduce the

vulnerability of soils to erosion, and to increase their capacity to absorb heavy rainfalls which might otherwise cause flooding.


80 Chapter 6

Tabl

e 4:

Lic

ense

d no

n-tid

al w

ater

abs

trac

tions

in E

ngla

nd a

nd W

ales

, 197

1-19

91. S

ourc

e: H

errin

gton

, 199

6

Year

Pu

blic

Wat

er S

uppl

y In

dust

ry

Spra

y Irr

igat

ion

Agr

icul

ture

El

ectr

icity

Gen

erat

ion

Fish

farm

ing

and

Tota

l w

ater

cres

s pr

oduc

tion

MI/d

%

of t

otal

M

I/d

% o

f tot

alM

I/d

% o

f tot

al

MI/d

%

of t

otal

M

I/d

% o

f tot

al

MI/d

%

of t

otal

M

I/d

1971

1434

533

.60

9214

21.5

868

0.16

170

0.40

1889

644

.26

4269

3

1972

1479

734

.97

9162

21.6

563

0.15

170

0.40

1811

842

.82

4231

0

1973

1525

236

.41

8658

20.6

758

0.14

156

0.37

1776

742

.41

4189

1

1974

1515

540

.55

7080

18.9

478

0.21

760.

2014

988

40.1

037

377

1975

1536

042

.86

6560

18.3

011

10.

3194

0.26

1371

438

.27

3583

9

1976

1500

942

.72

6655

18.9

416

10.

4696

0.27

1321

137

.60

3515

2

1977

1474

741

.72

6958

19.6

811

60.

3312

00.

3413

406

37.9

335

347

1978

1582

844

.94

6626

18.8

181

0.23

150

0.43

1253

935

.60

3522

4

1979

1626

745

.20

6762

18.7

910

60.

2914

00.

3912

710

35.3

235

985

1980

1618

646

.87

5034

14.5

892

0.27

133

0.39

1308

737

.90

3453

2

1981

1610

548

.05

4973

14.8

411

70.

3511

10.

3312

208

36.4

333

514

1982

1633

148

.21

4729

13.9

613

90.

4111

70.

3511

587

34.2

197

02.

8633

873

1983

1622

448

.06

4093

12.1

317

10.

5111

80.

3512

179

36.0

897

12.

8833

756

1984

1640

249

.05

3892

11.6

419

90.

6012

20.

3611

757

35.1

610

663.

1933

438

1985

1664

150

.95

3939

12.0

613

70.

4213

00.

4010

711

32.7

911

053.

3832

663

1986

1659

247

.69

4114

11.8

316

70.

4812

50.

3612

744

36.6

310

483.

0134

790

1987

1724

449

.16

3712

10.5

810

20.

2912

20.

3512

806

36.5

110

893.

1035

075

1988

1759

751

.21

3901

11.3

514

40.

4212

00.

3511

787

34.3

081

52.

3734

364

1989

1820

551

.64

3654

10.3

629

80.

8511

50.

3312

189

34.5

779

42.

2535

255

1990

1833

650

.66

3795

10.4

937

81.

0412

90.

3612

612

34.8

494

62.

6136

196

1991

1818

150

.78

3800

10.6

136

51.

0213

40.

3712

430

34.7

289

52.

5035

805


81Chapter 6

meeting even a modest anticipated increase indemand. The UKCIP02 scenarios all point to greaterwater deficits in summer and autumn months.

In terms of impact on overall demand for water, thehorticultural industry, gardens and golf courses are,and will continue to be, of minor importance.

Water used by farmers and commercial growers forirrigation increased erratically from 68 million litresper day (Ml/d) in 1974 to between 100-200 Ml/d inthe period 1974-1988 then rose rapidly to 365 Ml/din 1991. This increase was a result of general intensi-fication of farming, and the demand from the newsupermarkets for regular and predictable supplies ofhigh-quality vegetables, as well as the need forincreased irrigation in hot, dry summers. It is not pos-sible to separate out the effects of increased sophisti-cation of production systems from increased need tomatch higher evaporation levels resulting from cli-mate change, but the total increase represents achange from using 0.16% of the public water supplyfor irrigation in 1974 to using 1% in 1991. This is avery small proportion but a six-fold increase.

Domestic use in the south and east of England forlawn sprinkling increased from 0.1 litres per capitaper day (l/c/d) in 1976 to 4.3 l/c/d in 2001 and, ignor-ing any effect of climate change, is estimated to rise

to 8.7 l/c/d by 2021 (Table 5). The statistics for othergarden uses are 1.1, 4.8 and 7.2 l/c/d respectively.

This rapid rise is associated with increased owner-ship and use of sprinklers, but the extent to whichthis reflects climate change impacts, as distinctfrom a general increase in the standard of living(ability to afford sprinklers and time to use them)and in appreciation of gardens, is again uncertain.While the change from 1.2 l/c/d in 1976 to 9.1 l/c/din 2001 and 15.9 l/c/d in 2021 for lawn and gardenwatering represents a very large proportionalincrease it is, and will remain, less then use for per-sonal washing (33.5, 46.5 and 61.6 l/c/d) and rep-resents a change from 1% (1976) to 4.3% (1991) to8.9% (2021) of total domestic demand in the southeast, the driest part of the country.

In his analysis of domestic demand components ofwater supply for non-metropolitan south and eastEngland 1991/2021, Herrington (1996) calculatesthat, with a 1.1° rise in temperature by 2021, wateruse for lawn sprinkling will increase by 35% andfor other garden uses by nearly 20%.

Golf courses, with their need to create smoothgreens and the current cultural insistence on a lushgreen setting, are conspicuous consumers of waterbut, in a national context, not highly significant. An

Table 5: Domestic demand components of water supply for non-metropolitan south and east England in 1981 and 2021 incorporating climate change. Source: Herrington, 1996

Component 1991 2021 2021

Climate standardised No climate change +1.1°C warming

WC use 35.5 33.6 33.6

Showering 5.3 24.0 26.8

Other personal washing 41.2 37.6 37.6

Clothes washing 21.7 22.0 22.0

Dish washing 11.8 11.0 11.0

Waste disposal unit 0.4 1.5 1.5

Car washing 0.9 1.5 1.5

Lawn sprinkling 2.5 8.7 11.8

Other garden use 3.8 7.2 8.6

Miscellaneous use 23.9 31.3 31.3

Total domestic use 147.0 178.4 185.6


82 Chapter 6

average golf course using the public water supplyuses 2.7 million litres of water annually (Table 6).Assuming some increase in the number of golfcourses, Herrington (1996) estimates that waterdemand in the south east for irrigation of golfcourses might increase from 3.3 Ml/d (1992) to 4.8Ml/d (2021) in the absence of climate change.

A 1.1°C increase in temperature by 2021, and a2.1°C increase by 2051 (similar to temperaturechanges projected under the UKCIP02 mediumhigh emissions scenario) is expected to add 4% (by2021) or 8% (by 2051) to the requirements whichwould be expected in the absence of climatechange. This 8% increase compares with estimatesof 11.8% increase for agricultural irrigation, and37.5% increase for air-conditioning. The total of 5Ml/d estimated water use by golf courses in thesouth east in 2021 with moderate climate change,represents less than 0.1% of domestic water con-sumption and is therefore insignificant in terms ofthe total amount of water used.

These various horticultural uses of water are notlarge in relation to overall consumption then, butwhen seasonality of water supplies and peakdemands in water use are taken into account, a verydifferent picture emerges.

Not only are the levels of demand increasing rapid-ly, but the maximum demand for water for horticul-tural use occurs when water is least available. In ahot year, a golf course increases its water consump-tion by 40% over use in an average year

(Herrington, 1996). Calculations of garden use ofwater in the Thames and Lee Valley catchments,suggest that public water supplies will need toincrease by 1.2% to meet increases in demand relat-ed to climate change by 2050 on an annual basis,but this represents a 3-4% increase in demand forthe six months April-September, or 7-8% for June-July. In East Anglia, 3% of annual water use in anaverage household was used in the garden in thewet year of 2001 (Chivers, pers. comm.). This fig-ure was 6% in the dry year of 1996. Concentratedin the two driest months, the peak demand may riseto 25% above the average level of water use.

The situation is made worse by the fact that waterapplied to gardens, unlike water used in washingmachines, baths and other household uses, is notreturned quickly to replenish river flows. Althoughin the long term, water for horticultural (and agri-cultural) use is recycled via the hydrological cycle,replenishing the water table by infiltration orrecharging clouds by evaporation from plants, sub-stantial extraction of water for irrigation will leadin the short term to falling river levels.

The impacts of gardens on water demand as aresult of climate change will, therefore, be a mod-est increase in total demand for water, but a verymarked increase in peak demand in hot, dry sum-mers. As climate change continues beyond 2050,and as expectations of gardens continue to rise,water use for gardens may cease to be a minor pro-portion of total domestic demand. Sales of gardenwatering equipment have risen from £21 million to

Table 6: South and east England golf course water use in 1992 and 2021, without and with climate change (numbers of courses and demand for water). PWS = Public Water Supply; DA = Direct Abstraction. Source: Herrington, 1996

1992 No climate change 2021 No climate change 2021 With climate change

PWS only 368 @ 2.70 MI 546 + 55 @ 2.70 MI 546 + 55 @ 2.81 MI

DAs only 288 @ 3.64 MI 427 + 43 @ 3.64 MI 427 + 43 @ 3.79

MI Mixed: PWS 144 @ 1.35 MI 214 + 22 @ 1.35 MI 214 + 22 @ 1.40 MI

Mixed: DAs 144 @ 1.82 MI 214 + 22 @ 1.82 MI 214 + 22 @ 1.89 MI

Total no. of courses 800 1187 + 120 1187 + 120

Water use: PWS 1188 MI = 3.3 MI/d 4.8 + 0.5 MI/d 5.0 + 0.5 MI/d

Water use: DAs 1310 MI = 3.6 MI/d 5.3 + 0.6 MId 5.5 + MI/d


83Chapter 6

£61 million in the past four years but ownership inthe UK is still considerably below that of Franceand Germany, despite the higher proportion of peo-ple living in flats in those countries (Ofwat, 2002).

To adapt to these changes, either demand will needto be suppressed, by hosepipe bans or pricingstructures, for example, or supplies will need to beincreased, mainly by the construction of increas-ingly expensive reservoirs and in the face ofincreasing environmental opposition.

The impacts on gardens as a result of increasingdeficits of natural water supply will come bothdirectly, as a result of reduced rainfall andincreased evaporation within the garden, and indi-rectly as a result of reduced availability orincreased cost of water in the public supply system.Water shortage is likely to be the most serious sin-gle impact of climate change on gardens. In addi-tion to damage to plants, especially to mature trees,summer drying or serious depletion of lakes inlandscape parks is an increasingly frequent phe-nomenon, usually with serious ecological conse-quences as well as loss of visual amenity.

Deficits in natural supplies will occur in a contextof decreasing availability to the water suppliers andincreasing demand for water for other uses, primar-ily as a result of increased living standards.Gardeners could respond to these deficits in a num-ber of ways. Planting schemes could be adapted toincorporate drought tolerant species (see sections3.4, 4.2 and 6.3-6.8). Irrigation could be applied,although water for irrigation is likely to becomemore expensive and perhaps increasingly stringent-ly controlled. Water could also be stored during wetperiods to compensate for times of shortage

Water storage could take the form of improvedwater retention in soil, by mulching and increasingorganic matter content, or by installing rain waterbutts, recycling of ‘grey’ water from baths andwashing machines or, on a larger scale, by build-ing reservoirs. Increasingly, users of large vol-umes of water (farmers, golf courses, nurseries)are building private reservoirs so that they canstore (and in the case of nurseries recycle) waterfor use in times of shortage. Farm reservoir capac-

ity nearly doubled between 1984 and 1995, from33 million to 64 million cubic metres. This pro-vides about 40% of the current water supply need-ed for irrigation. A further 30 million cubic metresof storage capacity would cost between £13 mil-lion and £73 million depending on individual siteconditions (Orson, 1999).

Most such reservoirs are purely functional, regularin shape, steep sided and often fenced for safety. Ifthe land and resources are available, there is no rea-son why they should not be designed as visuallyattractive ponds and lakes that could also lookattractive (or at least acceptable) when the waterlevel drops during peak extraction periods. Theinvestment might well be worthwhile in large gar-dens threatened by water shortage. A 45,000m3

reservoir has recently (September 2002) been com-pleted in The Royal Horticultural Society’s gardenat Hyde Hall (Essex) as the central feature of a newenvironmental area.

Although the use of water butts in domestic gar-dens might seem a trivial response to climatechange, the widespread application of such conser-vation measures could have a significant impact inreducing peak demands for water. This dispersedstorage could also be more economic than cen-tralised provision of expensive and environmental-ly sensitive reservoirs (Entec, 2000).

Irrigation systems are already being installed inmany public and private gardens and are consid-ered to be an essential feature of any new golfcourse. Irrigation to reduce the impact of waterdeficits will be subject to availability of sufficientwater resources. Continued climate change is like-ly to result in increasingly stringent control ofextraction – perhaps even in the withdrawal ofextraction licences – higher cost of water, and thepossibility of restrictions on garden use in pro-longed droughts, when water is most urgentlyneeded. Trickle irrigation systems and leaky hose,or other surface and sub-surface systems, willeconomise on water loss by evaporation. But,reducing dependence on external water suppliesand prioritising key areas of the garden which mostneed irrigation, will be increasingly importantresponses as climate changes intensify.


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While reduced precipitation in the summer monthswill cause increasing shortages of water during thegrowing season, higher precipitation rates in wintermay cause unwanted surpluses. A recent flash floodat Wallington (Northumberland) for example result-ed in the collapse of a 17th century garden wall. Thesudden release of floodwaters built up behind thewall, then caused severe erosion of paths and bor-ders in the garden. On a wider scale, extensivefloods in the winter of 2000/01, which filled newsbulletins for many weeks, caused huge losses ofproperty and widespread damage to gardens. Theincreased volume and intensity of winter precipita-tion anticipated by the UKCIP02 scenarios is likelyto increase flood risk in future. The scale of damagecaused by recent flooding events suggests that itwould be wise to prepare for similar future events,while the experience gained is still fresh in the mind.

Adaptations might include physical protection ofthe garden, where possible, by earth mounding or,during Environment Agency flood alerts, preparingsand bags. Safety measures include ensuring thatpower points, water and gas taps, mower fuel andgarden chemicals are installed/stored above antici-pated flood levels. Any measures which will slowthe flow of water through a garden (encouraginginfiltration rather than run-off), or channel water toareas where it will cause least damage, will beworthwhile especially if the surplus can be storedfor future use. Where parts of a garden flood regu-larly, terracing or decking above expected floodlevels will allow access through the garden withouttreading on saturated lawns. Ensuring that drainagesystems (open or piped) are in good condition andthat the soil is in good physical condition to absorbwater and resist erosion, are sensible aspects ofgarden management, even if floods are not antici-pated. Reclamation of the garden in the aftermathof flooding is usefully described in theEnvironmental Agency booklet Flooding inGardens (Environment Agency, 2002).

Modification of the long term composition of thegarden in response to flood threats will usually notbe advisable, except in those situations in whichserious flooding has occurred in the recent past andthere is some evidence that the frequency of flood-ing is increasing. Most plants which are tolerant of

flooding are not tolerant of dry conditions, anddrought as a result of climate change poses a muchmore serious threat to gardens than does flooding.

6.2.2 WATER FEATURES

Water in the garden, whether a bird bath in a tinycourtyard or a lake in a Capability Brown land-scape, is a very attractive feature, both in itself andin the wildlife which is attracted even to the small-est water feature. Water features require a reliablesupply of water, natural or piped.

The main impact of climate change on all waterbodies will be increased evaporation from the sur-face. This will result in the need to top up waterlevels, by hosepipe or by borehole if permissible. Asecondary effect of using tap water to refill a pond,is that the nitrate content of the water will encour-age unwanted weed growth. Ponds and lakes rely-ing on a natural water supply to keep them full willbe at risk of drying out in hot, dry summers.

Higher summer temperatures present a furtherproblem, because oxygen is less soluble in water as

Water will be less available in summer, when it is

most needed to sustain plant growth, and more

abundant in winter. If water extraction increases

(for domestic use and for irrigation) the water table

will drop and steady river flows will be reduced.

The likely impact of climate change will be lower

summer flows in streams and rivers but sudden

increases in water level after heavy rain. This trend

will be exacerbated by urbanisation, and the damage

resulting from it will be increased if houses and

gardens are established on flood plains.

Although, as yet, a very minor component of total

water demand, water demand for irrigation of

gardens and golf courses will increase rapidly

(it increased six-fold between 1974 and 1991), and

concentration of demand into the hottest, driest

months may increase peak demands by 25% in the

south east. Where demand can not be met, the result

will be reduced pressure, or restrictions in use and,

in the longer term, higher costs to pay for new water

supply infrastructure.


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the temperature increases, but the demand for oxy-gen from aquatic organisms increases with temper-ature. Biological oxygen deficiency thereforeincreases sharply with temperature. In a balancedecosystem this de-oxygenation would normally becompensated for by higher rates of photosynthesisin submerged plants, releasing oxygen into thewater. However, when higher summer temperaturesare combined with excessive nitrates (from acceler-ated breakdown of soil nitrogen, from increasedleaching and erosion of agricultural soils and fromincreasing atmospheric pollution with nitrousoxides), the vigorous growth of a surface blanket ofalgae shades out submerged aquatics. As the algaeexhaust the nutrient supply and die, breakdown ofthe dead mass rapidly depletes dissolved oxygensupplies and the water will become eutrophic –stagnant, unhealthy and foul-smelling. The 1990sdroughts in Sheffield Park (Sussex), combined withan accumulation of silt, built up from leaf litter andpart caused by soil washed off surrounding farm-land in heavy storms, led to serious algal blooms.The cost of dredging to rectify the problem amount-ed to £40,000 (Calnan, pers. comm.; Owen, 2002).

Management of water bodies is a complex matter.Shading of the surface by tree planting will help toreduce surface temperatures, which could becomewarm enough in future to kill fish populations. Butautumn leaf fall from the trees could be detrimen-tal, increasing the amount of organic materialfalling into the pond or lake. In natural, unlined,ponds, trees will also extract water and exacerbatethe fall in water level.

Management of the edge to achieve a gentle gradi-ent and a shelving beach, or transition frommarginal vegetation (reeds for example) to deepwater, will disguise changes in water level, and thelatter especially will add to the biodiversity of thepond. Care will then be needed to ensure that thevegetation itself does not encroach too much on thewater surface. Where blanket weed growth andeutrophication become important problems, sur-face skimming of the water to remove algae willresult in immediate improvement, though at con-siderable effort on any substantial scale.Oxygenation of water using fountains or cascades,or air pumped through submerged porous pipes

where decorative treatment of water is consideredinappropriate, will improve water quality.

Running water bodies – streams and rivers – arealso at risk of falling water levels in periods ofreduced precipitation and high evaporation rates.The River Pang near Reading has dried out com-pletely in recent, hot summers because of reducedrainfall and increased abstraction, spoiling theappearance of gardens through which it runs, butwith much more serious loss to fish and other life.

On a garden scale, the problem of reduced watervolumes is not easy to counter, except by pumpingfrom elsewhere – usually at substantial cost andoften at the risk of depriving other areas of muchneeded water. Reshaping of the stream bed, or con-struction of weirs so that the stream forms a chainof small ponds as water levels drop, will providesome refuge for water life until such time as rain-fall restores the natural water flow.

Water bodies may also have to cope with periods ofexcess supply. Increases of water volume, throughmore intense and more prolonged rains, may causescouring of the stream bed, overflow of banks ordams, loss of marginal plants and fish, and floodingof adjacent land. There is also a risk that flooding ofgarden ponds could release exotic water plants intoadjacent streams, and thus into the wider landscape.Water flooding across farmland, the overflow of19th century combined storm and sanitary sewers,and the flooding of septic tanks, can cause pollutionof the water and contamination of water supplies.

The more natural the stream profile, with mean-ders, shallows and abundant marginal vegetation,the more able it will be to withstand fluctuations inwater throughput. Engineering efforts to clean outthe stream, to straighten and deepen the channeland to engineer the banks by removing vegetation,may solve a local problem but only by moving theproblem even more rapidly downstream. Impedingwater flow and diverting surplus water into areaswhich can safely be flooded temporarily (holdingareas, silt traps and balancing ponds), will providea far more effective and durable response. Suchareas can usually be designed to increase biodiver-sity and visual interest.


86 Chapter 6

The whole strategy of coping with the impacts ofclimate change, and especially in its effects onwater supplies and water bodies, will need to relyon learning lessons from nature rather than trying tooverrule it. Response to these alternating deficitsand surfeits of water will require careful manage-ment of water flow and water quality. Techniquesmight include impounding run-off, recycling irriga-tion water and using grey water where possible,combined with land contouring, improving soilstructure and better drain maintenance

6.2.3 WATER MANAGEMENT

i) Domestic gardensThe impacts of climate change on water supply tothe domestic garden will be significant, but can bereduced by sound water management using meth-ods described in section 6.2.2. Shortage of water inthe summer can be made good by irrigation, prefer-ably using stored water, and concentrating on themost important plants in the event of a prolongedhosepipe ban. Irrigation after dusk, using a timer orby staying up late, and irrigation using seep hose ortrickle irrigation, will reduce evaporative losses.

In well managed gardens, surplus water in wintershould infiltrate into good garden soil and run offdrives, paths and patios onto lawns or borders, orinto drains if levels are suitably designed. However,flood risk can be expected to increase in someareas in future (Hulme et al., 2002). Advice on howto cope with excess water in the garden is providedby the Environment Agency (2002).

In the long term, it will become advisable to adaptplanting schemes to the new climatic regime of theparticular area. It would certainly be wise, forexample, to replace any plants which consistentlysuffer from summer drought, but fluctuations ofweather will be more important than climatechange in the domestic garden, so major changesin planting from year to year as a result of lastyear’s weather damage, would be unwise.

Water bodies in domestic gardens will usually besmall in scale, most typically a garden pond.Reduced rainfall and the increased evaporationresulting from higher temperatures will necessitate

topping up of the pond. If this can be done usingstored rainwater, it will avoid the secondary prob-lem of nutrient enrichment which results if mainswater is used. Some reduction of evaporation maybe possible by ensuring that about 50% of thewater area is shaded by surrounding vegetation.

ii) Heritage gardensThe principal dilemma in managing water suppliesin heritage gardens is in deciding how closely it isnecessary to adhere to the status quo. Key areas ofthe garden may justify irrigation, in which case abalance must be struck between the high capital costof a sophisticated and automated system, and thehigh running costs of a hosepipe or watering can.

As in the domestic garden, maximum use of storedwater – water butts, underground water tanks suchas those in many old greenhouses or, where devel-opment is permissible, surface ponds or reservoirs– may be used.

Reference has already been made (in section 6.1.4)to the possible need for gullies and soakaways todirect excess surface water. Flooding, especiallyfrequent flooding, is not often a problem in her-itage gardens of the 18th century onwards as theircreators usually built above flood level. Whereflooding is occurring more frequently as a result ofclimate change or changes in land use, the onlyremedy is costly intervention to accommodatefloodwater, by improving drainage ditches, or todivert it from the most vulnerable areas with earthbunds or other techniques, including perhapspumping. Regular maintenance of drainage chan-nels is an essential part of garden management.Upgrading of the drainage and flood defence sys-tems may be called for in particular instanceswhere increased flood risk is evident.

The problem of water bodies in heritage gardens asa result of climate change impacts is considerablebecause, as attractive features, they usually occupykey locations in the landscape. Falling water levelsin summer may necessitate a pumped water supply.Barley straw is sometimes effective at reducingalgal blooms. Oxygenation by pumping air throughperforated hoses on the lake bed can improve waterquality and save fish stocks. When appropriate, the


87Chapter 6

creation of wetlands (such as reedbeds) or beaches,will buffer the visual impact of fluctuating waterlevels, increase the absorption of excess water intothe water table and localise the deposition of silt inareas where it can be more easily and cheaplyremoved than from the lake itself.

Water surplus may result in overtopping of lakes. Itis, therefore, very important to ensure regular inspec-tion of dams, sluices and spillways to conform to therequirements of the Reservoirs Act (1974), so that theexcess does not cause a threat to life or property.

6.3 Climate change impacts on trees

As the largest and longest-lived plants in a garden,trees are most vulnerable to the stresses induced byclimate change. As complex and long-lived organ-isms they experience climatic impacts over a longtime, sometimes centuries, and any impacts of thebenefits or injuries imposed by long term climatechange or short term fluctuations in the weather,will be reflected with compound interest as timepasses (Ceulemans, 1998).

Root suffocation of cedars in the National Trust’sgarden at Osterley Park (Middlesex), drowning ofbeech trees in the lower part of the RoyalHorticultural Society’s garden at Hyde Hall (Essex)and the loss of fifteen million trees across the southof England in the storm of October 1987 are con-spicuous reminders of the value of trees and of thethreats facing them. The one night of 16th October1987 saw a loss of trees equivalent to fifty years ofnatural tree decline (Rich, 1988).

Fire hazards, especially in coniferous windbreaks orwoodland, will increase sharply as temperaturesincrease and summer droughts extend, although heremany factors will interact. Evidence in recent yearsis that fire risk, not surprisingly, increases in hot, drysummers. The number of outbreaks also increaseswith increases in visitor pressure (also related to hot,dry summers), as most fires are started accidentallyby cigarettes or picnic fires. However, although thenumber of fires in recent dry summers has increased,the damage caused has declined, because outbreaksare reported and dealt with more rapidly since theadvent of the mobile phone.

The most serious threats facing trees are summerdrought, possibly winter waterlogging and highwinds. There is widespread concern, in particular,about the future prospects for beech (Fagus sylvat-ica). Beech is native to southern England and,under conditions of climate change the natural dis-tribution would be expected to move north and east(Berry et al., 2001; Broadmeadow, 2002a, b;Harrison et al., 2001) (see Figure 14a in section4.1). It was, however, extensively planted in theChilterns and on the North and South Downs fol-lowing the enclosure of downland in the early 19thcentury and the abandoning of arable cultivationafter 1850 (Piggott, 1988).

Unfortunately, it is unable to tolerate the increasingwater stress associated with climate change on theselight soils and hilltop situations. There is a strongnegative correlation between reduced rainfall in July,and crown density of beech in the following year(Cannell and Sparks, 1999), but the underlying caus-es of this correlation are complex. Low rainfall (andtherefore high light intensity) in one summer, stimu-late the beech to produce a heavy crop of seed (mast)in the following year. Whether this seed productionis the plant physiological equivalent of a panicresponse to drought, or a result of high carbohydratelevels in the sunny year enabling the tree to invest inreproduction in the following year is uncertain, butthe combined effects of drought stress in one yearand the drain on resources of fruiting in the follow-ing year is manifest in a thin canopy of small leaves.The tree can take several years to recover and, ifanother dry summer occurs before this recovery iscomplete, the tree will go into long term decline.

Decreasing natural flows in summer will result

in falling water levels in watercourses, ponds and

lakes. This will affect the appearance of the

landscape and have more serious consequences for

the environment and for fisheries. Average water

supplies may increase in winter, and major floods

will remain a risk.

Increasing temperatures will also affect water

bodies, decreasing dissolved oxygen levels and

increasing the risk of algal blooms. Water bodies will,

therefore, require more management in future.


88 Chapter 6

The impacts of summer drought can be overcometo some extent by irrigation and soil improvement,and especially by replacing highly competitivegrass with protective mulch beneath the canopy ofthe tree. The cost of this on a large scale, for scat-tered parkland trees or long avenues, for example,would be astronomical and the visual change fromtrees in grass to trees in large mulched circles willnot always be acceptable.

Winter waterlogging can also be reduced by goodsoil care and drainage. In most instances, drainageof wet sites will improve the tree’s root system,especially its depth of rooting, and make it more,not less, tolerant of drought. A particular problemwill arise in gardens in which higher winter rainfallis likely to result in root death, making the tree lessable to withstand reductions in summer rainfall.Careful attention to drainage will be needed insuch situations.

To minimise the impacts of high winds gardenerswill need to ensure good establishment of youngtrees and good shelter. In particular, planting open-ground stock rather than container grown trees,and planting trees as transplants, or even as seed,rather than as larger standards, will assist in thedevelopment of a wind-firm root system. Pit plant-ing (in a prepared planting hole), rather than slitplanting, has been shown in forestry planting toresult in much higher stability of trees

(Broadmeadow, 2002a). Improved drainage toreduce waterlogging will also reduce susceptibilityto wind-throw as soil strength and root anchorageare greatly reduced at high soil moisture contents.

However, the main strategy for protecting treesfrom the adverse effects of climate change mustlie in developing long term management andreplacement programmes. Storm damage in 1987was much higher in over-aged trees and in single-species plantations. Maintaining a good age struc-ture and, where appropriate, using a mixture ofspecies, will insure against massive storm dam-age. If historical precedent and historical signifi-cance do not constrain tree choice, there is muchscope for regenerating tree plantings with moreresilient species, as is already being carried out atSheffield Park (see section 6.3.1 below, and espe-cially Tables 3 and 4). Although old and decayingparkland and woodland trees are of great impor-tance in conservation of biodiversity, it is impor-tant to remember that the population of ‘veteran’trees depends, in the long term, on a flourishingpopulation of young trees which will become vet-erans in centuries to come.

There will be many instances in which historicalprecedent and the visual delight of a lofty, single-age and single-species avenue, for example, prevailagainst a mixed-age, mixed species approach but insuch cases it will be necessary to be aware of the

Figure 17a: Natural distribution of Serbian spruce (Picea omorika). Source: Tutin et al., 1964.


89Chapter 6

additional risks inherent in such situations, and tohave in place a policy for replacement when theavenue declines, as it inevitably will.

6.3.1 TREE SELECTION

In Britain, in contrast to many other parts of theworld, there are virtually no natural forests.Ancient woodland has been managed over cen-turies, sometimes millennia, to provide fuel,building materials and a habitat to attract deer andother animals to be hunted for food (Rackham,1993). Much of our current tree stock has beenplanted, usually for a combination of economicand aesthetic reasons. Changes in social aware-ness and changes in agricultural land use haveprovided opportunities for tree planting in recent

years. The shift away from planting monocultureof fast growing (many coniferous) species, toplanting mixed woodland species (White, 1994),initially for ecological and amenity reasons, willbe useful in insuring against total loss of a wood-land if a particular species proves unable to sur-vive climate change.

In deciding on suitable species for long term plant-ing, natural distribution maps of plant species areof limited use as indicators of how plants mightsurvive in gardens as the climate changes. Serbianspruce (Picea omorika), for example, has a verylimited distribution in the wild (Figure 17a) but isdescribed in Hillier’s Manual of Trees and Shrubs(n.d.) and by Bean (1976) as one of the most adapt-able spruces in cultivation.

Figure 17b: Natural distribution of Beech (Fagus sylvatica). Source: Tutin et al., 1964.


90 Chapter 6

Beech (Fagus sylvatica), on the other hand, has avery wide distribution from southern England toSicily (Figure 17b), but is found only at increasing-ly high altitudes towards its southern limits, andthen only on the well watered northern slopes ofmountains. Survival of beech in Sicily, therefore, isclearly no guarantee that it will survive at low alti-tude on the hot, dry chalk soils of the Chilterns.

The distribution of pedunculate oak (Quercus robur)is even wider, from the north of Scotland to Sicilybut this, too, is a result of differing altitudinal distri-bution, occupying higher altitudes in southern lati-tudes. There is also wide phenotypic variation with-in the oak population. Oak seeds collected from dif-ferent zones and germinated or grown in one siteshow wide variations in dates of leaf emergence, leafdrop, rate of growth, degree of branching and othercharacteristics. Clearly it will be possible to find‘oaks’ (in the broad specific sense) which, if plantedtoday, will tolerate the climate of Britain in a hun-dred years time, but these may not have the form thatis currently associated with the traditional Englishoak. Conversely, a particular phenotype of oakgrowing well in England now will probably not growwell after a century of accelerating climate change.

Similar problems arise with lime (Tilia x europea),a tree which exists in gardens as several distinctclones, each of which imparts its own particularcharacter to avenues, parks and gardens. It may benecessary to consider whether limes failing as aresult of climate change should be replaced byother clones of lime, which may have differentforms, or to use another species of more similarsilhouette.

Phenotypic variation will, however, be of some usein selecting tree provenances with greater toleranceof climatic changes. Such selection already formspart of the policy for redevelopment of the tree col-lections at Kew following the loss of many maturetrees in the 1987 storm.

In a garden context, it is also necessary to distin-guish between climatic tolerance and usefulness.Birch (Betula pendula and other species) isamongst the most tolerant of trees to a range ofenvironmental stresses, but they respond to droughtby losing their leaves. A near-leafless birch maysurvive increasing summer droughts, but will notmake an effective aesthetic contribution to thelandscape.

Table 7: Quality timber trees that should benefit from extra warmth in Britain. Source: White (1994).

Acer saccharinum Silver maple

Carya cordiformis Bitter nut

Cladrastis lutea Yellow wood

Corylus colurna Turkish hazel

Cupressus glabra Smooth Arizona cypress

Cupressus sempervirens Italian Cypress

Eucalyptus delegatensis Woollybutt

Fagus grandifolia American beech

Juglans nigra Black walnut

Ligustrum lucidum Tree privet

Liriodendron tulipifera Tulip tree

Paulownia tomentosa Foxglove tree

Platanus acerifolia London Plane

Prunus serotina Black cherry

Pyrus pyraster Wild pear

Table 8: Trees that are resistant to storm damage. Source: White (1994).

Acer pseudoplatanus Sycamore

x Cupressocyparis leylandii Leyland cypress

Magnolia (tree species) Magnolia

Ilex aquifolium Holly

Metasequoia glyptostroboides Dawn redwood

Robinia pseudoacacia Black locust

Sequoiadendron giganteum Wellingtonia

Taxus baccata Yew


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Table 7 lists trees which should benefit from high-er temperatures in the UK, and Table 8 trees whichare resistant to storm damage.

6.3.2 FRUIT TREES AND BUSHES

As discussed in chapter 4, many fruit trees and softfruit bushes have a winter chilling requirement tobreak dormancy before flowering (then fruiting)can occur. Higher winter temperatures could pose aserious problem to fruit growers. Problems arealready occurring with the blackcurrant crop afterunusually mild winters (Carew, pers. comm.). Theproblem may be overcome in the short term by sub-stitution of cultivars with smaller chilling require-ment and, in the longer term, by breeding or bychanging crops, from apples, pears and cherries topeaches. Clearly, this would represent a majorinvestment requiring long term planning and havinga significant impact on commercial fruit growers.

In heritage gardens, a particular problem may arisein growing historical cultivars of fruit trees. Manyof these were selected and grown originallybecause of their close adaptation to local climaticconditions, so additional care in cultivation will berequired when these conditions cease to exist.

6.3.3 TREE MANAGEMENT

i) Domestic gardensTrees in or near smaller gardens will inevitably benear to property, so great care is needed in theirmanagement. It is very difficult to balance thevisual and ecological value of a large oak on ahousing estate, with the real or perceived danger ofbranch loss or toppling in strong winds. The pres-ence of a house or houses will exacerbate theimpact on large trees of high temperatures, sum-mer drought and gusting winds, so it may be nec-essary to feed or water trees to maintain theirhealth. All operations connected with tree care inconfined spaces are expensive.

With newly planted trees, management is less com-plicated. The most important management consider-ation is to match the choice of species to site condi-tions. Most owners will plant small, ornamental treeswith limited lifespans, so long term considerations of

adaptation to climate change will not be an importantfactor in the choice of species. Large species are nev-ertheless important features of our urban landscapesand future planting policies need to perpetuate andenhance tree cover and ensure good management.

ii) Heritage gardensA very different situation applies to the manage-ment of trees in large gardens, and especially inthose heritage gardens which include parkland andornamental woodlands.

The average unit cost of dealing with trees dam-aged by weather extremes or weakened by climatechange, using skilled forestry or garden staff fromthe estate, will be much less than the cost of deal-ing with a tree in the urban setting of a domesticgarden using external contractors, although ofcourse the total cost of dealing with large numbersof trees (or with a large cedar in a courtyard) willbe very much higher.

Because of the generally high levels of knowledgeand skill in heritage gardens, the need to attend totrees will be recognised and measures such as soilaeration, irrigation, feeding and mulching are morelikely to be undertaken to arrest decline.

On average, again, a higher level of stress imposeddamage and decline may be tolerated in heritagegardens if the tree is seen at a distance, is unlikelyto create a serious hazard, or is one of many trees.Veteran trees are of historic and wildlife impor-tance, and they are often an intriguing feature.

Given the long term nature of heritage gardens,more attention will be given to the systematicreplacement of the tree population than to fire-fighting treatments on individual trees. The mainresponse to climate change impacts on trees in her-itage gardens will need to be directed at ensuring abalanced age range and, where possible, diversityof species, including species chosen to tolerate theanticipated climate changes for the next century.The long term advantages of using small plantingstock will usually prevail over the need to create aninstant impact with large trees which, when movedfrom the nursery, will have difficulty in establish-ing and in surviving hot, dry summer conditions.


92 Chapter 6

6.4 Shrubs and sub-shrubs and climate change

6.4.1 SHRUBS

Like trees, shrubs are vulnerable to summerdrought, waterlogging and wind damage, althoughthey are less susceptible to toppling by winds. Theenormous range of shrubs, such as roses, meansthat it would not usually be difficult to substituteone species for another where such substitution isacceptable. The life span of a shrub is such that cli-mate change during its lifetime is unlikely to short-en its useful lifespan. A particular problem canarise, as at the National Trust’s Sheffield Park(Sussex), when the canopy of mature trees is lost orthinned by storms, leaving shade demanding shrubsexposed to full sun and to the competition of vigor-ous weed growth in the wake of disturbance.

6.4.2 SUB-SHRUBS

Sub shrubs such as Fuchsia, Indigofera, Penstemonand some Ceanothus may become increasingly use-ful with climate change. Many are marginally hardy

because they originate in regions where the climatedoes not require adaptation to a cold season, and soallows more or less perpetual flowering. Givenincreasing temperatures in the UK, and especiallythe anticipated reduction in frost, sub-shrubs maymake a major contribution to the garden of thefuture. They are, though, often rather brittle andwill, of course, suffer badly in exceptionally severewinters. Most are not tolerant of waterlogging.

6.4.3 SHRUB AND SUB-SHRUB MANAGEMENT

i) Domestic gardensThe domestic gardener is likely to view shrubs aspart of a changing garden population. If a shrub iskilled by severe drought or by flooding, it will bereplaced, perhaps with something more tolerant butmore probably with something more interesting ornovel. The main factor determining the fate ofshrubs in domestic gardens is their size. As theyencroach too far beyond bounds they may bepruned, or removed and replaced. Shrubs areunlikely to be in place long enough to experienceany adverse impact of climate change.

High standards of husbandry – feeding, watering,pruning – will reduce impacts of adverse weatherwithin the smaller domestic garden, but the morepermanent the plant, the less likely it is to be recog-nised as needing or benefiting from such treatment.

Sub-shrubs have a promising role to play in domes-tic gardens as mean temperatures increase. Longflowering seasons and, in many species, some tol-erance of drought, make them useful substitutes forthe more demanding annuals. Many are also moredecorative than the majority of shrubs.

ii) Heritage gardensAs with other aspects of heritage gardens, theimpacts of climate change will depend on thedegree to which the status quo is considered impor-tant. In most heritage gardens carefully thoughtthrough management plans have had the effect ofmaking systematic pruning and/or replacement ofshrubs the norm, rather than allowing the structureof the garden to be swamped by excessive growth,then trying to rectify the damage by wholesaleclearance, perhaps prompted by widespread losses

Trees constitute the most vulnerable and most

visible component of the garden to face the impacts

of climate change. They are long -lived, so face the

longest period of exposure to climate change and the

highest risk of damage by infrequent events.

They also have the largest sail area exposed to the

elements. Beech will be especially vulnerable on

light soils and in dry areas of the south east, areas

where it has been extensively planted, is least

suited to, and which will suffer the largest degree of

climate change.

It will not be difficult to find trees suited to the

climates anticipated by the scenarios for the 2050s

or even the 2080s. Planting young trees from open

ground stock rather than container grown material,

and planting in autumn rather than spring, on well

prepared sites, will maximise the trees’ chances

of surviving climatic stresses. However, planting in

anticipation of continuing healthy growth for

centuries will not be possible, unless the rate of

climate change is abated.


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following drought, flood or storm damage. Wheresome freedom of interpretation is acceptable,shrub replacement may reflect conditions resultingfrom climate change. Otherwise, high standards ofhusbandry will be needed to retain authentic plant-ing until it becomes impossible.

If there is scope for innovation, then the potential foruse of sub-shrubs will be as for domestic gardens.

6.5 Herbaceous perennials

The range of herbaceous perennials is such that itis difficult to generalise on their responses to cli-mate change. The Beth Chatto garden in Essex,where conditions range within a few metres fromdry gravel to heavy clay, and where the plant rangevaries accordingly, is one of the best examples ofthis diversity (Chatto, 1994).

Many of the denizens of the traditional herbaceousborder will not adapt well to climate change, andespecially to water stress in summer. This is espe-cially so for the most highly developed herbaceousperennials such as aster (eg, Aster novi-belgii), del-phinium (Delphinium x cultorum), lupin (Lupinus xregalis) and phlox (Phlox paniculata) which havebeen selected and bred for garden use, on theassumption that soils will be deeply cultivated andwell supplied with nutrients and water (Martineau,1913; Roper, 1960). There is also a problem inselecting plants tolerant both of summer droughtand winter wet. Old cultivars of iris, for example,are tolerant of dry summer conditions but will bekilled by waterlogged conditions in winter.

As with most other plant forms, good soil cultiva-tion will help to ameliorate the adverse compo-nents of climate change. Improvement of soil

drainage and raising the soil level even by 10-15cmwill improve the chances of survival of plantsintolerant of wet conditions. Recent trends in theadoption of Dutch and German ideas in naturalis-tic planting of herbaceous perennials (vonSchoenaich, 1994) and gravel gardening or drygardening are already becoming popular in theUK. They may be tailored to suit a particular suiteof climatic conditions by appropriate plant choice,with modifications to the landform where neces-sary to improve drainage.

One characteristic of herbaceous perennials is thatmost establish very quickly, usually within a sea-son and certainly by the second year. If plantingschemes in general are increasingly threatened byregular swings from summer drought to winterfloods, a scenario which greatly exaggerates thechanges anticipated by UKCIP02 scenarios,increasing reliance on herbaceous perennials, pro-vides one attractive strategy for rapid reparation ofthe damage – as always, within the bounds of whatis acceptable in any particular garden.

It is possible that the need to stake plants willdecline if summer rainfall intensity declines andaverage wind speeds drop, as plants may also bemore robust as a result of higher carbon dioxidelevels. However, the uncertainties associated withstorm predictions and the potential impacts ofthese events are such that inherently unstableplants should continue to be supported.

6.5.1 HERBACEOUS PERENNIAL MANAGEMENT

i) Domestic gardensThe domestic gardener will have the freedom tochoose from a very wide range of perennials hav-ing the ability to cope with conditions created byclimate change in a particular place. The usual bal-ance will have to be struck between increasingmaintenance inputs to grow particular plants, orchoosing plants suited to a particular situation.

ii) Heritage gardensThe need to retain the essence, and perhaps thedetailed composition, of a traditional herbaceousborder will impose some management difficultiesin heritage gardens. However, given the high

Shrubs will be affected by climatic change in the

same way as trees but to a lesser extent because of

their smaller size and generally shorter lifespan.

Sub-shrubs may play an increasingly important role

in gardens. Their long flowering season is an

advantage and their marginal hardiness will become

less of a hindrance to use as temperatures increase.


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inputs needed in the past in soil cultivation, regu-lar lifting and replanting, staking and other opera-tions, the additional effort of adapting to climatechange will not be as great as with more perma-nent forms of planting.

6.6 Bulbs

Spring bulbs are strongly influenced in their devel-opment by temperature (Rees, 1972). The generalpattern is that they require low temperature (i.e.winter) to stimulate root development, rising tem-perature (spring) to stimulate leaf expansion andflowering and, in some cases, such as tulip, hightemperature (summer) to stimulate flower forma-tion in the newly developing bulb for the followingyear. Actual temperatures controlling this develop-ment vary from one genus to another, usuallyreflecting closely the natural climatic conditionswithin which the particular genus evolved.

Many spring bulbs are already flowering much ear-lier as a result of warmer winters and springs thanthey were 20-30 years ago and may continue to doso. However, if winters become too warm, root ini-tiation may fail and the plant may be severely weak-ened or killed. It will then be necessary to lift andstore bulbs in refrigerated stores (as is already doneto force early flowering in tulip and narcissus),rather than using them as permanent garden plants.Increasing soil and air temperatures may also upsetthe synchronised development of the bulb, forexample by causing leaves of hyacinths to expandmore quickly than the flower spike, thus reducingthe visual impact of the flower. Spring displays ofcarpets of naturalised bulbs, like daffodils and cro-cus may disappear as a garden feature.

Some spring bulbs (tulip, some crocus) are intoler-ant of flooding while others (many Narcissus) aremore tolerant, and others (Camassia) will grow invery wet soils. The natural distribution of the bulbgives a very good indication of its tolerance orintolerance of flooding.

Most summer and autumn flowering bulbs willflourish in higher temperatures. Temperatureincreases anticipated in the UKCIP02 scenariosmay permit an increasing range of these late flow-ering bulbs (including bulbs, corms, tubers andother plants with fleshy rootstocks) to be grown aspermanent inhabitants of the garden. They mayalso be grown more widely across the UK, but onlyif spared wet winter conditions, to which mostsummer bulbs are exceptionally sensitive.

6.7 Annuals and tender perennials

Many annuals, especially hardy annuals, willexhibit accelerated development and/or suffer fromwater stress, given the higher temperatures andreduced summer precipitation expected with cli-mate change. They will flower, seed and die earli-er, thus reducing their garden-worthiness. On theother hand, higher spring temperatures will permitearlier planting of half-hardy annuals and higherwinter temperatures will permit an increasingnumber of annuals to be grown as hardy, ratherthan half-hardy annuals, and will permit anincreasing number of hardy annuals to be sown inautumn for an earlier summer display. Many tenderperennials, especially the Pelargonium, are welladapted to hot dry summers and will flower morefreely in such conditions. All are removed at theend of the summer season, so tolerance of winterconditions is not an issue.

Herbaceous perennials are a very diverse group

so it will not be difficult to find some which will grow

in altered conditions. They are more or less

short-lived so will not need to adapt to any signifi-

cant degree of climate change in the course of their

life. Because they mature quickly, they could play a

useful temporary role where shrubs and trees have

had to be replaced either in phased renewal or as a

result of storm or flood damage.

Bulbs are strongly influenced by temperature so

climate change will affect their timing and sometimes

their healthy development. Most summer and autumn

flowering bulbs flourish at high temperatures and

will respond positively to dry summer conditions.

They will become increasingly useful, and

increasingly hardy as winter temperatures increase,

but are very intolerant of wet winter conditions.


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Spring flowering biennials such as wallflower(Cheiranthus cheiri) and forget-me-not (Myosotissylvatica) will benefit from higher winter temper-atures. In northern areas, where spring planting hasbeen the norm for wallflower, it will be increasing-ly possible under conditions of climate change tomove to autumn planting, and the quality of over-wintered plants should improve in most areas.Wallflower, in particular, is not tolerant of wetsoils. In areas in which increased winter rainfall isanticipated and the result is likely to be poor win-ter drainage, it will be necessary to avoid its use orto improve soil conditions by cultivation or, moreusefully, by raising soil level.

Strategies of response to climate change mightinclude improved cultivation (especially irriga-tion where possible), a change in the garden flora(replacing short-lived hardy annuals by the moredurable half-hardy annuals – which will becomeless half-hardy as temperatures increase) andperhaps moving in the mildest areas from twice-yearly replacement of spring then summer bed-ding, towards a permanent planting of plants pre-viously considered to be tender perennials(Owen, 2002).

In warmer parts of the world, such as the southernUnited States and Japan, it is common to use threebedding schemes per year – in spring, summer andautumn -, instead of two, where summer bedding isexpected to flower until late autumn. As summertemperatures in the UK rise and summer droughtcauses premature senescence of summer annuals,this practice could be employed in southern areas(Shaddick, 2000). This obviously has major costimplications if carried out on any substantial scale,but for most domestic gardens the extra cost islikely to be lower because of the smaller areasinvolved. Autumn bedding presents significantopportunities for nurseries and garden centres tostimulate sales at what is currently a very quiettime of year (see Chapter 8).

Overall, the advantages of climate change in rela-tion to cultivation of annuals might outweigh thedisadvantages: the opportunities to experimentwith new half-hardy plants which may eventuallybecome hardy as temperatures increase, are legion

(Shaddick, 2000). The great risk with annuals isthat, if weather conditions in a particular seasonare unfavourable to the plants being used in thatyear, the effect may be disastrous. On the otherhand, if the planting does fail to produce theplanned-for result, there is no permanent loss to thegarden. Good ground preparation and, where pos-sible, the availability of irrigation as a precautionagainst excessively dry summers, will maximisethe likelihood of favourable results.

6.8 Lawns and other grass areas

Grasslands are very characteristic of large areas ofthe UK’s landscape and a very important compo-nent of many gardens. A long history of grazinganimal husbandry and what has been a veryequable climate, has led to much of the countrysidebeing clothed in grasslands. These habitats oftenexhibit great species diversity. Visual delight in thisgreenness encouraged the cultivation of lawnswhich could be maintained with relatively littleeffort compared with most other parts of the world,and are much admired by overseas visitors.

The image of the UK as a ‘green and pleasant land’is an important factor in encouraging tourists tovisit, and it may be an increasingly important fac-tor in encouraging UK citizens to holiday at homeinstead of travelling to increasingly hot, dry desti-nations abroad. It is our view that the image couldbe damaged if, as seems inevitable in some parts ofthe UK, climate change leads to summer browningof the grass. There is a more direct economicimpact for large gardens and parks in which thegrass is used by grazing animals. Lower stocking

Annuals and other short-lived and temporary plants

will obviously not need to adapt to climate change

in their own lifetimes. Many hardy annuals will flower

earlier but seed and die more quickly in hot, dry

summers. Half-hardy annuals and tender perennials

will be favoured by higher temperatures but most

will need adequate water supplies to sustain lush

growth and free flowering. On balance, climate

changes will favour the exciting uses of annuals and

other ephemerals.


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rates and greater conservation of grass as hay orsilage may be necessary. On a regional scale,changes in farming practice in response to climatechange, such as the conversion of grassland toarable cultivation or the replacement of wheat bysunflowers (Wade et al., 1999), will also haveimpacts on the setting of the rural garden.

6.8.1 THE DIVERSITY OF GRASS AREAS

The lawn, meadow or parkland represents a partic-ular type of plant community created by regulardefoliation of the vegetation, by grazing animals orby cutting. Over many years (centuries in someinstances), the species composition adapts to localsoil and climatic conditions. Many grasslands con-tain a rich diversity of species. Some survivingfragments of species-rich grasslands have beendesignated as Sites of Special Scientific Interest(SSSIs) supporting a wide range of insects andother invertebrates, as well as many species ofplants. Old garden lawns, such as those atChatsworth (Derbyshire) or Charles Darwin’s gar-den at Down House (Kent) which have been regu-larly mown, can also be of considerable ecologicalsignificance.

On the other hand, it is possible to manage the lawnto favour only a small range of the finest-leavedgrasses and to eliminate especially the broadleavedplants, in order to achieve a fine, rich green lawn ofbowling green quality. It is also possible to modifythe lawn, by heavy feeding and careful attention todrainage, to make it tolerant of heavy wear,whether from sports activities or from casual useby garden visitors.

As a general principle, the shorter the grass, thegreater the desire for uniformity (especially theabsence of weeds), and the smaller the area of thelawn, the more effort will need to be invested in it.A first class bowling green may require mowingevery day, brushing, spiking, feeding at frequentintervals and irrigation, if dry weather persists formore than a few days. At the other extreme, a largeexpanse of grassland serving mainly as a green set-ting might, if not grazed, be cut once a year (evenless, on a poor soil) and receive no other treatment,but still continue to serve its purpose well.

As lawns are almost invariably maintained usingmowers powered directly or indirectly by fossilfuels, they constitute a visible contributor to cli-mate change. Calnan (pers. comm.) has calculatedthat the National Trust uses 82,000 litres of fueleach year to mow 30 square miles (77 sq km) oflawn at a cost of £136,000. In the context of the70,000 litres of fuel required to power a Boeing747 on a single journey across the Atlantic, thisannual consumption is insignificant, but if the prin-ciple of reducing fossil fuel use by changes in man-agement (see section 6.8.3) can be demonstrated inNational Trust and other heritage gardens, the mes-sage may be picked up more widely.

6.8.2 CLIMATE CHANGE IMPACTS ON THE GROWTH

OF GRASS

The traditional smooth, closely-shaved lawn of UKgardens will be disadvantaged by higher summertemperatures, drier summers and wetter winters.

Increasing temperatures, in particular, will lead totemporary and long term changes in the composi-tion of the grassland community. Jeffery (2001)found that annual meadow grass (Poa annua)decreased when turf temperature (and indirectlyair temperature) was increased by 3°C, whileclover (Trifolium repens), yarrow (Achillea mille-folium) and browntop bent (Agrostis tenuis)increased. In Australian research, extreme butshort term temperature increases decreased theproportion of cool season grasses, such as fescuesand increased the presence of warm season grass-es, such as Bermuda grass, but the effect was tem-porary (White et al., 2000).

Water deficits result in reduced growth. The pro-ductivity of grassland is inversely proportional toJuly and August temperatures and directly propor-tional to summer rainfall (Sparks and Potts,1999). This is not usually a disadvantage in itselfin gardens, but it can cause severe problems inparkland, where the grass is a food supply forgrazing animals as well as a visual amenity.Deficits also eventually result in discoloration ofthe grass. This in itself causes long term damageonly in very extreme circumstances. After thesummer-long drought of 1976, for example, it


97Chapter 6

took just ten days for lawns in the University ofReading’s teaching garden to return to a satisfac-tory state of greenness. However, the immediatedamage in visual terms is very serious for orna-mental lawns. Water stress will be especially seri-ous on close-mown areas, as continuous closemowing restricts root development (Bisgrove,1980).

Summer saturation of lawns after very heavydownpours poses a potentially bigger problem, inthat the grass will be very susceptible to com-paction and this will cause long term damage tothe lawn, unless extra effort is expended on aera-tion and other intensive maintenance practices. AtHidcote Manor (Gloucestershire), the cost of lawnrepairs and reinforcement is currently £3000 perannum and it is still necessary to close importantgrass paths at times, to prevent more serious dete-rioration from visitor pressure after heavy summerrains. Compaction problems may also arise inwarm, dry summers as visitor pressure can bedamaging when the soil is dry.

Increasing mean temperatures, a longer growingseason and increased rainfall during increasinglyfrost-free winters will result in greater productivityand more mowing, probably throughout the winterin the mildest areas. This has resource implica-tions, but also poses management problems, in thatmowing into the winter, combined with expecta-tions of higher winter rainfall, means that the riskof soil compaction by mowing equipment will beincreased, and permanent damage to the grass rootsystem is likely.

Increase of soil and air temperatures will almostinevitably also lead to higher incidence of pest anddisease outbreaks in lawns, because of the generalincrease in biological activity. Moss is only a majorproblem on most lawns in early spring when themosses, with their lower temperature thresholds forgrowth, are able to flourish in the absence ofactively growing grasses. In many regions, mosswill probably become more prevalent but paradox-ically, also less of a problem. Higher temperatureswill allow the moss to grow over a longer period inwinter, but earlier growth of grass in the spring willconceal and suppress the mosses.

6.8.3 GRASS MANAGEMENT

In order to explore potential responses to theimpacts of climate change on lawns and other grassareas, it is necessary to understand firstly thatgrassland results from regular defoliation and sec-ondly, that management inputs increase as the needfor a short and uniformly green surface increases.

One possible response to adverse climate impactswould be to accept different standards and thus topermit altered maintenance methods. A very likelyimpact of climate change will be increasingdrought stress and temporary discoloration of thelawn. Coping with the browning of lawns in dryperiods will require decisions as to whether to tol-erate the discoloration (to the visual detriment ofthe garden and perhaps the displeasure of payingvisitors) or to irrigate using increasingly scarcewater resources. Raising the height of cut, andreturning clippings to the lawn, could increase theresilience of lawns to drought, reduce the periodand degree of discoloration in summer and oftenreduce the incidence of moss in spring (Bisgrove,1980), but may lead to an increase in thatch prob-lems. In parts of the United States, lawns aresprayed with green dye to disguise the browningwhich occurs as a result of low temperatures inwinter or drought in summer.

In large gardens, the owner or manager could con-template greater development of more naturalistic‘meadow’ areas (Owen, 2002) with less frequentcutting, bearing in mind that long grass may poseincreased fire risk in dry summers and may even-tually harbour ticks and other undesirablewildlife, if temperatures continue to increase (seesection 7.1).

There is a further complication in decision-mak-ing in relation to frequency of grass cutting, in thata rotary mower uses much more fuel to cut a givenarea of lawn than does a cylinder mower. Anychange in mowing regime which results in theneed to change from cylinder to rotary mower (inorder to tackle longer grass), will not give the sav-ings in fuel consumption or time that one mightinitially expect, although it may produce a moreresilient turf.


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In small gardens, gardeners might respond to morefrequent browning of lawns by applying more irri-gation and fertiliser), or by replacing grass withpaving, gravel or ground cover.

Increasing variability of weather conditions willalso complicate management. Routine mowing oflawns during a summer of alternating drought,when growth ceases, and heavy rain, when growthis rapid but soil conditions unsuitable for mowing,could become questionable. The need to mow grassin increasingly warm but wetter winters becomesincreasingly probable. In long, wet periods it willbe necessary to decide whether to mow the grassand risk soil compaction or to accept greater vari-ability in height and tackle longer grass intermit-tently in the drier periods. Mowers could be devel-oped to help gardeners manage lawns through pre-dicted milder winters.

If mowing is done in-house (and especially indomestic gardens where the owners are ‘the staff’),a more flexible regime may be possible, switchingbetween mowing and other operations on a moread hoc basis, but the management of lawn areas bycontractors will be more difficult to specify andtherefore more costly. In any case, managementwill be more complex.

Some respite from increasing temperatures may beachieved by using newer cultivars of turf grassesdeveloped in the United States, though these usual-ly have the disadvantage of being bred for golfcourses and other highly managed landscapes, sorequire high nutrient and irrigation inputs.

Very substantial increases in temperature mayeventually necessitate the replacement or rein-forcement of the current range of cool temperate(C3) grasses with warm-temperate and subtropical(C4) grasses, as in much of the southern UnitedStates and southern Europe, but this is veryunlikely to be necessary by the 2080s even underthe high emissions scenario. Cool temperategrasses have an optimum air temperature forgrowth of 15-24°C and an optimum soil tempera-ture of 10-18°C, compared with 27-35°C and 24-29°C respectively for warm season grasses (Ward,1969), and warm season grasses discolour badly at

sub-optimum temperatures. Such substitutionwould represent a very major change in the quali-ty of lawns in the UK as warm season grasses aregenerally coarser and less tolerant of close mow-ing. Very considerable inputs in terms of irriga-tion, fertiliser application and disease controlwould then be required to maintain the fine tex-tured, short and soft turf which has been the hall-mark of the UK lawn.

i) Domestic gardensIn a small domestic garden, intensive managementof lawns is possible because of the small scale,even to the extent of using bath water for lawn irri-gation in times of hosepipe bans. More commonly,most domestic gardeners are accepting and movingto lower standards of maintenance, slightly longergrass cut by rotary rather than cylinder mower, andwith some acceptance of weeds and summer dis-coloration. Because of the regular time commit-ment imposed by lawns, perhaps combined withproblems resulting from increasingly frequentsummer droughts, small lawns in particular arebeing replaced by gravel, paving or decking.

ii) Heritage gardensThe specific character of different parts of a her-itage garden will call for particular standards ofmaintenance which may vary from bowling greento rough meadow. The skills are available for sen-sitive, appropriate and varied maintenance. Anextensive armoury of equipment for mowing, aera-tion, irrigation and other operations is available,though at a cost, and the scale of the garden maymerit investment in a range of such equipment.

Particular problems of compaction caused bymowing in wet weather or by increasing visitornumbers after wet weather may require sports turftype management, but the cost may be justified byincreased visitor capacity.

There may be some scope for conversion of finegrass surfaces to lower maintenance meadows,especially in those gardens where excessively highmaintenance is a recent departure from earlier con-ditions. In grazed areas, the problem of balancingstocking rates with varying productivity of park-land grass will need to be addressed.


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6.9 Paths, walls and garden structures

6.9.1 PATHS AND WALLS

Paths will need to be designed, constructed andmaintained to prevent washing out in storms and toavoid large volumes of water being dischargedfrom paved surfaces onto erosive soil during heavyrainstorms. It may also be necessary to choose pathand paving surfaces to avoid (or facilitate the treat-ment of) algal growth, especially if gardens areopened to the public early in the year as the flower-ing and garden visiting season advances.

6.9.2 GARDEN BUILDINGS AND STRUCTURES

Wind and sun damage to fabric, and increased wet-ness from driving rain are likely to be the main prob-lems associated with garden buildings (Jarman,2001). Deterioration of fences, pergolas and otherwooden structures will probably accelerate if highsummer temperatures dry the timber excessively,

leaving gaps for penetration by winter wet and fungaldecay organisms. Higher standards of construction(including larger gutters and downpipes) and mainte-nance, and more use of durable timbers from sustain-able sources where appropriate, will assist in over-coming these problems. There will be cost implica-tions though, and limitations to such adaptations inlisted and other important buildings. Antique statuarymay require special protective covers in winter toprevent damage from extreme weather events.

Above and below ground, archaeology may also bevulnerable to climate change impacts, such as ruinsin gardens exposed to flash floods. In recent years,flash flooding episodes have become more frequentat Studley Royal (Yorkshire), causing particularconcern for the surviving structure and foundationsof one of the garden’s picturesque ‘eye-catchers’ –Fountains Abbey.

Good ventilation of greenhouses and adequateshading will be necessary to avoid excessive build-up of heat in summer, for the benefit of the plantsand of the gardeners looking after them.

On the positive side, glasshouse heating costs in thewinter should be substantially lower in warmerwinters, and frost damage to stone and brickwork,sculpture and other features of ‘hard’ landscapeswill be reduced.

6.10 Garden staff

Those faced with the maintenance and manage-ment of gardens in the 21st century will face amore challenging task in dealing with climatechange. Physical working conditions shouldimprove throughout much of the year as tempera-

The lawn is a very characteristic feature of UK

gardens and is very likely to be adversely affected

by climate change. The more the lawn departs from a

natural meadow community towards a highly man-

aged, very short and weed-free green carpet, the

more vulnerable it will be to climate change impacts.

High summer temperatures and reduced

precipitation will reduce grass growth, sometimes

completely, and cause the lawn to go brown.

In winter, increasing temperatures and rainfall will

stimulate grass growth throughout the winter,

particularly in the southern UK. The mowing season

may shift with year-round mowing possible in the

mildest areas. The need to mow when the soil is wet

and therefore susceptible to compaction will be

difficult to manage.

Adaptation to climate change will require cultural

modifications (increased height of cut, timely

application of fertilisers, or acceptance of brown

summer lawns), or technical responses (irrigation

and perhaps new mowing equipment) or a

combination of these. Sensitive and responsive

management will be increasingly important.

High summer temperatures, heavy downpours and

driving rain, sun and wind damage, and increased

pest activity may accelerate deterioration of garden

structures. Higher standards of construction and

upkeep, and improved techniques of decay

prevention will be required to maintain garden

structures. On the other hand, frost damage and

glasshouse heating costs could decrease as a result

of climate change.


100 Chapter 6

tures increase in winter and rainfall decreases insummer, but very hot or very wet conditions willmake work more difficult at times. Care will beneeded to prevent dehydration in high temperaturesand more consideration will be needed in choosingprotective clothing suitable for use at high temper-atures. Care will be needed, too, to guard againstthe effects of higher UV light levels in summer.

Coping with climate change may involve more workand more stress, especially for those who have invest-ed years of their lives in establishing and caring fortheir gardens. Maintenance (especially mowing) willbe less predictable. Dealing with storm, flood or dis-ease damage to the garden is likely to be mentallystressful as well as physically demanding. The addi-tional ameliorative or remedial work required torespond to the impacts of climate change, such as soilcultivation, irrigation, mulching, more complicatedmowing and more active intervention to ensure ahealthy age range of trees, are all likely to add to thecost of managing gardens.

6.10.1 PERCEPTIONS OF CLIMATE CHANGE BY

GARDEN MANAGERS

When garden managers were asked about their per-ceptions of the potential impacts of climate change,most respondents saw a mean temperature rise of 2-5°C as an advantage as the range of plants whichcould be grown could be extended, and heating costsin glasshouses could be reduced. More extremechanges – a threefold increase in the number of daysabove 27°C – could enable arid zone plants to begrown in the garden and might increase visitor spendin shops, if they seek relief there from the outdoorheat. These comments indicate the diversity of fac-tors which garden managers need to consider.

Generally agreed negative aspects included theneed for increased watering, the possibility of newor increased pests and diseases, the need forincreased mowing and the possible loss of visitorsto the coast in sunny weather. The prospect of 5-10days each year with temperatures in excess of 40°Cwas seen by all respondents as a negative impact, asit could lead to plant damage, severe problemsunder glass, the need for irrigation and a probablereduction in visitor numbers.

In other respects there was an obvious regionalinfluence in the perceived impacts of climatechange. A longer growing season and a substantialreduction in the number of frost days were consid-ered to be beneficial, especially in northern gardens,but pest, disease and mowing problems were raised.

Responses to the prospect of a 20% increase inwinter rainfall were more varied. One respondentin the east of England saw a potential benefit inimproved tree health, but others saw only disad-vantages resulting from waterlogging, saturatedlawns, more damage, a reduction in available work-ing time and decreased visitor numbers.

The prospect of a decrease in summer rainfallreceived equally varied responses. Several respon-dents identified the need for increased irrigation asthe main potential disadvantage, with possibledamage to lawns and some plant loss as secondaryconcerns. Interestingly, the respondent from thedriest garden saw no problem as the challenge wasalready present, but saw the possibility of growingmore plants from semi-arid regions as an advan-tage. The respondent from one of the wettest gar-dens also saw possible advantages of reduced sum-mer rainfall in improved summer weather for visi-tors, and in creating better meadows.

Not surprisingly, the likelihood of more variablerainfall with more frequent droughts and heavy rain-storms was viewed as an entirely undesirable aspectof climate change, making planning of events andoperations more difficult. However, three of the tenrespondents suggested that uncertainty was alreadya normal feature of their operations and that any fur-ther change would make little difference. The pos-sible increased incidence of strong winds was alsoseen as wholly undesirable with the potential fordamage to plants and buildings, tree loss andincreasing the need for safety audits.

The prospect of a 50cm rise in sea level was con-sidered irrelevant to most respondents except theone manager gardening on the coast, whose gardenhad been under water for several months in the pre-vious winter. He noted from first hand experiencethat sea level rise would be devastating in its effectson plants and visitor numbers.


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The threat of new pests, salt damage, ultra-violetlight damage and managing the uncertainty ofchange were also identified as potential difficultiesor challenges. The potential inability to grow theexisting plant range was also raised as a problem byone gardener, as additional management inputwould be required to design new planting schemes.The fact that this point was raised just once sug-gests that managers are either confident of theirability to grow plants under future climates, or thatother problems associated with climate change,were considered more pressing.

When asked if actions were being taken to adapt to ormitigate climate change, six interviewees replied thatthey were engaged in or planning measures to reducethe environmental impact of their operations, includ-ing recycling, minimising carbon dioxide emissionsand reducing greenhouse heat losses. A seventh hadinstalled a new irrigation system to counter drought.No actions were planned by the other three respon-dents – in one case because none was affordable.

A majority of the responses to the individual ques-tions suggested that climate change would be dis-advantageous. Of the hundred individual responses(ten respondents with ten questions), 51 indicateddisadvantages, 27 advantages, 12 a balance and 10no impact. Paradoxically, though, when asked whatthe overall effect of these possible changes wouldbe, four replied “advantage”, three “balance” andthree “disadvantage”

6.10.2 RESPONSES OF GARDEN STAFF

i) Domestic gardensIn domestic gardens, the impacts of climate changeand responses to it will depend on the nature of theowner/gardener. The owner may choose to garden atdifferent times of day (in the cool of the lateevening), or to postpone maintenance in wet weath-er. They may also choose to exploit climate changewith more adventurous planting, to battle against itusing more intensive management, or to avoid itsimpacts by adopting low maintenance features.

ii) Heritage gardensIn heritage gardens, the impact of climate changewill depend on the nature of the garden. There may

be some sense in changing working practices, forexample an earlier start or later finish and longermidday break in very hot weather, but the managerwill need to operate within the constraints ofemployment law and contractual obligations. Themost difficult situation will apply when mainte-nance operations are contracted out, a situationwhich often applies in relation to grass mainte-nance. A horticulturally sensible approach of cut-ting grass when it needs cutting and when soil con-ditions are suitable for mowing is more difficult towrite into a standard contract than is the specifica-tion of a routine mowing regime. If such a specifi-cation is produced, the contractor will almostinevitably increase their quote to protect them-selves against the uncertainties within the task.

One of the important and costly impacts of climatechange on heritage gardens is that staff will need tobe increasingly highly trained to be able to contendwith impacts of change on the garden itself. Manyof the operations described above, although an inte-gral part of good garden management, will need tobe carried out more often and with greater thor-oughness, so increasing staff levels will be requiredif standards are to be maintained.

Climate change will have some benefits for garden

staff in terms of their working environment in

generally warmer and drier springs, summers and

autumns. Very high temperatures in summer

and wetter winter conditions will need to be

contended with.

Coping with the uncertainties and adverse

impacts of climate change and with damage

caused by extreme weather events may

increase job stress for gardeners. Most

respondents to the questionnaire felt that climate

change presented a mix of advantages and

disadvantages with their responses being

influenced by the geographical situation of their

garden. When asked about the overall impact of

climate change on their garden, responses

ranged very evenly across the spectrum of

“advantage” to “no overall impact” to

“disadvantage”.


102 Chapter 6

Box 6.1 Cost implications of climate change for gardens

It is impossible, in a study of this scope, to deal in detail with the costs of climate change in each and every type of garden,

and for each garden operation. However, the costs associated with managing the impacts of climate change on gardens fall into

six broad categories.

Plant growth. Higher growth rates over a longer season will result in a greater mass of vegetation. The removal and disposal

of surplus growth, whether lawn mowing or shrub pruning, will involve increased costs.

Plant failures. Removal and replacement of plants damaged by extreme weather events or by gradual failure over a sequence

of dry summers will be very costly. Experience of past events suggests that restoring gardens after major storms or floods could

mop up normal maintenance budgets of public gardens for several years.

Pest, disease and weed problems. Managing these problems will become increasingly resource intensive, particularly as

environmental concerns and tougher legislative requirements are reducing the available range of approved pesticides.

Reducing negative impacts. The equipment and resources required to reduce the impacts of climate change will have

significant costs. Irrigation water itself is likely to become more expensive, if it is available at all in times of severe drought.

Insuring for damage. Greater care will need to be taken to protect buildings and other garden structures against damage and

decay. This may involve increased maintenance, the use of more expensive building methods, and increasingly rigorous safety

inspections of trees and dams. Insurance cover against severe weather damage will be important, but may be increasingly

costly or difficult to obtain. Insurers are already taking steps to withdraw protection from some flood risk locations, unless policy

changes are made. Greater financial reserves may be required to cover loss.

Managing negative impacts. A major weapon against the adverse impacts of climate change will be sensitive management

with long term strategies for the care and phased repair and regeneration of all components of the garden. The increasing

complexity of dealing with climate change impacts will require more highly skilled staff, who may demand higher rates of pay if

such staff are forthcoming.


103Chapter 7

Climate change and garden visitorsThere are two distinct but related aspects to theinterrelationship between gardens and people: theeffect of climate change on people, including theirpropensity to visit gardens, and the effect of peopleon gardens as climate changes.

7.1 Impacts of climate change on garden visitors

Visitor numbers to gardens will be influenced bymany factors, of which climate change is onlyone. A recent press release from the EnglishTourist Council (26 August 2002) indicates thatvisitor numbers to tourist attractions in Englandhave decreased by 2% in the past year, in part asa result of the destruction of the World TradeCentre on September 11 2001 and the aftermathof the outbreak of Foot and Mouth disease.Visitors to many rural attractions decreased by25% or more. Visits to theme parks and gardensincreased, however, after several years of static orslightly declining numbers of garden visitors. Arecent survey of National Trust visitors con-firmed that nearly 60% came solely to visit thegardens, such is their popularity.

Scenarios of social change point to an ageing pop-ulation, increased leisure and increased mobility.Such changes could result in increasing numbersof people with the inclination and ability to visitgardens. A swing away from the foreign packageholiday, concerns over safety and the environmen-tal impact of air travel might discourage travelabroad. As hot areas of the world become hotterstill, there may be further disinclination to travel. Ifthese deterrents are combined with improvementsin UK facilities (cleaner beaches, refurbished holi-day resorts, higher standards of catering and morevisitor facilities in gardens for example), therecould be a positive swing towards holidays athome. The UK market could also benefit from anincrease in short holidays.

If, however, pensions are reduced and living stan-dards fall, and measures to limit private car use areimplemented without a commensurate improve-

ment in public transport services, visitor numberscould be expected to decline.

Predicting the number of visitors to gardens is fur-ther complicated by competition or synergy fromother attractions. The increase in visitor attractionsas a result of Millennium Commission funding anda general increase in the provision of leisure facil-ities has made it difficult to maintain visitor num-bers at older attractions, but there is potential forsynergy if nearby attractions cooperate in advertis-ing. This is clearly demonstrated in the success ofthe Cornish Gardens consortium. More recentlythe Eden Project, one of the most successfulMillennium Commission funded projects, hasdemonstrated its success in attracting visitors toCornwall and feeding them on into other Cornishgardens. The benefit to the local economy of theEden Project in its first year of operation, was £11million (Kendle, pers. comm.).

Climate change, although only one of many fac-tors, can be expected to influence visitor numbersin several ways. Improved weather should attractmore visitors to gardens. Evidence suggests thatgood weather, especially in spring, boosts visitornumbers (Entec, 2000) and that bad weather,especially at Easter, the traditional start of thegardening and garden visiting season, dramatical-ly reduces visitor numbers to gardens and gardencentres.

In the early and late parts of the year, the ‘shoul-ders’ of the holiday season, higher temperaturesand a longer growing season should encourage vis-itors (Entec, 2000). Spring temperatures and earlyflowering are already encouraging many gardens toopen earlier in the year. The UKCIP02 scenariosall point to a 10-20% reduction in autumn rainfall.Unless plants suffer from premature leaf fallbecause of summer drought, the higher autumntemperatures, sunnier conditions and the contrastbetween high day temperatures and cool nightswhich results from clear skies, should favour thedevelopment of autumn colour and further encour-age visitors. The pattern of daily visiting hours may


104 Chapter 7

also alter with climate change, as visitors may pre-fer to visit in the early morning and evening toavoid the heat of the day.

The effects of climate change on the incidence ofpests and diseases on plants have already been dis-cussed in Section 3.5, but the possible impacts ofclimate change are not limited to plant pests anddiseases. As summer temperatures increase, ticks,which are vectors of Lyme Disease in humans, andperhaps mosquitoes, could become more common(Department of Health, 2002). If the public per-ceive the risk of exposure to such insects to haveincreased, they may be less inclined to wanderthrough meadows or to picnic. This could nega-tively influence visitor numbers.

There are clearly many factors that influence vis-itor numbers to gardens, and each major gardenwill have its own set of parameters determiningits catchment area and the likely threats andopportunities arising from climate change.Climate change will undoubtedly affect all gar-dens in the UK to a greater or lesser extent.However, the most important influence on a gar-den’s attractiveness to visitors and on visitor num-bers will be marketing in its broadest sense. Ifgardens can continue to offer the high qualityenvironments that the public seek, they shouldsucceed in maintaining and increasing visitornumbers.

7.2 Impacts of visitors on gardens in achanging climate

The need to attract visitors is a very important fac-tor having an impact on heritage gardens in partic-ular. When garden managers were asked to identifythe main directions of development of their gardensand the main influences on forthcoming changes,visitor facilities were central in replies to the ques-tionnaire. The main categories of response were:

• to upgrade and/or expand facilities for visitors;• to increase visitor numbers, the range of

visitors and the visitor season;• to increase the educational value and use of

the garden;• to increase the diversity of the garden;• to increase operational efficiency.

Four of these five relate directly to increasing visi-tor numbers and visitor satisfaction. Other objec-tives included expansion of the garden (offeringmore to visitors), raising standards (making it moreattractive to visitors), integration into the sur-rounding landscape and expanding the scientificbasis of the (botanic) garden.

Most responses also reflected the need to meetbudgets (increase income) and to justify the exis-tence of the garden by increasing educational pro-vision, linking plants with science, and attractingmore visitors over a longer season.

Of the ten responses to the question “What is yournext major development?” five referred to newplanting schemes, one to the replacement of exist-ing planting (a hedge killed by flooding), two tobuildings (visitor facilities), one to the reopeningof an historic garden route, and the tenth to a lakefor water conservation. None of the plantingschemes reflected any explicit or obvious directresponse to, or awareness of, climate change,though the replacement of a hedge killed by flood-ing (too much water) and the lake to store water forirrigation (too little water) indicate the range ofproblems which might arise from climate change.

One effect of climate change is that significantincreases in visitor numbers might be expected at

Climate change is only one of many factors

which may influence visitor numbers to gardens.

Warmer weather could lead to increased visitor

numbers, but there may also be increased competi-

tion from the beach and other destinations. Warmer

and drier springs and autumns, may stimulate visitor

numbers, but very high summer temperatures may

discourage visiting.

Gardens will need to respond to changing climatic

conditions by providing, for example, adequate

visitor facilities to mitigate against the adverse

effects of poor weather. Ultimately, marketing in its

widest sense will be the most important factor

influencing future visitor numbers.


105Chapter 7

those times of the year when the weather is mostuncertain and potential damage to gardens mostsevere: a sunny weekend in February after a week ofrain could see large numbers of visitors arriving atrain saturated gardens, causing major problems ofsoil compaction and major discomfort in muddy carparks and on muddy or slippery paths. This hasalready occurred at Nymans in Sussex, for example.

Weather will remain highly variable in future, soinvestment in weatherproof facilities such as gar-den shelters, glasshouses and other indoor visitorattractions may be needed to sustain visitor num-bers and to encourage repeat visits.

Visitor expectations are equally difficult to predict.Expectations of high material standards such asgreen lawns and air-conditioned restaurants mayincrease the difficulty of adapting to impacts of cli-mate change. On the other hand, visitor expecta-tions of high ethical standards with respect to sus-tainable management of gardens may make it easi-er to introduce some modifications, such as accept-ing browner lawns, and more difficult to introduceothers, such as irrigation systems, made necessaryby climate change. Visitor education and interpreta-tion, and the conspicuous implementation of goodenvironmental practices, will be critical so that peo-ple may understand, accept and hopefully, adopt forthemselves, a more sustainable way of gardening.

All the garden managers responding to the ques-tionnaire survey indicated that increased visitornumbers were important to the funding of the gar-den and, in some, to meeting the educational objec-tives of the garden. Higher visitor numbers willexert greater physical pressures on gardens. Grasswas seen as especially vulnerable. Climate changemay result in an increasing number of days onwhich the weather is suitable for garden visitingbut when the ground is wet or exceptionally dry.Both circumstances can lead to severe soil com-paction beneath lawns. Intensive managementtechniques developed initially for sports turf canassist the garden manager to cope by invigorating,reinforcing, protecting (in exceptionally wetweather), or replacing grass. Such techniques arealready widely used in heavily visited gardens.There are cost implications and environmental

implications in, for example, mowing heavily fer-tilised grass or replacing grass with paving, but ifincreased visitors bring in increased income, thesolutions may be self financing.

It is evident that gardens are useful small models ofthe environment as a whole, and that some of thechallenges presented by climate change occur in,and can be remedied on, a garden scale. The abili-ty of a garden to demonstrate good practice in itsresponse to climate impacts may engender increas-ing support for gardens. Many gardens with aneducational aim are focusing on sustainability asthe major thrust of their activities. Gardens whichhave not in the past developed this role may be ableto boost visitor numbers and income, by imple-menting sustainable practices and marketing themas exciting and educational attractions.

Increasing visitor numbers will have impacts on

gardens. Shelters, wet weather facilities and

air-conditioned restaurants may be required and may

change the character of the garden – for better or

worse. Gardens could be most severely affected by

increased visitor numbers when the soil is too wet or

too dry to withstand compaction. Increased

management and maintenance inputs, and perhaps

changes of design, will be needed to reduce impacts.

Changing climate could also impact on staff and

staffing. In a hot, dry summer for instance, the

garden may be at its best in the early morning or

evening, but opening the garden at these times

would have significant impacts on staffing.


107Chapter 8

Climate change will have significant impacts forgarden-related industries such as nurseries and gar-den centres in relation to two aspects: operationaleffects and market opportunities.

8.1 Impacts on operations

Insofar as these enterprises are growers of plants,they will meet the much the same type of opportu-nities and risks as those facing gardens.

Higher mean temperatures will allow a wider rangeof plants to be grown (see Table 9, page 110), butextreme weather events may cause more damage.This damage will be exacerbated where plants aregrown in plastic tunnels or glasshouses, which arethemselves vulnerable to hail storms and strongwinds, and in containers. Container grown plantswith their root systems confined, often in a blackcontainer, are more susceptible to root damagefrom exceptionally high or low temperatures, thanare established plants in gardens. Notcutt (pers.comm.) has pointed out that there has not been anexceptionally severe winter since 1962/3, at whichtime container plant production was in its infancy.A severe winter now that container plant produc-tion is the norm, could have devastating conse-quences for nurseries and garden centres.

Reduced availability of water in the summer willhave major implications for nursery stock produc-tion. Even if not rationed, the cost of mains water islikely to increase as will the cost of boreholes,reservoirs and water treatment plants which may benecessitated by tighter regulation of water supplies.Container grown plants are much more susceptibleto moisture stress in hot, especially windy, condi-tions than are plants in the open ground, so a reli-able water supply is essential regardless of cost.Water conservation, recycling and adoption of moreefficient irrigation systems will help to offset theincreasing cost and decreasing availability of water.Novel work by Horticulture Research International,

East Malling, uses restricted water supply toimprove the quality of container grown plants(Cameron, pers. comm.). Reduction of wind speedon the nursery, using natural (hedges and trees) orsynthetic windbreaks, will also reduce water loss.

Although not strictly a result of climate change, theincreasing legislative and societal pressures arisingout of general concern for the environment, andtherefore stimulated by debate about climate change,will also pose challenges in terms of pesticide andother chemical use, restrictions on peat use, taxationof fuel, recycling of waste materials and tighter safe-ty regulations, all of which will be reflected in high-er production costs. Working conditions will deterio-rate on very hot days, especially in polythene tunnelsand glasshouses, possibly necessitating a midday restfor staff and improved clothing for spray operators orstructures with increased ventilation.

More unsettled conditions arising from climatechange will also have impacts on plant production.Container grown trees and shrubs, in particular, areinherently unstable and will fall over in high winds orheavy rain. Although a nursery will not usually facethe type of wind-blow risk which faces the owner ofa garden with mature trees, young trees will be dam-aged if containers are blown over. The cost of settingtrees upright after a gale, or of staking to preventblowing over, or of writing off damaged stock is con-siderable. More attention to shelter will reduce winddamage. Howard’s Nursery in Norfolk uses livingwindbreaks of Miscanthus sacchariflorus, whichalso has some potential as biomass for fuel.Increasing the weight of compost, although introduc-ing other problems, would make plants more stable,and probably better able to adjust from the nursery tomore normal, mineral soils. Container design couldalso be developed to improve stability.

Nursery infrastructure will also be affected by cli-mate change. A hailstorm at Notcutts nursery inAugust 1987 caused damage to polythene tunnels

Climate change and garden related industries


108 Chapter 8

and glasshouses, as well as to plants, with costsexceeding that of all frost damage in the past 50years. Notcutts garden centre in Peterborough wasflooded at Easter 1998 causing widespread loss toproperty and stock, and a flood during the sameperiod at the National Trust’s Bodiam Castle (Kent)destroyed the gift shop’s entire stock.

Indirect losses as a result of adverse weather condi-tions can be even more devastating. The flooding atNotcutts Peterborough centre resulted in loss of tradeat a crucial time of year for sales. At present, 50% ofgarden centre sales are concentrated into a few weeksof spring and early summer. Hilliers Nurseries selltwo-thirds of their stock in March, April and May(Woodhead, pers. comm.) Drought also reduces inter-est in gardening and has a major impact on the pur-chase of plants, lawn mowers and other equipment.

Perhaps the most significant implications of cli-mate change for garden industries will arise fromextreme weather events, such as droughts andfloods. These events may affect garden industriesmore severely than they affect gardens. The loss oftrees in a storm will be a devastating but temporarysetback in the lifespan of a garden, incurring extracosts and reducing visitor numbers during closure,but the garden has considerable cultural momen-tum. Suitably informed, visitors might be encour-aged to visit in larger numbers to see the devasta-tion from a safe vantage point and to support repairefforts. The loss of stock or of structures on a com-mercial nursery engenders no such sympathy and,without adequate reserves or costly insurance,there is a serious risk of bankruptcy.

The key to survival will be adaptability and manage-ment of risk. Irrigation equipment may be necessaryas temperatures rise and summer rainfall declines butit may be less acceptable as water shortages becomemore severe. A company which continues to produceonly hosepipes might run into trouble. One whichdiversifies into water butts, grey water treatment andmulch mats would probably flourish.

To survive, businesses will need to understand that theonly constant is change. For instance, plants weregrown in non-peat composts for centuries before peatbecame widespread less than half a century ago.

Among the benefits of historic garden conservationhas been the rediscovery of more sustainable methodsof production and the realisation that it is possible tomake a profitable enterprise out of demonstratingthese methods. The use of sun frames rather thansophisticated mist units in plant propagation, tradi-tional techniques of composting and organic produc-tion, and biological control of pests, have all led tomore sustainable plant production systems and oftenreduced costs of production. In addition to the directadvantages which these developments produce, thesuccess of demonstrating such techniques at the‘Lost’ Garden of Heligan (Cornwall), for example,shows the commercial advantages of adopting them.

8.2 Impacts on marketing opportunities

The warmer temperatures that should result from cli-mate change may stimulate the enthusiasm for gar-dening and the use of gardens. The sale of gardenproducts, such as garden furniture, tools and equip-ment for an outdoor lifestyle, may therefore increase.

Climate change also offers the opportunity forintroducing new exotic species onto British patiosand possibly even the open ground, benefiting bothsuppliers and customers. However, successfulestablishment of exotics depends crucially on ade-quate hardening off. For example Citrus ‘Meyer’sLemon’ has been cultivated outdoors in the UK forseveral years. The plant is grown by UK raisersunder glasshouse conditions, but needs carefulhardening off if it has a reasonable chance of sur-

Growers will be affected in the same way as gardens

will be by very high temperatures, water deficits and

other climatic changes, but because the nursery

stock and garden centre industry relies on rapid

throughput of plants, it will be particularly vulnerable

to extreme weather events. Awareness of the risk

climate change poses is important. Coping with risks

will require business decisions, balancing reserves

against increased profits or increased investment, as

much as any physical response. The costs, savings

and benefits of climate change adaptations should

be calculated in relation to the costs of increased

productivity or decreased damage.


109Chapter 8

viving outdoors. Many retailers are importing exot-ic new species from warmer countries. Withoutsuitable hardening off, it is likely that such import-ed plants may not survive the transition from grow-ing in a warm climate to a cool one.

Similarly for many potential new garden species,there exist in the UK selected clones which havedemonstrated their hardiness. If the same speciesare obtained from abroad it is likely that less hardyclones will be selected with the resulting poorestablishment. Clonal selection of exotic speciesfor growing in the UK is therefore important.

As gardeners become more adventurous, the loss ofgarden plants from drought, floods or late frostsshould potentially be beneficial for the gardenindustries if they can themselves manage to escapethe worst effects of extreme weather events.

Some idea of what may be possible in terms ofplanting in warmer conditions can be seen in gar-dens such as Tresco on the Isles of Scilly, or inmany Cornish gardens, which enjoy exceptionallymild climates. Nurseries such as Hilliers areexpanding their range of Mediterranean plants andpalms (Woodhead, pers. comm.) and others areexploring the boundaries of what is possible inrelation to the introduction of exotic new speciesinto British gardens (Emmett, pers. comm.).

A sophisticated international network of breedersand growers is feeding new plants into UK gardencentres, especially half-hardy bedding container,and hanging basket plants from Australia,California, Germany and elsewhere. Decrease insize of new gardens, an increasing emphasis onoutdoor leisure and disinclination to struggle tomaintain small lawns is leading to an explosion ofdemand for decking and containers.

Encouragement to change the contents of contain-ers several times each year could stimulate anincrease in year-round sales of plants and perhapsreduce the risks currently incurred in heavy depen-dence on spring sales. Indeed, autumn planting ingeneral may become more popular. Before theadvent of container plant production the majority ofplanting in UK gardens was carried out with bare

root plants in October, November and December.The availability of container plants has shifted themain planting season to the spring when morepleasant planting conditions usually prevail. A trendto warmer, drier autumns and increasingly mildwinters could regenerate interest in autumn plant-ing, a move which would also reduce the risk oflosses of newly (spring) planted material in summerdroughts. A particular opportunity for nurserymenwill arise, as summer temperatures increase andsummer rainfall decreases, of focussing customerattention on autumn gardening as a sign that theheat of summer is over in much the same way thatspring sales reflect delight in the end of winter. Thiswill require research and development of new cropsand crop schedules and a considerable marketingeffort, but potential rewards are substantial.

Table 9, overleaf, indicates species that have poten-tial as new garden plants in the future conditions ofclimate change.

Quite often it is not the ability to survive winter coldthat limits the introduction of new species into UKgardens, but their ability to tolerate wet conditions.For example many cactus species can tolerate tem-peratures well below freezing, but will not toleratewet conditions. Retailers may introduce new specieswhich are cold tolerant, but may be quite intolerantto the wet conditions which inevitably occur duringUK winters. As well as the possibility of introduc-ing new exotic species into gardens many ‘lessexotic’species are likely to do better under condi-tions of climate change. For example, cyclamen willthrive in milder winters and drier summers.

Climate change is likely to alter and expand the

range of plants which can be grown in the UK. There

is potential to encourage gardeners to use their

gardens more and to invest in them more, whether

on new outdoor furniture, new exotic plants or water

conservation equipment.

As with impacts of climate change on garden

visitors, so with impacts on garden expenditure,

there will be some direct and obvious effect of

climate change but marketing will be the main factor

in shaping the garden industry market.


110 Chapter 8

Table 9: Plants likely to perform better in a warmer climate. (Source: Emmett [pers. comm.])

The following genera (listed by family) are currently of interest to enthusiasts of exotic gardening because theycontain species that are currently on the borderline ofcold/wet tolerance in milder regions of the UK.

Agavaceae Agave

Cordyline (Torbay palm)

Yucca

Aizoaceae Carpobrotus (Hottentot fig)

Delosperma

Drosanthemum

Aloaceae Aloe

Araceae Alocasia

Arecaceae Chamaerops )

Trachycarpus) Palms

Phoenix )

Bromeliaceae Fascicularia

Puya

Cannaceae Canna (Indian shot)

Crassulaceae Aeonium

Crassula

Echeveria

Sedum

Musaceae Ensete

Musa (Banana)

Myrtaceae Callistemon (Bottle brush)

Metrosideros

Oleaceae Olea (Olive)

Proteaceae Banksia

Leucadendron

Grevillea

Protea

Restionaceae Elegia

Restio

Zingiberaceae Hedychium (Ginger)


111Chapter 9

This desktop study has brought together materialfrom a wide range of sources to explore the poten-tial impacts of climate change on gardens. Ofnecessity, its focus has been wide more than deep,and in many areas its conclusions have raised asmany questions as they have provided answers.

One of the main objectives of this report was toidentify gaps in our information on the impacts ofclimate change on gardening, heritage gardensand the garden industry and, from these gaps, todefine a future research agenda. While this is notan exhaustive list, several areas for further inves-tigation have been identified. These are outlinedbelow.

9.1 Climatological research

9.1.2 CLIMATE CHANGE SCENARIOS

Climate impacts on particular gardens will bestrongly influenced by their regional climate.Scenarios at a higher spatial resolution would facil-itate climate impacts assessments for gardens.

9.2 Horticultural research

9.2.1 FROST SENSITIVITY

There is some doubt as to whether plants will bemore or less damaged by frost with climate change.Further research is required to establish whetherplants will be more likely to suffer damage as aresult of frosting of precocious growth, or less like-ly to suffer damage because of reduced frequencyand severity of frosts.

9.2.2 THE EFFECTS OF WARMER WINTERS ON

DORMANCY AND PLANT DEVELOPMENT DURING

DORMANCY

There is a substantial literature on dormancy, butthe impacts of higher winter temperatures ongrowth, flowering and fruiting of garden plantsneeds more attention, as does its effects on winterhardiness. The possible interactions with elevated

carbon dioxide concentrations also merits furtherstudy. Defra is looking towards funding research inthis area.

9.2.3 AUTUMN COLOUR

Summer drought may result in premature leafsenescence and higher autumn temperatures indelayed leaf fall. The potential impacts of climatechange on these precursors of winter dormancyhave not been studied in any detail, unlike springemergence from dormancy, but they could haveconsiderable implications for the appearance ofgardens in the autumn.

9.2.4 PLANT HARDINESS

Hardiness is determined not just by innate toler-ance to freezing temperatures, but also by the con-ditioning of the plant in the previous summer orautumn. More refined maps of hardiness zoneswould enable informed judgements to be made onappropriate adaptations to climate change.

9.2.5 CARBOHYDRATE CONCENTRATIONS IN PLANTS

The effects of increasing carbohydrate concentra-tions in plants on flowering, autumn colour, andsusceptibility to pests and diseases needs furtherinvestigation.

9.2.6 PESTS AND DISEASES

The incidence and virulence of pests and diseasesin future, needs further investigation, with particu-lar emphasis on phenology, monitoring and envi-ronmentally sensitive control.

9.2.7 WEEDS AND POTENTIAL WEEDS

The introduced plants which are currently causingconcern in the UK, notably Japanese knotweed(Fallopia japonica) and Rhododendron ponticum,were in cultivation for a century or more beforethey became problems. It is important to under-stand why this should be, to review the various

Research and further actions


112 Chapter 9

causes of exotic plant infestations in other parts ofthe world, and to establish which plants have thepropensity to become problems in the UK as aresult of climate change.

9.2.8 THE RELATIONSHIP BETWEEN PLANTS AND

SYMBIOTIC SOIL FUNGI AND BENEFICIAL MICRO-

ORGANISMS

Climate change may have significant impacts onmycorrhizal and other symbiotic associations, bothdirectly and indirectly by influencing the hostplant. A greater understanding of these relation-ships may result in the ability to offset adverseeffects of climate change on trees, for example, byincreasing mycorrhizal activity.

9.2.9 LAWNS

Lawns already receive considerable research input,mainly because of the importance of grass surfacesfor sports. Most of this research is focused, however,on highly managed surfaces such as golf greens andfootball pitches. More research is needed, and theresults of previous research on agricultural grasslandand ecologically important grassland communitiesneed to be reinterpreted, to meet the needs of gar-deners seeking to adapt to the particular conditionsimposed by climate change. Ways of managinglawns in wet conditions during late autumn, winterand early spring require specific investigation.

Mower manufacturers would do well to look at pos-sible changes in mowing technology, to reducecompaction risk and perhaps to reduce the impactsof mowing on the environment.

9.3 Research on soils and water

9.3.1 SOILS

Soil is, in every sense, fundamental to the garden.Research into the fluctuations and fate of soil nitro-gen and the dynamics of the relationship betweennitrate release and uptake by plants or loss byleaching will have far reaching implications forgardens. Research into the dynamics of gain andloss of soil organic matter should also be consid-ered a priority.

Mitigation of climate change is not within thescope of this present study, but it must not be for-gotten that management of soil carbon (as organicmatter) will have impacts on mitigating the effectsof climate on gardens and on the mitigation of cli-mate change itself.

9.3.2 WATER SUPPLY AND DEMAND

Further analysis of the potential demand for waterin gardens on a regional basis will highlight prob-lems of supply, and foster examination of possiblemethods of reducing or meeting water demand

Further research and dissemination of good prac-tice is also needed to prevent loss of water qualityin ponds and lakes as a result of climate change andto explore the possibilities of storage and recyclingof rain and ‘grey’ water in domestic gardens.

9.3.3 WATERLOGGING

Amelioration and/or remediation of winter water-logging and its effects on the growth of mediter-ranean plants requires further invest-igation. Predicted warmer winters and hotter, driersummers will encourage gardeners to grow moremediterranean plants, which are increasingly popu-lar, but wetter winters will pose problems for theseplants, which are generally intolerant of winterwaterlogging. The suggested topic is a particularaspect of the wider issue of water management andone which is a practical issue for amateur garden-ers, who will want to exploit the opportunities togrow a wider range of exotic plants in an environ-ment of increasingly variable water supply.

9.4 Economic research

9.4.1 GARDEN VISITOR PATTERNS

Greater awareness is needed of visitor behaviour ina garden, so that gardens may better cater for visi-tors’ needs. Better understanding of the factorsinfluencing visiting is also required to inform mar-keting strategies in present and future climates.


113Chapter 9

9.4.2 INDUSTRY RELEVANT RESEARCH

Technical investigations into improved productionsystems, including improved water management,ventilation or cooling systems for greenhouses,management and timing of production of novelcrops, is required. Market research is also neededto gain a better understanding of customerbehaviour and requirements.

9.4.3 HERITAGE SECTOR

More research on climate change effects for her-itage gardens and landscapes is needed. EnglishHeritage has commissioned UCL’s Centre forSustainable Heritage to undertake a scoping studyto investigate likely risks and potential mitigationand adaptation strategies for the historic environ-ment. The scoping study will be published in 2003.Climate change impact monitoring is also identi-fied in the heritage sector’s forthcoming annualState of the Historic Environment report.

Specific heritage garden research will also beneeded to identify resources and level of invest-ment required to maintain the quality and integrityof these gardens and the criteria for the manage-ment of significant plant collections on a nationalbasis. The National Trust, for example, plan toassess the climate change risks facing its ownparks and gardens. The value of conservation man-agement plans as tools for managing change needsto be promoted; and the training and education ofprofessional gardeners needs to supported toensure the availability of skilled staff to managethese sites in the future.

9.5 Networks

9.5.1 A GARDEN NETWORK

The literature reviewed for this study suggeststhat the outcomes of much of the research out-lined above will be varied and dependent on com-plex interactions. Interdisciplinary research at thelevel of the whole plant and plant community isneeded. More needs to be understood about therole of gardens and parks in relation to biodiver-sity, nationally and internationally and how they

might provide ‘connectivity’, in the form of greencorridors, to ensure wildlife migration as climatezones shift.

A garden network is needed to exchange and coor-dinate observations, ideas and actions, and to com-municate the effects of climate change on gardenswidely. The network should highlight solutions tomanagement problems and identify areas for fur-ther research relating specifically to the impact ofclimate change on gardens.

9.5.2 TOWARDS A HORTUS EUROPEUS

Flora Europea (Tutin et al., 1964) maps the natu-ral distribution of plant species in Europe (seeFigure 17 for examples). The parallel developmentof distribution maps for selected garden plants as abasis for (and result of) phenological mappingwould provide a valuable resource in determiningthe actual and potential response of garden plantsto climate change. It would also facilitate studies ofplant hardiness and refinements to the concept ofhardiness zones referred to in section 9.2.4 above.

There is clearly scope for, and great benefit in,relating garden plant phenological records withthose of native plants, many of which contribute togardens as well as to the wider landscape. Linkswith the International Phenology Garden Network,established in 1957, would be particularly useful inenabling comparisons with countries which cur-rently have climates similar to those anticipated forthe UK by the UKCIP02 climate change scenarios.

Data on responses of garden plants to changes inweather and climate are continually collected byUK gardeners. Systematic gathering of even asmall proportion of this information through pro-fessional organisations would add significantly toour understanding of what will grow, and where.

9.5.3 AN INDICATOR SPECIES LIST

A list of genetically uniform plants which can begrown in a wide range of gardens should be devel-oped to monitor the effects of climate change ingardens. The survival (tolerance of low and per-haps of high temperatures) and phenological data


114 Chapter 9

of these species should be collected. Plants whichmight not be expected to survive the winter inmany situations should be included in the list toinvestigate hardiness.

9.6 Policy development

The garden network (above) should be used to dis-seminate an understanding of the importance ofgardens in the national culture, so that they receivea rightful place in policy formulation. Nature con-servation is achieved by a range of statutory desig-nations and controls, but garden conservation doesnot receive such attention. The environmental andcultural benefits inherent in sensitive managementof the garden heritage underline the importance ofgardens in responding to the impacts of climatechange. It is, therefore, important to ensure thatgardens in all their diversity receive proper atten-tion when matters of national and European policyare being developed.


115Chapter 10

ConclusionsClimate change is not something about to happen.It is something which is already here and is accel-erating at a rate not previously experienced in theEarth’s recent history. Current concerns about thepotential effects of climate change on gardens havearisen not out of a theoretical possibility of change,but because we are already facing the consequencesof climate change and extreme weather events.

The great storm of October 1987 marked a water-shed in thinking about trees and gardens, with therealisation that nothing is immortal. Plant popula-tions in gardens require constant management,development and renewal if they are to survive.The timespan within which major decisions needto be taken in gardens, will result in climate changebecoming a significant factor in determining thedirection and objectives of management inputs.With the exception of historically important gar-dens with closely prescribed planting schemes,gardens will be able to adapt to climate change andsome will benefit, but the cost of adaptation maybe considerable.

In discussing the impacts of climate change on UKgardens, it is important to distinguish between longterm trends and the occurrence of sudden extremeevents. The two main trends are steady increases inmean temperature and a reduction in annual pre-cipitation, particularly in the summer, but the scaleof change varies by region across the UK. Thecombination of heat and drought will be most dam-aging to large trees growing on light soils in thesouth of England, but its effects will be felt to agreater or lesser extent throughout the UK. In thenorth, for example, more rapid growth of vigorousannual weeds will threaten gardens where slowgrowing plants are particularly susceptible to com-petition, and gardens on steep slopes with shallowrocky soils will be very susceptible to drought.

The more dramatic impacts on gardens come fromextreme weather events, particularly droughts,gales and floods. These are unusual and unpre-dictable events but they help to emphasise theimportance of sustained management of the garden

heritage, of not allowing a garden to slip into seniledecay, in which condition it is vulnerable to even amodest gale.

The symbol of a garden as Paradise, a Garden ofEden, is an ancient but still a powerful one. Theimage of the UK as one large garden, a green andpleasant land, is a source of pride, pleasure andhealthy exercise for many of its inhabitants and amagnet for tourists. Our garden heritage is a valu-able national resource and warrants continuedinvestment.

The role of gardens and parks as innumerable com-ponents in a green web, supporting and at timesreplacing the fragile network of natural ecosys-tems, has been little explored in this report.However, these millions of landscapes, large andsmall, will have a vital role to play in reinforcing asystem of ecological corridors through whichwildlife can migrate in response to climate change.

Lastly, the beneficial effects of good soil manage-ment and maintenance of a healthy plant cover incoping with climatic extremes in gardens providesa model which, if followed on a national and inter-national scale, will do much to slow the pace ofclimate change and to reduce its impacts.

Adapting gardens to the impacts of climate changewill incur additional expense and labour. However,by investing in gardens and by adopting good gar-dening principles on a wider scale, the UK will beable to address the implications of climate changefor our gardens, and give future generations theopportunity of experiencing the pleasure of sittingunder a tree that is some 200-300 years old.


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Thoday, P (pers. comm.) Former senior lecturer inAmenity Horticulture, University of Bath,Consultant to the Lost Gardens of Heligan andEden Project

Tolley, LC and Strain, BR (1985) Effects of CO2enrichment and water stress on gas exchange ofLiquidambar styraciflua and Pinus taedaseedlings grown under different irradiance levelsOecologia 65: 166-172

Tunnard, C (1938) Gardens in the ModernLandscape Architectural Press

Tutin, TG, Heywood, VH, Burges, NA, Moore,DM, Valentine, DH, Walters, SM and Webb, DA(1964) Flora Europea Cambridge UniversityPress 5 vols 1964-1980

Vines, G (2002) Weathering changes Kew(Spring): 16-19

Visser, ME & Holleman, LJM (2001) Warmersprings disrupt the synchrony of oak and wintermoth phenology Proceedings of the Royal Societyof London – Series B: Biological Sciences 268:1464: 289-294

Wade, S, Hossell, J, Hough, M and Fenn, C (Eds)(1999) Rising to the Challenge: the impacts of climate change in the South East in the 21st century WS Atkins, Epsom

Ward, CY (1969) Climate and adaptation Hanson,AA and Juska, FV Agronomy 14: TurfgrassScience American Society for Agronomy,Madison, Wisconsin

Wheeler, D and Mayes, J (1997) RegionalClimates of the British Isles Routledge, London

Wheeler, TR, Morison, JIL, Ellis, RH and Hadley,P (1994) Effects of CO2, temperature and theirinteraction on the growth and yield of carrot(Daucus carota L) Plant Cell and Environment17: 1275-1284

Wheeler, TR, Ellis, RH, Hadley, P and Morison,JIL (1995) Effects of CO2, temperature and theirinteraction on the growth, development and yieldof cauliflower (Brassica oleracea L Botrytis)Scientia Horticulturae 60: 181-197

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Woodhead, JD (pers. comm.) Director, HilliersNurseries Ltd

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126 References

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127

Appendices


129Appendix 1: Questionnaire

Climate Change and Gardens

1. What is the planning time-frame within which you operate? How far ahead do you plan for the garden?

2. What are the general objectives of any forthcoming changes? Expansion, upgrading of facilities, increased diversity of facilities, to attract more visitors, to widen visitor base, more economy /efficiency of operation, increased historical authenticity etc

3. What are the major influences on forthcoming changes? Budgetary restraints, need to attract visitors, difficulties in staffrecruitment, new historical evidence, increasing emphasis on education etc.

4. What is your next major development?

5. To what extent does sustainability feature in your planning? Please tick one:

a. Central driving force

b. Important consideration

c. Of some importance, but other factors more important.

d. Minor importance

e. No importance

6. To what extend does the need to respond to climate change feature in your planning? (Please tick one)

a. Central driving force

b. Important consideration

c. Of some importance, but other factors more important

d. Minor importance

e. No importance

7. Can you cite recent (past 5 years) examples of extreme weather events which have affected your garden?

8. In your experience, are extreme weather events more or less frequent than they used to be?

Continues overleaf

Climate change questionnaire

Nick JarvisAlfriston Clergy House, Sussex

Professor John ParkerCambridge Botanic Garden

David LockChirk Castle and Garden, Wrexham

Professor Charles StirtonNational Botanic Garden of Wales, Llanarthne

Peter HallPowis Castle and Garden, North Wales

Chris BailesRosemoor, Devon

Dr David RaeRoyal Botanic Garden, Edinburgh

Dr Nigel TaylorRoyal Botanic Gardens, Kew, Surrey

Sarah CookSissinghurst Castle Garden, Kent

Barry ChampionTrelissick, Cornwall

Jim GardenerWisley, Surrey

The questionnaire below was sent to fourteen gardens. Replies were received from:


130 Appendix 1: Questionnaire

9. Below are some predictions about climate change over the next 20-50 years based on the UKCIP98 climate change scenarios. What positive and/or negative effects on your garden do you anticipate from these predictions if they prove to be true?

a. A mean temperature rise of 2-5°C. (giving the south of England a climate similar to that of Bordeaux, and the north of England a climate similar that of Surrey).

b. A 3-fold increase in the number of hot days (above 27°C or 80°F)

c. 5-10 days each year with temperatures above 40°C or 105°F

d. A longer growing season – perhaps ten days earlier start and later finish

e. Substantial reduction in number of days with frost (to near nil in parts of Southern England).

f. 20% increase in winter rainfall

g. 20% decrease in summer rainfall

h. Increasing uncertainly of rainfall: alternation of droughts and heavy rainstorms

i. Increased incidence of strong winds

j. 50cm rise in mean sea level

k. Any other direct impacts of climate change

10. Do you see an overall benefit, an overall deficit, a balance of advantages and disadvantages or no impact for your garden in each of these scenarios? (Please write “ben”, “def”, “bal” or “no” as appropriate below.

a. A mean temperature rise of 2-5°C (giving the south of England a climate similar to that of Bordeaux, and the north of England a climate similar that of Surrey).

b. A 3-fold increase in the number of hot days (above 27°C or 80°F)

c. 5-10 days each year with temperatures above 40°C or 105°F

d. A longer growing season – perhaps ten days earlier start and later finish

e. Substantial reduction in number of days with frost (to near nil in parts of Southern England).

f. 20% increase in winter rainfall

g. 20% decrease in summer rainfall

h. Increasing uncertainly of rainfall: alternation of droughts and heavy rainstorms

i. Increased incidence of strong winds

j. 50cm rise in mean sea level

k. Any other direct impacts of climate change

11. Would the overall impacts on your garden of the climate change phenomena described above be beneficial, damaging or neutral?

12. Are you (or are you likely to be) taking any steps in your garden to adapt to anticipated impacts of climate? If so, what steps?

13. Are you (or are you likely to be) taking any steps to reduce the causes of climate change?

14. Do you maintain records of;

a. climatic data?

b. phenological data?

c. visitor numbers?

Many thanks for your time. The results of this questionnaire will be published, as part of a report on the impacts of climate change on gardens, in 2002. Richard Bisgrove


131Appendix 2: Glossary

Acclimation: literally “becoming adapted to”, aswhen plant response to higher carbon dioxide con-centrations decrease because the plant has adaptedto the new situation. Acclimation is used in plantphysiology in preference to “acclimatisation” whichstrictly means “becoming used to a new climate”.

Albedo: reflectivity of a surface. A surface (forexample a dark soil) which has an albedo of zeroabsorbs all the light falling on it. A surface with analbedo of one (for example, fresh snow) reflects allthe light falling on it.

Arthropod: animals (usually small animals) withsegmented bodies and jointed limbs. Arthropodsinclude insects, mites and millipedes, for example.

Assimilates: in the context of plants, materialssuch as carbohydrates and proteins, which the plantsynthesises and then incorporates into its tissues.

C3/C4 plants: photosynthesis in C3 plants (mostplants from temperate regions) involves 3-carbonmolecules building into sugars. Some (mainlytropical) plants have a different mechanism involv-ing 4-carbon molecules, with the advantage thatthey are able to store then release carbon dioxidewithin the leaf so that photosynthesis can takeplace while stomata are closed. These are referredto as C4 plants. A full explanation cannot be givenin simple terms. The topic of C3/C4 plants hasbeen referred to only in passing in the text, becausetemperatures much higher than those anticipated inthe UKCIP02 scenarios would be needed to makesignificant use of C4 plants in UK gardens.

Clone: a population of genetically identical plants(and now animals). Clonal plants are producedasexually, by vegetative propagation, rather thanfrom seed.

Development: see Growth

Dicotyledon(ous plant) (abbr. Dicot): there aretwo major divisions in the plant kingdom. Thosewhich emerge from the seed with two seed-leaves

(cotyledons) are known as dicotyledons or dicots.Most of these have broad true leaves. Those whichemerge from the seed with one cotyledon areknown as monocotyledons or monocots. Most ofthese, such as grasses and bulbs (narcissus, crocus,lily) have narrow leaves.

Dry matter: the substance (including carbohy-drates and proteins) of plants as measured afterdrying the plant in an oven to remove all water. Drymatter includes the assimilates (q.v.) within theplant and the structural material of the plant.

Evaporation: conversion of (liquid) water to watervapour. Evaporation from plant leaves is oftentermed transpiration; the combination of evapora-tion from soil and leaf surfaces, and transpirationfrom within the leaf is termed evapo-transpira-tion. The amount of water which could theoretical-ly be evaporated in particular climatic conditions isthe potential evaporation, or potential evapo-transpiration. Actual evaporation is usually lessthan potential evaporation because the amount ofwater available for evaporation is reduced as thesoil dries and plants become stressed.

Growth: the increase in size of the plant.Development refers to changes of state within theplant, such as the production of leaves, the formationand expansion of flowers or the onset of dormancy.

Legume/leguminous plant: a plant of the familyLeguminosae, with pea or bean-like flowers.Legumes include herbaceous plants such as sweetpea, shrubs such as broom (Cytisus) and gorse(Ulex), and trees such as Robinia and Laburnum.Legumes have root nodules in which symbioticbacteria (q.v.) live. These are capable of convertingatmospheric nitrogen into nitrates.

Monocotylendon(ous plant) (abbr. Monocot): seeDicotyledon

Glossary


132 Appendix 2: Glossary

Mycorrhiza (plural Mycorrhizae): fungi whichlive in association with plants (especially manytrees). The fungus receives assimilates (q.v.) fromthe plant roots and supplies the roots with mineralsobtained from the soil through its extensivehyphae. Mycorrhizae are especially important invery acid or very poor soils as they are capable ofextracting minerals which would be unavailable tothe tree itself.

Multivoltine: having several generations eachyear.

Nematode: eelworm. A microscopic, worm-likecreature. Some nematodes are harmful to plants,causing deformation of roots, stems or leaves.Others are beneficial, attacking pests such as vineweevil.

Phenology: the study of organisms as affected byclimate, especially the timing of seasonal phenom-ena such as leaf emergence or flowering of plants,or seasonal arrival of migrant birds and butterflies.

Phenotype: the observable characteristics of aplant or animal produced by the interaction of itsgenetic make-up and the environment.

Photoperiod: length of the light period, usually inrelation to a 24 hour day. ‘Long-day’ plants willrespond (for example by flowering or emergenceform dormancy) to photoperiods above a criticalvalue. ‘Short-day’ plants will develop in responseto short or declining photoperiods.

Pneumatophore: a woody outgrowth on the rootsof swamp cypress (Taxodium distichum) whichgrows above the surface of water or wet soil toconduct oxygen down to the root system.Pneumatophores are commonly called ‘knees’.

Potential evaporation: see Evaporation

Ppm(v): parts per million. Gas concentrations areusually measured as parts per million by volume(ppmv) while concentrations of substances in solu-tion or in mixtures of solids are measured in partsper million by mass (ppmm).

Precipitation: the amount of water reaching thesoil surface, whether as rain, hail, snow or settle-ment of water droplets from mist and fog. A moreinclusive term than ‘rainfall’.

Provenance: the location from which seed isobtained. Provenance is important because therecan be wide variations within a species as a resultof evolution in different climatic zones. Sprucetrees from seed obtained from a population inAlaska, for example, will be much slower growingand have a shorter growing season, but be muchmore cold tolerant than seed collected from thesame species in more southerly regions when bothare grown in the same place.

Sink: plant physiologists use the term source torefer to the sites within the plant where assimilates(q.v.) (especially products of photosynthesis) areformed and sink to refer to the sites to which assim-ilates are transported for use. The presence of anactive sink in the plant (such as actively growingtips or developing flower buds or seeds) will stimu-late the plant to photosynthesise more rapidly.

SSSI: Site of Special Scientific Interest. A statuto-ry designation of land, sometimes an extensive areaof a particular habitat and sometimes a small areacontaining a rare plants or animals.

Stoma (plural stomata): small pores in the plantleaf (and to a lesser extent on other surfaces),through which water vapour and gases such as car-bon dioxide and oxygen diffuses into and out of theplant. The stoma is bordered by guard cells whichopen and close the pore in response to changes inwater vapour and carbon dioxide concentration.Stomatal aperture is the size of the pore.

Symbiosis: coexistence for mutual benefit.Mycorrhizae (q.v.) and trees have symbiotic rela-tionships, as do the bacteria in root nodules ofleguminous plants (q.v.) with their host plant.

Synchrony: (= same time). Two events which areorganised so that they happen together are said tobe synchronous, or to exhibit synchrony. In biolo-gy, synchrony evolves to ensure that a food sourceis available in suitable condition to sustain another


133Appendix 2: Glossary

organism, as when the hatching of a caterpillar issynchronised with the emergence of the leaves onwhich it needs to feed.

Thermal growing season: the longest period with-in a year that satisfies the twin requirements of (i)beginning at the start of a period when the dailyaverage temperature is greater than 5.5°C for fiveconsecutive days and (ii) ending on the day prior tothe first subsequent period when the daily averagetemperature is less than 5.5°C for five consecutivedays. The actual growing season will depend on thetype of plant, on its emergence from dormancy andother factors but thermal growing season providesa precise meteorological measure of the way inwhich growing conditions vary from year to year.

Thermal time: the ‘amount of heat’ received by aplant. Thermal time is the product of the number ofdays and the number of degrees above a particularminimum for the process under consideration. Thetiming of many plant growth responses, such asbreaking of dormancy or initiation of flowers, isdetermined by thermal time.

Transpiration: see Evaporation

Xerophyte (adj. Xerophytic): a plant adapted tovery dry conditions, by its compact form, thick,waxy leaf surface or other characteristics.

Xeriscape: the use of xerophytic plants to creategardens which have low water requirements. ‘Drygardening’.


135Appendix 3: Examples of climate change scenarios for three case study gardens

In the main report, the impacts of climate changeare dealt with component by component, and interms of regional scenarios. What matters in a gar-den is the combination of factors at a particularspot. In order to give some feel for these localimpacts, three locations have been chosen, in thenorth, east and south west of the UK. For each loca-tion, basic meteorological data for 1961-90 are pre-sented (Wheeler and Mayes, 1997) and changesanticipated by the medium high emissions scenariofor the 2080s estimated by reference to theUKCIP02 scenarios (Hulme et al., 2002).

To choose a representative site for the north of theUK, in Scotland, is impossible because the deeplyindented coastline and generally steep topographymean that the local influence of the sea and ofchanging altitude is always present, as anyone whohas travelled the 24km (15 miles) from the coldrocky summit of Ben Eighe National NatureReserve to the palm filled garden at Inverewe willappreciate. However, Ardtalnaig, south of BenLawers on Loch Tay has been selected as an inlandsite at low altitude on the southern edge of thehighlands. Its climate is moderated by Loch Tay,which is of sufficient depth that it never freezes,but in other respects it is as ‘average’ as it is possi-ble to find in this varied landscape.

Cambridge is in the driest part of England, but itsclimate is representative of a broad swathe of east-ern England.

Torquay is a coastal location, but has been chosenas more representative of the south west as a wholethan would be a more westerly site. It is also morecharacteristic of the gardened south west than areplaces farther inland, where the high moors, thinsoils and atypically severe climate impose theirlocal limitations on gardening.

Tables A3.1, A3.2 and A3.3 show January and Julydata for temperatures and precipitation at eachlocation, and the changes to these which are antic-ipated by the UKCIP medium high emissions sce-nario for the 2080s. Current January and July pre-cipitation figures are added to give some indicationof changes in annual rainfall (data have not beencalculated separately for every month of the year)and the ratio of January to July precipitation is pre-sented to give some indication of the change in sea-sonal distribution of rainfall.

To avoid tedious repetition of qualifications, in thefollowing descriptions the terms “now”, “currently”or “at present” are used, slightly incorrectly, to referto 1961-90 baseline conditions. References to futureconditions throughout the text apply to the UKCIP02medium high emissions scenario for the 2080s.

A3.1 Ardtalnaig, Loch Tay, CentralScotland

In Ardtalnaig the winter temperature, currently notdissimilar to that of Cambridge, is anticipated torise by 2°C. The minimum may also increase froman average of 0.4°C, barely above freezing, to2.4°C, somewhat cooler than Torquay now. In muchof Scotland temperature rise could result in meanminimum temperatures changing from negative(below freezing) to positive.

Present July temperatures are 2-3°C below those ofCambridge and Torquay. With an anticipated rise ofabout 3°C, July temperatures in the 2080s inArdtalnaig would be warmer than are Cambridgeand Torquay at present.

In common with most of the western half of theUK, precipitation in Ardtalnaig falls mainly in thewinter months. Current January precipitation is

Examples of climate change scenarios for three case study gardens


136 Appendix 3: Examples of climate change scenarios for three case study gardens

more than twice that of July. With an anticipated20% increase in precipitation in an already wetwinter, but a 35% decrease in a relatively dry sum-mer, total precipitation (currently 50% higher thanTorquay and 250% higher than Cambridge) wouldbe little altered but the January:July ratio couldchange from 2.3:1 to 4.2:1.

Although the magnitude of these changes is lessthan in the east or south west, the decrease in pre-cipitation in an already dry summer, combinedwith increasing temperatures, is likely to have asignificant impact, especially in gardens on steepslopes and with shallow, rocky soils, where contin-uous throughput of water is necessary for thegrowth of the drought sensitive plants which arecharacteristic of Scottish gardens.

Whether increased winter precipitation is a bonus(replenishing soil moisture depleted in summer)or a problem (saturating soils and causing erosionor flooding) will depend on the management of

water within the garden. The total amount ofwater is likely to be virtually unchanged but theamount available to plants will diminish becauseof higher evaporation rates at higher tempera-tures. The need to conserve winter supplies inorder to compensate for summer deficits willtherefore be very important.

A3.2 Cambridge, Eastern England

In Cambridge, January temperatures, and especial-ly January minima, are currently similar to those inArdtalnaig. An increase of about 2.8°C in wintertemperatures by the 2080s would result in Januarytemperatures higher than those currently experi-enced in Torquay.

Current July temperatures are similar to those ofTorquay, though as a result of slightly higher max-imum temperatures and slightly lower minimumtemperatures in Cambridge’s more continental cli-mate. An increase of 4.5°C in summer tempera-

Table A3.1: Ardtalnaig, Loch Tay, Central Scotland

1961-90 mean Change in med high Estimated figuresscenario for 2080s for the 2080s

January Winter

Max temp (°C) 5.2 7.2

Min temp 0.4 2.4

Mean 2.8 +2°C 4.8

Precipitation (mm) 159 +20% 190

% of annual total 12.7%

July Summer

Max temp (°C) 18.6 21.9

Min temp 10.0 13.3

Mean 14.3 +3.3°C 17.6

Precipitation (mm) 69 -35% 45

% of annual total 5%

Precipn: Jan+July 228 235

Precipn: Jan:July 2.3 4.2



tures will not change that comparison. In terms oftemperature, Cambridge and Torquay will be simi-larly affected.

In common with much of the eastern half of theUK, precipitation is distributed evenly throughoutthe year. July, with 8.7% of the annual precipita-tion, is slightly wetter in terms of precipitation thanJanuary, with 7.8%, although much higher evapo-ration rates with high summer temperatures meanthat water is less available to plants in July.

By the 2080s, winter precipitation is anticipatedto increase by 25%, but this could only mean an11mm increase and would be insufficient to com-pensate for increased evaporation, given themean temperature change from 3.65 to about6°C. Summer precipitation would be nearlyhalved. Total annual precipitation decreases by asmall total amount, but the ratio of winter:sum-mer precipitation could increase from 0.9:1 toabout 2:1.

The combination of higher temperatures andreduced precipitation will result in severe droughtconditions in many years and accentuate themediterranean climate which Cambridge alreadyexperiences to some extent.

What is not shown in the table of climate data isthe fact that much of Cambridgeshire, Norfolk andLincolnshire is very low lying, some of it at orbelow sea level. The landscape, and especiallywater resources, have been intensively managedfor agriculture. However, as fertile peat soils havebeen lost by oxidation or wind-blow and as sealevels rise, land use and nature conservation poli-cies have moved to planned retreat from the mostvulnerable areas.

A3.3 Torquay, Devon

Torquay has a mild climate, several degrees warmerin winter than Cambridge and Ardtalnaig. MinimumJanuary temperatures in Torquay are similar to mean

Table A3.2: Cambridge, Eastern England


January Winter

Max temp (°C) 6.5 9.3

Min temp 0.8 3.6

Mean 3.65 +2.8°C 6.45



July Summer

Max temp (°C) 21.5 26.0

Min temp 11.7 16.2

Mean 16.6 +4.5°C 21.1

Precipitation (mm) 48 -45% 26.5


Precipn: Jan+July 91 80.5



138 Appendix 3: Examples of climate change scenarios for three case study gardens

January temperatures in Cambridge. In summer thedifferences are much less. Maximum July temper-atures are lower than in Cambridge, though themean is very slightly higher.

An anticipated 2.5°C rise in winter temperatures(compared with about 2.8°C in Cambridge) repre-sents some closing of the difference but Torquayremains 2-4°C warmer than the other two places.Anticipated summer increases of about 4°C wouldresult in 2080s summer temperatures again verysimilar to those of Cambridge.

As in Ardtalnaig, precipitation in Torquay fallsmainly in the winter months. January, with114mm, has 2.5 times the July precipitation of46mm. A 20% increase in anticipated winter pre-cipitation would result in an extra 23mm of rain,but increased evaporation would mean that soilmoisture content in winter may increase by only 0-4%. July precipitation in Torquay is similar to thatin Cambridge and, as in Cambridge, the 2080s sce-

nario anticipates nearly halving this amount. Soilmoisture deficit could increase by 40-50%.

The overall effect of moderate decrease in the rela-tively high winter precipitation and a substantialdecrease in the much smaller summer precipitationis to leave the total unaltered, but the ratio of win-ter:summer precipitation could increase from 2.5:1to 5.5:1. On thin, shallow soils and free drainingslopes especially, water shortage in summer will besevere. In the past, lack of major aquifers or riversto supplement precipitation and the extra demandon supplies caused by a marked increase in summerpopulation of holiday makers has often led to watershortages. The building of new reservoirs has beenstrenuously opposed because of their impacts on abeautiful landscape. These problems can onlyincrease as climate change continues.

One positive aspect of climate change in the southwest is that soil moisture content is expected toincrease by only a very small amount, perhaps 0-

Table A3.3: Torquay, Devon


January Winter

Max temp (°C) 8.8 11.3

Min temp 3.4 5.9

Mean 6.1 +2.5°C 8.6



July Summer

Max temp (°C) 20.6 24.8

Min temp 13.1 17.3

Mean 16.85 +4.2°C 21.05

Precipitation (mm) 46 -45% 25


Precipn: Jan+July 160 162




4% above current levels. Soil saturation which hasbeen a feature of recent exceptionally wet wintersand which has led to root rot of many plants, espe-cially magnolias, seems unlikely to become thenorm. Gardeners will not be facing a losing battleif they improve soil drainage to contend with theproblems caused by what should be infrequent verywet winters.

Another possible advantage is that the summer cli-mate, although posing serious challenges for thecultivation of gardens, will improve the prospectsof the south west as a holiday destination andimprove the prospects for major gardens relying ontourism for their livelihood.

A3.4 Summary

In all three locations, the main challenge will be incoping with much drier conditions in the summer.In the north, higher winter rainfall will assist inovercoming the problem if the winter surplus canbe stored. In the east, precipitation will decreasethroughout the year and higher evaporation rateswill exacerbate water shortages. In the south west,annual precipitation will be little altered by climatechange but higher evaporation rates will reduce theamount of available water. If the UKCIP02 scenar-ios are reasonable approximations of climatechange, it is unlikely that excessive winter wet willbe an increasing problem, although there will, ofcourse, be some very wet winters (and indeed somevery wet periods in some summers) as in the past.