old stone walls as an ecological habitat for urban trees in hong kong

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Old stone walls as an ecological habitat for urban trees in Hong Kong

C.Y. Jim *

Department of Geography and Geology, University of Hong Kong, Pokfulam Road, Hong Kong, China

Received 22 January 1998; received in revised form 13 April 1998; accepted 17 April 1998

Abstract

Urban growth in Hong Kong is constrained by rugged topography resulting in grave shortage of developable land. Besidesforming new land by reclamation from the sea, hillsides have been extensively cut into terraces to accommodate densely-packed roads and buildings. To maximize useable area and to provide geotechnical stability, stone retaining walls were widelybuilt between platforms. Such vertical habitats constitute a unique opportunity for spontaneous colonization by a diversi®edhumid-tropical ¯ora, including large trees up to 20 m tall. The walls-cum-vegetation, many exceeding 100 years old, furnish aprecious natural-cum-cultural heritage and decorate some otherwise drab neighborhoods. Recent city redevelopmentunfortunately has damaged beautiful walls and their living companions. A city-wide survey was conducted to establish a

microcomputer database to assess wall and tree characteristics and to identify candidates for conservation. Some 505 wallswith 1275 trees (>1 m tall) were found mainly in residential areas. A broad range of stone types, wall dimensions, constructionmethods and wall age were recorded. The 30 tree species, largely native, are dominated by Moraceae (Mulberry family), eightof which contribute 88% of the population. About 10% of the trees are >9 m tall, providing conspicuous and pleasantlandscape elements. Some tree attributes are associated with wall characteristics. Many trees had been heavily pruned to meetvehicular clearance needs and perceived safety concerns. The absence of an of®cial policy to preserve champion-caliber treesand walls need to be urgently recti®ed to prevent further loss of an irreplaceable community asset. # 1998 Elsevier ScienceB.V. All rights reserved.

Keywords: Stone wall; Wall tree; Urban tree; Urban ecology; Wall preservation; Hong Kong

1. Introduction and preamble

A continual quest for developable land underscored150 years of urban-expansion in Hong Kong. Thepredominantly rugged terrain is dominated by steepslopes which constrain city growth in the old circum-harbor core. Rapid population and economic growthdemand land which has to be created without respiteby reclamation from the sea and cutting the hillslopes

to form platforms. These two modes of site formationover the years have molded the direction and pace of urban development which must be preceded by labor-ious creation of usable land. The city's morphology,being prevalently high-density and high-rise, has beendictated by this inherent limitation of land scarcity andits high production cost.

Lands acquired by reclamation from the sea presentfew constraints to development, and were rapidly®lled with buildings and roads leaving little openspaces. Lands carved from hillslopes, however, arebeset by geotechnical constraints. To ensure that dis-

Landscape and Urban Planning 42 (1998) 29±43

*Tel.: +852 2859 7020; fax: +852 2559 8994; e-mail:[email protected]

0169-2046/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved.P I I S 0 1 6 9 - 2 0 4 6 ( 9 8 ) 0 0 0 7 2 - 3



turbed slopes would remain stable has been a keyconsideration in terracing hillsides. Cutting slopes intoa ¯ight of platforms, likened to giant steps, requires

much efforts to stabilize the intervening overstee-pened sections. Sometimes well vegetated pocketsof residual slopes are left in the built-up matrix astelltale legacies of the landform disturbances. Tomaximize usable land, adjacent platforms are oftenseparated by vertical faces which must be protectedand reinforced against failure, unless there is soundsolid rock. In the past, stone retaining walls (Geotech-nical Control Of®ce, 1979; Teng, 1980) were com-monly constructed to serve this purpose.

Continual urban sprawl up the foothills has createdmany stone walls. The hilly northern part of HongKong Island, with a long development history, hasmost of the oldest representatives including manyolder than 100 years. Some are conspicuously situatedat roadsides, whereas others are sequestered behindbuildings and in obscure lanes. Over the years, themethod and the style of construction graduallyevolved, resulting in a wide variety of walls withelaborate masonry occurring in equally varied envir-ons. As soon as these arti®cial vertical habitats wereformed, nature's cliff-hangers would begin to colonizethe vacant niches. Most walls nurture a rich comple-ment of ¯ora of different growth forms, includingmosses, lichens, ferns, herbs, shrubs, climbers andtrees, together with diverse wildlife. Of the assorteddenizens, the biomass and ecological±environmentalbene®ts of large trees deserve particular attention.

The landscape of the hilly neighborhoods, other-wise monotonous and sterile, has been decorated bystone walls and their living companions. The oldercrop of walls is often blanketed by dense vegetationwith large trees up to 20 m tall. The ecological andcityscape value of such natural ingredients deserve tobe treasured, yet they have been largely taken forgranted if not neglected. Recent widespread redeve-lopment of the hilly districts have damaged andobliterated many ®ne walls. New city extensions intofringe slopes carve huge platforms separated by engi-neered slopes rather than retaining walls. New retain-ing structures, occasionally needed, are invariablymade of reinforced concrete (to meet stringent geo-technical requirements) rather than stone. Old stoneswalls in Hong Kong have become a threatened heri-tage.

This study aims at assessing comprehensively thisdiminishing resource, ruderal vegetation on a specialruderal habitat (Frenkel, 1970). Intrinsic and extrinsic

wall variations present a plethora of chances forvegetation. An empirical evaluation of both wallsand trees can throw lights on the ecological relation-ship between the two intimate partners. As most wallvegetation studies were carried in the temperate lands(e.g. Segal, 1969; Darlington, 1981), this humid-tro-pical study can provide comparative data. Analysis of detailed information gleaned in the ®eld helps to: (1)understand the nature, distribution and environs of stone walls, for plant-growth substrate in the city; (2)to enumerate the species composition and relativeabundance of the attached trees; (3) to evaluate thegrowth condition and problems of the wall trees; (4) toidentify candidate walls and trees for conservation; (5)to explore the long-term management strategy of oldstone walls and trees.

2. Study area and methods

The location of walls were initially pinpointed on1:1000 large-scale maps. A reconnaissance of 100walls in different districts provided basic knowledgeabout walls and trees to design two detailed ®eldrecord forms to facilitate systematic data collection.The wall form assembles data on location, material,stone dimensions, surface smoothness, surface moist-ure, weathering status, joint type and condition, wallaspect, inclination and exposure, wall environs, vege-tation cover, integrity and threats to wall existence.The tree form assesses tree position on the wall,species, tree height, surface root length and density,crown spread, integrity and restriction, and overall treeperformance rating species. Each wall form is tied toone or more tree forms. The possible answers werelargely of a closed-ended nature to minimize subjec-tive judgement and to facilitating microcomputerdatabase treatment.

A pilot study on 30 walls and 60 trees was con-ducted to test the applicability of the record forms andto gather feedbacks to re®ne them. The study area wasthen divided into small enclaves each approximatelyequivalent to 1 day of ®eld work. Most walls werefound on northern Hong Kong Island with the oldestcity core dating back to 1840s. The ®eld survey

30 C.Y. Jim / Landscape and Urban Planning 42 (1998) 29±43



focused on this study area where walls over 1 m talland wall tree over 1 m tall, attached on them, wereevaluated. Only the retaining walls (holding back soil

materials) were included; free-standing ones were not.A wall tree was de®ned as one with most of its rootsspreading on or penetrating through the wall face, andwith the trunk base situated within the con®nes of awall. A tree overhanging above a wall but not physi-cally attached to it, and a tree with trunk base and mostroots located outside a wall's boundaries, did notqualify.

A 3-month intensive ®eld study was conducted inthe summer (June±August) of 1996 to gauge the treesin their prime growing season. A total of 505 walls and1275 trees were assessed. Wall locations were plottedon 1:5000 maps which was the working scale. Twocomputer ®les, namely for wall and tree data, werecreated with Microsoft Excel Version 5.0. Statisticalanalysis was performed with SPSSPC for WindowsVersion 6.1. Botanical nomenclature follows those of Agriculture and Fisheries Department (1993) and Jim(1990).

3. Stone walls as vegetation habitat

The enumerated 505 stone walls are found in theolder hilly neighborhoods built on Hong Kong Island.Two districts have a signi®cant clustering of walls,taking up 40% of the total number. There is a markedconcentration in several winding two-lane roads thatrun largely along the contour where most large walltrees (<6 m) are attached. The old urban-sprawl modetended to terrace hillslopes into small pockets sepa-rated by stone retaining walls; recent developmentsformed large tracts bounded by cut and ®ll slopesrather than walls. By land use, the low- to medium-density residential areas contain most walls and asso-ciated trees. Hilly lands are mainly earmarked forbetter-quality private housing. Other land uses, excepthigh-density residential, have few and scattered walls.

The walls are found within 20±400 m altitudinalrange, with the majority in the 20±200 m bracket.Most walls are highly or moderately exposed, andface north which is the main slope direction in thestudy area. This factor carries both positive and nega-tive connotations. Exposed walls are subject to moredirect sunshine and wind, and may be more prone to

desiccation. Exposure, however, can allow walls toreceive more rainwater by lateral impaction due towind. Furthermore, exposure provides opportunities

for the wind vector to admit extraneous propagulesand disseminate resident propagules.

Only a few walls incline over 10 degrees from thevertical; the remaining are vertical or nearly so. Otherthings being equal, inclination may allow marginallyeasier lodging of seeds. Deviation from the verticalcan facilitate capture of rainwater. Wall stones aremainly granite (60%) which are lighter in color, andsecondarily volcanic which are much darker. Thelatter contains more ma®c minerals which uponweathering can release more basic nutrients. Its dark color absorbs more radiant energy and attains a higherstone temperature which accelerates drying of walls.No wall is constructed with a mixture of stones. Bothrock types are commonly found in Hong Kong.

Most stone faces have either smooth or moderatelyso ®nish, with only a few roughly-hewn. Uneven stonesurface could furnish more landing opportunities forpropagules and more secure root anchorage. Thestones show different degrees of incipient weathering,re¯ecting wall age which for many exceeds a century.The most weathered ones are likely to be pre-weath-ered. Weathering rate in the humid-tropical condition,in an subaerial rather than subterranean environment,is unlikely to attain an advanced stage within the 150years maximum tenure of the oldest walls (Ollier,1984). The weathered minerals can release nutrientsfor wall vegetation (Woodell, 1979), supplementing aresource in restricted supply.

Moisture availability is a crucial factor that impartslife on walls. Some 57% of the walls have dry surfacesdespite surveying during the summer rainy season.The remaining walls are moist, with some havinglocalized seepage and others pervasively moisture-laden. The back-of-wall land use is a pivotal controlof moisture status. In®ltrated rainwater soaking thesoil behind walls supplies seepage out¯ow throughweep holes and joints. Extensive stretch of vegetatedslope above walls provides a ready source of moisturereplenishment. Walls situated on the upper edge of thecity have such a favorable disposition. Occasionally,in times of heavy downpour at an intensity exceedingsoil in®ltration capacity, surface runoff may spreadfrom the abutting slope onto or cascade down wallfaces, and water may shoot out of weepholes.

C.Y. Jim/ Landscape and Urban Planning 42 (1998) 29±43 31



T a b l e 1

D i s t r i b u t i o n o f t r e e s s p e c i e s o n s t o n e w a l l s

o f d i f f e r e n t d i m e n s i o n s

S p e c i e s

W a l l w i d t h ( m

)

W a l l h e i g h t ( m )

W a l l a r e a ( m

2 )

< 2 5

2 5 ± 5 0

5 0 ± 7 5

7 5 ± 1 0 0

> 1 0 0

< 2 . 5

2 . 5 ± 5

5 ± 7 . 5

7 . 5 ± 1 0

> 1 0

< 2 5 0

2 5 0 ± 5 0 0

5 0 0 ± 7 5 0

7 5 0 ± 1 0 0 0 > 1 0 0 0 T o t a l

A l a n g i u m c h i n e n s e

0

2

0

0

0

0

0

2

0

0

0

2

0

0

0

2

A l b i z i a l e b b e c k

1

0

0

0

0

0

1

0

0

0

1

0

0

0

0

1

A p o r u s a c h i n e n s i s

0

0

1

0

0

0

1

0

0

0

1

0

0

0

0

1

B a u h i n i a b l a k e a n a

0

0

1

0

0

0

1

0

0

0

1

0

0

0

0

1

B r i d e l i a m o n o i c a

3

6

1

2

0

0

4

6

2

0

6

4

1

1

0

1 2

B r o u s s o n e t i a p a p y r i f e r a

1 2

2 3

9

5

5

6

2 7

1 8

3

0

2 8

2 1

1

1

3

5 4

C a r i c a p a p a y a

0

1

0

0

0

0

1

0

0

0

1

0

0

0

0

1

C a s s i a s u r a t t e n s i s

3

3

0

0

0

0

3

3

0

0

3

3

0

0

0

6

C e l t i s s i n e n s i s

2 0

1 9

1 1

4

5

5

3 1

1 1

1 2

0

3 7

1 3

9

0

0

5 9

C r a t o x y l u m l i g u s t r i n u m

0

4

1

0

0

0

1

4

0

0

1

4

0

0

0

5

D a l b e r g i a b a l a n s a e

0

3

0

0

0

0

0

3

0

0

0

3

0

0

0

3

D e l o n i x r e g i a

2

0

0

0

0

0

0

2

0

0

2

0

0

0

0

2

F i c u s h i s p i d a

1 5

6 4

8

1 2

6

2

5 3

3 7

1 3

0

6 4

2 7

1 1

2

1

1 0 5

F i c u s m i c r o c a r p a

1 2 8

1 6 9

1 2 8

8 4

1 2 8

6 1

3 0 2

1 4 3

1 2 7

4

2 8 3

2 1 7

7 6

1 4

4 7

6 3 7

F i c u s r e l i g i o s a

1 0

1

0

1

1

0

1 1

1

1

0

1 1

1

0

0

1

1 3

F i c u s s u p e r b a

5 7

4 9

2 0

2 1

3 7

1 9

8 2

4 1

4 0

2

1 0 6

2 9

1 6

1 1

2 2

1 8 4

F i c u s v a r i e g a t a

1 5

1 9

1 1

0

5

1 2

2 5

1

1 2

0

4 3

1

4

1

1

5 0

F i c u s v i r e n s

2 3

2 3

1 4

6

8

7

3 6

1 0

1 9

2

4 5

1 6

6

4

3

7 4

L i g u s t r u m s i n e n s e

9

1

4

4

0

3

9

2

4

0

1 3

5

0

0

0

1 8

L i q u i d a m b a r f o r m o s a n a

1

0

0

0

0

0

0

0

1

0

1

0

0

0

0

1

L i t s e a g l u t i n o s a

0

7

4

0

0

0

6

5

0

0

6

5

0

0

0

1 1

L i t s e a m o n o p e t a l a

0

1

0

0

0

0

1

0

0

0

1

0

0

0

0

1

M a c a r a n g a t a n a r i u s

4

0

2

1

1

5

0

2

1

0

5

1

2

0

0

8

M a e s a p e r l a r i u s

1

4

1

1

8

0

4

9

2

0

4

2

1

0

8

1 5

M a l l o t u s p a n i c u l a t u s

0

4

2

0

0

2

2

2

0

0

4

2

0

0

0

6

N e r i u m i n d i c u m

1

0

0

0

0

0

0

0

1

0

1

0

0

0

0

1

P u n i c a g r a n a t u m

0

0

0

0

1

1

0

0

0

0

0

0

1

0

0

1

S a p i u m s e b i f e r u m

0

0

0

1

0

0

1

0

0

0

0

1

0

0

0

1

S c h e f f l e r a o c t o p h y l l a

0

0

1

0

0

0

1

0

0

0

0

1

0

0

0

1

T h e v e t i a p e r u v i a n a

0

0

0

1

0

0

0

1

0

0

0

0

1

0

0

1

T o t a l

3 0 5

4 0 3

2 1 9

1 4 3

2 0 5

1 2 3

6 0 3

3 0 3

2 3 8

8

6 6 8

3 5 8

1 2 9

3 4

8 6

1 2 7 5




For the few walls adjacent to well-wooded compa-nion slopes, the high humidity environment allowedcapture of atmospheric moisture in a process similar to

fog straining. A well vegetated slope is more able tostore water and release it gradually over a longerperiod between rainfalls. Moreover, a site with luxur-iant vegetation cover has more organic matter andmore fertile soils, thus generating more eutrophicout¯ows. The nutrients thus conveyed to walls, acontinual material input into the mural ecosystem,constitute an important means of sustenance for vege-tation. Stones can hardly store water and the tinyvolume of soil in the joints has meager storage capa-city. For small plants with limited root spread, thetimeliness and adequacy of water supply is a criticalsurvival determinant.

Surface area of walls is limited, with 79% in the<250 m 2 small class, and only a few in the large 750±1000 and >1000 m 2 categories. The largest wallattains 2340 m 2 . By linear dimensions (Table 1 andFig. 1), most walls are 2.5±5.0 m in height and 25±

50 m width. The longest wall reaches 310 m and thetallest 12 m. The wide walls are dominated by granite,whereas the tall walls are both granitic and volcanic.

In line with the species-area concept, large walls wereexpected to accommodate more species. Besides pre-senting a large physical surface for plant attachment,they are likely to furnish more microvariations inhabitat conditions, hence a diversity of niches. Theyare also more conspicuous if not imposing urbanlandscape features. Tall walls, however, are morelikely to be construed as a safety threat especially if they are situated close to buildings.

Most stones are 30±40 cm height and 20±60 cmwide; the widest stones reach over 200 cm, and thetallest over 60 cm (Fig. 2). Walls made of large stonesare mainly granitic. Small stone means more gapsbetween individual ones, hence a better chance forseepage, propagule lodging and root anchorage. Some77% of the walls have regularly-shaped stones, eithersquare or rectangular, and the remainder are irregular(subrounded or subangular). Irregularity provides

Fig. 1. Frequency of stone walls by categories of wall height and width.




more and wider inter-stone gaps furnishing bene®tsanalogous to small stones. Most joints are mortaredwith ¯ush or ribbon pointing. Only 3.6% of the wallshave open joints, being the oldest representatives.Most joints are either intact/original or intact/repaired;few walls have broken mortar at the joints. With openor broken joints, advantages similar to small stonesmentioned above can be achieved.

4. Characteristics of wall trees

The surveyed walls contain vegetation of differentgrowth forms, ranging from mosses and lichens, toferns and herbs, and to shrubs and trees. The wallplants are due to spontaneous colonization unassistedby human actions. As mimicry of nature's rockyprecipices, the wall habitat is expected to harbor asimilar plant assemblage. The humid-tropical envir-onment of Hong Kong, with an inherently rich native¯ora enriched by exotic components, has many can-didate species with different degrees of suitability towall existence. Some 49.5% of the walls harbor trees,a total 1275 individuals from 30 species, which are

conspicuous by virtue of biomass, ecological andenvironmental signi®cance (Table 1 and Table 2). Incomparison, roadside trees in urban Hong Kong reach149 species (Jim, 1996), and the total tree species of the territory is about 300 (Agriculture and FisheriesDepartment, 1993). In relative terms, the tree speciesdiversity on walls is subdued. In terms of habitat stresslevel and limited surface area, that some 30 speciescan establish despite the heavy odds is an impressiverecord. No data on wall vegetation in other tropicalareas is available for comparison. Temperate-latitudewalls are dominated by non-woody species of herbs,ferns and lower plants, with trees as minor or rareelements (Segal, 1969).

The tree stock is dominated by a small number of common species, accompanied by uncommon torare ones (Table 2). The top three species take up72.7% and the top 12 as high as 96.7%. Some 18species have frequencies below 10, of which 11 aresolitary specimens. Most walls have two to ®vespecies each; those with less than two or morethan ®ve species are less common. Native speciesare predominant, mainly Moraceae (Mulberryfamily), and to a lesser extent Ulmaceae (Elm family),

Fig. 2. The height and width of stones which constitute the stone walls.




Oleaceae (Olive family), Myrsinaceae (Ardisiafamily), Euphorbiaceae (Spurge family), and Laura-ceae (Laurel family). Exotic trees are rarely found,constituting merely 1.89% from six species. Only oneof the top 12 species is exotic; the remaining ®veexotic species are sparsely represented.

By botanical family, Moraceae is overwhelmingdominant with seven species and 87.6% of the trees.The genus Ficus contributing six species, ®veof whichare native, is the undisputed arboreal leader on walls.The evergreen Ficus microcarpa alone occupies 50%of the wall-tree population. The species is naturallyfound in local woodlands of the lowland evergreen

variety (Zhang et al., 1989). It is commonly planted asamenity trees in the city (Jim, 1990), furnishing thelargest and oldest urban specimens including high-caliber champion trees (Jim, 1994a, b). It is renownedfor huge ®nal size up to 20 m in height and crownspread, for longevity easily over 500 years, and forenvironmental tolerance to persevere in stressful urbansites. The species stocks most walls and most of thelargest wall trees. They are the wall-tree par excel-lence of the city, providing important landmark fea-tures in the hilly neighborhoods. Moreover, they serveas wildlife habitats especially for birds, adding faunaldiversity to the city's biotic community.

Table 2Species composition and frequency of trees growing on stone retai urban Hong Kong

Scientific name Common name Family Tree frequency

Count %

Ficus microcarpa Chinese banyan Moraceae 637 49.96Ficus superba Superb fig Moraceae 184 14.43Ficus hispida Rough-leaf stem-fig Moraceae 105 8.24Ficus virens Big-leaved fig Moraceae 74 5.80Celtis sinensis Chinese hackberry Ulmaceae 59 4.63 Broussonetia papyrifera Paper mulberry Moraceae 54 4.24Ficus variegata Red-stem fig Moraceae 50 3.92 Ligustrum sinense Chinese privet Oleaceae 18 1.41 Maesa perlarius (nil) Myrsinaceae 15 1.18Ficus religiosa * Peepul tree Moraceae 13 1.02 Bridelia monoica Pop-gun seed Euphorbiaceae 12 0.94

Litsea glutinosa Pond spice Lauraceae 11 0.86 Macaranga tanarius Elephant's ear Euphorbiaceae 8 0.63Cassia surattensis a Sunshine tree Caesalpiniaceae 6 0.47 Mallotus paniculatus Turn-in-the-wind Euphorbiaceae 6 0.47Cratoxylum ligustrinum Yellow-cow wood Hypericaceae 5 0.39 Dalbergia balansae South China rosewood Papilionaceae 3 0.24 Alangium chinense Chinese alangium Alangiaceae 2 0.16 Delonix regia a Flame of the forest Caesalpiniaceae 2 0.16 Albizia lebbeck Lebbeck tree Mimosaceae 1 0.08 Aporusa chinensis Aporusa Euphorbiaceae 1 0.08 Bauhinia blakeana Hong Kong orchid tree Caesalpiniaceae 1 0.08Carica papaya

a Papaya Cariaceae 1 0.08 Liquidambar formosana Sweet gum Hamamelidaceae 1 0.08

Litsea monopetala Persimmon-leaf litsea Lauraceae 1 0.08 Nerium indicum a Oleander Apocynaceae 1 0.08Punica granatum Pomegranate Punicaceae 1 0.08Sapium sebiferum Tallow tree Euphorbiaceae 1 0.08Schefflera octophylla Ivy tree Araliaceae 1 0.08Thevetia peruviana

a Yellow oleander Apocynaceae 1 0.08

Total 1275 100.00a Exotic species.




The exceptional ability of Ficus microcarpa toconquer wall habitat is related to its evolutionaryhistory and adaptive trait. It evolved as a strangler

in the tropical forest (Hills, 1967), with proliferationof aerial roots which upon reaching the soil canbecome woody and fuse together to form a sturdyweb-like matrix. The species has a natural growthhabit pre-adapted to grip tightly onto other trees, and itis equally at home gripping stone faces. Besidesbeginning life as a normal seedling on the forest ¯oor,its surface-clinging ability also allows spontaneousgrowing on cliffs and rocky faces in general (Corner,1988). On local stone walls, most F. microcarpa sendout a massive amount of roots which grip the stoneface and penetrate the gaps between stones and weepholes to explore the soil behind the stone facade. Forthe species, growing on a stone wall is analogous togrowing on a host tree, except that the wall cannot bestrangled. In some respect, wall trees behave like giantepiphytes with the necessary adaptations for a stead-fast attachment, except that they were not speci®callyevolved for this purpose.

Other Ficus species include F. superba , F. hispida ,F. virens , F. variegata , and F. religiosa . They arepresent on most walls, and owe their common occur-rence mainly to aggressive roots and ef®cient seeddispersal mainly by birds. F. superba , F. virens andF. religiosa in particular are by growth habit similarto F. microcarpa . On the other hand, F. hispida , F.variegata and Broussonetia papyrifera (the remainingmember of Moraceae) attach themselves by sendingroots into the soil behind the wall facade, with littlesur®cial roots on the stone face. Wall trees thus havetwo means of anchorage, namely, sur®cial roots withcommon self-grafting forming a strong basket-likeframework, as well as penetrating roots spreading intothe soil retained behind the stones. Species adopting adualistic anchorage mode tend to attain a higherfrequency and reach the largest biomass category(>9 m tall and >9 m crown spread; Table 1). The mostcapable species is F. microcarpa which has the highestroot density, longest root spread, and constitutes 80%of the 137 largest wall trees. A few managed to reach20 m tall which is close to the maximum potential sizefor the species. Other species with a single mode of attachment, only penetrating roots, have restrictedbiomass usually much smaller than their respectivebiological potential dimensions. Overall, rooting habit

plays a cardinal role in molding the capability of treesto colonize walls and to attain their ®nal size.

The remaining 23 species are non-Moraceae, most

of which have low frequencies, in total contributing12.4% of the wall trees. The only signi®cant non-Moraceae is Celtis sinensis of Ulmaceae, a native andcommon in local woodlands with a strong and search-ing root system adopting the penetrating-root attach-ment mode. A few trees managed to reach over 9 mtall. Other trees with >10 frequencies include Ligus-trum sinense , Maesa perlarius , Bridelia monoica and Litsea glutinosa . The rest of the tree composition ismade of rare species, 11 of which are solitary speci-mens. Unlike frequent tree species with special adap-tations to thrive on walls, the uncommon ones do nothave speci®c surface-clinging traits. The occurrenceof such rarities on walls could be incidental if notaccidental.

Many uncommon species are ruderals (Jim, 1994a,b) which spontaneously invade disturbed and arti®cialhabitats which abound in the city. Cases in point are Aporusa chinensis, Bridelia monoica, Ligustrumsinense, Litsea glutinosa, Litsea monopetala, Macar-anga tanarius, Sapium sebiferum , and Schef¯era octophylla . They have opportunistic and fugitivecharacters allowing invasion of open unstable sitessuch as roadside cut and ®ll slopes and vacant buildinglots. Some are found in the early successional stage of local woodlands (Zhang et al., 1989). It is possible thatthey had spread from adjacent woodlands onto walls.

A few unusual and unexpected species, namely ®ve¯owering trees ( Bauhinia blakeana , Cassia suratten-sis, Delonix regia, Nerium indicum and Thevetia peruviana ) and two fruit trees ( Carica papaya andPunica granatum ), are garden escapees. They arenormally cultivated in gardens and roadside greenstrips in the city. In this group, the ability of exoticspecies to grow on walls indicates a tendency tobecome naturalized; their degree of naturalization islimited as they do not venture into natural habitats.Together with the proper wall trees, an interestingspecies assemblage taking advantage of the uniqueopportunity for plant life has been nurtured. Theyfurnish an additional dimension to the ruderal diver-sities of urban ecological habitats.

The small trees (1±3 m tall) take up some 56% of the wall trees. Large trees (>9 m tall) contribute only10.7%, but are conspicuous due to biomass as well as




environmental-landscaping impacts (Fig. 3). Thatthey hang on walls above the ground tend to empha-size their imposing stature. The largest trees arealmost invariably Ficus microcarpa (80% of the>9 m category; Table 1), accompanied by some F.virens , F. superba , F. variegata and Celtis sinensis .The small trees are also commonly represented bythis species group, indicating an ample supply of saplings and young trees with the potential in duecourse to become sizable wall trees. This phenomenonsuggests that the colonization of walls by trees isstill actively proceeding despite deleterious urbaniza-tion impacts. Crown spread measurements provide asimilar physical con®nement with dominance by alimited 1±3 m span. Some 54.7% of the trees haveincomplete crowns due to various growing-space andphysiological constraints, pruning and other damagingactivities.

5. Ecological association between walls and trees

Wall dimensions apparently do not affect tree fre-quency. Overall, large walls do not necessarily harbormore trees or more species (Table 1; Figs. 4 and 5).

The smaller walls are often the best endowed with tree¯ora. Most trees are attached to walls with stonesslightly to moderately weathered, implying anincreased af®nity for old walls. Alternatively, itimplies that it takes time for walls to be graduallycolonized by trees, thus an old wall can accumulatemore trees. Highly weathered walls tend to attractmore Ficus variegata and F. virens . Walls with stonesof irregular size and shape and with wider gapsbetween stones attract more trees, and uncommonto rare species are more likely to dwell on them. Suchwalls have features that deviate from the harshestconditions, or rather more akin to normal habitats,thus they are more amenable to invasion by specieswithout special wall-hugging adaptations.

The positions of trees on walls, de®ned as the spotwhere the trunk base is attached, tells something aboutseed lodging and establishment. Most trees (68.7%)are attached somewhat away from thewall base or top.For a large tree to be adequately supported on avertical surface, the amount of biomass and resourcedevoted to anchorage is substantial. Comparing withground-growing trees, this extra burden drains heavilyon the tree's limited sustenance. It is reasonable tohypothesize that a wall tree does not outgrow the

Fig. 3. Frequency of trees in different tree-height and wall-height categories.




ability of its root system to support it. The mechanicalprinciple of tree architecture suggests an equilibriumbetween shoot loading and root holding capability(Mattheck and Breloer, 1994). That very few treestoppled from walls despite occasionally typhoononslaught (the few that fell were traced to deleterioushuman activities such as root severance) lend supportto this assumption. For trees that emanate from thewall top or the base, the need to support tree weight is

partly shouldered by the horizontal or nearly horizon-tal surfaces, with a subdued problem of attachmentand stability.

Wall moisture status in the wet summer months haslittle in¯uence on long-term tree growth. Most treesare found on quite dry walls, although some speciesare slightly more favored where seepage occurs. The®ne tree roots responsible for moisture and nutrientuptake (Perry, 1994) penetrate the wall face to enter

Fig. 4. Frequency of trees on walls of different categories of height and width.

Fig. 5. Species diversity index on walls of different categories of height and width.




the shielded soils. The availability of water in theretained soil in the dry winter months is more impor-tant in regulating tree survival. Whether the joint

pointing is of the ¯ush or ribbon types has no sig-ni®cant in¯uence on wall-tree colonization. A fewspecies surprisingly are more common on ¯ush ratherthan ribbon or recessed pointings, although the lasttwo types are supposedly more amenable to receiveseeds and support seedling growth. In terms of mate-rial composition, granite walls favor more of Ficusmicrocarpa , F. religiosa , F. variegata and Celtis sinen-sis . The volcanic walls are more accommodating toFicus hispida , F. superba , Broussonetia papyriferaand Ligustrum sinense .

Wall exposure affects the growth of some species.The exposed walls are more frequently invaded by Broussonetia papyrifera and Bridelia monoica ,whereas the sheltered walls favor Ficus hispida , F.superba and F. virens. Tolerance of the more stressfulhabitat conditions associated with exposure is neededfor survival. Regarding wall verticality, the inclinedones accommodate trees that are not normally foundon walls, including the ruderal and garden escapeegroups discussed in Section 4. The angled face facil-itates seed lodging and subsequently growth, anddemands less on anchorage.

The ability to venture into the soil trapped behindthe wall is essential for trees to absorb water andnutrient and to ascertain anchorage. That Ficus micro-carpa and other related species can reach phenomenaldimensions on walls, up to 20 m tall and 1.2 m trunk diameter, is attributed to the aggressive and spreadingroots which greatly expand the rooting room. Thus thesuccessful wall trees enlist the stone face mainly forinitial landing and physical attachment, with the bulk of the water, nutrient and anchorage needs for sustain-able growth ful®lled by the soil lying behind. Aretaining wall with its backing soil, both in situ andback®ll (Mohoney, 1994), form a partnership forvegetation establishment. A stone wall with portals,in the forms of between-stone joints and weepholes, ispertinent for effective communication between treeand soil. The wall lying between trees and soil formsan intermediary providing a rigid yet penetrable struc-ture to connect the two end members. The better theconnection, the better is the tree growth. In a sense, themost successful wall trees are the most able emulatorof normal ground-grown trees in capturing resources

for long-term survival, with a better chance to reachtheir growth potential.

6. Vicissitudes of wall vegetation

In nature, the uncommon vertical surfaces aremainly rocky cliffs with trying conditions for survival,mainly the absence of an effective soil layer and theimpermeability of stones. This is compounded byshortage of available moisture and nutrients, andexposure to wind and insolation. In cities, arti®cialvertical faces abound on buildings and other struc-tures, yet most are too inhospitable for even the mosttolerant species. Stone retaining walls, however, arehuman emulation of nature's cliffs created usingnatural materials in an arti®cial organization. Thephysical contiguity with behind-wall soil provides aready source of surface and subsurface moisture whichcontains nutrients. The gaps between individual stonesfurnish portals for nutrient-laden water to seep out andfor plant roots to reach the retained soil. The samegaps also afford micro-platforms for propagules toland and subsequently germinate. Weep-holes,designed to facilitate drainage of water away fromthe held-back soil to reduce loading on walls, have apleasant side-effect of enhancing plant life.

A vertical face is a dif®cult substrate with fewniches for propagules to lodge. Few seeds may bychance land on the minuscule ledges. The ensuing lifeis prone to a host of risks. Insuf®cient water andnutrients severely limits germination which is the mostcritical life stage. The young saplings have to eke outenough resources to send out roots to grip the largelysealed surface. Resources have to be diverted towardsgrowing supporting roots to maintain the foothold.The normal geotropic responses of the shoot and rootsare distorted by habitat verticality. Soil trapped behindthe stone facade could be accessed only if they are notsecluded by cement, lime or other mortar and pointingmaterials. Additionally, pollination and propagationare dif®cult due to shortage of suitable vectors.

Stone walls are further subjected to atmosphericstresses. In exposed sites, direct solar radiation heatsup the stones which have higher thermal capacity thanthe soil. The temperature range of stones is wider thanthe normal soil environment, with daytime heat accu-mulation reaching harmful levels. The energy loading




could also dry up the stones and the soil in thecrevices. Wind can accelerate the desiccation process.Trees on fully exposed walls could be snapped or

extracted by strong wind and typhoon. At sites trappedbehind high-rise buildings, heavy shading may reducemoisture loss. The lack of sunshine, however, maycheck photosynthetic rate. Arti®cial street lightingmay interfere with plant photoperiod response andother physiological functions.

Besides physical stresses, wall chemistry can berestrictive to plant life. The mortar jammed betweenstones contains lime which upon dissolution releasescalcareous substances and raise the pH of the substrateinto the alkaline range (Darlington, 1981). Smallplants such as herbs and ferns with limited root spreadhave to be rooted mainly in the alkalized materials. Asmost humid-tropical species are adapted to an acidicsubstratum, many plants ®nd the chemical environ-ment alien. The roadside location of many walls opensthem to air pollution.

Walls in built-up areas are increasingly regarded asa safety threat (Table 3). Wall plants, particularlytrees, are often construed as potentially disruptiveelements that may compromise wall stability(Fig. 6) despite the lack of relevant evidence (Woo-dell, 1979). With safety a paramount consideration,they are habitually pruned or removed in routinehighway and building maintenance. Recent heigh-tened concern on geotechnical stability due to severalfatal failure of disturbed slopes has increased wallrepair and vegetation clearance. Selected walls havebeen strengthened with new concrete structures addedas attachments to old walls, resulting in massivedestruction of the companion greenery. Some wallshave been demolished, either partly but often com-pletely, and replaced by new reinforced concrete wallswhich are inhospitable to vegetation. The widespreadredevelopment of old buildings is the most prevalentthreat to walls which are usually removed to lower thesite level to that of the adjacent road. New retainingwalls are no longer built with the traditional masonrytechnique which requires much manual skill and labor,and which does not satisfy stringent modern safetystandards. Some new concrete walls were given a thinstone veneer to visually mimic old ones but are devoidof ecological necessities for plant life.

Wall trees beside narrow roads are regularlytrimmed for safe pedestrian and vehicle (including

Table 3Habitat constraints of stone wall in cities

Factor Constraint

A. Intrinsic1. Verticality Propagule lodging

Germination stressEstablishment difficultyGeotropism adjustmentAnchorage problem

2. Sealed surface Rooting restrictionMoisture storageMoisture supplyNutrient deficiencyTemperature stress

3. Shading Photosynthetic rateShade sensitivity

4. Exposure DesiccationDeformationBreakageUprootingTyphoon selection

5. Substrate alkalinity Species admissionNutrient availability

B. Extrinsic6. Air pollution Growth dampening

Selective force7. Artificial lighting Photoperiod response8. Reproduction Pollination vector

Propagation opportunity9. Tree care Inadequate attention

Access problem10. Vehicular clearance Tree pruning

Tree removalGrazing injury

11. Wall maintenance Tree pruningVegetation removalSurface-root clearancePointing repairJoint sealingSurface srcubbing

12. Wall instability ButtressingReinforcementPartial reconstructionTotal reconstruction

13. Utility trenching Root severanceWall foundation disturbance

14. Construction activities Vegetation removalShoot physical damageRoot physical damageSoil (behind wall) contaminationParticulate pollutionBuilding foundation intrusionBuilding awning intrusionWall demolitionWall reconstruction

40 C.Y. Jim/ Landscape and Urban Planning 42 (1998) 29±43



double-decker buses) clearance. Some estate manage-ment regard vegetation on walls as a sign of poorupkeep, and impose a diligent purging regime. Theseperiodic culling is catastrophic to existing muralplants, ushering a dim prospect for urban landscapein the degraded neighborhoods. For old walls thatescape the scourge thus far, they have recentlyreceived ubiquitous maintenance treatments such asre-doing or adding mortar and pointing. Such largelycosmetic tinkering may bring psychological solacevis-aÁ-vis safety, but are detrimental to small plantsand tree roots. For annuals and biennials, there may bethe chance to complete a life cycle before they areheavily injured or eradicated. For perennials, suchperiodic havoc is inimical to their continued survival.The heavy crown reduction and branch removal leavethem weak and deformed. Too many wall plants withan initial promise fail to reach maturity.

7. Long-term management strategy andconclusion

In Hong Kong, intensive and relentless urban devel-opment in a dif®cult terrain over 150 years has

bequeathed many ®ne stone retaining walls. Theytestify the elaborate masonry work of a bygone era,entailing much hard manual labor and proud work-manship. Stone walls were built to stabilize a verticalengineered surface. Any subsequent plant coloniza-tion is a coincidental byproduct, a result of the inter-actions between nature and culture. A harsh and coldvertical wall face is softened by greenery and renderedless visually obtrusive. As a geographical reality andnecessity, the population has accustomed to livingnear them and to appreciating their landscape con-tributions.

The co-existence between walls and people over theyears unfortunately has recently been upset byincreasing safety concern and widespread redevelop-ment. Massive destruction and inappropriate proppingmeasures have been proceeding with little regard totheir ecological and landscape values. With no newwalls built in the traditional way, the continual ravageof a shrinking legacy needs to be arrested. Old stonewalls, especially those with more than a century of tenure, should be treated as a cultural heritage of thecommunity.

The spontaneous colonization of walls by diverseand vigorous vegetation, a pleasant spin-off of the

Fig. 6. Tree biomass index in different categories of threat to wall existence and wall integrity.

C.Y. Jim / Landscape and Urban Planning 42 (1998) 29±43 41



engineering endeavors, has greatly augmented theirvalue in the cityscape. The traditional wall-construc-tion method has laid down ample chances for nature to

explore and exploit. Walls constitute a highly specia-lized arti®cial habitat for the proliferation of ruderalvegetation which are in effect followers of humans andunconscious consequences of human activities. Naturehas attempted to claim the vacant niches of suchhabitats which happen to afford the basic sustenancefor life. The tropical environment has provided bothabiotic and biotic ingredients for a rich wall ¯ora,signi®cantly more so in comparison with temperatewalls.

The internal variabilities of this habitat is faithfullyechoed by equally varied permutations of ¯oristicassemblages, making it particularly interesting in bothecological and landscape terms. The species composi-tion also re¯ects the plasticity of wall plants towardsadverse habitat conditions. The pleasant green veneeradds interesting and vivid life to the otherwise lifelessstone facades. A wide range of species representingdifferent botanical families and growth forms man-ages to gain a foothold on the apparently dif®cultruderal sites. They arrive and establish without humanintervention, require no maintenance, hence qualify asthe highly-priced and genuine nature-in-the-city com-ponents, assiduously enriching the city ¯ora and land-scape. The hanging vegetation, not taking up preciousground-level space, furnishes a unique landscape ele-ment for the dull city environment, often contributingthe main if not the sole greenery in the hilly neighbor-hoods. The largest specimen trees are believed to be asold as the century-aged walls. In these regards, thewalls-cum-trees truly constitute a treasure of natural-cum-cultural heritage of the city.

Unfortunately, the inexorable pressure of urbanredevelopment in recent years has threatened theexistence of this valuable heritage. Many ®ne oldwalls together with their sylvan doyens have beendamaged or removed. Urban renewal, including reju-venating the buildings, roads and associated infra-structure, often disregards the conservation worth of stone walls. The best examples of this irreplaceableheritage, as shown by the present ®ndings on wallconstruction methods and their vegetative compa-nions, should be identi®ed and preserved for posterity.An inventory of old walls and associated trees shouldbe established and they should be declared as the

community's heritage under the provisions of theAntiquities and Monuments Ordinance. All listedwalls can be equipped with a plaque placed nearby

to signify their importance to the city's cultural andnatural history, to be accompanied by publicity andother modes of information so as to raise awarenessand respect.

The statutory umbrella should be accompaniedby a rigorous program of protection and enforcementto ensure their long-term welfare. All constructionactivities that may affect them directly or indirectlyshould be carefully scrutinized by an appropriateauthority, and clear technical guidelines should beprescribed to minimize deleterious impacts. Wallsand their constituent living companions should betreated as an integral package. Unless it can be provenabsolutely necessary by objective evidence, theheritage walls should not receive damaging ordis®guring treatments. Research should be conductedto develop a reliable scienti®c method to detectwall instability. Research should also probe the meansof stabilizing walls without degrading their heritagequalities. Any new walls can be designed to invitevegetation colonization or be installed with recepta-cles to accommodate plants. To accomplished theabove goals, the active support of developers andrelevant professionals both within and without thegovernment is essential.

Acknowledgements

I would like to convey my gratitude to the CaltexGreen Fund for ®nancial support of this project. Thehelp in ®eld work provided by my student researchassistants Wing-yee Ho, Oi-man Lee, Shun-wa Lee,Esther Li, Jeannette Liu, Wai-ling Tam and May Wongis gratefully acknowledged.

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old stone walls as an ecological habitat for urban trees in hong kong

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