dissertations - core · leaves towards the end of the experiment, but did not affect relative...

Publications of the University of Eastern Finland Dissertations in Forestry and Natural Sciences

Publications of the University of Eastern Finland

Dissertations in Forestry and Natural Sciences

isbn 978-952-61-0832-2

Sami K. Mörsky

Long-term Effects of ElevatedOzone and UV-B Radiationon Vegetation and MethaneDynamics in NorthernPeatland Ecosystems

The aim of this thesis was to

assess whether chronically

elevated tropospheric ozone

concentration and UV-B radia-

tion, studied separately, affect

northern peatland ecosystems.

In particular, the effects on

common peatland vegetation

and methane dynamics were

studied. The thesis increases

knowledge of the long-term

ozone and UV-B effects on peat-

land vegetation and methane

dynamics in realistic open-field

conditions.

dissertatio

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ong-term E

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Sami K. Mörsky

Long-term Effects of ElevatedOzone and UV-B Radiationon Vegetation and Methane

Dynamics in NorthernPeatland Ecosystems

SAMI K. MÖRSKY

Long-term Effects of Elevated Ozone and UV-B Radiation on Vegetation and Methane

Dynamics in Northern Peatland Ecosystems


No 75

Academic Dissertation

To be presented by permission of the Faculty on Sciences and Forestry for public examination in the Auditorium L22 in Snellmania building at the University of

Eastern Finland, Kuopio, on June, 15th, 2012, at 12 o’clock noon.

Department of Environmental Science

Kopijyvä Oy Kuopio, 2012

Editors:

Research director Pertti Pasanen Lecturer Sinikka Parkkinen

Distribution:

Eastern Finland University Library / Sales of publications P.O.Box 107, FI-80101 Joensuu, Finland

tel. +358-294-45-8145 http://www.uef.fi/kirjasto

ISBN 978-952-61-0832-2

ISSN 1798-5668 ISBN 978-952-61-0833-9 (PDF)

ISSN 1798-5676 (PDF) ISSNL 1798-5668

Author’s address: University of Eastern Finland Department of Environmental Science P.O.Box 1627 FI-70211 KUOPIO FINLAND email: [email protected]

Supervisors: Professor Toini Holopainen, Ph.D.

University of Eastern Finland Department of Environmental Science P.O.Box 1627 FI-70211 KUOPIO FINLAND email: [email protected] Professor Pertti J. Martikainen, Ph.D. University of Eastern Finland Department of Environmental Science P.O.Box 1627 FI-70211 KUOPIO FINLAND email: [email protected]

Reviewers: Docent Minna Turunen, Ph.D.

University of Lapland Arctic Centre P.O.Box 122 FI-96101 ROVANIEMI FINLAND email: [email protected] Associate professor Thomas Friborg, Ph.D. University of Copenhagen Department of Geography and Geology P.O.Box 1353 COPENHAGEN K DENMARK email: [email protected]

Opponent: Senior scientist Lucy Sheppard, Ph.D. Centre for Ecology and Hydrology Edinburgh Bush Estate, Penicuik Midlothian EH26 0QB UNITED KINGDOM email: [email protected]

ABSTRACT

In the stratosphere, ozone (O3) forms an effective barrier against high energy ultraviolet radiation (UV), which is harmful to living cells. Despite the stratospheric O3 layer re-covering due to international agreements, seasonal O3 deple-tion periods with high UV-B levels, may still occur, especially in the Polar regions. In the troposphere, O3 is a significant greenhouse gas contributing to global warming and also causing oxidative stress to animal- and plant cells. Global tropospheric O3 concentration has approximately doubled during the last century and the same trend is expected to con-tinue.

Northern peatlands are sinks of atmospheric carbon diox-ide (CO2) and sources of the powerful greenhouse gas meth-ane (CH4). Two multi-year open-field experiments were con-ducted to study the effects of elevated O3 concentration and UV-B radiation on peatland vegetation and CH4 dynamics in Finland. Peatland microcosms were used in the O3 experi-ment and the UV-B exposure study was conducted on a natu-ral fen.

Elevated O3 concentration significantly increased leaf cross-sections and the total number of Eriophorum vaginatum leaves towards the end of the experiment, but did not affect relative length growth, stomatal density or volume of aeren-chymatous tissue of leaves. Elevated O3 did not affect relative length growth of Sphagnum papillosum shoots either. Concen-trations of chlorophylls or carotenoids in E. vaginatum leaves or in S. papillosum shoots were not changed under elevated O3. During the first growing season, elevated O3 concentra-tion decreased methanol-extractable, UV-absorbing com-pounds in E. vaginatum leaves. Elevated O3 increased concen-trations of organic acids and microbial biomass (estimated by phospholipid fatty acid biomarkers) in peat during the third growing season. In the first growing season net CH4 emission was temporarily decreased by elevated O3 concentration. However, the temporary decrease in CH4 emission was not

consistent and towards the end of the experiment O3 tended to increase net CH4 emission.

Enhanced UV-B radiation did not affect leaf anatomy or senescence rate of Eriophorum russeolum leaves. In addition, there were no UV-B effects on the total chlorophyll or carote-noid concentrations in E. russeolum leaves during the current study. UV-B radiation transiently increased the amount of cell wall bound UV-absorbing pigments in E. russeolum leaves. Organic acid concentrations in peat were slightly higher in the UV-B treatment compared to ambient control treatment. Elevated UV-B radiation did not reduce net CH4 emission during the three consecutive growing seasons nor did it affect wintertime CH4 emission rates.

The results of this thesis indicate that almost doubled am-bient O3 or moderately enhanced UV-B would not reduce vi-tality of peatland vegetation in the near future. However, these stress factors can change carbon allocation below ground, which would increase net CH4 emissions from northern peatlands in the longer term.

Universal Decimal Classification: 504.7, 546.214, 547.211, 582.323, 632.151 CAB Thesaurus: methane; mosses; Eriophorum; Sphagnum; ozone; ultra-violet radiation; peatlands; fens; vegetation; leaves; shoots; chlorophyll; ca-rotenoids; organic acids; plant pigments; microbes; stress factors; carbon; Finland Yleinen suomalainen asiasanasto: metaani; otsoni; ultraviolettisäteily; sammalet; suot; kasvillisuus; hiili; Suomi

Dedicated to my loving family

Acknowledgements

The present study was carried out at the Department of Envi-ronmental Science at the University of Eastern Finland in Kuopio and at the Arctic Research Centre of the Finnish Me-teorological Institute – FMIARC in Sodankylä. I thank the Department and the FMIARC for providing high quality fa-cilities for the scientific work.

The study was financially supported by the Academy of Finland, the Graduate School in Forest Sciences - GSForest, the Environmental Risk Assessment Centre - ERAC, the Fin-nish Cultural Foundation, the Kone Foundation, the Niemi Foundation and the University of Kuopio.

I express my gratitude to my unique and skilful supervi-sors: Professors Toini Holopainen and Pertti J. Martikainen and Docent Jouko Silvola. Especially, Toini and Pertti, when-ever I needed, you always had time for me - often outside the working hours.

I am grateful to Timo Oksanen and the staff at the Univer-sity of Eastern Finland Research Garden in Kuopio for main-taining the ozone experiment in Ruohoniemi and for techni-cal assistance. I thank Professor Esko Kyrö, Hanne Suoka-nerva and the staff of the Arctic Research Centre of the Fin-nish Meteorological Institute for maintaining the exposure field throughout the study and for the environmental data.

I am thankful to Senior scientist Dr. Lucy Sheppard for her acceptance of the invitation to act as an opponent in public defence. I thank the pre-examiners of the thesis, Docent Minna Turunen and Associate Professor Thomas Friborg, for their valuable comments. I also thank Dr. James Blande for revising the English language of the manuscripts.

I owe my sincere thanks to my colleague Jaana K. Haapala for conducting measurements in Sodankylä and co-writing the articles. I would like to thank several graduate students

and trainees: Päivi Tiiva, Marjo Valtanen, Saara Patronen, Nina Kontinaho, Ulla Räty, Britta Scharf, Merja Mustonen and Tuulia Venäläinen for their assistance with field and laboratory work. I thank Jaana Rissanen for analysing plant samples in the laboratory and Virpi Miettinen from BioMater Centre for preparing the light microscopy samples. I also thank all my colleagues and friends at the Department for scientific and non-scientific discussions and sport activities. I am grateful to Juha Heijari and Janne Räsänen for your friendship while we shared an office.

I express my sincere gratitude to my family members, es-pecially, my parents for all your love and support you have given to me.

Finally, I would like to express my gratefulness to my wife and to my lovely children. You mean everything to me. Kuopio, May 2012 Sami K. Mörsky

ABBREVIATIONS

•O2- superoxide

•O2H hydroperoxide •OH hydroxyl radical

a.s.l. above sea level

AOS active oxygen species

AOT40 accumulated exposure over a threshold of 40 ppb

AUC area under curve

BVOC biogenic volatile organic compound

C:N-ratio carbon:nitrogen-ratio

CFC chlorofluorocarbon

CH3COOH acetic acid

CH4 methane

CO carbon monoxide

CO2 carbon dioxide

DNA deoxyribonucleic acid

DOC dissolved organic carbon

Fv/Fm quantum efficiency of photosystem II

H2O2 hydrogen peroxide

NaCl sodium chloride

NEE net ecosystem exchange

NOx nitrogen oxides

O3 ozone

OTC open-top chamber

PAR photosynthetically active radiation (400-700 nm)

Pg gross photosynthesis

PLFA phospholipid fatty acid

ppb parts per billion, nl l-1

Rtot total respiration

Rubisco ribulose-1,5-bisphosphate carboxylase-oxygenase

UV ultraviolet radiation

UV-A ultraviolet-A radiation (315-400 nm)

UV-B ultraviolet-B radiation (280-315 nm)

VOC volatile organic compound

δ13C stable carbon isotope ratio (13C:12C)

LIST OF ORIGINAL PUBLICATIONS This thesis is based on data presented in the following arti-cles, referred to in the text by the Roman chapter numbers II-V.

II Mörsky SK, Haapala JK, Rinnan R, Tiiva P, Saarnio S, Silvola J, Holopainen T, Martikainen PJ. (2008) Long-term ozone effects on vegetation, microbial commu-nity and methane dynamics of boreal peatland micro-cosms in open field conditions. Global Change Biology 14: 1891-1903.

III Mörsky SK, Haapala JK, Rinnan R, Saarnio S, Silvola

J, Martikainen PJ, Holopainen T. (2011) Minor effects of long-term ozone exposure on boreal peatland spe-cies Eriophorum vaginatum and Sphagnum papillosum. Environmental and Experimental Botany 72: 455-463.

IV Mörsky SK, Haapala JK, Rinnan R, Saarnio S, Suo-

kanerva H, Latola K, Kyrö E, Silvola J, Holopainen T, Martikainen PJ. (2012) Minor long-term effects of ul-traviolet-B radiation on methane dynamics of a su-barctic fen in Northern Finland. Biogeochemistry 108: 233-243.

V Mörsky SK, Haapala JK, Rinnan R, Saarnio S, Kyrö E,

Silvola J, Martikainen PJ, Holopainen T. (201x) Tran-sient effects of elevated UV-B radiation on UV-absorbing pigments but no effects on leaf anatomy of a sedge Eriophorum russeolum. Submitted manuscript.

The original articles have been reprinted with permission of the copyright holders.

AUTHORS’ CONTRIBUTIONS II Sami K. Mörsky designed and modified the equip-

ment (e.g. water collector, microscopic photograph-ing), contributed to the field work, analyzed CH4 samples, determined potential CH4 production and oxidation, prepared samples for microscopic analyses, conducted light microscopic analyses, data analyses and wrote the paper. Jaana K. Haapala contributed to the writing. Riikka Rinnan designed the study, per-formed the PLFA laboratory analyses and contributed to the writing. Päivi Tiiva contributed to the field work and laboratory analyses. Sanna Saarnio, Jouko Silvola, Toini Holopainen and Pertti J. Martikainen designed the study and contributed to the writing.

III This paper was conducted as a joint project with Jaana

K. Haapala and it is also included in her thesis. Sami K. Mörsky designed and modified the equipment (e.g. growth measurement sticks), contributed to the growth measurements, pigment analyses, membrane permeability tests, prepared samples for microscopic analyses, conducted light microscopic analyses, data analyses and wrote the corresponding chapters. Jaana K. Haapala contributed to the data analyses, con-ducted transmission electron microscopic analyses, C:N-ratio measurements, and wrote the correspond-ing chapters. Riikka Rinnan and Sanna Saarnio de-signed the study and contributed to the writing. Jouko Silvola designed the study. Pertti J. Martikainen and Toini Holopainen designed the study and contributed to the writing.

IV Sami K. Mörsky designed and modified the equip-

ment (e.g. water collection cell), contributed to the field work, analyzed CH4 samples and stable isotopes, worked with a mass spectrometer, conducted data

analyses and wrote the paper. Jaana K. Haapala con-tributed to the field work and writing. Riikka Rinnan and Sanna Saarnio designed the study and contrib-uted to the writing. Hanne Suokanerva designed and constructed the UV-exposure field and contributed to the data handling. Kirsi Latola and Esko Kyrö de-signed the UV-exposure field. Jouko Silvola, Toini Holopainen and Pertti J. Martikainen designed the study and contributed to the writing.

V Sami K. Mörsky designed and modified the equip-

ment (e.g. filtration system for UV-pigment samples), contributed to the field work, analyzed plant pig-ments, prepared samples for microscopic analyses, conducted light microscopic analyses and analyzed digital photographs, conducted data analyses and wrote the manuscript. Jaana K. Haapala contributed to the field work, laboratory analyses, data analyses and writing. Riikka Rinnan and Sanna Saarnio de-signed the study and contributed to the writing. Esko Kyrö designed the UV-exposure field. Jouko Silvola designed the study. Pertti J. Martikainen and Toini Holopainen designed the study and contributed to the writing.

Contents

1 General Introduction .............................................................. 19

1.1 Northern peatland ecosystems ........................................... 19 1.1.1 Special characteristics of peatland ecosystems ...................... 19 1.1.2 Peatland vegetation ............................................................... 20 1.1.3 Effects of environmental changes .......................................... 21

1.2 Tropospheric ozone .............................................................. 23 1.2.1 Ozone concentrations in Northern Europe .......................... 23 1.2.2 Harmful ozone effects on plants............................................ 23

1.3 Solar UV-B radiation ............................................................ 27 1.3.1 Stratospheric ozone depletion ............................................... 27 1.3.2 UV-B effects on plants .......................................................... 27

1.4 Methane as a greenhouse gas .............................................. 30 1.4.1 Sources and atmospheric concentration ............................... 30 1.4.2 CH4 fluxes from peatlands .................................................... 31 1.4.3 Contribution of vascular plants to CH4 emissions ............... 32

1.5 Summary of the current experiments and other studies related to the thesis ..................................................................... 33

1.6 Objectives and hypotheses of the present study .............. 41

References ................................................................................... 43

II Long-term Ozone Effects on Vegetation, Microbial Com-munity and Methane Dynamics of Boreal Peatland Micro-cosms in Open Field Conditions ............................................. 51

III Minor Effects of Long-term Ozone Exposure on Boreal Peatland Species Eriophorum vaginatum and Sphagnum papillosum .................................................................................... 67

IV Minor Long-term Effects of Ultraviolet-B Radiation on Methane Dynamics of a Subarctic Fen in Northern Finland .......................................................................................... 79

V Transient Effects of Elevated UV-B Radiation on UV-absorbing Pigments but no Effects on Leaf Anatomy of a Sedge Eriophorum russeolum in a Long-term Field Expo-sure Study .................................................................................... 93

6 General Discussion ............................................................... 111

6.1 Ozone and UV-B effects on peatland plants and belowground processes ........................................................... 111 6.1.1 Relative length growth, stomatal density and aerenchymatous tissue ............................................................................................ 111 6.1.2 Plant chemistry .................................................................. 113 6.1.3 Belowground processes ....................................................... 115

6.2 Ozone and UV-B effects on CH4 dynamics ..................... 116

6.3 Methodological considerations ......................................... 118

6.4 Conclusions ......................................................................... 120

References ................................................................................. 121

Dissertations in Forestry and Natural Sciences No 75 19

1 General Introduction

Tropospheric O3, “bad ozone”, concentration is constantly in-creasing and stratospheric O3, “good ozone”, depletion can cause intensive UV-B radiation periods, especially in the Po-lar regions (McKenzie et al. 2011). Northern peatlands play an important role in atmospheric carbon balance forming a sink for atmospheric CO2, but simultaneously are a source of the powerful greenhouse gas CH4. Peatland vegetation, espe-cially the vascular plants and mosses, is a link between CO2 fixation, CH4 production and gas transport through the peat. So far, impacts of O3 and UV-B stresses on peatland vegeta-tion and biogeochemistry have mostly been studied in growth chambers, greenhouses or open-top chambers with experiments lasting no more than one growing season. In this thesis two long-term experiments have been conducted in open-air conditions to clarify the effects of increasing tropo-spheric O3 concentration and UV-B radiation, studied sepa-rately, on vegetation and functioning of northern peatland ecosystems.

1.1 NORTHERN PEATLAND ECOSYSTEMS

1.1.1 Special characteristics of peatland ecosystems Peatlands form a remarkable part of the landscape in the Northern Hemisphere. For example, in Finland one third of the total land area can be classified as peatland (Tomppo & Henttonen 1996). Peat formation is possible when soil is wa-ter saturated as a result of suitable hydrological conditions (geomorphology, soil has low water permeability, precipita-tion exceeds evapotranspiration). Additionally, there must be specialized plants, such as mosses, that can adapt to the high water availability (Charman 2002). The majority of the peat is

Sami K. Mörsky: Effects of O3 and UV-B on Vegetation and Methane Dynamics

20 Dissertations in Forestry and Natural Sciences No 75

formed from the remains of various moss species that even-tually sink down below the water table. In anoxic conditions decay is slow and thus primary production overcomes de-composition.

Peatlands can be divided into two major groups, based on hydrological conditions and nutrient availability (Vasander 1996). In nutrient poor ombrotrophic peatlands (bogs) vege-tation only gets water from precipitation and nutrients from precipitation and dry deposition. In addition, minerotrophic peatlands (fens) also get water and nutrients through leach-ing from surrounding ecosystems. Peatland development generally starts as a fen, which then develops towards the bog type in peatland succession (Vasander 1996).

Water table and increase in peat bulk density divides peat profile into two distinguishable layers called acrotelm and ca-totelm (Ingram 1978). Living plants form the upper, oxic acrotelm, and are responsible for the peatland’s primary pro-duction. Below the acrotelm is the catotelm where decaying processes are slower and anoxic conditions enable CH4 pro-duction (Charman 2002).

1.1.2 Peatland vegetation Vegetation in northern peatlands consists of Sphagnum mosses, sedges, dwarf shrubs and trees (Gorham 1991; Va-sander 1996). Trees and dwarf shrubs prefer dryer peatland surfaces whereas Sphagnum mosses can survive well in wet, acidic and nutrient poor conditions. Living without a root system, these mosses absorb water and nutrients straight into the leaves and stem (Brown 1982). Sphagnum mosses compete with other plant species by producing and excreting organic acids. Furthermore, animals do not usually graze Sphagnum mosses (Vasander 1996).

The species studied in the O3 experiment of this thesis was Sphagnum papillosum, which is one of the most important peat formers in boreal peatlands (Vasander 1996). S. papillosum ex-ists in oligo- or oligomesotrophic peatlands, often together with Sphagnum magellanicum. S. papillosum prefers high pre-

General Introduction


cipitation and higher temperature than Sphagnum fuscum, which occurs over a wider climatic range (Gignac 1994).

Even though Sphagnum mosses are common in peatlands, sedges also contribute to the peat formation process, espe-cially in fens. In this thesis two Eriophorum species, E. vagi-natum and E. russeolum have been studied. Sedges use a dif-ferent strategy than mosses to survive in harsh peatland eco-systems. Roots and rhizomes of these plants penetrate deep into anoxic peat and can survive there because of the special aerenchymatous tissue typical to sedges (V, Fig. 2), which transports oxygen from the atmosphere to the rhizosphere (Schütz et al. 1991). Simultaneously, aerenchymatous tissue acts as a conduit for the greenhouse gas CH4 to be released from peat into the atmosphere (Whiting & Chanton 1996; Au-lakh et al. 2000).

1.1.3 Effects of environmental changes The climate is warming more rapidly in the boreal and su-barctic regions (> 60°N) than at the lower latitudes (ACIA 2005). The scientific community has agreed that most of the warming observed over recent decades is based on human activities. Increasing concentrations of CO2 and other green-house gases, primarily from fossil fuel burning and land use, are projected to cause additional arctic warming of 4-7 °C over the next 90 years (ACIA 2005). Generally, increased pre-cipitation, shorter and warmer winters, and decreases in snow and ice cover are very likely to persist for centuries (IPCC 2007). Obviously, these changes will also have an im-pact on northern peatland ecosystems. However, it is still unknown how increasing precipitation and evapotranspira-tion will alter the water table in northern peatlands (ACIA 2005).

Higher air temperature is expected to increase ecosystem production (Strack et al. 2006), but also increase evapotran-spiration and thus decrease water table level in peatlands (Roulet et al. 1992). If water table decreases, productivity of peatlands will increase due to a shift in vegetation towards



higher order plants (Weltzin et al. 2003; Dorrepaal et al. 2004). Methane emissions are positively correlated with peat tem-perature, thus methanogenesis would increase in warmer climate conditions (Christensen et al. 2003) if water the table remains high enough (Nykänen et al. 1998). Additionally, higher peat temperatures will accelerate decomposition of organic matter. The decomposition chain produces organic acids, including acetate, which is an important substrate in CH4 production (Christensen et al. 2003).

If water table decline occurs with global warming, overall vegetation composition of peatlands is expected to change (Strack & Waddington 2007). Sphagnum moss cover will de-crease in hummocks, but increase in hollows. Wetter lawns will be invaded by sedges, which are linked with the CH4 transportation discussed earlier. More shrubs and trees will be growing in dryer hummocks (ACIA 2005; Breeuwer et al. 2009).

Increased nitrogen (N) deposition and enhanced nutrient availability in soils following climate warming may also cause greenhouse gas exchange affecting changes in peatland ecosystems. The results of N and phosphorus (P) fertilization experiments in oligotrophic bogs showed that both gross primary production and ecosystem respiration were in-creased by N addition when background N deposition was low (Lund et al. 2009). Gross primary production was stimu-lated by P addition in the high N deposition site. However, the treatments had no effect on the CH4 exchange. Similarly, only a slight increase in annual CH4 emissions was found in a two-year-long N addition study of a boreal oligotrophic mire (Saarnio et al. 2000). In another study of a poor fen lawn, Granberg et al. (2001) studied the effects of increased air tem-perature in combination with increased N and/or sulphur (S) deposition on CH4 emission. The nitrogen addition decreased CH4 emission when the sedge cover was high and had no or slightly positive effect with the low sedge cover. Sulphur ad-dition had a negative effect on CH4 emission at ambient tem-perature, but had no effect at raised temperatures.



Changes in hydrology and air temperature will be the main factors affecting peatland vegetation and biogeochemi-cal cycles in a changing climate (Roulet et al. 1992; Moore et al. 1998). However, tropospheric O3 concentration and en-hanced UV-B radiation can cause stress to plants and thus also affect the biogeochemical cycles.

1.2 TROPOSPHERIC OZONE

1.2.1 Ozone concentrations in Northern Europe Ozone is essential in the stratosphere, protecting life on Earth from harmful UV-radiation. Conversely, in the troposphere, it is a greenhouse gas and a major secondary air pollutant formed in photochemical gas-phase reactions in which vola-tile organic compounds (VOCs), CH4 and carbon monoxide (CO) are oxidised in the presence of catalyzing nitrogen ox-ides (NOx) (Kleinman 1994; Finlayson-Pitts & Pitts 1997). The global tropospheric O3 concentration has approximately dou-bled (daily mean from 20 to 40 ppb) during the last century (Vingarzan 2004). The same trend is expected to continue in the future (IPCC 2007).

Tropospheric O3 concentration has also been rising in Finland. Laurila et al. (2004) suggested that O3 concentration in Finland will slightly increase until 2050, but the longer term trend is unclear, and will depend on emissions of O3 precursors. Even though the lifetime of molecular O3 is short, long-lived O3 precursors can reach remote areas (Hakola et al. 2006), such as peatlands. Thus, it is possible that remote peat-lands are also exposed to elevated O3 concentration.

1.2.2 Harmful ozone effects on plants In higher order plants intact cuticle protects leaves from O3. However, O3 affects the structure of epicuticular waxes caus-ing a significant shift towards lower molecular weight chains (Kerfourn & Garrec 1992). O3 is taken up through the open stomata into the sub-stomatal cavity and hence primary



damage is confined to the leaf mesophyll cells (Kangasjärvi et al. 1994). The guard cells of the stomata are not fully pro-tected by cuticle, and thus will be exposed to the highest con-centrations of O3 as it diffuses into the leaf (Long & Naidu 2002). Stomatal conductance is closely linked to photosyn-thetic rate. Therefore, stomatal conductance could decline in-directly when O3 causes damage to the mesophyll cells and directly when O3 has a negative impact on guard cells (Long & Naidu 2002).

In the apoplast, O3 dissolves into wet surfaces and forms highly reactive free radicals, containing one or more un-paired electrons, such as superoxide (•O2-) and hydroperox-ide (•O2H). Further reactions produce hydrogen peroxide (H2O2), hydroxyl radicals (•OH) and singlet oxygen (Long & Naidu 2002). These active oxygen species (AOS) may attack all organic components of the plasmalemma. These reactions may promote up- and down regulation of various genes causing activated defence, accelerated senescence and pro-grammed cell death (Kangasjärvi et al. 1994). To prevent or minimize the damage of AOS, plants produce antioxidants such as phenolics, flavonoids and carotenoids (Long & Naidu 2002).

Photosynthesis is an early target of O3 exposure, and sometimes the only physiological symptom of damage dur-ing chronic exposure of leaves (Long & Naidu 2002). In fact, O3 has been shown to be capable of damaging or inhibiting almost every step of the photosynthetic process from light capture to starch accumulation (Farage et al. 1991). O3 par-ticularly affects the dark reaction of photosynthesis by de-creasing Rubisco (ribulose-1,5-bisphosphate carboxylase-oxygenase) content and activity (Farage et al. 1991; McKee et al. 1995). Often, tropospheric O3 concentration decreases plant growth, but harmful effects at cellular level (e.g. de-creased chloroplast size, increased number and size of plas-toglobuli) can already be seen earlier before visible symp-toms or growth suppression occur (Oksanen et al. 2004).



Many northern plant species, for example trees, have shown O3 sensitivity (e.g. Manninen et al. 2009; Oksanen et al. 2009). Scots pine and downy, silver and mountain birches have negatively responded to elevated O3 concentration (e.g. Mortensen 1999; Riikonen et al. 2004). Data from 23 different laboratory, open-top chamber, and free-air fumigation ex-periments with birch and aspen in Finland, showed that de-creased root growth appeared to be the most vulnerable tar-get of enhanced O3 concentration (Oksanen et al. 2009). Growth reductions were accompanied by increased visible foliar injuries, formation of defence compounds, reduced carbohydrate contents of leaves, impaired photosynthesis processes, disturbances in stomatal function, and earlier au-tumn senescence. However, in both families large genetic variation exists (Oksanen et al. 2009).

In common with northern tree species, meadow plants suffer from O3 stress. In an open-top chamber study (1.5 x ambient, 3 growing seasons) total aboveground biomass of harebell (Campanula rotundifolia) and tufted vetch (Vicia cracca) was significantly reduced by elevated O3 concentra-tion (Rämö et al. 2006). Furthermore, Power & Ashmore (2002) reported that elevated O3 concentration (AOT40 10000 ppb h, 23 days) reduced above- and below-ground biomass of Cirsium arvense. There was a significant positive correlation between stomatal conductance and the magnitude of the O3 effect on root biomass. A short-term 14CO2 pulse and chase study with spring wheat showed that elevated O3 concentra-tion increased root exudation to the rhizosphere (McCrady & Andersen 2000). However, these O3 induced effects below ground cannot be directly generalized since there is inconsis-tency in the literature resulting from species differences and experimental protocols (Andersen 2003). An important point is that carbon flux to the rhizosphere is altered, affecting in-teractions between roots and rhizosphere organisms.

In wet peatlands, vascular plants, such as E. vaginatum, can keep the stomata open through the growing seasons and could thus take up O3 continuously (Bungener et al. 1999). In



a growth chamber study with rather high O3 concentration (100 ppb, 4-5 weeks) elevated O3 reduced chloroplast size and the amount of starch in E. vaginatum leaves (Rinnan & Holo-painen 2004). Even though the data set of the study was lim-ited, E. vaginatum showed O3 sensitivity. However, in a two-year-long mesocosm study in Northern England, elevated O3 exposure (OTCs; 8 h day-1, 49 ppb in summer; 10 ppb in win-ter) had no significant effect on above ground biomass of E. vaginatum or S. papillosum (Toet et al. 2011).

In contrast to higher order plants, mosses are not able to regulate O3 uptake and thus can be exposed to high O3 doses during growing seasons. Although Sphagnum mosses play an important role in northern peatland ecosystems, there are only a few studies concerning the effects of O3 on these plants (Gagnon & Karnosky 1992; Potter et al. 1996b; Niemi et al. 2002a; Rinnan & Holopainen 2004). Gagnon & Karnosky (1992) reported adverse effects of O3 (OTCs, 80 ppb, 10 weeks) on the chlorophyll concentration of S. magellanicum but not in Sphagnum rubellum. In addition, Potter et al. (1996b) studied the responses of four Sphagnum species to acute O3 fumigation (150 ppb, 6 hours, 5°C) in growth chambers. O3 exposure caused significant reduction in photosynthesis and increased membrane leakiness of S. recurvum, but S. capilli-folium, S. cuspidatum and S. papillosum showed O3 tolerance. Furthermore, Niemi et al. (2002a) found that elevated O3 con-centration (growth chambers, 50 ppb, 4 weeks) increased membrane permeability of Sphagnum angustifolium in autumn conditions, but in summer conditions such an effect was not apparent. Rinnan & Holopainen (2004) showed that O3 in-duced alterations in cell organelles in several Sphagnum moss species are comparable to typical O3 stress symptoms of higher order plants.



1.3 SOLAR UV-B RADIATION

1.3.1 Stratospheric ozone depletion The stratospheric O3 layer, which protects all living organ-isms from harmful UV radiation (UV-B 280-315 nm), has be-come thinner due to human activities. Long-lived chlorofluorocarbons (CFCs) and other manmade chemicals are causing stratospheric O3 depletion (ACIA 2005). The stratospheric O3 layer has been depleted above the Antarctic and above the Arctic causing enhanced UV-B exposure (Taalas et al. 2000). The largest Arctic O3 reductions have been monitored in spring, with average losses of 10-15% since 1979 (ACIA 2005). O3 depletion was more severe at earlier times (minimum 1992-1993), but because of the Montreal Protocol and its amendments the O3 layer has gradually recovered in the stratosphere (Austin & Wilson 2006; IPCC 2007). Recent studies, however, have highlighted the possibility of inten-sive O3 depletion periods in the future (Rösevall et al. 2007; McKenzie et al. 2011).

There is an indirect relationship between climate change and stratospheric O3 depletion. Greenhouse gases (e.g. CO2, CH4 and O3) warm the troposphere and simultaneously cool the lower stratosphere favouring the conditions for polar stratospheric cloud formation and thus O3 destruction as stratospheric clouds are the main sites of O3 depletion reac-tions (Shindell et al. 1998; WMO 2007).

1.3.2 UV-B effects on plants Terrestrial plants absorb photosynthetically active radiation (PAR) wavelengths to drive photosynthesis in chloroplasts. Most of the incident UV-B is absorbed by leaves, and only minor fractions are reflected or scattered at the leaf surface (Caldwell et al. 1995). The primary targets of UV-B in plants are DNA (e.g. formation of cyclobutane dimers), membranes (e.g. peroxidation of unsaturated fatty acids) and damage af-fecting photosynthesis (e.g. disruption of thylakoid mem-branes) (Rozema et al. 1997; Jansen et al. 1998). These harmful



effects of UV-B on the key targets in plants can be avoided by effective filtering of UV-B by phenolic compounds localized in epidermal cells. Additionally, defences against UV-B dam-age consist of DNA repair, scavenging of radicals and pro-duction of polyamines (Rozema et al. 1997).

Early studies reported that elevated UV-B caused reduc-tion in plant growth or reproductive yield (Teramura 1983). However, most of these studies were conducted in growth chambers or greenhouses under unrealistic light conditions. In fact, controls excluded UV-B radiation entirely and studied plants were exposed to low background UV-A and visible light (Caldwell et al. 1994). It has been shown by Caldwell et al. (1994) that UV-A and visible wavebands are important in mitigating harmful UV-B effects on plants via induction of UV-B-absorbing compounds.

Different taxa have shown different sensitivity to en-hanced UV-B, but there is great variation in the sensitivity within genera and even between genotypes within a species (Teramura 1983). In their meta-analysis concentrating on bryophytes and angiosperms living in the Polar regions, Newsham & Robinson (2009) reported that elevated UV-B in-creased concentration of UV-B absorbing pigments, reduced aboveground biomass and plant height, and increased DNA damage. However, carotenoid or chlorophyll concentration, net photosynthesis, total biomass or leaf area showed no UV-B response. A few years earlier, Searles et al. (2001) reported similar results in a meta-analysis concerning UV-B effects on vascular plants at lower latitudes.

When peatland microcosms were exposed to enhanced UV-B (30% above ambient) in open-field conditions for a sin-gle growing season in Central Finland, leaf cross-section area and the percentage of aerenchymatous tissue in E. vaginatum leaves were significantly reduced (Niemi et al. 2002c). How-ever, morphology of E. vaginatum leaves was not affected when the UV-B intensity was lower in a cloudy summer (Niemi et al. 2002b). Enhanced UV-B increased the amount of UV-B-absorbing compounds in S. papillosum, but decreased



those in S. angustifolium (Niemi et al. 2002c). Additionally, in a summer with lower UV-B intensity, enhanced UV-B in-creased the concentration of chlorophyll and carotenoid pig-ments in Sphagnum balticum (Niemi et al. 2002b).

The effects of enhanced UV-B radiation on carbon alloca-tion and root exudation of Eriophorum angustifolium and Nar-thecium ossifragum were studied in an 8-week-long open-air study in southern Scandinavia (Rinnan et al. 2005). Enhanced UV-B increased rhizome biomass and decreased dissolved organic carbon (DOC) and monocarboxylic acid concentra-tion in the pore water of N. ossifragum mesocosms. By con-trast, enhanced UV-B tended to increase monocarboxylic acid concentration in the rhizosphere of E. angustifolium and its root:shoot-ratio. Additionally, microbial biomass carbon was increased by enhanced UV-B in the surface water of the E. angustifolium mesocosms.

A UV-filtering approach (10%, near-ambient; 80%, re-duced) was used in a 3-year-long field study in an ombrotro-phic bog in Southern Argentina (Searles et al. 2002; Robson et al. 2003). Morphology of vascular plants (Empetrum rubrum, Nothofagus antarctica and Tetroncium magellanicum) was not af-fected, but there was a 10-20% decrease in the amount of UV-B-absorbing compounds of T. magellanicum in the 80% re-duced UV-treatment (Searles et al. 2002). However, during the fourth to sixth growing seasons, stem growth and branch-ing frequency of E. rubrum and branching frequency of N. antarctica were decreased under near-ambient UV-B (Robson et al. 2003).

UV-B radiation can affect the decomposition of plant litter by altering the chemical characteristics of the plant material (Cybulski et al. 2000; Pancotto et al. 2005). UV-B radiation in-creases the concentration of UV-B absorbing pigments in leaves, which can reduce the decomposability of the leaf litter (Gehrke et al. 1995; Brandt et al. 2007). UV-B radiation can photodegrade plant material and change structure and activ-ity of the decomposer community (Austin & Vivanco 2006; Brandt et al. 2007). Photodegradation may increase the de-



composition rate, especially in arid ecosystems where micro-bial activity is limited by low soil water content (Brandt et al. 2007; Smith et al. 2010). In wetter conditions, the negative ef-fect of UV-B on microbial activity becomes more pronounced and the decomposition rate can be decreased by supplemen-tal UV-B (Moody et al. 2001; Pancotto et al. 2003; Smith et al. 2010).

1.4 METHANE AS A GREENHOUSE GAS

1.4.1 Sources and atmospheric concentration Atmospheric CH4 originates from natural and anthropogenic sources. Methane is emitted from wetlands, oceans, forests, wildfire, termites, ruminants and geological sources. The an-thropogenic sources include rice cultivation, livestock, land-fills and waste treatment, biomass burning, and combustion of fossil fuel (IPCC 2007). Natural wetlands (174 ± 51 Tg(CH4) yr-1) are the largest of all natural CH4 sources (199 ± 45 Tg(CH4) yr-1). Anthropogenic CH4 sources (341 ± 51 Tg(CH4) yr-1) are over 70% greater than the natural ones.

The major sinks for atmospheric CH4 are oxidation by hy-droxyl free radicals (•OH) in the troposphere, biological oxi-dation in soil, and loss to the stratosphere (IPCC 2007). Oxi-dation by •OH radicals forms the most important CH4 sink (481 ± 36 Tg(CH4) yr-1) of the total sink (550 ± 38 Tg(CH4) yr-1). The primary formation of •OH in the troposphere is con-trolled by the UV radiation flux, dependent on the overhead atmospheric O3 column as well as the local O3 and water va-pour concentrations (Lelieveld et al. 2004; Hofzumahaus et al. 2009). Thus, •OH levels are highest in the tropics where the stratospheric O3 layer facing the orthogonal sun rays is thin-nest and the absolute humidity is highest (Lelieveld et al. 2002).

Biogenic CH4 formation is a result of complex biogeo-chemical reactions in anaerobic environments. In anaerobic fermentation reactions organic macromolecules are converted



to acetic acid (CH3COOH), other carboxylic acids, alcohols, CO2 and hydrogen (H2). Acetate, H2 and CO2 act as substrates for methanogenic Archaea (Conrad 1996).

Stable carbon isotopic signatures can be used to quantify the relative contribution of the major methanogenic path-ways (H2/CO2-dependent and acetate-dependent) to total CH4 production (Conrad 2005). However, determination of δ13C (13C:12C-ratio) in CH4 and CO2 give only coarse informa-tion on the methanogenic pathways. More accurate estima-tion requires determination δ13C in the methyl group of ace-tate as well.

1.4.2 CH4 fluxes from peatlands CH4 produced in anoxic peat moves to the atmosphere by diffusion, ebullition, and via plant-mediated transport (Charman 2002). Part of it is oxidized to CO2 by methanotro-phic bacteria before reaching the atmosphere. Methanotrophs are most effective in peat layers, close to the water table, where both CH4 and oxygen exist (Sundh et al. 1994; Ket-tunen et al. 1999).

There are many uncertainties in the future projections of ongoing climate change and its effects on peatlands (ACIA 2005). It has been suggested that a change including an in-crease of 4°C in temperature and a small and persistent in-crease in annual precipitation, would reduce water table in northern peatlands. This would favour the growth of schrubs and other higher order plants and thus have the potential to shift northern peatlands from net carbon sources to net car-bon sinks (ACIA 2005). However, if the rate of decomposition exceeds enhanced photosynthesis, CO2 emissions from peat-lands will increase. Furthermore, a combination of tempera-ture increase and elevated water table could result in in-creased CH4 emissions (ACIA 2005).

Tropospheric O3 concentration and enhanced UV-B radia-tion can affect peatland vegetation (1.2.2 and 1.3.2). Because CH4 production and emission are linked to the function of plants, it is likely that CH4 fluxes are also affected by UV-B



and O3. In a short-term (6 week) growth chamber study, Niemi et al. (2002a) exposed peatland microcosms (Ø 10.5 cm, depth 40 cm) to elevated O3 concentration. In this study the CH4 emission was increased by O3 fumigation, especially at high concentration (100 ppb). In contrast, CH4 emission was not affected in another similar growth chamber study where peatland microcosms were exposed to elevated O3 concentra-tion for seven weeks (Rinnan et al. 2003).

Southern temperate regions have peatlands which can be affected by high O3 concentrations year-around. In a two-year-long open-top chamber study in Northern England, peatland mesocosms were exposed to elevated O3 (49 ppb or 10 ppb above ambient in summer and in winter, respec-tively). Methane emission was significantly reduced (about 25%) by elevated O3 during midsummer periods of both years, but no significant effect of O3 was found during the winter periods (Toet et al. 2011).

In a microcosm study representing a minerogenic, oligotrophic low-sedge pine fen, for one growing season in outdoor conditions, Niemi et al. (2002c) reported significant reduction in CH4 emission under elevated UV-B radiation (30% above ambient). However, in another microcosm study with a similar peatland type, conducted in a cloudy summer, CH4 emission was not affected by elevated UV-B (Rinnan et al. 2003).

1.4.3 Contribution of vascular plants to CH4 emissions Vascular plants, like sedges, have several functions in CH4 dynamics in peatlands. Firstly, their root system and hollow aerenchymatous tissue serve as a conduit for produced CH4 moving towards the atmosphere but also for downward shift of O2 that takes part in CH4 oxidation in the rhizosphere. The positive correlation between aerenchymatous tissue and CH4 dynamics has been shown by Aulakh et al. (2000) with rice cultivars and by Whiting & Chanton (1996) using aquatic macrophytes. Additionally, Rinnan et al. (2003) reported that at the end of the UV-B experiment, CH4 efflux was 12-fold



higher in microcosms with intact vegetation, dominated by E. vaginatum and Sphagnum spp., compared to microcosms lack-ing E. vaginatum shoots. Stomatal conductance of vascular plants is one of the processes regulating CH4 flux from the anoxic peat into the atmosphere (e.g. Morrissey et al. 1993; Garnet et al. 2005).

Secondly, the rhizosphere of vascular plants serves fresh labile carbon as root exudates for microbes, including methanogens. It has been shown that CH4 emission correlates positively with net primary production (Whiting & Chanton 1993; King & Reeburgh 2002). When studying the relation-ship between photosynthesis and root exudation of peatland plants, Ström et al. (2003) showed that when photosynthesis was limited by shading, the amount of acetate in peat pore water was remarkably reduced.

1.5 SUMMARY OF THE CURRENT EXPERIMENTS AND OTHER

STUDIES RELATED TO THE THESIS

The thesis is based on four publications originating from two open-field experiments (II-V, Table 1). The first experiment was conducted in an open-air O3 exposure field (Ruoho-niemi) at the University of Eastern Finland, Kuopio Campus Research Garden (Figure 1) and the other in a UV-B exposure field at Halssiaapa in Sodankylä (Figure 2) close to the Fin-nish Meteorological Institute (Figure 3).

Peatland microcosms, used in O3 exposure studies (II, III), originated from Salmisuo mire complex in Mekrijärvi (Figure 3). The microcosms were cored from the lawn of a minero-genic, oligotrophic, low-sedge S. papillosum pine fen (for more details, see Saarnio et al. 1997). The vegetation of the microcosms consisted of a dense Sphagnum moss cover, dominated by S. papillosum, and a sedge E. vaginatum. Addi-tionally, a few shoots of S. balticum, S. magellanicum, Carex limosa, Carex pauciflora, Andromeda polifolia, Vaccinium oxycoc-cus and Drosera rotundifolia were present in the microcosms.



The UV-B experiment (IV, V) was conducted in Hals-siaapa, Sodankylä. The study site was established on a natu-ral subarctic mesotrophic flark fen ecosystem. Halssiaapa fen consists of hummock strings with wet flarks in between. A sedge E. russeolum and a moss Warnstorfia exannulata domi-nate the vegetation in the study area. Additionally, shoots of C. limosa, Scheuchzeria palustris, Carex magellanica, Menyanthes trifoliata, A. polifolia and V. oxycoccos were present. Both open-field experiments lasted for several years and the present data was collected during 3-4 consecutive growing seasons (2003-2006).

In addition to the studies included in this thesis, there have been other researchers working with the peatland mi-crocosms under elevated O3 concentration in Ruohoniemi, Kuopio and with the UV-B study plots in Halssiaapa, Sodan-kylä. Firstly, the effects of elevated O3 concentration on gross photosynthesis (Pg), total respiration (Rtot), and chlorophyll fluorescence have been reported by (Haapala et al. 2011). In addition, isoprene emissions were studied by (Tiiva et al. 2007b). Secondly, the effects of enhanced UV-B radiation on carbon balance (Haapala et al. 2009), CO2 assimilation rate, chlorophyll fluorescence, and fine structure of the dominat-ing plant species (Haapala et al. 2010) have been studied. Ad-ditionally, Tiiva et al. (2007a) measured isoprene emissions and Rinnan et al. (2008) studied UV-B-related belowground processes and carbohydrate concentrations in E. russeolum. Furthermore, Martz et al. (2011) analyzed total soluble pheno-lics in E. russeolum leaves. Moreover, BVOC (biogenic volatile organic compound) emissions including some measurement campaigns of CH4 emissions were reported by Faubert et al. (2010). The main results of these studies are summarized in the Table 2.

Tabl

e 1.

Sum

mar

y of

the

expe

rim

ents

of t

he th

esis

. Rom

an c

hapt

er n

umbe

rs r

efer

to th

e or

igin

al p

ublic

atio

ns w

hich

con

tain

det

aile

d ex

peri

-m

enta

l des

crip

tions

. Loca

tion

V

ari

ab

les

Tim

e

Peatl

an

d

Tre

atm

en

t C

hap

ter

Ele

vate

d O

3,

Kuopi

o

(62°5

3’N

,27°3

7’E

)

net

CH

4 e

mis

sion

pote

ntial

CH

4 p

roduct

ion a

nd o

xidat

ion in p

eat

mic

robi

al c

om

munity

com

posi

tion

in p

eat

(PLF

A)

org

anic

aci

d c

once

ntr

atio

ns

in p

eat

pore

wat

er

str

uct

ure

of E.

vagin

atum

lea

ves

o le

af d

ensi

ty

o st

om

atal

den

sity

o a

eren

chym

a (%

)

2003-

2006

S.

pap

illosu

m

pin

e fe

n

1.7

-1.9

x a

mbi

ent

O3

conce

ntr

atio

n

II

r

elat

ive

length

gro

wth

of

E.

vagin

atum

lea

ves

and S

.

pap

illosu

m s

hoot

s

chlo

roph

yll an

d c

arote

noi

d c

once

ntr

atio

ns

in E

. va

gi-

nat

um

lea

ves

and S

. pap

illosu

m s

hoots

met

han

ol e

xtra

ctab

le c

om

pou

nds

in E

. va

gin

atum

leav

es

mem

bran

e pe

rmea

bili

ty o

f S. pap

illosu

m s

hoot

s

lea

f cr

oss

-sec

tion

are

a

str

uct

ure

of E.

vagin

atum

lea

ves

and S

. pa

pill

osu

m

shoots

o num

ber

of m

itoch

ondr

ia p

er c

ell

o ch

loro

plas

t si

ze

o am

ount

of s

tarc

h

o num

ber

of pl

asto

globuli

o gra

num

sta

ck t

hic

knes

s (o

nly

E. va

gin

atum

)

C:N

-rat

io o

f S.p

apill

osu

m

2003-

2006

S.

pap

illosu

m

pin

e fe

n

1.7

-1.9

x a

mbi

ent

O3

conce

ntr

atio

n

III



Con

tinue

d

Loca

tion

V

ari

ab

les

Tim

e

Peatl

an

d

Tre

atm

en

t C

hap

ter

Ele

vate

d U

V-B

,

Sodan

kylä

(67°2

2’N

,26°3

9’E

)

net

CH

4 e

mis

sion

(su

mm

er a

nd w

inte

r)

org

anic

aci

d c

once

ntr

atio

ns

in p

eat

pore

wat

er

sta

ble

iso

topi

c co

mposi

tion

of CH

4 (

δ13C-C

H4)

2003-

2005

Subar

ctic

flar

k fe

n

21-6

3%

abov

e am

bi-

ent

UV-B

rad

iation

IV

s

olu

ble

and c

ell w

all bo

und

com

pounds

in E

. ru

sseo

-

lum

lea

ves

chlo

roph

yll an

d c

arote

noi

d c

once

ntr

atio

ns

in E

.

russ

eol

um

lea

ves

str

uct

ure

of E.

russ

eolu

m lea

ves

o le

af c

ross

-sec

tion

are

a

o st

om

atal

den

sity

o ae

rench

yma

(%)

sen

esce

nce

and d

ecom

pos

itio

n r

ate

of E

. ru

sseo

lum

leav

es

C:N

-rat

io o

f E.

russ

eolu

m lea

ves

2003-

2005

Subar

ctic

flar

k fe

n

21-6

3%

abov

e am

bi-

ent

UV-B

rad

iation

V

(()

Tabl

e 1.

Con

tinue

d





Figure 1. Ozone fumigation experiment in Ruohoniemi, Kuopio, Finland. (A) An octagonal experimental field (n = 4) with vertical, perforated O3 releasing pipes. (B) A partially soil embedded peatland microcosm equipped with a water groove.

Figure 2. UV-B experimental field (n = 10) in Halssiaapa, Sodankylä, Finland. (A) UV-lamps (UV-A control, UV-B treatment) and wooden frames (ambient control) were placed above peat surface in a natural subarctic fen. (B) An aluminium collar with a water filled groove used in gas exchange measure-ments.



Figure 3. Closed symbols show the locations of the experimental sites of this thesis and an open symbol indicates the origin of the peatland micro-cosms used in O3 experiments in Ruohoniemi, Kuopio.



Table 2. Summary of the main results of the other studies conducted, to-gether with the studies presented in this thesis, either in Ruohoniemi, Kuo-pio (O3, peatland microcosms) or in Halssiaapa, Sodankylä (UV-B, natural fen) experimental sites.

Experiment Time Variables Effect Reference

O3 microcosm

experiment in

Ruohoniemi

2003-

2006

Pg ↓* Haapala et al.

2011

Rtot, Fv/Fm in E.

vaginatum leaves

→

2004-

2005

isoprene emission → Tiiva et al.

2007b

UV-B experi-

ment in

Halssiaapa

2006,

2008

BVOC emission

(toluene, 1-

octene)

↑* Faubert et al.

2010

CH4 emission ↑

2003-

2005

NEE ↑ Haapala et al.

2009

Pg →

Rtot ↓

2003-

2005

CO2 assimilation

rate in E. russeo-

lum and W. exan-

nulata

→ Haapala et al.

2010

fine structure of E.

russeolum and W.

exannulata

→

Fv/Fm in E.

russeolum leaves

↑*

2006-

2007

total soluble phe-

nolics in E. russeo-

lum leaves

↑ Martz et al.

2011

phenolic composi-

tion in E. russeo-

lum leaves

→

2005 sucrose and starch

concentrations in

E. russeolum

leaves

↓ Rinnan et al.

2008

sucrose concentra-

tion in E. russeo-

lum rhizome

↑

carbohydrate

concentrations in

E. russeolum roots

→

Continued



Experiment Time Variables Effect Reference

total phenolics

concentration in E.

russeolum roots

↑

2004-

2006

isoprene emission ↑ Tiiva et al.

2007a

↑ increase, ↓ decrease, → no effect * transient effect

Table 2. Continued



1.6 OBJECTIVES AND HYPOTHESES OF THE PRESENT

STUDY

Prior to the studies presented in this thesis, there was already some knowledge of the O3 and UV-B effects on northern peatland plants and CH4 dynamics (Niemi et al. 2002a; Niemi et al. 2002b; Niemi et al. 2002c; Rinnan et al. 2003). However, all of these studies were conducted using peatland micro-cosms and lasted at maximum of one growing season and none of the O3 studies were conducted in open-air conditions.

The aim of the thesis was to assess whether chronically elevated tropospheric O3 concentration (II, III) and UV-B ra-diation (IV, V), studied separately, affect northern peatland ecosystems. The effects on vegetation and CH4 dynamics of natural peatland were studied in particular. The thesis in-creases knowledge of the long-term O3 and UV-B effects on peatland vegetation and CH4 dynamics in realistic open-field conditions. The following hypotheses were formulated: 1) Elevated O3 concentration does not affect morphology

(relative length growth, stomatal density, cross-section area, aerenchymatous tissue) of E. vaginatum leaves but E. russeolum leaves can be affected by enhanced UV-B radia-tion

2) Elevated O3 and enhanced UV-B increase organic acid concentration in peat via function of peatland vegetation

3) Elevated O3 does not change plant chemistry of E. vagi-natum and S. papillosum, but elevated UV-B may affect UV-B-absorbing pigments in E. russeolum leaves



4) Elevated O3 increases CH4 emission of northern peatlands, but enhanced UV-B has the opposite effect



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6 General Discussion

6.1 OZONE AND UV-B EFFECTS ON PEATLAND PLANTS AND

BELOWGROUND PROCESSES

6.1.1 Relative length growth, stomatal density and aerenchymatous tissue

Elevated O3 concentration did not affect relative length growth of E. vaginatum leaves (III), which is in accordance with the hypothesis of the present thesis. However, temporal effect, indicating natural seasonal variation, appeared to be significant, but interaction between the treatments did not ex-ist. Additionally, Haapala et al. (2011) reported no permanent O3 effects on Pg measured simultaneously from the same peatland microcosms. These results show that there were no significant long-term treatment effects that could be inte-grated to relative length growth during the four consequent growing seasons. Contrary to the earlier results, elevated O3 concentration did not affect plant morphology (stomatal den-sity, volume of aerenchymatous tissue) or ultrastructure of E. vaginatum leaves (III). In their growth chamber study with higher O3 exposure levels, Rinnan & Holopainen (2004) showed negative O3 responses in sensitive structural parame-ters such as size of the chloroplasts and the amount of starch. Even though, the relative length growth of E. vaginatum leaves was not affected, elevated O3 significantly increased (8%) leaf cross-sections (III) and the total number of sedge leaves towards the end of the current experiment (II).

Elevated O3 concentration did not affect relative length growth of S. papillosum shoots either (III), which is in line with the hypothesis of the present thesis. However, in the lat-ter part of the experiment, elevated O3 tended to decrease the mean relative length growth of S. papillosum. The result is similar to those reported for Sphagnum recurvum and Poly-

General Discussion


trichum commune, exposed to 70-80 ppb O3 for 6-9 weeks (Pot-ter et al. 1996a). In contrast, at the beginning of the growing seasons when the O3 concentration was the highest, the mean relative length growth of S. papillosum was slightly higher under O3 treatment (III). The result was not supported by the Pg results reported in Haapala et al. (2011), probably because E. vaginatum dominated in the microcosms. Early phase growth stimulations by elevated O3 have been reported for P. commune (Petersen et al. 1999), Mentha aquatica (Power & Ashmore 2002), Molinia caerulea (Franzaring et al. 2000), Pinus sylvestris (Manninen et al. 1998), and Betula pendula (Oksanen et al. 2009). Petersen et al. (1999) suggested that production of growth controlling hormones, like cytokinin, may have been affected by O3.

Contrary to the hypothesis of the present thesis, elevated UV-B did not affect leaf anatomy or senescence rate of E. russeolum leaves (V). Despite the UV-B exposure in the pre-sent experiments being higher (21-63% above ambient) than in the earlier microcosm studies (24-27% above ambient; Niemi et al. 2002b; Niemi et al. 2002c), there was no clear UV-B response in terms of the number of stomata, cross-section area or the amount of aerenchymatous tissue of the E. russeo-lum leaves. Additionally, Haapala et al. (2009) reported a slight increase in net ecosystem exchange (NEE), decrease in Rtot but no change in Pg under elevated UV-B in the same ex-periment. In contrast, Niemi et al. (2002c) reported UV-B-related reductions in leaf cross-section area and the percent-age of aerenchymatous tissue in E. vaginatum leaves. How-ever, in another study with lower UV-B intensity, morphol-ogy of E. vaginatum leaves was not affected (Niemi et al. 2002b). All of these studies were conducted in open-air condi-tions, but only E. vaginatum living in microcosms, represent-ing boreal peatland, responded clearly towards elevated UV-B.



6.1.2 Plant chemistry Elevated O3 concentration did not affect concentrations of chlorophylls or carotenoids in E. vaginatum leaves during the present study (III), which supports the hypothesis of the pre-sent thesis. However, elevated O3 slightly increased total chlorophyll concentration, especially at the beginning of the second growing season. The result is in accordance with those reported for S. rubellum (Gagnon & Karnosky 1992). Furthermore, increased chlorophyll concentrations may be associated with the early phase growth stimulations under elevated O3 discussed earlier (6.1.1).

During the first growing season, elevated O3 concentration decreased methanol-extractable, UV-absorbing compounds in E. vaginatum leaves (III). After that the amount of metha-nol-extractable compounds tended to increase towards the end of the study in O3 treated plants. A similar trend was seen both in UV-B and UV-A wavelength regions. A meta-analysis of tree leaf chemistry revealed that concentrations of secondary metabolites, phenolics and terpenes, were signifi-cantly increased by elevated O3 concentration when the expo-sure lasted more than five months (Valkama et al. 2007). In contrast, a shorter exposure period caused the opposite re-sult. These findings suggest that long-term O3 exposure can alter formation of chemical defence compounds and thus carbon allocation to chemical defence also in E. vaginatum leaves.

Similar to E. vaginatum, elevated O3 concentration did not change chlorophyll or carotenoid concentrations in S. papillo-sum shoots during the present study (III). However, during the third growing season, elevated O3 slightly decreased total chlorophyll and carotenoid concentrations. These results agree with those of Gagnon & Karnosky (1992) who reported adverse O3 effects on chlorophyll concentration of S. magel-lanicum. When responses of four Sphagnum species were studied using acute O3 fumigation, membrane leakiness of S. capillifolium, S. cuspidatum and S. papillosum was not affected

General Discussion


(Potter et al. 1996b). In contrast, O3 fumigation caused signifi-cant reduction in photosynthesis and increased membrane leakiness of S. recurvum. Furthermore, Niemi et al. (2002a) re-ported increased membrane permeability of S. angustifolium under O3 fumigation treatment in autumn conditions, but a similar effect was not seen in summer conditions. In contrast to earlier studies, conducted in growth chambers or OTCs, membrane leakiness of S. papillosum was not affected by ele-vated O3 concentration in the present open-field study (III).

Elevated UV-B radiation did not affect total chlorophyll or carotenoid concentrations in E. russeolum leaves during the present study (V). However, elevated UV-B tended to de-crease chlorophyll and carotenoid concentrations during the first two growing seasons when UV-B dose was higher than during the third growing season. These results are in accor-dance with the hypothesis of the present thesis. Surprisingly, in a summer with low ambient UV-B levels, UV-B treatment increased the concentration of chlorophyll and carotenoid pigments in S. balticum (Niemi et al. 2002b). In contrast, two meta-analyses, representing field studies, concerning UV-B effects on vascular plants (Searles et al. 2001) and bryophytes and angiosperms (Newsham & Robinson 2009) indicated that enhanced UV-B radiation does not affect chlorophyll concen-trations.

Elevated UV-B radiation transiently increased the amount of cell wall bound UV-absorbing pigments in E. russeolum leaves during the present study (V), which is in accordance with the hypothesis of the present thesis. The increase, seen both in UV-B and UV-A wavelength regions, was significant at the end of the first growing season when the UV-B dose was highest. However, there were no treatment effects on the amount of methanol-extractable pigments in E. russeolum leaves. In contrast, the results by Martz et al. (2011) from the Halssiaapa experimental field showed significant UV effects in E. russeolum leaves. Total soluble phenolics were higher in the leaves exposed to enhanced UV-B radiation and in the UV-A control than in the ambient control leaves, but the



phenolic composition was not significantly modified. Similar results of E. russeolum leaves were reported for a UV-B exclu-sion experiment conducted in an oligotrophic tall-sedge Sphagnum flark fen in Vuotso, Finland (Soppela et al. 2006). Niemi et al. (2002c) showed that enhanced UV-B increased the amount of methanol-extractable UV-B-absorbing com-pounds in S. papillosum, but decreased those in S. angusti-folium.

6.1.3 Belowground processes Elevated O3 increased concentrations of organic acids in peat pore water during the third growing season (II), which is in accordance with the thesis hypothesis. The result is also con-sistent with those of slightly increased DOC concentrations measured from O3 treated peatland mesocosms in Northern England (Toet et al. 2011). In the present study concentration of acetate, the key substrate for the acetoclastic methano-genes, was the highest of the organic acids analyzed. Acetate is the primary organic terminal product of the anaerobic de-composition chain in bogs (Duddleston et al. 2002). However, based on the stable isotope analyses of the produced CH4, it was evident that acetate was not the primary substrate for methanogenesis in the present peatland microcosms. At the end of the third growing season, microbial biomass (esti-mated by phospholipid fatty acid biomarkers) was higher in peat exposed to O3 than in peat of the control microcosms (II). These results were in line with those of organic acids in peat. The results of the present study indicate that elevated O3 con-centration support the general growth of microbes in peat by increasing their substrate availability by altering the function of plants and carbon allocation below ground (Ashmore 2005). It is unlikely that O3 could directly alter microbial populations belowground. Recent 18O labelling studies showed that the penetration of O3, and any reaction products, into wet soil is limited to the top few millimetres (Toet et al. 2009).

General Discussion


Similar to the scenario under elevated O3, organic acid concentrations in peat were slightly higher in the UV-B treatment than the ambient control treatment (IV). These re-sults support the hypothesis of the thesis. The highest con-centrations were measured at the beginning of the growing seasons when the plants were growing rapidly. These results agree with those of E. angustifolium (Rinnan et al. 2006) and suggest that root exudation of Eriophorum spp. is altered by elevated UV-B. Surprisingly, Pg estimated from NEE results and leaf level photosynthesis of E. russeolum, also measured from Halssiaapa fen, were not affected under elevated UV-B (Haapala et al. 2009; Haapala et al. 2010). Thus, higher concen-tration of organic acids in the UV-B treatment must be be-cause of enhanced carbon allocation below ground. However, there were no UV-B effects on microbial biomass that could have increased peat decomposition and thus increased labile carbon in the peat profile (Rinnan et al. 2008).

6.2 OZONE AND UV-B EFFECTS ON CH4 DYNAMICS

In the first growing season net CH4 emission was temporarily decreased by elevated O3 concentration (II), which did not confirm the hypothesis of this thesis but was in line with the results of Toet et al. (2009) concerning peatland mesocosms. This observation contrasted with the earlier results that rela-tively high O3 concentration (100 pbb), more than doubled CH4 emission (Niemi et al. 2002a). However, the temporary decrease in CH4 emission at the beginning of the present ex-periment was not consistent and towards the end of the ex-periment O3 tended to increase net CH4 emission. Elevated O3 slightly increased the total number of sedge leaves in micro-cosms but that was taken into account by using leaf density as a covariate in statistical analyses. The temporary decrease in CH4 emission most likely occurred because of the coring and transporting processes, which have temporarily affected



intact vegetation in microcosms and partly exposed deeper peat layers to oxygen.

Elevated O3 concentration did not affect CH4 production or oxidation potentials, determined at the end of the third growing season (II). In fact, elevated O3 increased concentra-tions of organic acids, substrates for methanogenesis, in peat that could explain slightly enhanced net CH4 emission under elevated O3. However, stable isotope analysis of δ13CH4 of produced CH4 indicated that hydrogenotrophic CH4 produc-tion dominated in the peat studied. Even though CH4 pro-duction potential did not respond to higher concentrations of organic acids in peat (6.1.3) it is possible that O3-induced belowground processes could increase net CH4 emission in longer term exposures.

Contrary to the hypothesis of the present thesis elevated UV-B radiation did not reduce net CH4 emission during the three consecutive growing seasons (IV). In general, the high-est emissions were measured from the UV-B-treated plots but after normalization by leaf density of E. russeolum the differ-ence appeared to be insignificant. These results are in line with the results of CO2 exchange measured simultaneously (Haapala et al. 2009). Elevated UV-B did not affect wintertime CH4 emission rates, which were less than 10% of the summer-time emission rates. In their microcosm study, Niemi et al. (2002c) found significant reduction in CH4 emission under elevated UV-B. However, similar measurements during a cloudy summer did not show significant differences between the treatments (Rinnan et al. 2003).

In the present study there were no UV-B treatment effects on CH4 emission during the first three growing seasons (2003-2005) in the Halssiaapa experimental field. However, during the fourth (2006) and sixth (2008) growing seasons net CH4 emission was 43% and 45% higher under elevated UV-B compared to the ambient control, respectively (Faubert et al. 2010). This increase in the net CH4 emission could be associ-ated with altered microbial community composition in the UV-B plots (Rinnan et al. 2008) that changes the CH4 dynam-

General Discussion


ics after several years of exposure. Based on these results long-term enhanced UV-B radiation can affect CH4 emission, but there are no short-term effects. This is in accordance with the results of long-term UV-B effects on peatland plants in Tierra del Fuego summarized by Aphalo (2003).

6.3 METHODOLOGICAL CONSIDERATIONS

The O3 fumigation experiment was conducted in an open-air O3 exposure field at the University of Eastern Finland, Kuo-pio campus Research Garden (62°53’N, 27°37’E, 80m a.s.l.) for four consecutive growing seasons (2003-2006). Ruohoniemi exposure field (n = 4) has successfully been used for various O3 fumigation studies since 1990 (e.g. Karnosky et al. 2007; Oksanen et al. 2009). In the present study peatland micro-cosms (II, Fig. 1b), cored from Salmisuo, Ilomantsi (62°47’N, 30°56’E, 145m a.s.l.), were embedded in the ground leaving the uppermost 4cm of the core above the soil surface. When embedded into the soil a more natural temperature gradient was formed inside the microcosms compared to the earlier microcosms studies (Niemi et al. 2002a; Niemi et al. 2002b; Niemi et al. 2002c; Rinnan et al. 2003). Although the micro-cosms used in the present thesis were placed fairly close to the side of the octagonal O3 fumigation rings (1.5, Figure 1), there were only a few peak O3 concentration periods moni-tored at the peatland vegetation level (III). Thus, the present O3 fumigation study was conducted in representative open-air conditions using realistically elevated O3 concentrations (Laurila et al. 2004).

The present O3 study lasted for several growing seasons and during the winters in between the microcosms were cov-ered by a natural snow layer. The water table of microcosms was kept artificially constant by adding rain or distilled wa-ter when needed. Additionally, the microcosms were isolated from the surrounding soil and no additional nutrients were added during the experiment. The microcosm approach does



have restrictions compared to natural mire ecosystems, espe-cially concerning horizontal water movements. In this re-spect, the absolute results of the present study must be com-pared with others acknowledging the special characteristics and restrictions of peatland microcosms.

The UV-B exposure study of this thesis was conducted in a natural subarctic fen (Halssiaapa, 67°22’N, 26°39’E, 179m a.s.l.) in Sodankylä, close to the Finnish Meteorological Insti-tute of Sodankylä. The UV-B exposure study lasted for sev-eral growing seasons and the present thesis focuses on the first three years (2003-2005). The experimental setup con-sisted of 30 study plots (n = 10), representing ambient control, UV-A control and UV-B treatment. Aluminium collars (60 x 60 x 30cm), embedded into the study plots were used in the gas exchange measurements. At the beginning of the grow-ing seasons and during the autumn rains the exposure field was flooded. Lateral water flow maximum was estimated with a sodium chloride (NaCl) tracer experiment (IV). The re-sults showed that lateral water flow velocity in the shallow peat layer (0-30 cm) was slow (20 ± 7 cm d-1). The rather slow water flow velocity indicated that the waters between the study plots were not particularly mixed, especially during the growing seasons when the water flow velocity was much slower than in the early summers and late autumns. How-ever, the present experiment lasted for three years and thus it is possible that lateral water flow may have suppressed the treatment effects, for example those of organic acids.

Irradiation treatments were conducted by using fluores-cent tubes (4 per plot), placed 120 cm above the peat surface. Even though all electrical installations were made water re-sistant, harsh humid mire conditions caused occasional prob-lems. The target level for the UV-B enhancement level was set to 46% above ambient, but due to these technical difficulties, the UV-B enhancement varied between 63 and 21% above ambient. In long-term field studies functionality of electric equipment should be checked often to make sure that the UV-B enhancement follows preset values. It should be em-

General Discussion


phasized that in their meta-analysis Newsham & Robinson (2009) showed UV-B effects on UV-B absorbing compounds in experiments conducted with filter screens and natural fluc-tuations while experiments with UV lamps failed to elicit the response. This is probably due to the fact that UV-B en-hancement achieved using UV lamps is markedly lower than the reduction of UV-B levels when using the screens (Searles et al. 2001; Newsham & Robinson 2009).

6.4 CONCLUSIONS

The present study emphasizes the importance of O3 and UV-B exposure studies in natural open-field conditions. It is rela-tively easy to control environmental factors in growth cham-bers and solar domes but the results gained from those condi-tions should be also verified in more natural conditions. Natural ecosystems are complex and have a capacity to ac-climatize to various stress factors. Therefore the results from growth chambers or solar domes may be overestimated.

The results of the present study suggest that the predicted increase in O3 concentration will not have great effects on bo-real or sub-arctic peatland plants. Some changes in pigments concentrations were detected and these changes may affect long-term plant litter decomposition processes. Slowly de-veloping belowground changes such as increasing amount of organic acids and changes in microbial community composi-tion may slightly increase net CH4 emission in longer time-scale. The expected effects are very similar under elevated UV-B radiation. Three growing seasons with enhanced UV-B did not affect net CH4 emission, but after that the emission was higher compared to the ambient UV-B. It is evident that O3 and UV-B radiation affects carbon allocation belowground in the studied peatland ecosystems. Exact pathways and mechanisms of carbon flow are still unknown and should be studied further using the tools of isotope chemistry.



Increasing temperature and changes in water table (in-crease or decrease) will be the main factors affecting CH4 emissions from northern peatlands in future. Other factors re-lated to global change (i.e. O3 concentration, UV-B radiation and N deposition) will definitely have an effect, but not of the same magnitude those caused by changes in hydrology and temperature.

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General Discussion


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Publications of the University of Eastern Finland

Dissertations in Forestry and Natural Sciences

isbn 978-952-61-0832-2

Sami K. Mörsky

Long-term Effects of ElevatedOzone and UV-B Radiationon Vegetation and MethaneDynamics in NorthernPeatland Ecosystems

The aim of this thesis was to

assess whether chronically

elevated tropospheric ozone

concentration and UV-B radia-

tion, studied separately, affect

northern peatland ecosystems.

In particular, the effects on

common peatland vegetation

and methane dynamics were

studied. The thesis increases

knowledge of the long-term

ozone and UV-B effects on peat-

land vegetation and methane

dynamics in realistic open-field

conditions.

dissertatio

ns | 075 | S

am

i K. M

ör

sky | L

ong-term E

ffects of Elevated O

zone and UV

-B R

adiation on Vegetation and M

ethane Dynam

ics in ...

Sami K. Mörsky

Long-term Effects of ElevatedOzone and UV-B Radiationon Vegetation and Methane

Dynamics in NorthernPeatland Ecosystems

dissertations - core · leaves towards the end of the experiment, but did not affect relative...

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