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R Aspects of the carbon cycle in terrestrial ecosystems of Northeastern Småland Torbern Tagesson Geobiosphere Science Centre Physical Geography and Ecosystems Analysis Lund University February 2006

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R Aspects of the carbon cycle in terrestrial ecosystems of Northeastern Småland Torbern Tagesson Geobiosphere Science Centre Physical Geography and Ecosystems Analysis Lund University February 2006 Svensk Kärnbränslehantering AB Swedish Nuclear Fuel and Waste Management Co Box 5864 SE Stockholm Sweden Tel Fax ISSN SKB Rapport R Aspects of the carbon cycle in terrestrial ecosystems of Northeastern Småland Torbern Tagesson Geobiosphere Science Centre Physical Geography and Ecosystems Analysis Lund University February 2006 Keywords: Carbon fluxes, Soil respiration, Primary production, Climate change, Ecosystem modeling, LPJ-GUESS, Simpevarp. This report concerns a study which was conducted for SKB. The conclusions and viewpoints presented in the report are those of the author and do not necessarily coincide with those of the client. A pdf version of this document can be downloaded from Foreword This master thesis of 20 points was the finishing degree project in my studies in Environmental Science at Lund University and it was done at the department of Physical Geography and Ecosystem analysis. The Swedish Nuclear Fuel and Waste Management Co. (SKB) commissioned the project and all field measurements has been taken place at one of their sites investigated for a future deep repository of nuclear waste, the Simpevarp Investigation area. The master thesis passed with distinction and is now published in the site investigation report series by SKB to be further spread to the public. First I like to thank: SKB for the funding and all support. I am also grateful to the landowners in the Simpevarp investigation area for letting me do my field measurements on their properties. I like to thank my superadvisors: Torben Christensen, Professor at the Institute of Physical Geography and Ecosystems analysis, Lund University for advises, help with planning the project, method of carbon fluxes measurements, analysis of the results, references and control reading the text. Erik Wijnbladh, Ecologist at SKB, Svensk Kärnbränslehantering AB, for help in the field, climate parameters to the DGVM, all work done up in the Simpevarp investigation area, references and control reading the text. Annett Wolf, Postdoc. at the institute of Physical Geography and Ecosystem Analysis, Lund University for all help with the DGVM, references and control reading the text. I also like to thank: Lena Ström for help with a method to field measurement of soil organic acids, the analysis of soil samples in laboratory, help with references and control reading text. Denis Koca for the parametrization of the species in the DGVM and help with references. Fredrik Lagergren for help with a method to field measurements of tree layer carbon pool, equations for biomass calculations of tree layer carbon pool and references. The Balance project for the modelling of the temperature used as a climate parameter in the DGVM. Maj-Lena Lindersson, Thomas Hickler and Ben Smith for help with climate parameters and adjustments of the DGVM. Tobias Lindborg at SKB, Maria Olsrud and Torbjörn Johansson for help with references, and finally Bernt Tagesson and Johan Thorngren for control reading the text. Abstract Boreal and temperate ecosystems of the northern hemisphere are important for the future development of global climate. In this study, the carbon cycle has been studied in a pine forest, a meadow, a spruce forest and two deciduous forests in the Simpevarp investigation area in southern Sweden (57 5 N, E). Ground respiration and ground Gross Primary Production (GPP) has been measured three times during spring 2004 with the closed chamber technique. Soil temperature, soil moisture and Photosynthetically Active Radiation (PAR) were also measured. An exponential regression with ground respiration against soil temperature was used to extrapolate respiration over spring A logarithmic regression with ground GPP against PAR was used to extrapolate GPP in meadow over spring Ground respiration is affected by soil temperature in all ecosystems but pine, but still it only explains a small part of the variation in respiration and this indicates that other abiotic factors also have an influence. Soil moisture affects respiration in spruce and one of the deciduous ecosystems. A comparison between measured and extrapolated ground respiration indicated that soil temperature could be used to extrapolate ground respiration. PAR is the main factor influencing GPP in all ecosystems but pine, still it could not be used to extrapolate GPP in meadow since too few measurements were done and they were from different periods of spring. Soil moisture did not have any significant effect on GPP. A Dynamic Global Vegetation Model, a DGVM called LPJ-GUESS, was downscaled to the Simpevarp investigation area. The downscaled DGVM was evaluated against measured respiration and soil organic acids for all five ecosystems. In meadow, it was evaluated against Net Primary Production, NPP. For the forest ecosystems, it was evaluated against tree layer carbon pools. The evaluation indicated that the DGVM is reasonably well downscaled to the Simpevarp investigation area and it was used for future predictions of soil respiration, tree layer carbon pool and fast decomposing soil organic carbon pool, NPP was also predicted for meadow. Two different climate scenarios were used. The fast decomposing soil organic carbon pools and soil respiration increased for all ecosystems, the tree layer carbon pools increased for the forest ecosystems and NPP increased in meadow in both scenarios, Sammanfattning De boreala och tempererade ekosystemen på den norra hemisfären har en stor betydelse för hur klimatet kommer att utvecklas i framtiden. I denna undersökning har kolets kretslopp undersökts i en tallskog, en betesmark, en granskog och två lövskogar i Simpvarps undersökningsområde (57 5 N, E). Markrespiration och markskiktets bruttoprimärproduktion (GPP) har mätts i fält vid tre olika tillfällen under våren Det har skett med hjälp av en infrarödgasanalysator som kopplats till en plexiglaskammare. Jordtemperatur, jordfuktighet och den del av solljuset som fotosynteserande växter tar upp (PAR) har även mätts. Jordtemperaturen användes till att beräkna markrespiration under våren 2004 och PAR användes till att beräkna markskiktets GPP i betesmarken under våren Jordtemperaturen påverkar markrespiration i alla ekosystem utom tallskogen, men den förklarar bara en liten del av variationen i markrespirationen. Jordens fuktighet påverkar markrespirationen i granskogen och i en av lövskogarna. Jordtemperaturen kan användas till att beräkna markrespiration under vårens gång. PAR är den främsta faktorn som påverkar GPP i alla ekosystem utom tallskogen. PAR kan dock inte användas till att beräkna GPP under vårens gång eftersom för få mätningar gjordes vid vart mättillfälle. Det går inte att använda data från olika mättillfällen vid beräkningarna eftersom det är skillnad på yttre faktorer som t ex vegetation och temperatur. Jordens fuktighet har ingen signifikant påverkan på GPP. En Dynamisk Global Vegetations Modell, en DGVM som kallas LPJ-GUESS användes även till att göra simuileringar för de olika ekosystemtyperna. Klimatdata för Simpevarp fördes in i DGVMn och sedan kontrollerades den gentemot fältmättningar från de fem ekosystemen. Den kontrollerades mot markrespiration, mängd kol i trädskikt och mängd snabbnedbrytbara organiska syror i marken i alla fem ekosystem samt nettoprimärproduktion (NPP) i betesmarken. Inom en rimlig gräns gav DGVMn samma resultat som fältundersökningarna gjorde. DGVMn användes till framtidssimuleringar av de kolflöden och de kolpooler som den var utvärderad gentemot. Två olika framtidsklimatscenarier användes. I båda scenarierna ökade NPP i betesmarken, mängden kol i trädskiktet, mängden snabbnedbrytbart kol i marken och jordrespirationen. Contents 1 Introduction Aims and hypotheses 9 2 The carbon cycle in terrestrial ecosystems Gross Primary Production, GPP Respiration Net Primary Production, NPP Vegetation carbon pool Soil organic carbon pool Human influences 13 3 Description of DGVM Abiotic parameters Individual properties NPP Soil organic matter and litter decomposition 16 4 Method Site description Carbon fluxes at ground ecosystems Data treatment Tree layer carbon pool Field measurements Data treatment Fast decomposing soil organic carbon pool Field measurements Temperature controlled high-speed centrifugation Organic acid analysis Data treatment DGVM simulations Climate parameters Carbon dioxide Soil texture Vegetation parameters Disturbances DGVM 2004 simulations DGVM evaluation DGVM future simulations Data treatment 23 5 Result Effect of soil temperature and soil moisture on respiration Respiration calculated over spring Effect of PAR and soil moisture on ground GPP GPP in meadow calculated over spring Evaluation of DGVM NPP in meadow Tree layer carbon pool Soil respiration Fast decomposing soil organic carbon pool 6 Discussion Effect of soil temperature and soil moisture on respiration Respiration calculated over spring Effect of PAR and soil moisture on GPP GPP in meadow calculated over spring Evaluation of DGVM NPP in meadow Tree layer carbon pool Soil respiration Soil organic carbon pool Conclusions 40 7 References 43 1 Introduction The average temperature on earth has increased 0.8 C since The increase is probably due to human emissions of greenhouse gases, among them carbon dioxide. /IPCC 2001/ predicts an increase in average global temperature by C in the coming century, Precipitation patterns will also be altered /IPCC 2001/. Boreal and temperate forests of the northern hemisphere are important for the future development of global climate. They are today considered the most important terrestrial carbon sinks and contain a large soil organic carbon pool /Denning et al. 1995/. The largest increase in air temperature is expected at high latitudes /IPCC 2001/. The combination with a large soil organic carbon pool and an increase in air temperature can result in a change in the boreal and temperate forests from being carbon sinks to become sources /Kirschbaum 1995/. In the study of the carbon cycle forests have been in focus due to their large productivity, while grasslands have received less attention /Valentini et al. 2000/. Grasslands are important for the global carbon cycle since approximately 40% of the world s surface is grassland /White et al. 2000/. Most grasslands are grazed and it is therefore important to understand the carbon cycle of meadows /LeCain 2002/. Especially soil respiration can be increased due to global warming /Kirschbaum 1995/. Soil carbon fluxes represent around 70% of total forest ecosystem respiration /Janssens et al. 2001/. All respiration in grasslands comes from the ground level. Ground carbon fluxes are therefore an important part of the total carbon exchange with the atmosphere. During daytime, soil respiration is diminished by photosynthesis of the ground vegetation /Widén 2002/. Where there are sufficient light conditions for photosynthesis by ground vegetation, ground carbon fluxes are affected /Widén 2002/. Different methods to calculate annual carbon fluxes of forest floors have been used, and among them different regression equations with carbon fluxes against abiotic factors /Janssens et al. 2003, Olsrud and Christensen 2004/. /Janssens et al. 2003/ found that regression functions, with respiration against soil temperature and soil moisture gave similar results as total annual flux. /Olsrud and Christensen 2004/ used Photosynthetically Active Radiation (PAR) to calculate Gross Primary Production (GPP). More advanced modelling than simple regression calculations is required to simulate responses of ecosystems to long-term climatic change. Dynamic Global Vegetation Models (DGVMs) have been developed for this type of predictions and these include both vegetation dynamics and biogeochemical processes /Cramer et al. 2001/. 1.1 Aims and hypotheses The study sets out to investigate the carbon cycle of boreal/temperate ecosystems of Northeastern Småland. There are three general aims. First, the influence of abiotic factors on ground carbon fluxes in boreal/temperate forests and a meadow is to be analysed. Second, to see if abiotic factors can be used to calculate ground carbon fluxes over spring Third, to use a Dynamic Global Vegetation Model to study changes in ground carbon fluxes and carbon pools in the boreal/temperate ecosystems Eight specific hypothesises are set up. 1. Previous studies have indicated that soil temperature and soil moisture has an effect on ground respiration /Lloyd and Taylor 1994, Swanson and Flanagan 2001, Morén and Lindroth 2000, Davidson et al. 1998, Davidson et al. 2000/. The first specific hypothesis is that ground respiration is affected by soil temperature and soil moisture. 2. It has been shown that simple regressions are sufficient to model respiration over a longer time period /Janssens et al. 2003, Olsrud and Christensen, 2004/. The second specific hypothesis is that soil temperature can be used to calculate respiration over spring Precipitation and photosynthetically active radiation (PAR) are abiotic factors influencing photosynthesis /Lambers et al. 1998/. The third specific hypothesis is that GPP is affected by PAR and soil moisture. 4. It has also been shown that simple regressions can be used to calculate GPP over a longer time period /Olsrud and Christensen 2004/. The fourth specific hypothesis is that PAR can be used to calculate GPP in meadow over spring It have been shown in previous simulations with future scenarios that global warming and an increase in atmospheric concentration of carbon dioxide will result in an increase in NPP /Pussinen et al. 1997, White et al. 2000, Cramer et al. 2001/. The fifth specific hypothesis is that NPP in meadow will increase Simulations with different types of DGVM and different futures scenarios have shown an increase in the vegetation carbon pool over the next century /White et al. 2000, White et al. 1999, Cramer et al. 2001, Pussinen et al. 1997/. The sixth specific hypothesis is that the tree layer carbon pool will increase Global warming will increase the temperature and this increases soil respiration /Kirschbaum 1995/, which has also been shown in simulations with future scenarios /White et al. 2000, White et al. 1999, Cramer et al. 2001, Cox et al. 2000, Pussinen et al. 1997/. The seventh specific hypothesis is that soil respiration will increase Earlier future simulations have shown that global warming and an increase in soil respiration can result in a decrease in soil organic carbon pools /Cox et al. 2000, White et al. 2000/. The eighth specific hypothesis is that fast decomposing soil organic carbon pool will decrease The study is divided into two parts, field measurements and DGVM simulations. First, ground carbon fluxes and abiotic factors will be investigated in a pine forest, a meadow, a spruce forest and two deciduous forests in the Simpevarp region in Northeastern Småland. To investigate the first and the third hypothesises measured ground carbon fluxes will be analysed against abiotic factors. Soil temperature will be used to calculate ground respiration in all ecosystems over spring 2004 and PAR will be used to calculate GPP. Comparing the calculated results with the field measurements will test the second and fourth hypothesis. In the second part a DGVM (LPJ-GUESS) will be downscaled to the Simpevarp region and hereby be valid for the same ecosystems measured at. Results from the downscaled DGVM with simulations for spring 2004 will be evaluated against field-measured results for spring Simulations for will be done to test the fifth, sixth, seventh and eight hypotheses. 10 2 The carbon cycle in terrestrial ecosystems All living tissues are composed of carbon and all life on Earth is depending on processes in the carbon cycle. Photosynthesis and respiration are together with mortality and different disturbance regimes (fire, storms, drought etc) the processes of main importance for the carbon cycle /Schlesinger 1997/. 2.1 Gross Primary Production, GPP The total uptake of carbon through photosynthesis is called Gross Primary Production, GPP. Photosynthesis is the biogeochemical process that transfers carbon from the atmosphere and its oxidized form, carbon dioxide, into the biosphere and its organic form, carbohydrates. 6 CO H 2 O + sunlight = C 6 H O 2 It is the process capturing sun light, which results in plant growth and provides life with energy. The photosynthesis provides the atmosphere with the oxygen necessary for all animal life. Figure 2-1. Flowchart over carbon cycle in terrestrial ecosystems. Squares are carbon pools; arrows and circles are processes moving carbon between the pools. 11 2.2 Respiration Energy stored by photosynthesis is later used for maintenance, growth or reproduction by living organisms. The process responsible for the breakdown of the carbohydrates is respiration. C 6 H O 2 = 6 CO H 2 O + energy The plants use about half of GPP for their own maintenance and the carbon dioxide is then released back to the atmosphere through autotrophic respiration /Schlesinger 1997/. Twenty percent of GPP is consumed by herbivores and becomes a part of the animal carbon pool /Cyr and Face 1993/; this carbon is either released to the atmosphere through heterotrophic respiration or transported to the soil through mortality. The rest of the carbon taken up by plants is either released to the atmosphere through disturbance, such as fire, or transported to the soil through mortality of the vegetation. Part of the carbon transported to the soil is decomposed and released to the atmosphere through heterotrophic soil respiration. Soil respiration rate varies as a function of soil temperature, soil moisture and chemical composition of material to be decomposed /Schlesinger 1997/. Soil respiration and soil temperature has an exponential relationship in the soil temperature range found in the field; higher soil temperature gives more soil respiration /Widén 2001/. Soil respiration and soil moisture has different relationships at different moisture ranges /Davidson et al. 2000, Janssens et al. 2003/. In dry soils there is a positive linear relationship. Soil respiration can be inhibited due to dryness. In waterlogged soils decomposition is reduced due to anaerobic conditions and there is a negative linear relationship between soil moisture and soil respiration. In between these conditions is a plateau where soil respiration is not affected by soil moisture /Heal 1981/. Nitrogen and lignin content in litter will speed up respectively slow down the breakdown processes /Yao 2003/. In soil organic matter there are different acids that are more or less easy to decompose /Schlesinger 1997/. 2.3 Net Primary Production, NPP Net Primary Production, NPP, is here defined as the rate at which plants accumulate carbon in their living tissues, GPP minus autotrophic respiration. NPP = GPP R p R p = autotrophic respiration NEP Net Ecosystem Production is net primary production on an ecosystem level. NEP = GPP R t R t = (R p + R h + R d ) R t = total respiration, R d = heterotrophic respiration, R h = herbivore respiration. In ecosystems being young or exposed to disturbances, most of the NEP goes to the production of new plant tissues /Giese et al. 2003/. In old and stable ecosystems GPP mainly goes to the maintenance of the vegetation and most of NEP will be allocated to the soil organic carbon pool /Giese et al. 2003/. Temperature, precipitation and photosynthetically active radiation (PAR) are abiotic factors influencing primary production /Lambers et al. 1998/. High temperature gives a longer growing season increasing annual production of the ecosystems /Hasenauer et al. 1999/. 12 A raise
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