Land-atmosphere interaction of a subarctic palsa mire
Sammanfattning: The main objective of this thesis was to quantitatively estimate the gaseous exchange of carbon dioxide (CO2) and methane (CH4) between a subarctic wetland ecosystem and the atmosphere. Additionally some initial estimates of the carbon input to the atmosphere of nonmethane hydrocarbons (NMHCs) were also obtained by combining different techniques in a joint setting. The data presented were collected at the Stordalen mire, a palsa type complex located in the northernmost part of subarctic Sweden (68°20’N, 19°03’E, alt. 363 m asl). This site represents an ecosystem type that is highly sensitive to climatic conditions, especially to the ongoing processes of climate warming. Recent studies performed in this area have shown that those ecosystems in northern Scandinavia are subject to dramatic changes, as the distribution of permafrost is diminishing and vegetation is changing, which has been triggered by warming over recent decades. These regional landscape changes are affecting the ecosystems carbon balance, and might determine whether these ecosystems will be classified as either greenhouse gas sources or sinks. In addition to the obtained estimates of the emission of the two major greenhouse gases (CO2 and CH4), the quantitative studies of NMHCs were derived by simultaneously performed chamber measurements coupled with total hydrocarbon (THC) analyses, and by applying a disjunct eddy covariance (DEC) technique to measure specific emissions of isolated biogenic volatile organic compounds (BVOCs) such as isoprene and methanol. The measured eddy covariance tower net ecosystem exchange (NEE) showed a strong and consistent annual uptake signal. During the years 2004 to 2008, with relatively comparable climates the annual NEE amounts to -89 g CO2-C m-2 y-1, with very small variation. The eddy covariance (EC) tower acquired CH4 flux measurements and showed a strong positive input to the annual carbon budget in the range of 18-22 g CH4-C m-2 y-1, reducing the studied ecosystem sink strength by almost 20%. Additional positive budget components in the form of emission of NMHCs constitute roughly an additional 15% to the CH4 fluxes. This fraction may be compared with the EC CH4 flux measurements emission rates and the results of the isoprene and methanol flux measurements obtained by the DEC method. Here the proportional atmospheric emission is estimated as 4% and 1% respectively for the above-mentioned components, indicating that other compounds must be responsible for a large proportion of the NMHC flux. By applying a global warming potential factor and calculating the annual balances expressed in CO2 equivalent numbers converted according to a 100 year time horizon, the degrading parts of the Stordalen mire appear as a greenhouse warming source with a level of emission estimated at +93 g CO2equiv.-C m-2 y-1. This strong greenhouse warming source demonstrates the necessity of conducting detailed carbon budgeting experiments that will lead to an improvement of our understanding of how global carbon cycling interacts with climate.
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