Processes controlling carbon fluxes in the soil-vegetation-atmosphere system

Increasing emissions of carbon dioxide (CO2) to the atmosphere have a direct impact on global warming and climate change. The ability of ecosystems to sequester CO2 is therefore of great importance. The Boreal forest contains one of the world’s largest carbon stocks after having acted as a net sink of CO2 for thousands of years. Carbon is stored within the forest in the form of biomass and soil carbon and has accumulated largely due to the cold climatic conditions which limit respiration for much of the year. However with a changing climate and land use, the fate of the large carbon stores and future sequestration capabilities within the boreal zone are threatened and not fully understood. This thesis aims to improve the understanding of the processes that control fluxes of CO2 into and out of terrestrial ecosystems within the boreal zones. Long term continuous high frequency measurements of carbon dioxide fluxes were done using the eddy covariance technique on a variety of forest and agricultural sites within Sweden. This thesis includes studies from six sites: a managed northern boreal forest, a primary unmanaged boreal forest, two agricultural fields of which one was cultivated and one was set aside, and two sites of a chronosequence of pine forests where the carbon use efficiency of mycorrhiza was studied. Four out of six study sites were seen to have a net loss of CO2 to the atmosphere. Ecosystem and site specific variables such as natural disturbance, change in ecosystem functioning and tillage were found to be strong drivers influencing the carbon fluxes. One common finding from all studies however was that the net carbon balance of an ecosystem is highly sensitive to small changes in either gross photosynthesis or gross ecosystem respiration. Overall this thesis shows that carbon fluxes are driven by a number of different processes and variables which are highly dependent upon ecosystem type and site conditions such as development stage and disturbances. We further found that site characteristics such as forest heterogeneity and prevailing wind directions could affect the accuracy of annual net budgets of CO2 measured by the eddy covariance technique. In addition it highlights the importance in correcting for bias within the measurement technique and suggests correction schemes