Land-atmosphere exchange of carbon in a high-Arctic wet tundra ecosystem
Abstract: Arctic ecosystems play a key role in the terrestrial carbon (C) cycle, but spa-tially explicit data on the C exchange is scarce in these remote areas. The global warming is especially dominant in the Arctic, and these areas are vul-nerable to climate change. It is therefore important to quantify and understand the processes affecting the C dynamics in these regions. In this thesis, the land-atmosphere exchange of C, with an emphasis on methane (CH4), was measured in a high-Arctic wet tundra ecosystem (Rylekærene) in Zackenberg, north-eastern Greenland. Rylekærene was a heterogeneous area concerning the source strength of CH4 and concerning which environmental variables that best determine the CH4 fluxes. For the central parts of Rylekærene, temporal variability in CH4 fluxes was well correlated with soil temperature, gross primary production (GPP), active layer thickness and soil organic acids. The spatial variability in CH4 fluxes was closely correlated to water table depth (WtD), species compo-sition and soil organic acids. The CH4 fluxes remained low during autumn and early winter of both 2008 and 2009. The central parts of Rylekærene acted as a C source during the warmer and wetter measurement season of 2008, whereas it was a C sink during the colder and drier measurement season of 2009. At a site at the edge of Rylekærene, CH4 emissions were lower than at the central parts of the fen, most likely because of lower WtD and different plant species composition. At this site, the variability in CH4 flux was not explained by any of the above-mentioned factors. Increases in CH4 fluxes coinciding with soil freezing after the growing seasons were observed here in 2007, 2009 and 2010. Changes in peak growing season GPP 1992 to 2008 was investigated by combining satellite data with ground-based GPP measurements. The modeled results show a substantial increase in peak growing season GPP during this period. The GPP increase was accompanied by a strong increase in air tem-perature, possibly indicating that the increase in GPP was climate-driven. Changes in CH4 fluxes 1997-2010 was studied by combining satellite data with CH4 flux measurements. During 1997-2010, there were no major changes in modelled CH4 fluxes in Rylekærene, and during this period no trend in soil temperature at 10 cm depth and WtD were seen. However, as changes both in temperature and hydrology are expected as global warming continues, it can be assumed that such changes will have strong effects on the land-atmosphere exchange of C in wet tundra ecosystems in the future.
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