| Summary: | Arctic ecosystems hold massive amounts of the global carbon in their soils and are of great importance for the global terrestrial exchange of greenhouse gases to the atmosphere. The arctic region has in general been acting as a C sink for the past 10000 years, however with climate change the C balance in some areas is shifting towards becoming a C source. Herbivory are an important part of many ecosystems and have been found to have an important impact on the C balance. This seemingly important aspect of the C balance in arctic ecosystems are however very rarely considered. In this project, the interactions between plants, soil and herbivores have been studied in a high arctic mire in Zackenberg, northeast Greenland. The aim of the project was to study the impact of the large herbivores muskoxen on the vegetation composition and density, CO2 and CH4 fluxes and substrate availably for CH4 production. Over a time period of three years three in-situ field studies were conducted. In paper one and two of this thesis treatments were applied that simulate plausible responses of the muskoxen population with climate change. In paper I muskoxen was excluded from part of the Zackenberg mire, thus representing a decrease in the population. In paper II the grazing pressure was increased, by clipping of plots twice each summer. In paper IV the main in-situ drivers of the spatial variability of CH4 flux was studied. Further, a laboratory study (paper III) was conducted to make an in-depth study on the C allocation pattern in the area and the consequences of clipping. To our surprise, the results from paper I and II showed that the ecosystem responded in similar ways to changes in grazing pressure despite the contrasting treatments. Both increased and decreased grazing resulted in a decrease in the density of vascular plants, in particular of Eriophorum scheuchzeri (Erioph), and in a substantial decrease in CO2 and CH4 fluxes. The third year into the exclusion experiment in paper I the mean Net Ecosystem Exchange (NEE) of CO2 ...
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