Plant-soil-herbivore interactions in a high Arctic wetland - Feedbacks to the carbon cycle

Abstract: 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 had decrease with 47%, while the CH4 emission had decreased with 44%. In the clipping experiment in paper II NEE decreased already in year one of the experiment and had after three years decreased with on average 35%. A change in CH4 emission was apparent in year two and over the last two years CH4 emission decreased with on average 26%. In the laboratory study, several factors indicated that clipping altered the C allocation pattern and resulted in more C allocation to above ground vegetation and more root exudation. The results from paper IV showed that there is a strong dependence of the spatial variability in CH4 flux on productivity and C input to vegetation and pore water. The main driver of this carbon input is the vegetation composition, with high number of Erioph tillers leading to high input. Consequently, since both decreased and increased grazing pressure resulted in a decrease in Erioph density both these treatments ultimately resulted in lower productivity and decreased CH4 fluxes. In conclusion, the results from this thesis clearly show that herbivores are of great importance for the C balance of this wet arctic ecosystem, the driving forces behind this effect being interactions between plants, soil and herbivores. The results from this thesis further points to the importance of considering the impact of herbivory when the past, present or future C balance in the arctic is discussed.