| Summary: | The grazing optimization hypothesis predicts that net primary production (NPP) and nitrogen levels within vegetation will be highest with moderate grazing levels. In the Canadian High Arctic, muskoxen are one of two major herbivores; they prefer to graze in wet sedge meadow plant communities. To test the grazing optimization hypothesis in these plant communities, two studies were initiated in 2007. The first study spanned two years and compared grazed and ungrazed sedge meadows. The grazed meadows had higher belowground biomass in 2007 and graminoid net primary production was larger in 2008. The ungrazed meadows had greater quantities of dead biomass. Nitrogen concentrations in Carex aquatilis ssp. stans and Eriophorum angustifolium ssp. triste and soil ammonium availability were higher at the grazed site. In the second study, we created two experimental grids with clipping and litter removal treatments. Aboveground net primary production, ecosystem respiration, and shoot carbon concentrations decreased due to clipping. However, shoot nitrogen concentrations increased in C. membranacea and E. triste as clipping frequencies increased. Soil moisture levels also rose with clipping frequencies. Litter removal did not effect aboveground net primary production or soil moisture content. We conclude that the grazing optimization hypothesis applies to High Arctic wet sedge meadows because of the higher aboveground NPP and belowground biomass at the grazed site. However, decreased aboveground NPP in the clipping experiment indicates that muskoxen stimulate primary production in these plant communities by accelerating the nitrogen cycle by the addition of nutrients to the soil from their excrement.
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