| Summary: | 101 p. Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2008. The change in use of Norwegian alpine rangelands from mixed species herds to herds dominated by sheep and reindeer has raised concerns for the potential effect on vegetation and native animal communities. Intensive grazing by large mammals can have significant impacts on plant community composition, species diversity, and nutrient cycling and may also affect densities of small mammals. I used a set of experimental treatments to evaluate the impact of sheep, arvicoline rodents, and fertilization on rangeland in the alpine zone in Hessdalen, central Norway. I established 5 replicated experimental treatment combinations with exclosures and fertilization from which I collected samples of 4 species of common forage plants preferred by sheep: the shrub Salix herbacea , the sedge Carex bigelowii, and the grasses Deschampsia flexuosa and D. cespitosa. I analyzed samples to look for effects of the treatments on indicators of plant quality, including total nitrogen, acid detergent fiber, total phenolic content, tannin content, protein inhibitor activity and ratio of protein inhibitor to soluble plant proteins. My results indicated a positive effect of fertilization but little effect of mammalian herbivory on plant quality, perhaps because of the relatively low overall densities of herbivores at my study site. I also investigated the influence of low to moderate densities of sheep on density and choice of food by field voles. To do so, I assessed use of experimental plots manipulated to change density of sheep (control, exclosure, fertilization, and salt licks) by live trapping voles in summer and by counting nests and runways used in winter. I also conducted cafeteria trials with the 4 species of plants preferred by sheep to determine if voles preferred grazed or un-grazed plants. I found no significant effects of sheep on vole density or on food choice of voles during my 3-year study, but I could not rule out long-term effects owing to changes in vegetation composition. To investigate the potential for common forage plants to respond to herbivory with increased production of defensive chemicals, a clipping experiment was set up in the greenhouse. Only a few of the species tested responded to clipping and the direction and intensity of the response varied between species. It is not likely that sheep grazing will cause a widespread deterioration in forage quality due to increased secondary chemicals.
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