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The objective of this thesis was to investigate the effects of mammalian grazers, such as microtine rodents and reindeer, (top-down effects) and nutrient availability (bottom- up effects) on the plant community of a tundra heath. I conducted a large-scale fertilization experiment and studied the impact of grazers using exclosures. I measured the effects of fertilization and grazing on soil microbial activity and nutrient cycling. I investigated the responses to fertilization of the invertebrate community, I studied the effects on the quality of bilberry as food for mammalian herbivores, and I looked at how concentrations of nutrients and carbon-based secondary defences against herbivory fluctuated between seasons in unfertilized and fertilized treatments. The results of my thesis show that the plant community investigated is exposed to a strong top-down control by mammalian herbivores. On the fertilized and grazed areas the aboveground biomass of the vascular plant community did not increase compared to unfertilized areas. However, the productivity of the plant community was clearly nutrient- limited. During the eight years of the experiment, on the fertilized areas plant biomass was significantly increased inside the herbivore exclosures In my study mammalian herbivores at comparatively low densities and grazing outside the growing season were sufficient to control the biomass of a heterogeneous plant community. Microtine rodents (Norwegian lemmings and grey-sided voles) preferred the fertilized areas for overwintering. The food plant quality of bilberry for grey-sided voles was improved on the fertilized areas throughout the year. Grazing decreased the nitrogen storage in the aboveground plant biomass. Reindeer and rodents had also important indirect effects on the plant community by decelerating soil nutrient cycling and soil microbial activity. This effect may be accelerated by the impact of herbivore on plant species composition. Graminoids, which contained the highest nitrogen concentrations in their tissues, increased rapidly on the fertilized areas, but their abundance was significantly lower on grazed fertilized areas. The invertebrate community was detritus-based and received their energy indirectly from the litter via soil microbes and detritivores. Fertilization increased the biomass of invertebrate carnivores, but had no effect on the biomass of invertebrate herbivores. Apparent competition between detritivores and invertebrate herbivores, mediated by carnivorous invertebrates predating on both of them, is supposed to keep the densities and grazing pressure of invertebrate herbivores low. Grazing damage by invertebrates was very low and only 0.021 % of the total vascular plant biomass was removed. Diss. (sammanfattning) Umeå : Umeå universitet, 2001, härtill 6 uppsstser. digitalisering@umu