This study investigated the impacts of fertilization and grazing by Norwegian lemmings (Lemmus lemmus), grey‐sided voles (Clethrionomys rufocanus), and reindeer (Rangifer tarandus) on a diverse tundra plant community dominated by deciduous shrubs. Four out of eight study areas, having a size of 2500 m2 each, were fertilized with a N‐P‐K fertilizer and four areas served as unfertilized controls. Two types of exclosures were used within each study area, one to exclude solely reindeer, and one to exclude both rodents and reindeer. Open, grazed plots served as controls. During 5 years following the fertilization event the changes in vegetation inside and outside the exclosures were monitored using a point frequency method. The densities of rodents on the fertilized and unfertilized areas were investigated by live trapping and by counting nests of overwintering individuals. Reindeer do not graze on the study area during the growing season but migrate through this area in autumn and spring. Fertilization increased the abundance of vascular plants while grazing by reindeer and rodents decreased the abundance of vascular plants significantly on both fertilized and unfertilized areas. Rodents preferred clearly the fertilized areas during winter, decreasing the abundance of Vaccinium myrtillus and Vaccinium vitis‐idaea, while very little grazing occurred during summer. Graminoids showed the strongest positive response to fertilization and dominated the plant community on ungrazed plots, while winter grazing by both reindeer and rodents significantly decreased the abundance of graminoids. Deciduous shrubs (Betula nana, Vaccinium myrtillus) increased slightly but significantly due to fertilization and evergreen dwarf shrubs showed no response to fertilization. However, the use of functional growth forms for predicting the responses of nutrient enrichment and grazing must be questioned, as responses to fertilization as well as preferences by herbivores were shown to be species‐specific rather than uniform within functional groups based on plant growth forms.

According to the exploitation ecosystems hypothesis (EEH), productive terrestrial ecosystems are characterized by community-level trophic cascades, whereas unproductive ecosystems harbor food-limited grazers, which regulate community-level plant biomass. We tested this hypothesis along arctic-alpine productivity gradients at the Joatka field base, Finnmark, Norway. In unproductive habitats, mammalian predators were absent and plant biomass was constant, whereas herbivore biomass varied, reflecting the productivity of the habitat. In productive habitats, predatory mammals were persistently present and plant biomass varied in space, but herbivore biomass did not. Plant biomass of productive tundra scrublands declined by 40% when vegetation blocks were transferred to predation-free islands. Corresponding transfer to herbivore-free islands triggered an increase in plant biomass. Fertilization of an unproductive tundra heath resulted in a fourfold increase in rodent density and a corresponding increase in winter grazing activity, whereas the total aboveground plant biomass remained unchanged. These results corroborate the predictions of the EEH, implying that the endotherm community and the vegetation of the North European tundra behaves dynamically as if each trophic level consisted of a single population, in spite of local co-occurrence of >20 plant species representing different major taxonomic groups, growth forms, and defensive strategies.

We tested the hypothesis that herbivory accelerates nutrient cycling through nutrient‐rich soils and decelerates nutrient cycling through nutrient‐poor soils in a well‐drained arctic tundra heath by measuring effects of herbivory on soil and plant properties in control areas and areas treated with NPK fertilizer. The impact of herbivores was studied with two types of exclosures, one excluding reindeer and the other excluding both rodents and reindeer. We predicted that herbivores would enhance soil microbial processes in the fertilized areas, but retard them in the unfertilized ones. Microbial respiration and microbial C were significantly lower in grazed areas than in herbivore exclosures in both unfertilized and fertilized treatments, indicating that herbivores limit the C available for the soil microbes. Microbial N was significantly increased in the exclosures in the fertilized treatment, but there were no effects in the unfertilized one. This reflects both an increase in resources by fertilization and the effect of herbivores on microbial N acquisition: there was surplus N in the fertilized areas to be immobilized by microbial biomass only when access by herbivores was prevented. Thus, mammalian grazers affect the resource coupling between the plant and microbial trophic levels. Fertilization did not affect the soil inorganic, organic, or microbial N contents, or litter decomposition rates, but it significantly increased soil and microbial P. Microbial C was significantly decreased by fertilization, contradicting some earlier fertilization studies in the arctic. One reason for deceleration of soil nutrient cycling in our study area may be that herbivory by both reindeer and rodents occurs mainly outside the growing season; hence, mammalian waste products do not contribute to soil nutrient availability at the times of highest nutrient demands by plants. In addition, grazing during reindeer migrations is likely to cause net resource output from the system.

We tested the hypothesis that herbivory accelerates nutrient cycling through nutrient-rich soils and decelerates nutrient cycling through nutrient-poor soils in a welldrained arctic tundra heath by measuring effects of herbivory on soil and plant properties in control areas and areas treated with NPK fertilizer. The impact of herbivores was studied with two types of exclosures, one excluding reindeer and the other excluding both rodents and reindeer. We predicted that herbivores would enhance soil microbial processes in the fertilized areas, but retard them in the unfertilized ones.Microbial respiration and microbial C were significantly lower in grazed areas than in herbivore exclosures in both unfertilized and fertilized treatments, indicating that herbivores limit the C available for the soil microbes. Microbial N was significantly increased in the exclosures in the fertilized treatment, but there were no effects in the unfertilized one. This reflects both an increase in resources by fertilization and the effect of herbivores on microbial N acquisition: there was surplus N in the fertilized areas to be immobilized by microbial biomass only when access by herbivores was prevented. Thus, mammalian grazers affect the resource coupling between the plant and microbial trophic levels. Fertilization did not affect the soil inorganic, organic, or microbial N contents, or litter decomposition rates, but it significantly increased soil and microbial P. Microbial C was significantly decreased by fertilization, contradicting some earlier fertilization studies in the arctic.One reason for deceleration of soil nutrient cycling in our study area may be that herbivory by both reindeer and rodents occurs mainly outside the growing season; hence, mammalian waste products do not contribute to soil nutrient availability at the times of highest nutrient demands by plants. In addition, grazing during reindeer migrations is likely to cause net resource output from the system.

Bilberry shoots are one of the main food sources for grey‐sided voles Clethrionomysrufocanus during winter. This study examined the relation between plant quality variables of bilberry Vacciniummyrtillus shoots, overall vegetation height, and the winter consumption of bilberry by grey‐sided voles. Results show that only the phenolic concentration correlated significantly with the amount of bilberry biomass consumed, and bilberry consumption decreased with an increasing phenolic concentration. The best predictor of herbivory on bilberry was not plant quality but rather the overall vegetation height. This may be because the quality of bilberry shoots is high in all habitats and that predation is a greater mortality risk than nutritional deficiencies during winter.