Regardless of numerous studies on global warming affecting tundra vegetation, only very recently herbivores have been acknowledged as important factors interacting with or even counteracting warming. The Arctic is encountering remarkable temperature increase, and knowledge on the combined effects of warming and grazing on vegetation is needed for better management plans. Shrubs - the dominant growth form and key food source on tundra - are rapidly responding to warming. This study analyzes the effects of long-term warming (open-top chambers) on abundance and nutritional quality of four dominant shrub species, aiming at better understanding how food resources available for tundra herbivores respond to warming climate. We used a multisite experiment in forest-tundra ecotone along 900 km gradient from central to northern Norway. Warming increased shrub abundance in all locations, only Vaccinium myrtillus decreased in the southernmost location due to heavy competition with Empetrum hermaphroditum. Nitrogen and phosphorus concentrations of Betula nana decreased, but remained unchanged in the other shrub species. Total soluble phenolics did not respond to warming. Tannins varied greatly among species, locations and habitats and showed idiosyncratic responses to warming. Our findings suggest that abundance of palatable shrubs will increase especially in northern tundra habitats, whereas less-palatable Empetrum is highly increasing in nutrient-rich alpine forests. The large changes in shrub abundance will probably be more important for herbivores than the minor, and partly idiosyncratic, effects of warming on plant quality. Future warming is thus expected to be favorable to tundra herbivores, at least if only food availability is considered.

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