Reindeer herbivory reduces willow growth and grouse forage in a forest-tundra ecotone

Browsing mammals strongly modify the structure of vegetation of forest-tundra ecotones. We investigated the impact of reindeer browsing on growth and morphology of an arctic willow, Salix glauca, by studying plants inside and outside fenced areas in a tundra habitat at the tree line. We also studied if reindeer feeding has an effect on the forage availability for willow grouse, a herbivore sharing the same food plant. Analyses of 6-year data show that reindeer strongly reduces the growth of tundra willow and changes plant morphology to a stunted growth form. Intense reindeer browsing on willow limited the forage availability for willow grouse and grouse fed less on reindeer-browsed willow than willows protected from reindeer browsing. The results of this study imply that herbivores can counteract the increase of shrubs in forest-tundra ecotones, as has been predicted in some studies discussing the effects of climatic warming on vegetation. Furthermore, trophic interactions should be incorporated in modelling vegetation changes as a response to increased temperatures.