Divergent roles of herbivory in eutrophying forests

Abstract

Ungulate herbivore populations are increasing across Europe with important implications for forest plant communities. Concurrently, atmospheric nitrogen (N) deposition continues to eutrophy forests, threatening many rare plant species. These pressures may critically interact to shape biodiversity as in grassland and tundra systems, yet any potential interactions in forests remain poorly understood. Here we combined vegetation resurveys from 52 sites in 13 European countries to test how changes in ungulate herbivory and eutrophication drive long-term changes in forest understorey communities. Changes in herbivory increased temporal species turnover, however, identities of winner and loser species depended strongly on N levels. Under low level N-deposition, herbivory favored threatened and small-ranged species, while reducing non-native and nutrient-demanding species. Yet all these trends were reversed under high levels of N-deposition. Herbivores also reduced shrub cover, likely exacerbating N effects by increasing light levels in the understorey. Eutrophication levels may therefore determine whether herbivory acts as a global change catalyst for the “N time bomb”, or as a conservation tool in forests.