CANCELLED - Eutrophication-dependent roles of herbivory in temperate forests: faunal fuse of N time bomb or conservation actor?

Background/Question/Methods

Aerial nitrogen (N) deposition continues to eutrophy temperate forests, reducing their diversity and threatening many rare species. Concurrent widespread increases in large herbivore populations represent a separate pressure that could either dampen or exacerbate N effects on these communities, yet these interactions remain poorly understood. We used vegetation resurvey data from 52 forest sites across 13 European countries over timescales of up to 64 years to study how herbivory and eutrophication, alone and together, drive long-term changes in forest understory communities.

Results/Conclusions

Increases in herbivory have accelerated species turnover and affected the understory light regime through shrub removal. Diversity effects of increasing herbivory, however, strongly depend on nutrient conditions. Under low levels of N-deposition, herbivory benefits threatened and smaller-ranged species while discouraging non-native and nutrient-demanding species. Yet all these trends are reversed under high levels of N-deposition. Such contrasting outcomes highlight the divergent roles that herbivory playsunder different nutrient contexts and document how herbivores can either catalyze accelerating impacts from N-deposition or act as a potential facilitator of threatened species. Efforts to conserve and restore forest biodiversity should therefore account for the possibility of a “N time bomb” release under these twin pressures.