The relative importance of nitrogen deposition as a driver of Racomitrium heath species composition and richness across Europe

Nitrogen (N) deposition is a major global threat to biodiversity and ecosystem function. Increased deposition rates are linked to reduced species richness and vegetation change in a wide range of habitats. Here, we use gradients of N deposition and climate across thirty-six European sites to examine impacts of N deposition relative to those of climatic, soil and grazing conditions, on composition of Racomitrium heath, a moss-dominated mountain summit community of high nature conservation value. Nitrogen deposition was the second most important driver of community composition after climatic conditions at this European scale, explaining 15% of variability, and was more important than soil factors or current grazing. Along a gradient of 0.6-39.6 kg N ha−1 y−1 species richness declined by 5 species m−2 and there was a 30% shift in cover from mosses to graminoids. Such large changes in dominant plant functional type have potential to impact on other biota and on habitat functions including nutrient cycling and hydrology. Importantly, changes in community composition were seen across the whole range of N deposition, with impacts on sensitive components such as lichens seen at rates of N deposition only slightly above background. No evidence was found for a threshold deposition rate or critical load below which no change in composition occurred. This study demonstrates the powerful role of N deposition as a driver of plant community change relative to other factors, and highlights the sensitivity of alpine habitats adapted to low background N deposition levels.