The role of nitrogen deposition in widespread plant community change across semi-natural habitats

Experimental studies have shown that deposition of reactive nitrogen is an important driver of plant community change, however, most of these experiments are of short duration with unrealistic treatments,and conducted in regions with elevated ambient deposition. Studies of spatial gradients of pollution can complement experimental data and indicate whether the potential impacts demonstrated by experiments are actually occurring in the ‘real world’. However, targeted surveys exist for only a very fewhabitats and are not readily comparable. In a coordinated campaign, we determined the species richness and plant community composition of five widespread, semi-natural habitats across Great Britain in sites stratified along gradients of climate and pollution, and related these ecological parameters to major drivers of biodiversity, including climate, pollution deposition, and local edaphic factors. In every habitat, we found reduced species richness and changed species composition associated with higher nitrogen deposition, with remarkable consistency in relative species loss across ecosystem types. Whereas the diversity of mosses, lichens, forbs, and graminoids eclines with N deposition in different habitats, the cover of graminoids generally increases. Considered alongside previous experimental studies and survey work, our results provide a compelling argument that nitrogen deposition is a widespread and pervasive threat to terrestrial ecosystems.