Direct and indirect effects of roads and road vehicles on the plant community composition of calcareous grasslands

Exposure of plants to vehicle exhaust emissions and road-induced changes to soil biogeochemistry and hydrology can lead to shifts in plant composition in calcareous grasslands. Mixed effects models were used to identify relationships between plant community composition and a suite of measured and modelled environmental variables along transects away from roads at eight calcareous grasslands. Ellenberg pH, moisture and nitrogen (N) scores increased nearer roadsides, however, only Ellenberg N scores were associated with their respective measured or modelled values highlighting NO2 deposition as a likely driver of change. Forb abundance and diversity increases nearer roadsides were also associated with NO2 deposition, with increases seen in the abundance and diversity of typical edge species rather than species characteristic of calcareous grasslands. Grazing, removal of invasive species and the use of barriers to intercept transport-derived air pollution may help to reduce the detrimental effects of roads across these diverse but threatened landscapes.