This report presents the largest analysis of national vegetation data sets in relation to nitrogen (N) deposition carried out to date. The study statistically analysed eight independent national vegetation surveillance datasets using a consistent approach, to identify evidence of N deposition impacts in four habitat types; acidic and calcareous grassland, heathland and bogs.

This new analysis was the largest of its kind, using data from the Vascular Plant Database (1930-1969 and 1987-1999), BSBI Local Change Survey (1987-1988 and 2003-2004), British Bryological Society, British Lichen Society, Plantlife Common Plant Survey, and three country grassland datasets. The project took four widespread habitats, calcareous and acid grasslands, heathlands, and bogs, and for each, looked for evidence of vegetation change caused by nitrogen deposition, after accounting for other factors that could have an impact.  Spatial analysis, and where possible temporal analysis, was conducted to analyse the relationship between N deposition and summary variables (Ellenberg scores, canopy height, specific leaf area), species diversity (species richness, Shannon-Wiener diversity index) and individual species occurrence.  

The study clearly demonstrated significant changes in the abundance of individual species and variables describing habitat structure and function as a result of N deposition.  This, together with previous evidence from surveys and experiments, provides clear evidence of species-level impacts of N deposition in all four of the habitats investigated. 

Results are presented individually for each of the datasets examined.  The discussion section draws the results together examining them for each habitat.  Key results are provided in the results section of the report.  A complete set of all significant results are presented in the Electronic Appendix.

The analysis presented in Report 447 represents the first part of a large study of nitrogen impacts on vegetation.  In a second part of the project, reported in JNCC Report 449, the results of this new analysis, together with other sources of independent evidence, are interpreted in respect of implications for “conservation commitments”.

A project summary, presenting the key messages and summarising the impacts, is also provided.