Evaluation of nitrogen indicators on ombrotrophic acid bogs: observations from a nitrogen manipulation study.

• Among Natura 2000 ecosystems, ombrotrophic bogs/peatlands are likely to be one of the most sensitive to reactive N deposition. • This paper briefly examines methods of evaluating species, growing on an ombrotrophic bog, to different N forms using a case study from Whim bog to illustrate peatland vegetation response to wet and dry N deposition. • Vegetation cover data are used to examine typical N indicators in terms of species richness, diversity, functional group response, Ellenberg indicator values, species cover and National Vegetation Classification (NVC). • In addition an ordination technique is applied to the data and principal response curves (PRC) generated to examine temporal community response to the N deposition. • The N indicators tested here of species richness, diversity, Ellenberg indicator values and NVC were insensitive to the apparent effects of the N deposition treatments. This is despite significant responses at both the species and community levels detected using a weighted cover index and PRC. • In addition responses varied with N form and particular functional groups e.g. species from the ericoids or Sphagnum show differential responses: Calluna responds negatively to ammonia whereas Vaccinium myrtillus and Empetrum nigrum have responded positively, the hummock forming S.capillifolium is negatively affected by N while the wetter loving S. fallax increased in response to N additions. • The results emphasise that indicators represent a range of sensitivities and a considered approach to the choice of indicator needs to be applied with reference to the ecosystem under question and form of nitrogen applied. • Future research efforts should concentrate on developing predictive indicators with sufficient sensitivity to show species level change that can be used to detect and predict loss of important peatland species or groups.