NP stoichiometry of low-alpine heathland: Usefulness for bio-monitoring and prediction of pollution impacts

Plant communities in the low-alpine zone are exposed to relatively high levels of nitrogen (N) deposition compared to surrounding lowland areas and this has the potential to alter community composition and ecosystem function. N content and nitrogen: phosphorus (P) ratio of plant tissues have been used as bio-monitors of N deposition impacts in sub-alpine systems but their suitability in alpine habitats is unknown. We investigated spatial variation in N content and N:P in Calluna vulgaris and Racomitrium lanuginosum in low-alpine heathlands across Scotland and related this to nitrogen deposition, climate and soil chemistry. N content in C. vulgaris and R. lanuginosum were not well predicted by N deposition estimates and it appears that altitude and climate also have strong influences on plant chemistry which must be accounted for before using these parameters as bio-indicators of deposition. N:P ratios in C. vulgaris suggested that most sites are currently co-limited by N and P or P limited. The degree of P limitation was strongly related to P content of the soil and hence to underlying rock type. We suggest that this high degree of co-limitation may be the result of accumulated long term N deposition in mountain regions. Vegetation on soils with low P content and strong P limitation on growth is least likely to sequester N inputs in additional biomass production and so may be the most vulnerable to N saturation and breakthrough of N into surface waters.