Can the foliar nitrogen concentration of upland vegetation be used for predicting atmospheric nitrogen deposition? Evidence from field surveys.

The deposition of atmospheric nitrogen can be enhanced at high altitude sites as a consequence of cloud droplet deposition and orographic enhancement of wet deposition on hills. The degree to which the increased deposition of nitrogen influences foliar nitrogen concentration in a range of upland plant species was studied in a series of field surveys in northern Britain. A range of upland plant species sampled along altitudinal transects at sites of known atmospheric nitrogen deposition showed marked increases in foliar nitrogen concentration with increasing nitrogen deposition and altitude (and hence with decreasing temperature). For Nardus stricta L., Deschampsia flexuosa (L.) Trin., Calluna vulgaris (L.) Hull, Erica cinerea L. and Hylocomium splendens (Hedw.) Br. Eur. on an unpolluted hill, foliar nitrogen increased by 0.07, 0.12, 0.15, 0.08 and 0.04% dry weight respectively for each 1 kg ha(-1) year(-1) increase in nitrogen deposition. Most species showed an approximately linear relationship between foliar nitrogen concentration and altitude but no trend with altitude for foliar phosphorus concentration. This provided evidence that the tissue nutrient status of upland plants reflects nutrient availability rather than the direct effects of climate on growth. However, differences in the relationship between foliar nitrogen concentration and atmospheric nitrogen deposition for N. stricta sampled on hills in contrasting pollution climates show that the possibility of temperature-mediated growth effects on foliar nitrogen concentration should not be ignored. Thus, there is potential to use upland plant species as biomonitors of nitrogen deposition, but the response of different species to nitrogen addition, in combination with climatic effects on growth, must be well characterised.