The importance to upland vegetation of enhanced nitrogen deposition at high altitude
Semi-natural upland plant communities in the U.K. characterised by calcifugous grasslands and montane dwarf shrub communities, with low N requirements, are thought to be particularly vulnerable to the effects of N pollution. N deposition can be enhanced at high altitude sites via cloud droplet deposition and the seeder-feeder effect and the consequences of this for upland vegetation were investigated. Sampling of a range of upland plant species on altitudinal transacts at sites of known N deposition showed that the foliar N concentration increased with altitude and appeared to reflect the enhancement of N deposition with altitude at these sites. Transplant experiments with Nardus stricta in pots and turves showed that although this species reflected N deposition to some extent, the relationship was modified by temperature. This should be taken into account in any attempt to biomonitor N deposition using higher plants. A two year experiment in which upland plant species were misted with N at a rate typical of more polluted sites in the U.K. (60 kg N ha-1 yr-1) produced increased root and shoot tissue N concentrations in all species and differential effects on growth and nutrient allocation, and flowering. The dwarf shrubs Vaccinium vitis-idaea and Erica cinerea showed a decrease in root:shoot ratio as a consequence of increased shoot growth, as did the sedge Eriophorum vaginatum, but in the grass Nardus stricta both root and shoot growth were stimulated with the result that there was no change in root:shoot ratio. The magnitude of the response of foliar N concentration to N deposition was species specific and greatest in slow growing species, such as dwarf shrubs, which showed preferential allocation of N to the shoots.