Differential responses of UK upland plants to nitrogen deposition

Native upland species, Nardus stricta, Eriophorum vaginatum, Erica cinerea and Vaccinium vitis-idaea were given 3 or 60 kg N ha(-1) yr(-1), over 2 yr, applied as a mist (NH4NO3). The high N treatment increased above-ground bi omass in all four species, but only significantly in E. cinerea, E. vaginat um and N. stricta. Biomass increases in E. vaginatum and N. stricta resulte d from enhanced tiller production rather than shoot elongation. Root growth increased in N. stricta, so that root:shoot ratio in this species was unch anged by N. Root growth in E. vaginatum, E. cinerea and V. vitis-idaea did not respond to N and their root:shoot ratios decreased. Tissue N concentrat ions increased in both shoots and roots of all species in response to S. Th e accumulated foliar N did not increase the proportion of N allocated to Ru bisco and the photosynthetic capacities of S, stricta, E. vaginatum and V. vitis-idaea were unchanged. Thus growth responses to N were due to altered allocation rather than increased rate of photosynthesis per unit leaf area. The high N treatment increased flower production significantly in E. ciner ea but not in the other species. Although in this experiment dwarf shrubs w ere more responsive than graminoids to N, in the field at current N inputs the enhanced tillering of the graminoids ma? be more competitively advantag eous, especially where gaps develop in the canopy. Thus increasing N deposi tion may lead to increased grassiness of upland heath, and in particular, a spread of N. stricta.