Interactive effects of nitrogen deposition, fire and grazing on diversity and composition of low-alpine prostrate Calluna vulgaris heathland.
Low-alpine heathlands world-wide have high biodiversity value but are increasingly impacted by the effects of nitrogen deposition as well as fire and grazing. We conducted a 5-year fertilization experiment to examine the interactive effects of these three factors on vegetation diversity and species composition. Factorial combinations of nitrogen addition (0, 10, 20 and 50 kg N ha-1 year-1), burning (burned once or not burned) and grazing (annual clipping, 0% or 12% of current year shoots removed) were applied to internationally important prostrate Calluna-Cladonia heathland in the Cairngorm Mountains, Scotland. Species diversity (higher plants, mosses and lichens) and percentage cover were recorded annually. Fire had a large effect on vegetation diversity and composition, but both were quick to recover. Species richness recovered within 4 years and vegetation composition was predicted to recover within 7 years. Vegetation composition appeared resilient to the effects of disturbance and there was little invasion by graminoids. Nitrogen deposition interacted with fire. Burned plots showed no significant effect of nitrogen treatment on species diversity, while the diversity of unburned plots was significantly reduced only 1 year after treatment with 50 kg N ha-1 year-1. After 5 years, significant diversity reductions were seen in the 10 kg N ha-1 year-1 treatment. Impacts of nitrogen on species richness were primarily through reductions in lichen diversity. Severe winter browning of Calluna vulgaris in plots receiving 50 kg N ha-1 year-1 followed early snowfall in 2002-03. This interaction affected only Calluna vulgaris and may have the potential to trigger species composition changes by reducing the dominance of this species. Synthesis and applications. Although resilient to small-scale fires, species richness of low-alpine heaths is reduced by exposure to low levels of nitrogen deposition (background+10 kg N ha-1 year-1). The lichen component of the vegetation is most sensitive to additional nitrogen, although higher plants can be affected via interactions with climate. These data support the current critical load of nitrogen for this community of 5-15 kg N ha-1 year-1 and suggest that lichen diversity could be a useful indicator of nitrogen deposition impacts in alpine habitats. Effective conservation of biodiversity in low-alpine heathland will require action at national and international levels to reduce nitrogen deposition in the many areas where the critical load is currently exceeded.