Summary1. 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. ), 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. 3. 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. 4. 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. 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. 5. 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. 6. 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.
Summary1. Agrostis capillaris±Festuca ovina-dominated communities are widespread in the uplands of Great Britain. They are agriculturally productive but little is known about how to manage this community for speci®c goals. Vegetation trajectories were examined in this plant community under dierent sheep grazing management regimes at two sites in Scotland. One site had a substantial presence of moorland species, the other was characterized by a more productive vegetation. Management consisted of maintaining sward heights of 3, 4´5 or 6 cm during the growing season, or complete exclusion of grazing stock. 2. Changes in species composition were small over the 7 years of the experiment. Few species invaded or were lost during the course of the study. The observed changes were largely as a result of shifts in abundance of the dominant species. 3. Maintenance of sward height at low levels (3 or 4´5 cm) during the growing season resulted in the spread of Nardus stricta where present. Where N. stricta was absent, the sward developed a higher content of mosses, speci®cally Hypnum jutlandicum and Rhytidiadelphus squarrosus. 4. Removal of grazing resulted in an increase of cover of grazing-intolerant species, such as Deschampsia¯exuosa and Molinia caerulea, and in the cover of dwarf shrub species where present. 5. The two sites diered in the treatment that resulted in the smallest change in species composition. At the more productive site, maintenance of the sward at 4´5 cm resulted in the smallest overall change in species composition. At the less productive site, grazing the sward to 6 cm resulted in the smallest shift in vegetation composition. Grazing at this height appeared to prevent the spread of both M. caerulea and N. stricta. 6. The study demonstrates that sustainable grazing regimes for upland Agrostis± Festuca grasslands need to take into account both the initial composition of the vegetation, speci®cally the presence of species capable of replacing A. capillaris and F. ovina and of achieving dominance, and the overall productivity of the site.
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