Manipulation of nutrients and grazing levels on heather moorland: changes inCallunadominance and consequences for community composition

Summary Experimental studies of the combined effects of herbivory and the availability of nutrients on semi‐natural communities remain relatively scarce. Here we report the effects of 6 years of nutrient addition (N, P and K) and protection from grazing on moorland plant communities in the Scottish uplands, particularly on the cover of the dominantCalluna vulgaris. We also recorded the cover of vascular plants and bryophytes, to assess the impact of changes inCallunadominance on competing species. Grazing in combination with nitrogen addition caused the greatest decline inCallunacover, typically 40–50%, but nitrogen addition did not cause a significant decline inCallunaon plots protected from grazing. MoreCallunashoots were browsed on nitrogen‐treated plots than on unfertilized ones, presumably because grazing animals preferred fertilizedCalluna. Nitrogen addition allowed grasses to increase in cover, especially on grazed plots. However,Nardus stricta,Festuca ovinaandAgrostissp. all declined in fenced areas but increased in grazed plots, whereasDeschampsia flexuosaandFestuca rubraincreased in fenced plots. The effects of grazing and nutrient addition varied markedly between sites, possibly because of differences in soil moisture and organic matter. Nitrogen addition, however, increased soil nitrogen mineralization rates in both glens. Fencing increased the cover of grazing‐intolerant plants with low nutrient demands (as classified by Ellenberg and suited species scores) that were categorized as competitive plants by Grime's CSR model. Plots receiving nitrogen and phosphorus had more nutrient‐demanding plants able to tolerate high grazing pressure that were often classified as ruderals. The impact of nitrogen addition on the cover ofCallunaand on competing grass species in the community critically depends on the level of grazing. Changes in community composition caused by grazing and fertilizer addition can be explained in terms of the ecological tolerances of individual species, allowing us to predict the types of plants that are likely to increase or decrease in cover.