Manipulation of nutrients and grazing levels on heather moorland: changes in Calluna dominance and consequences for community composition
S. E. Hartley (tel. 01273 872751; fax 01273 678433; e-mail S.Hartley@sussex.ac.uk).
- 1Experimental 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 dominant Calluna vulgaris. We also recorded the cover of vascular plants and bryophytes, to assess the impact of changes in Calluna dominance on competing species.
- 2Grazing in combination with nitrogen addition caused the greatest decline in Calluna cover, typically 40–50%, but nitrogen addition did not cause a significant decline in Calluna on plots protected from grazing. More Calluna shoots were browsed on nitrogen-treated plots than on unfertilized ones, presumably because grazing animals preferred fertilized Calluna.
- 3Nitrogen addition allowed grasses to increase in cover, especially on grazed plots. However, Nardus stricta, Festuca ovina and Agrostis sp. all declined in fenced areas but increased in grazed plots, whereas Deschampsia flexuosa and Festuca rubra increased in fenced plots.
- 4The 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.
- 5Fencing 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.
- 6The impact of nitrogen addition on the cover of Calluna and 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.