Effects of nutrient supply,light availability and herbivory on the growth of heather and three competing grass species

Grasses are becoming more abundant in areas in NE Scotland which until recently were dominated by heather (Calluna vulgaris). However, it is not clear if grasses are aggressive competitors which are now able to outcompete the dwarf shrub due to changes in environmental factors (such as grazing pressure and increasing nutrient inputs), or just opportunistic invaders, occupying gaps in the canopy which occur when heather reaches the degenerate stage. Experiments in turves and in field plots were carried out in order to investigate the performance of three grass species, Nardus stricta, Deschampsia cespitosa and Deschampsia flexuosa growing in competition with heather. These three species were selected because they differ in their nutrient requirements, palatability to herbivores and tolerance of shading. The grasses were planted in heather canopies of different structure, either turves of heather of different height and age, or moorland plots with or without heavy grazing by sheep and deer. Fertiliser (NPK) was applied to half the experimental plants. The growth of the grass species and the heather in response to the fertiliser and grazing treatments was measured, together with the light levels penetrating the canopy and received by the grass plants.Results indicated that heather was likely to be outcompeted by grasses only when there are gaps in the canopy, resulting either from heavy grazing or from the heather being in the mature or degenerate phase. Fertiliser enhanced plant growth whereas fencing out herbivores led to strong competition for light as the heather canopy closed. It is concluded that grasses require gaps in the canopy to successfully invade heather moorland, or they tend to be shaded out. Thus better management of heather moorlands to maintain a dense canopy structure may help to preserve heather cover even under increasing nutrient inputs.