Response of bilberry Vaccinium myrtillus L. stands in the Derbyshire Peak District to sheep grazing, and implications for moorland conservation

Abstract

Exclosure systems were set up at two sites in Derbyshire, England, in 1990 to compare summer, winter, year-round and zero grazing. Initially the sites were similar in bilberry Vaccinium myrtillus extent, with cover means c. 70%, but they differed in heather Calluna vulgaris extent, with cover means 1 and 25%. Sheep occupance was estimated from counts of faecal-pellet groups six or seven times a year and utilisation of bilberry and heather from assessments in permanent positions four times a year. Plant cover was measured each August from 1990 to 1996. It was found that the sheep consistently chose to graze the bilberry swards much more heavily in autumn than the rest of the year. Dung deposition rates were quite similar at the two sites, but the winter-only grazed plots had significantly greater dung deposition at the heather-bilberry site than the pure bilberry site. Shoot utilisation reflected sheep occupance. Winter protection reduced utilisation, significantly so at the pure bilberry site, but differences were minimised because of bursts of heavy usage following the reopening of the plots to grazing in April. At the pure bilberry site the height of bilberry stayed remarkably constant under all three grazing regimes, but increased sharply in the absence of grazing. Bilberry cover was unaffected by season of grazing, but crowberry Empetrum nigrum appeared to benefit from winter protection. At the heather-bilberry site, bilberry was significantly reduced in cover under all the grazing treatments, whereas heather increased significantly in cover and height despite an average grazing pressure of 1·4 sheep ha −1. This suggests that the desired light stocking rates presently being advocated to conserve heather moorland will cause losses of bilberry moorland, and that for the conservation of a diversity of vegetation types in a block of moorland some zonal variation in grazing pressure is required.