The Responses of Blanket Bog Vegetation to Controlled Grazing By Hill Sheep

Abstract

(1) The effects of grazing by sheep on blanket bog vegetation in south-west Scotland were studied over 11 years in an experiment in which three stocking rates were provided at each of three sites. (2) One site was treated as though it were part of an off-wintering system (no grazing from October to May), and two sites as though they were part of year-round grazing systems. Stocking rates in grazing days ha-1 p.a. on the low (L), intermediate (I) and high (H) treatments, respectively, were 136, 296 and 484 (off-wintering) and 237, 494 and 810 (year-round). (3) Grazing treatments were begun in August 1971 and terminated in July 1982. Harvests to record above-ground biomass and green-shoot biomass of higheter plants were made at 3-yearly interval (1971, 1974, 1977 and 1980) and floristic composition (% cover) was recorded annually each July. (4) total biomass increased in successive harvests as the dwarf shrubs aged and accumulated wood; the biomass on plots allocated to the different stocking rates was similar initially, but was significantly reduced on H compared with L and I plots at all subsequent harvests. Green-shoot biomass on H compared with L plots was reduced on average by 21.7% in 1974, 25.5% in 1977 and 48.5% in 1980. (5) Cover of Calluna vulgaris (L.) Hull and that due to Eriophorum vaginatum L. for year-round systems, was reduced and the area of bare ground increased on H compared with L and I plots. There was little evidence of differences between L and I plots prior to 1979-80, but subsequently deleterious trends became noticeable on the I plots. Rainfall data suggest that increased wetness was a contributing factor. (6) The sensitivity of the bog vegetation to grazing was greatly influenced by initial species composition and age of the stand. (7) The results of this and other grazing studies and their management implications are discussed.