The responses of blanket bog vegetation to controlled grazing by hill sheep.
The effects of grazing by sheep on blanket bog vegetation (mainly Sphagnum spp., Eriophorum vaginatum, Calluna vulgaris and Erica tetralix) in SW Scotland were studied on 3 sites during 1971-82. One site was treated as part of an off-wintering system (no grazing from Oct. to May), and 2 sites as part of year-round grazing systems. Stocking rates were 136, 296 and 484 (off-wintering) and 237, 494 and 810 (year-round) grazing days/ha annually on the low (L), intermediate (I) and high (H) treatments, resp. 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 av. by 21.7% in 1974, 25.5% in 1977 and 48.5% in 1980. Cover of C. vulgaris and E. vaginatum for year-round systems, was reduced and the area of bare ground increased on H compared with L and I plots. There was little difference between L and I plots prior to 1979-80, but subsequently C. vulgaris was reduced on I plots. Rainfall data suggest that increased wetness was a contributing factor. The sensitivity of the bog vegetation to grazing was greatly influenced by initial sp. composition and age of the stand. The results of this and other grazing studies and their management implications are discussed.