LONG-TERM EFFECTS OF EXCLUDING SHEEP FROM HILL PASTURES IN NORTH WALES

1. From 1957 to 1968 a series of nine experiments was established to determine the effects of excluding sheep from hill pasture in Snowdonia. Each experiment consisted of three ungrazed plots and six grazed control plots. Species abundance was recorded non-destructively by cover pins during 1958-81. In autumn 1982, two of the experiments were completely fenced; vegetation changes in the formerly grazed plots were recorded by herbage sampling from 1981 to 1990. 2. Successional changes depended strongly on the soil and initial vegetation of the site. Compositional change in grasslands initially dominated by Agrostis capillaris and Festuca ovina was smaller than in vegetation initially dominated by Nardus stricta and Festuca ovina, and was less obviously driven by coarser plants overtopping short ones. 3. There was almost no invasion of new plant species, the only exceptions being Sorbus aucuparia, which was associated with bird perches on fences, Dryopteris dilatata, which emerged in tussocky grassland, and Solidago virgaurea, which may have spread to one site from nearby cliffs. The invaders played no important part in the succession. 4. Early changes were relatively rapid, with the character of the vegetation altering markedly over about 7 years. Low-growing plants such as Danthonia decumbens, Juncus squarrosus, Trifolium repens and Polytrichum commune declined consistently, as did the biennial Cirsium palustre. 5. Longer-term changes in the herbaceous vegetation were few. Suppressed shrubs of Calluna vulgaris and Erica cinerea grew into full-sized bushes, which at one site flourished for 20 and 12 years, respectively, before degenerating, but at other sites coalesced to achieve local dominance. 6. In the absence of sheep, voles became the dominant herbivore and caused large year-to-year variation in herbage biomass. Agrostis vinealis and Holcus mollis were less damaged by voles than A. capillaris. Established tussocks of Molinia caerulea were also little affected. In years of high vole abundance the pleurocarpous mosses Hylocomium splendens and Pleurozium schreberi increased greatly among mats of dead grass.