An experimental study of the effects of sheep grazing on vegetation change in a species-poor grassland and the role of seedling recruitment into gaps.
Abstract
An experiment was set up in 1986 on a species-poor grassland in Oxfordshire to determine the effect of sheep grazing management on vegetation change after cessation of fertilizer applications. Three seasons of grazing (winter, spring and summer) were applied, each with two grazing intensities. Point quadrat surveys in 1990 showed that the grassland vegetation was dominated by perennial grasses and that the frequency distribution of species was highly skewed. Dicotyledonous species ('dicots') were extremely rare, having an overall frequency of only 0.43%. The frequencies of 8 of the 10 dominant grasses were significantly affected by grazing intensity although these effects depended on the grazing season, were species-specific and were generally small. Intensive surveys of the dicots in 1990-91 discovered 40 species although most of these were rare. The dicots exhibited stronger and more consistent responses than the grasses, their abundances being significantly increased by increased grazing in one or more grazing periods. Dicot species number was significantly increased by increased grazing intensity in all periods. The potential was studied for seedling establishment in gaps to bring about vegetation change. Regular monitoring of the natural recruitment of seedlings into artificially created gaps was carried out in each paddock. Comparison between the species composition of seedlings emerging in gaps where the soil had been replaced with a sterile loam and that of gaps formed over the original soil showed no evidence of a persistent seed bank and that all seeds were probably derived from recent seed rain. No species novel to the vegetation emerged in the gaps and the species composition of seedlings in the gaps was significantly and positively correlated with that of the vegetation in a majority of the paddocks. However, some species differences in the contribution to the seed rain was noted. In particular, the dicots were over-represented. The number of grass seedlings in the gaps was decreased by increased summer grazing. It is concluded that grazing had complex effects on vegetation change. Change is likely to be slow, especially while fertility is high, because of the small responses of the grasses to the grazing treatments and the lack of input of novel species from a seed bank. However, the dicots may continue to increase under increased grazing because of their high seed production and the effects of grazing in increasing gap frequencies.