Will climate change counteract the benefits of sheep-grazing in conserving a rare alpine dwarf-herb community?

Abstract

Scottish alpine vegetation is subject to constantly changing pressures from grazing and from climatic shifts but there has been no long-term field study of the consequences. In 1987, permanent plots were set up at Ben Lawers in the Scottish Highlands to examine the effects of protection from sheep-grazing on the population size of the rare annual Gentiana nivalis L. and on the alpine dwarf-herb vegetation among which it grows. The botanical composition of grazed and ungrazed vegetation was recorded annually from 1987 to 1996. Then grazing was restored on the ungrazed plots. The vegetation was recorded again in 2010 and the results compared with the 1996 data. Decreases in vegetation height and the amount of plant litter as well as increases in the cover of annual forbs and of bryophytes followed from the restoration of grazing in 1996. This is consistent with the hypothesis that sheep-grazing may help to maintain the dwarf-herb vegetation. However, in continuously grazed vegetation, the cover of graminoids, annual forbs and bryophytes increased, while that of bare ground decreased between 1996 and 2010. This finding suggests that an increasingly warm and prevailingly wet climate is linked with the spread of graminoids and bryophytes and eventually may alter the characteristics of the dwarf-herb vegetation.