Studies in the Grazing of Heather Moorland in North-East Scotland. II. Response of Heather

Abstract

Trends in heather cover, height and standing crop were observed at 32 sites for periods of at least 4 yr. Occupance by larger herbivores (cattle, deer, grouse, hares, rabbits, sheep) was monitored from the volumes of dung deposited. Heather trends were regressed on deposition rates and other factors likely to affect response. Transforming percentage cover by logits and including an index of growth improved goodness-of-fit substantially. The estimated rates of dung deposition causing zero heather trend were little changed by the addition of regression variables, but confidence limits narrowed. Zero trend in cover resulted from a lower deposition rate than zero trend in height and standing crop. Various ways of sub-dividing total dung were examined to expose possible differences between herbivores or seasons. The most satisfactory treatment giving stable regression coefficients was to partition non-cattle deposition around threshold levels. Cattle had the most severe impact, and heavy usage by other herbivores also significantly reduced heather, but light usage had a significant positive effect. Selectivity in defoliation between food plants accounted for only a modest part of the variability in heather response. Its importance seemed greatest in lagomorphs, but cattle affected heather in ways unrelated to the observed utilization. The stocking rates above which heather declines are estimated as 0.2 cattle and 2.7 sheep ha-1, given average growth of 4.7 cm yr-1 and a normal amount of light usage from other herbivores. With 1.2 cattle ha-1 cover would decline by 32% in 4 yr and in 10 yr from 80 to 5%; a stocking of 5 sheep ha-1 would cause a decline of only 9% over 4 yr.