Prediction of the shoot production of heather under grazing in the uplands of Great Britain

An empirical model for the annual shoot production of heather Calluna vulgaris in the absence of grazing in the uplands of Great Britain is presented. Production is modelled as a function of the growth phase of the heather and weather conditions during the summer growing season and the previous winter.
The effects of grazing, principally by sheep, on changes in heather cover after burning are reviewed. A simple model of annual cover change is proposed, and fitted to data from dry and wet heaths in Scotland.
A model of the influence of grazing on annual heather shoot production is developed for dry heath.
The models indicate that previously published estimates of the utilization levels that heather can tolerate without a reduction in shoot production may be too high. For pioneer and early building heather, estimated threshold utilization levels are 16% in summer and 24% in autumn. Furthermore, it is suggested that any level of utilization will reduce the rate of increase of heather ground cover after burning.