An example of the seasonal impact of sheep on colonisation by deciduous trees

Summary

Seasonal grazing treatments were begun in 1990 on two sites of bilberry moorland that lay 60–200 m from a strip of riverside woodland in Northern England. Treatments were summer grazing, winter grazing, year-round and no grazing, fences being moved in mid April and mid October each year to open or close plots. The main plant species, Calluna vulgaris, Empetrum nigrum and Vaccinium myrtillus, showed negligible response in cover or height to season of grazing, but colonisation by deciduous trees was much affected. On plots given summer protection (total area 2000 m²) 249 Sorbus aucuparia saplings were counted in 2000, together with one Quercus robur sapling, but no saplings were found in summer-grazed and year-round-grazed plots. Each winter the rowan saplings in the summer-protected plots were severely browsed by sheep, but they recovered in the next growing season. Their increasing trunk diameter and shoot increment up to 2000 suggested that some would soon escape herbivore control. But the fence moves did not take place in 2001,and in April 2002 these saplings were found to have been very heavily browsed. Nearly all survived but regrowth was much poorer in summer 2002 than summer 2000, so several more years of recovery are needed before it can be decided if summer protection from grazing allows succession to woodland at these sites.