An evaluation of thinning to improve habitat for capercaillie (Tetrao urogallus)

Abstract

In Scots pine (Pinus sylvestris) forests the composition of the ground flora can be affected by the amount of light reaching the forest floor, influencing the balance between the three common ericaceous shrubs bilberry (Vaccinium myrtillus), cowberry (Vacciniumvitis-idaea) and heather (Calluna vulgaris). A pinewood ground flora with more than 20% bilberry cover is considered good habitat for capercaillie (Tetrao urogallus), a large forest grouse of considerable conservation interest throughout Europe. Old, semi-natural Scots pine woodland is considered its prime habitat in Scotland, although this is limited in area compared to twentieth century planted forests. Action to manipulate environmental conditions within Scots pine plantations by altering light levels to favour bilberry through thinning and felling could potentially increase greatly the area of available capercaillie habitat in Scotland. We implemented a five year study to look at bilberry response to variable intensity thinning in two Scots pine plantations, where thinning followed management guidance for capercaillie available at the time. Bilberry was present in both forests but a positive response to thinning was not universal; although at both sites bilberry cover increased significantly over five years with levels >20% cover reached, this could only be attributed to the thinning treatment at one of the sites. A treatment of small patch clearfelling did not lead to losses in bilberry. Management guidance published after the trial had begun, identified the appropriate intensity of thinning for enhancing bilberry cover at our study sites, indicated by the relative increase of bilberry in the plots where the prescription had been followed. Although there was no significant treatment effect by year five, 42% average bilberry cover was reached at one site tested. However the format of this guidance, a range of stem density-tree height combinations, was difficult to apply using typical forest management data and we explore redefining the guidance as a post thinning stand basal area range. We suggest >22 to <31m2ha−1 basal area would be appropriate in Scots pine plantations established at normal spacing and subject to the commonest form of selective thinning regime. This range in basal area can be achieved without conflicting with management for timber production. Our results also support small patch clearfelling as a method of diversifying plantation age structure which is compatible with maintaining capercaillie brood habitat.