Long-term effects of leader browsing by deer on the growth of Sitka spruce (Picea sitchensis)

This study aimed to show the long-term consequences of leader browsing by deer on growth of Sitka spruce in plantations and took place in Glenbranter Forest in western Scotland. Browsing and other leader damage were monitored at 11 sites until age 11 years when nearly all trees were too tall to have leaders browsed. Impact on girth growth was examined up to 15-20 years later. The occurrence of multiple-trunk trees was strongly related to browsing frequency, and their main trunks had smaller mean girth than single-trunk trees. There was less leader browsing and hence fewer multiple-trunk trees at higher tree stocking densities. Relationships between tree girth and several factors were investigated in separate analyses for single-trunk trees and all trees. The initial height of trees always had the largest significant effect on tree girth, being positively related; trees in older stands significantly benefited from edge position. In all-tree runs, the effect of multiple trunking was negative and usually highly significant, being second to initial height in explaining final girth. Our results suggest that planting at high stocking density for good-quality timber is likely to reduce deer browsing and multiple trunking compared with the strategy of less-dense planting required for pulp.