Leader Browsing by Red and Roe Deer on Young Sitka Spruce Trees in Western Scotland. II. Effects on Growth and Tree Form

Abstract

Some 2000 trees at 14 main sites in Glenbranter Forest, Argyll, were monitored during 1978–1990 for damage. Tree state (whether leaderless or with single or multiple leaders or trunks) and tree size were regularly recorded. Browsing of leaders depressed height increment by 5–10 cm in the year of ocurrence compared with undamaged trees; the rapid development of new leaders minimized the check in growth. Trees initially leaderless had equal height increment over a year to trees initially with leaders if no damage occurred in the year. At a subsidiary site where some trees were planted in small exclosures, the trees protected from deer reached a safe height after five seasons of growth, unprotected trees reaching equal height 1 year later. The girths attained when trees were 10–15 years old were on average only slightly depressed by earlier browsing damage to leaders, reductions at most equalling one year's increment. Many trees became multi-trunked, there being a significant relationship between the percentage incidence of multi-trunking and the number of occurrences of leader damage. Unbrowsed trees mostly became single-trunked. We judge that the present high incidence of multi-trunking (45 per cent in our 9–15 year-old stands) results largely from deer browsing.