Natural regeneration of Sitka spruce in the Forest of Ae: development over 25 years.

摘要

Dense natural regeneration of Sitka spruce is an increasingly common feature of recently clearfelled sites in upland forests of the British Isles. Some guidance on an appropriate management strategy for these stands is provided by a pilot experiment established in 1973 in the Forest of Ae in regeneration 1.6 m tall with an average stocking of 27 000 stems ha-1. The experiment contrasted unthinned controls with selective or mechanical respacing to a target density of 2,500 stems ha-1. After 25 years all treatments had densities of around 2,000-2,500 stems ha-1 as a result of self-thinning and/or respacing. There was similar height growth between treatments except where chemical application to control stump regrowth temporarily depressed tree vigour. Mean tree diameter was consistently larger in the selectively respaced treatments. There were no differences between treatments in terms of stem straightness, branching, and estimated wood density. Evaluation of snow damage in 1996 indicated that smaller diameter trees were more vulnerable than larger ones. Evaluating the impact of these treatments upon stand stability using the ForestGALES model suggested that respacing would reduce the risk of windthrow compared to the control with consequential gains in tree volume. An appropriate strategy for managing dense stands of natural regeneration should take account of the wind risk on a given site and be based upon a combination of natural self-thinning in the early stages of stand establishment and selective respacing to favour the best quality stems. No respacing should occur before the average height is 2-3 m tall and on stable sites maximum use should be made of self-thinning to reduce stand density. On less stable sites, a single intervention to a target density of 1,750-2,000 stems ha-1 should provide sufficient improvement in mean tree diameter without compromising timber quality.