Wind-driven gap development in Birkley Wood, a long-term retention of planted Sitka spruce in upland Britain

Management of planted forests in Britain is changing to incorporate biodiversity and other nontimber values by lengthening rotations, seeking alternatives to clear-felling, and identifying stands for nonintervention. However, there are particular uncertainties over stand development of exotic species on areas of high windthrow hazard. This study considered the effect of strong winds between 1987 and 2000 on stand development of Sitka spruce (Picea sitchensis (Bong.) Carrière) planted in 1923. Windthrown gap area increased eightfold through at least 38 separate events. Windthrow of trees assessed as stable occurred during periods of higher wind speed than those with windthrow of trees assessed as vulnerable, and there was a nonlinear relationship between wind speed and increase in gap area. Gap expansion contributed more to the increase in gap area than new gap formation, and the centre of the largest gap migrated downwind. Nevertheless, a substantial proportion of the stand remains undisturbed to an extent not predicted. Therefore, a complex stand structure may develop, including a partial wave of natural regeneration of Sitka spruce in the expanding gaps, but species diversification will be slow due to the absence of local seed sources. The results provide insights for stand dynamics in windy, maritime environments.