Abstract

The nature of wind damage as a risk to managed forests is reviewed, based primarily upon experience in Britain. Risk assessment is required to guide site selection, choice of techniques to counter the damage, and plans to respond to damage. Assessment may simply rank sites but prediction of timing of damage is also needed for many decisions. Subjective assessments can be unreliable where there is little history of forest management in that locality or when the damage is determined by many factors. Objective assessments are preferred but are not easy to derive. The existing British windthrow hazard classification is readily applied to sites and can be mapped. However, both its treatment of the wind climate and its estimates of timing of damage are deterministic. The potential to improve the system is discussed using a conceptual model of the assessment of risk which highlights a number of important research issues. Wind damage occurs as a result of an interaction between a variable wind climate and changing tree vulnerability. Vulnerability includes persistent, progressive and episodic components. There can be substantial variability in the frequency and magnitude of damaging storms which is not adequately represented by the conventional separation into storms causing endemic and catastrophic damage. The change in vulnerability with time can have a very marked effect on the frequency of damage. Recent validation has indicated that the estimates of damage based on the hazard classification of damage tend to be pessimistic, and that there is more variability in rate of windthrow than the classification allows. The variability requires a stochastic model and necessitates flexible forest planning; the latter conflicts with many pressures on forest managers.