Important extreme events affecting forests include wind storms, frost, excessive snowfalls, glaze, drought and temperature anomalies (such as an unusually warm period in mid-winter). These tend to result in the mortality of individual trees, and may be highly selective of species or even individuals. Individuals also vary in their responses, although the relative importance of genotype and site characteristics is often difficult to separate. Extreme climatic events tend to be much more important in initiating change in forest ecosystems than average climatic conditions. The changes created by such disturbances provide the gaps which will enable species better suited to the current environmental conditions to become established. Consequently, a forest at any given time will consist of a mosaic of patches, each occupied by species that were favoured at the time of gap formation. Models of forest succession under a changing climate need to pay much greater attention to the role of extreme climatic events.