Windthrow after group and dispersed tree retention in eastern Canada

Although green tree retention has been proposed as a means of maintaining forest structural attributes normally found after stand replacing disturbances, there is some concern about mortality and fall rates of residual trees. Very little data regarding windthrow after green tree retention is currently available for Quebec, eastern Canada. The present study documents windthrow after two types of retention in two regions with contrasting biophysical characteristics, and identifies the most influential factors. A retrospective survey of windthrow was conducted in dispersed and group retention cuts conducted two to five years previously. In the Abitibi region of northwestern Quebec, the level of windthrow was comparable between group and dispersed tree retention. In the North Shore region of eastern Quebec, windthrow was higher in dispersed retention and levels of mortality were generally lower than in the Abitibi region. Windthrow probability was best explained by edaphic variables, tree species, slenderness ratio, tree height or diameter at breast height, and sapling abundance. Variables such as regional mean wind speed, retained group area, simple fetch and topographical exposure (Topex to distance) were not included in the best models. Although mortality rates increased after green tree retention, it remains to be established whether this increase has detrimental effects in terms of maintaining biodiversity.

► Windthrow after green tree retention can be significant.
► Windthrow can be especially high after green tree retention in old even-aged stands.
► Windthrow probability generally increases with tree size and varies with species and soil characteristics.