Modelling the vulnerability of balsam fir forests to wind damage

Wind causes significant damage to forests in central Quebec, and with the current move to more refined silvicultural practices, concerns are increasing. This study investigates the mechanisms of windthrow resistance in stands dominated by balsam fir (Abies balsamea (L.) Mill.) with a minor component of white spruce (Picea glauca (Moench) Voss.), representing the dominant feature of a large section of the Eastern Canadian boreal forest. The aim was to compare the stability of trees of both species on mesic and rich mesic sites and then to compare the windthrow vulnerability of the entire stands on the same sites. Stability was measured by static tree-pulling tests. The resulting critical turning moments were calculated at the base of the stem and were related to a series of tree descriptors. Stem mass and tree mass were the variables showing the highest correlation coefficients. The effects of the site and the species on the critical turning moments were non-significant (P > 0.05). Influence diagnostics did not reveal the need for separating the stability of the pulled trees from their mode of failure. A similar analysis also showed that the amount of rot at the base of the stem did not affect significantly the vulnerability to windthrow in this study. Windthrow vulnerability was modelled using the methodology developed for the British GALES model. By using the relationship between the drag of the air on a surface and its aerodynamic roughness, the wind speed over a forest canopy was transformed into a resulting turning moment at the base of the stem. Critical wind speeds at which trees would overturn or snap could then be inferred for typical balsam fir stands growing on two site indices corresponding to those where the tree-pulling study was conducted. The immediate effect of a thinning operation was simulated by reducing the number of stems in the stands. The critical wind speeds were invariably higher for stem breakage than for overturning. In the later case, they were close to 20 m s(-1) on both sites before declining at senescence. A thinning treatment removing 30% of the basal area would reduce this speed by approximately 4 m s(-1). (C) 2004 Elsevier B.V. All rights reserved.