Factors influencing windthrow in balsam fir forests: from landscape studies to individual tree studies

Windthrow hazard depends upon the interaction of numerous factors whose relative importance has yet to be assessed in the specific context of Eastern Canadian boreal forests. This paper presents results from three studies looking at their relative importance at different scales in the context of balsam fir stands. First, factors involved in a catastrophic windthrow are examined in order to assess their importance at a regional and local scale. Wind speeds were estimated from a numerical model and overlayed with windthrow, soil and stand maps. The two regions that were damaged presented many predisposing factors: high occurrence of shallow soils, of vulnerable species and of overmature stands. Moreover, both regions have been identified as high wind areas. No consistent effect of wind speed on windthrow could be demonstrated, but the increasing vulnerability of stands with increasing proportion of balsam fir in one region and increasing age in balsam fir stands in both regions was significant. In one region, shallow tills experienced more damage in comparison with deep tills but not in the other. The second study looks at windthrow in riparian buffer strips at a local level. It includes st wind tunnel study and a field monitoring in stands of similar species composition and age. Results have shown that windthrow after 7 years was very variable and did not differ with strip width or thinning in the strip. In fact, it was more closely related to locations exposed to stronger winds stressing the importance of wind exposure estimation. It also confirms the greater vulnerability of balsam fir. The last study addresses the identification of external indicators of mechanical weaknesses on individual trees. It involves dissections and winching of trees with and without defects. Results tend to indicate a reduction in resistance when cracks are present on balsam fir. These three studies provide some information to begin considering windthrow hazard in the management of balsam fir forests. (C) 2000 Elsevier Science B.V. All rights reserved.