Consistent influence of tree diameter and species on damage in nine eastern North America tornado blowdowns

Abstract

Are tree damage patterns in natural-forest windthrows predictable? Here, I synthesize published and unpublished findings from nine North American forest sites that were disturbed by tornadoes, to ask how well tree damage patterns might be predicted on the basis of tree diameter and species identity. All sites were sampled by the author and assistants, using generally similar methodology, thereby avoiding many of the barriers to direct cross-site comparison of wind damage. In almost all cases, there is a consistent pattern of steady increase in risk of treefall with tree diameter. In most sites, uprooting was substantially more common than trunk breakage, although their relative frequencies varied with tree diameter, and among species and sites in complex ways. Species differed substantially in risk of treefall, even when controlling for diameter. However, ranking among species in risk of treefall was consistent, suggesting that among species differences are general across sites and forest types. In only two out of nine sites was there a significant influence of wood strength properties, independent of species identity. Type of treefall was weakly related to tree diameter, although differences among species were not consistent among sites. Direction of treefall was significantly more eastward for large compared to small trees in six out of eight sites. Stand-level characteristics of height variation, and density, were not consistently related to level of damage at the plot scale. At the scale of individual trees, a moderate level of predictability exists for treefall risk, sprouting, and direction of fall on the basis of easily recorded tree diameter and species identity; type of damage remains poorly predicted on the basis of these traits.