Can pruning reduce the likelihood of tree failure?

Brian Kane [Department of Environmental Conservation, University of Massachusetts]

Trees growing in cities and towns provide many benefits, but since they grow in proximity to targets, they inherently present a degree of risk. Actual risk depends on the likelihood of tree failure, the likelihood of a failed tree impacting a target, and the severity of the consequences of the target impact. As part of tree risk assessment, arborists often prune trees to reduce the likelihood of failure (or eliminate it entirely, by removing defective branches or trees). Simple mechanical theory provides a conceptual understanding of the factors that determine the likelihood of tree failure and how pruning may or may not alter those factors. While some factors (like the rate of wound closure and extent of decay that forms after pruning) have been studied for many years, other factors (such as drag, drag-induced bending moment, and sway characteristics - frequency and damping ratio) have only recently been investigated. Pruning affects all of these factors: reducing drag and drag-induced bending moment roughly in proportion with pruned biomass; increasing the extent of discoloration with improper pruning cuts. Per unit pruned biomass, there are expected differences between pruning types: pruning types that shorten the crown and remove proportionally more leaf areas more effectively reduce drag and drag-induced bending moment. Studies are limited by the consideration of comparatively few species and growing conditions and testing mostly smaller trees (results of which cannot easily be scaled up to larger trees). Neither have many studies considered the long-term effects of pruning on the likelihood of failure. And despite the demonstrated effect of pruning on mechanical parameters relevant to the likelihood of tree failure, very few studies of failed and standing trees following a storm have revealed differences in the likelihood of failure directly attributable to pruning.

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