Arboriculture & Urban Forestry 40(6): November 2014 compromising other structural, aesthetic, or physi- ological elements of the tree? It is a basic tenant of fracture mechanics that once a fracture occurs, it cannot be stopped. This is particularly true in trees. The insertion of rods across the fracture zone can stabilize the fracture, but does it alter the ability of the tree to respond to additional mechanical loads? How does cabling alter canopy and branch development and does it significantly alter the ability of a tree to absorb wind energy and reduce oscillation via damping? Does cabling sig- nificantly alter branch and branch union strength similar to how staking compromises tree stability to wind and ability to respond to future loading? Answers to these important questions can provide an improved understanding of how arboricultural practices impact tree strength and resilience. Some of the critical information needed to address these questions already exists in part, but is likely to be found in the forestry, ecology, and tree physiology literature. Combined with well-designed research studies and careful field observation, existing data could be applied to accurate models of tree growth and response to mechanical loads to determine the limits to which we should treat our trees. Such models might even provide a better predictor of how long it will take for a tree to regain its stability. FOCUS AREA 5: EFFECTIVE- NESS OF MITIGATION PRACTICES The biomechanical implications of many arboricul- tural practices are not well understood. Arboricul- tural practices, such as pruning and cabling, have mechanical consequences on trees and are oſten intended to reduce the likelihood of tree failure. Changes to drag, moments (the product of force and lever), stresses, oscillatory frequency, and damping all may result from pruning and cabling, but too few empirical data exist to draw robust conclusions and validate theoretical or numerical models. A greater breadth and depth of understanding of tree biomechanics exists for trees of excurrent form (ex- emplified by forest- or plantation-grown conifers), but investigations considering trees of decurrent form are less common and oſten involve small trees. Larger trees do not necessarily have the same al- lometry or behave the same mechanically as smaller trees, hence scaling up results can be problematic. Mitigation Research Priorities * Will pruning to alter crown architecture reduce the likelihood of failure? * Can crown pruning alter root/soil plate movement? * Does cabling reduce the failure rate of codominant stems? * Are support systems properly designed to mitigate biomechanical weaknesses in trees? * Does pruning enhance safety? There is general agreement that regardless of the location from which biomass is removed pruning reduces drag in approximate proportion with biomass removed. However, the location from which biomass is removed can alter the effect of pruning on bending and twisting moments. Crown raising can increase the lever on a tree, while crown reduction decreases it. Consequently, the bending moment, which is the product of drag and lever, is more effectively decreased through reduction pruning, as experiments have shown. Since the effect of thinning on the shape of a crown is usually negligible, it decreases bending moment primarily by decreasing drag. Several questions remain, however, since the mechanical analysis cannot ignore biological reality. Reduc- tion pruning—while effective in reducing drag, bending moment and bending stress—may ulti- mately increase stress if trees sprout vigorously and decay rapidly in response to the pruning. Using a static analysis, which has been histori- cally more common, cannot easily investigate the true effectiveness of pruning on reducing the like- lihood of failure because the interaction of wind and tree is dynamic. Consistent with dynamic beam theory, the natural sway frequency of trees of excurrent form can be approximated by the ratio of DBH to the square of tree height. This model, however, does not hold for trees of decurrent form, presumably because the complex crown architecture of such trees induces a multi-modal dynamic response. Mass participation of the trunk and larger branches can affect the frequency. Predicting the damping ratio of trees of either excurrent or decurrent form has been gener- ©2014 International Society of Arboriculture 315
November 2014
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