316 ally attributed to mass or structural damping (the movement of branches within a crown indepen- dent of the trunk) and aerodynamic drag on leaves. Of the few studies that have investigated the effect of pruning on the sway response of trees, changes to the mass and aerodynamic drag of the tree alter the frequency in somewhat predictable ways. The effect of leaves, however, supersedes the effect of pruning. Similarly, leafless trees experience greater reduction in drag and bend- ing moment than typical pruning doses achieved. The effect of pruning on trees of decurrent form requires considerably more empirical work, which should help validate existing finite element models. None of the work on pruning has specifically investigated movement of the root plate and soil. Installing support systems is another common arboricultural practice intended to reduce the likelihood of tree failure, but there is minimal research to confirm this. Installing brace rods can change the location of failure of codominant stems (i.e., forked branches nearly equal in size, often with included bark) to immediately above the hardware, but this effect has not been con- firmed with studies on cables or large trees. Some opinions suggest that installing support systems (especially those made of steel) alters tree growth in accordance with the hypothesis of acclimative growth, but a recent experiment testing steel and synthetic cables did not support this specula- tion. Installing steel cables in red oaks increased natural frequency, but not damping. Once again, the effect of leaves superseded the effect of the cable, and leaves did increase damping, regard- less of whether a cable had been installed. Many of the conventions of the biomechanical effects of arboricultural practice are based on simple mechanical approaches which, while intuitive, neglect to consider the complex crown architec- ture of a typical open-grown tree. For example, leaves increase damping and drag, but decrease frequency. These outcomes theoretically offset one another to some degree: reduced frequency and greater drag mean greater bending stress, but greater damping allows trees to shed wind energy. A recent finite element model using Monte Carlo simulations illustrated the complexity of such competing effects: of two modeled sugar maples (Acer Saccharum), one was more likely to fail ©2014 International Society of Arboriculture Dahle et al.: Tree Biomechanics White Paper while leafless, while the other was more likely to fail when it was in leaf. The simple mechanical approaches are conceptually useful, but with- out substantially more experimental observa- tions, their practical application is tenuous. ADVANCING TREE BIOMECHANICS RESEARCH There are a growing number of talented scientists interested in tree biomechanics research, but as is so oſten the case in arboriculture and urban forestry, resources to support research are oſten limited. Collaboration, technology and knowledge transfer, and funding are strongly linked and are important to generating new knowledge through research. Collaboration Collaboration is the fastest and most efficient way to develop and build our understanding of tree biomechanics. Teams working together can oſten accomplish more than the same individuals working separately. Collaboration encourages exploring and sharing new ideas, and larger, more comprehensive projects. This collaboration will benefit arborists, researchers, the tree care industry, property owners, and trees. A Tree Biomechanics Working Group or collaboration needs to be developed to advance large projects. This would be best housed in an existing arboricultural or academic association. Joint research projects, conferences, and events that allow sufficient time for informal discus- sions have opened communication channels and fostered further collaboration among tree bio- mechanics researchers within the field. However, greater attention does need to be given to work- ing and creating links with researchers from other disciplines, such as forestry, engineering, physics, forest biology, wood products, biofuels, storm water, horticulture, and pomology. The benefits of multi- disciplinary research were recently illustrated by the successful application of National Aeronautics and Space Administration-developed technology in measuring surface strains to evaluate stress in trees and branches. Beyond collaboration among academics, the inclusion of practicing arborists as contributors to the research process is just starting to be explored by researchers. This mix of researchers
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