310 research. Two broad themes permeated the planning and hosting of the event. First, there is a need to improve our current understand- ing of tree failure. Secondly, this knowledge must serve as a foundation for improving our best management practices for tree risk assess- ment and maintenance to mitigate tree failure. At the close of the summit, participants gener- ally agreed that economic damages and loss of life due to weather induced tree failure could be reduced by improving arboricultural prac- tices once research improves our understanding of tree stability and storm-induced failure. To improve current risk assessment and miti- gation efforts, researchers and industry profes- sionals must understand tree structure, stability, and biomechanics. Trees with and without signs of defects or weakness can fail, and assess- ment methods must consider both. The par- ticipants at the Tree Biomechanics Summit identified five critical focus areas requiring research: 1. Assessing the likelihood of failure in trees 2. Modeling the impact of mechanical loading on trees 3. Understanding the mechanisms and modes of tree failure 4. Tree growth response to mechanical loads 5. Effectiveness of mitigation practices FOCUS AREA 1: ASSESSING THE LIKELIHOOD OF FAILURE IN TREES Likelihood of failure is determined from the mo- ment the capacity of the tree (which is based on geometry and material properties) as well as the anticipated loads [which are related to the tree (e.g., leaf area, drag coefficient) and weather (e.g., wind velocity, gust frequency, and the type and amount of precipitation)]. Ideally, arborists would be able to assess some combination of potential applied load, tree form, strength (material properties), and deg- radation in stability and estimate the associated like- lihood of failure for the parameters observed. This formal assessment would follow a methodological sequence, allowing the arborist to evaluate the col- lective strengths and weaknesses of a tree in order to estimate the likelihood of failure for the whole tree or its specific parts. Unfortunately, we have lim- ited knowledge on many key questions, including: ©2014 International Society of Arboriculture Dahle et al.: Tree Biomechanics White Paper * What type of average and extreme loads (static and dynamic) will a tree encounter? * What is a desired form or structure of a given tree? * How strong does the wood need to be? * How extensive can decay be prior to failure? Mechanical Failure Research Priorities * Can we correlate observable tree defects with likelihood of failure? * What is the relationship between root architecture and tree stability? * Can existing data (tomography, etc.) be analyzed collectively to identify tree haz- ards confidence? * What is the link between static and dynamic methods to evaluate tree strength and behavior * Can or should we apply engineering speci- fications to trees? Furthermore, we are limited by our ability to directly measure many aspects of it is uncertain if tree stability or instability. Questions remain regarding the proper methods for assessing trees and their parts. Similarly, formal protocols can be developed that link perceived strengths and inherent weaknesses (e.g., defects) directly to an increased or decreased probability of failure. The arboriculture community has identi- fied defects (e.g, decay, cavities, included bark, codominant branches, crack, splits) that provide a visual indication of an elevated risk of tree failure. Beyond this, little research exists to quantify the rates at which a defect increases the likelihood of failure. The ability to estimate the likelihood of failure from the severity of specific defects would improve the practice of tree risk assessment. The ability to measure concealed defects in trees is constrained by the rate of technological advance- ment within arboriculture and urban forestry and our ability to adapt and apply technologies cur- rently utilized by other fields for our purposes. The development of new tools should always be a goal for researches and practitioners. One promising technology currently used by some in the industry is tomography. As application of this technology by
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