Arboriculture & Urban Forestry 40(6): November 2014 313 Failure Mechanism Research Priorities * What are the most common mechanisms of failure? * What are the failure mechanisms associated with specific defect types? * Is there species variation in failure mecha- nisms? * How do the environmental factors affect failure mechanisms? * What factors are involved in Sudden Branch Drop? * Can we calculate a safety factor for anchor- age that accounts for variations in soil moisture conditions? Urban soil conditions, such as soil compac- tion, poor soil structure, and lack of water, oxygen, and nutrients severely limit root system growth. Barriers, such as curbs, pavement, and retaining walls prohibit horizontal root expansion. Life- long structural weaknesses in root systems can begin in the nursery as a consequence of produc- tion practices. For example, lateral root growth in container-grown trees is impeded by the con- tainer walls. As they grow outward, roots are deflected down in a circling pattern or kink back in toward the trunk. As roots grow and become woody, permanent defects are created that reduce stability. When planted in the landscape, these compact, circling root systems are partially responsible for uprooting during strong storms. Decay compromises the structural integrity of trees and predisposes them to failure. Susceptibil- ity to decay and the rate of decay progression in trees varies greatly by species and environmental conditions. Future research should investigate how cultural practices, such as pruning, affect likeli- hood of attack by decay, causing fungi at the species level, and attempt to quantify strength loss based on the species of tree and decay-causing organism. Poor tree structure, including defects such as codominant stems and included bark, increase the likelihood of branch and crown failure. Targeted pruning can improve tree structure and reduce crown exposure to natural forces. Reduction prun- ing of branches slows their growth rate. This helps decrease the branch-to-trunk diameter ratio (aspect ratio) over time. Branches with a lower aspect ratio tend to be strongly attached to the trunk. Conversely, branches with a large aspect ratio and codominant stems are more weakly attached to the tree. Codom- inant stems oſten have a narrow angle of attachment and tend to develop bark inclusions, which further reduces attachment strength. Codominant stem fail- ure is one of the most common types of crown and branch failure of urban trees. More work is needed to develop models that help practitioners predict attachment strength based on visible external char- acteristics such as aspect ratio and included bark. Numerous studies have catalogued tree damage following storms. Most have focused on categorizing species susceptibility to failure by ranking them into damage categories (e.g., high, medium, and low). Although reports of susceptibility by species vary greatly, certain factors, such as weak wood (i.e., low modulus of elasticity/modulus of rupture) and poor form (e.g., trees with a tendency to pro- duce codominant stem or have branches with nar- row angles of attachment), may predispose some species to storm damage. Post hoc storm damage surveys are oſten limited, however, because impor- tant characteristics, such as branch orientation, morphometric data, and presence of decay are oſten lost during clean-up operations. Capturing this data would help researchers improve method- ologies in studies that examine failure mechanisms. Summer branch drop (SBD) (aka sudden branch drop) is the phenomenon where seemingly healthy, undamaged branches break and fall from trees, usu- ally on hot, dry, calm summer aſternoons. Break- age usually occurs away from the branch union. The exact mechanism for SBD is unclear, though it is believed that water loss through transpiration is involved. The literature is apparently devoid of empirically tested SBD, presumably because the random nature of the phenomenon makes it almost impossible to predict when it will occur. Having a better understanding of the failure mechanisms involved with SBD could lead to preventive treat- ments that help minimize this phenomenon. Although researchers have employed static and to measure forces on trees, dynamic tests there is little information on how these forces trans- late into natural forces that act on landscape trees. Research relating the forces exerted on trees from ©2014 International Society of Arboriculture
November 2014
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