Arboriculture & Urban Forestry 48(4): July 2022 a second storm, at the same location. Furthermore, studies utilizing more sophisticated statistical tools and multiple storms or multiple location model vali- dation methodologies are needed. Factors Which Influence Failure Across all methodologies, certain factors which con- tribute to tree failure have been illuminated. In this section we will discuss the factors that have been related to tree failure across all methodological approaches, including: tree stems, tree crowns and branches, root systems, soil type and properties, pre- cipitation, and wind (Figure 2). Stems Post-storm study literature has suggested failures are more likely as tree size and wind speed increase (Duryea et al. 2007). Kane (2008) observed an increase in likelihood of failure of trees with a greater diameter as well as taller trees. Peterson (2007) also observed that as tree diameter increased so did the risk of tree failure. Additionally, Kabir et al. (2018) found that the probability of failure for a tree increased for tall trees, though the height used to determine “tall” was not provided. The model used by Kabir et al. (2018) found that trees with smaller DBH were more likely to experience failure, which is incongruent 249 with most current literature (Kabir et al. 2018). Kabir et al. (2018) also found that height and DBH had large influences on a model’s predictions, whereas the removal of nearby trees had a relatively small effect on the model’s prediction (Kabir et al. 2018). Despite the general correlation of increased tree size and increased likelihood of failure, multiple stud- ies have found that tree size and wind-gust speeds by themselves cannot explain the variation in failure rates for different tree species (Francis and Gillespie 1993; Kane 2008). Yet, despite the unexplained vari- ation within species, simplified methods for estima- tion of uprooting and stem breakage have been described (Gardiner et al. 2008; Kane 2008). In addi- tion, Lundström et al. (2007) found that 75% of the variation of the turning moment in the soil-root plate was explained by tree mass, trunk mass, trunk diam- eter, or tree height, either alone or in combination, during static loading. Decay is a major component of the likelihood of failure of a given tree (Smiley et al. 2017). Decay causes moment capacity loss in tree branches and stems (Dahle et al. 2006; Ciftci et al. 2014b), and the severity and location of decay are the factors which determine the effect of decay on likelihood of failure (Luley et al. 2009). The study by Kane (2008) found that most trunk failures (76%) involved a defect and that about half (56%) of the trunk failures were visi- ble prior to failure. However, currently, the detection of decay through remote-sensing means does not appear to be feasible, and as such the full relationship of decay and likelihood of failure will not be reviewed here. Instead, see Dahle et al. (2014), Ciftci et al. (2014b), or Kane (2008) for a more complete review. Crown and Branches Figure 2. Common factors in studies that investigated tree failures. Crown size and shape has been generally found to play a significant role in how trees resist wind, snow, and ice loads (Niklas and Spatz 2000; Gaffrey and Kniemeyer 2002). Furthermore, stem taper, canopy shape, and canopy size have a more significant effect on wind-induced stem-stress intensities than the shape of the wind-speed profile (Niklas and Spatz 2000). Gaffrey and Kniemeyer (2002) found that a crown-volume reduction of 50% reduced sail area by 18%, which caused a stress reduction of 15% to 24% (Gaffrey and Kniemeyer 2002). Yet, in the same study, an asymmetric crown (Figure 3) reduction resulted in a mid-crown increase in stress of up to 25%, which may have implications for UVM ©2022 International Society of Arboriculture
July 2022
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