Arboriculture & Urban Forestry 48(4): July 2022 properties, while partially confounding the results due to the effect of that individual tree’s crown and root architecture. In conclusion, a tree’s biophysical properties, includ- ing stem, crown, and root characteristics, have been found to dictate how trees resist loads, whether from self-weight or wind, snow, or ice loads (Niklas 2000; Niklas and Spatz 2000; Peterson and Claassen 2013). These biophysical properties have a more significant effect on wind-induced stem-stress intensities than the shape of the wind-speed profile (Niklas and Spatz 2000). Furthermore, the literature has suggested fail- ures are more likely as tree size increases (Reilly 1991; Duryea et al. 2007; Peterson 2007; Kane 2008). Root systems also play a vital role in tree stability, and decay is a major component of the likelihood of failure of a given tree (Smiley et al. 2017). Soil Type and Properties Soil type and soil conditions are factors which affect the load-bearing capacity of a tree’s root system (Moore 2000; Dupuy et al. 2005; Ji et al. 2006; Ow et al. 2010). The most crucial region appears to be the soil-root plate, and its depth is particularly important in sandy or clay soils (Ji et al. 2006; Dupuy et al. 2007). In the trenching study by Smiley (2008), the side of the tree where the roots were cut had an influ- ence when soil was water saturated, but not under dry conditions. This demonstrates the importance of soil conditions (e.g., type, texture, and moisture content) in the process of windthrow and how soil plays an integral role in the soil-root plate and tree stability. Precipitation Saturated soils exacerbate wind-caused failure rates (Peterson 2007). Thinning (pruning) of an individual tree helps prevent snow and/or ice damage but may have repercussions related to wind regimens and the wind exposure of neighboring trees (Peltola et al. 1999; Kane 2008; Peterson and Claassen 2013). Snow and ice loads cause the static loading of trees and may help explain the vast difference in likelihood of failure of deciduous trees, due to phenological dif- ferences of leaf-on load interception and leaf-off load interception (Ciftci et al. 2014a; James et al. 2014; Dahle et al. 2017). When snow or ice loads are inter- cepted in tandem with wind loading, elevated likeli- hoods of failure are to be expected. Research has incorporated both wind and snow/ice loads into their models, but there is little empirical evidence detailing 251 the relationship of combined wind and snow/ice loads (Peltola et al. 1999; Niklas and Spatz 2000; Gaffrey and Kniemeyer 2002; Luley and Bond 2006; Ciftci et al. 2014a). Wind The literature has suggested failures are more likely as tree size and wind speed increase (Duryea et al. 2007). Niklas (2000) suggested that wind is likely the most common causal factor of tree failure, and wind was described as the most prevalent dynamic force on trees in the terrestrial environment (Niklas 1992). Wind gusts may initiate more failures than a constant wind speed, since gusts cause additional crown dis- placement (Milne 1988). Additionally, changes in the local wind regimen, through the removal or failure of neighboring trees in the stand, will result in higher likelihood of failure of remaining trees due to increased exposure to wind forces (Peltola et al. 1999; Kane 2008; Peterson and Claassen 2013). Further- more, stem taper, canopy shape, and canopy size also possess a more significant effect on wind-induced stem-stress intensities than the shape of the wind- speed profile (Niklas and Spatz 2000). The fluid pressure of wind increases with the square of wind velocity (Francis and Gillespie 1993). Thus, the severity of wind damage to trees can be explained by relatively small increases in wind speed (Francis and Gillespie 1993). Instantaneous wind speeds are rarely available and average wind speed may be calculated over either 10-minute or 1-hour intervals (James et al. 2014). Wind-gust speed is described as an average wind speed, though taken over a 3-second interval (Holmes 2007). The lack of consistent reporting methods and measures of wind can be an obstacle to disseminating knowledge for practical tree-risk management (Cullen 2002). Predictive mechanistic modeling studies have shown the CWS for a vast number of tree species to exist between 36 and 234 km/h, with many species failing by roughly 180 km/h (Suzuki et al. 2016; Virot et al. 2016). Francis and Gillespie (1993) observed that wind-induced tree damage was not present below about 60 km/h, damage increased rapidly as gust speeds increased from 60 to 130 km/h, and beyond 130 km/h variability in damage increased dramati- cally. The wind speed necessary to cause tree failure will vary depending on tree species, growth pattern, and location (James et al. 2014). Yet, trees generally cannot weather violent storms with mean wind speeds ©2022 International Society of Arboriculture
July 2022
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