260 Johnson et al.: Boulevard Tree Failures Table 1. Beaufort Wind Scale (abbreviated). National Weather Service—National Oceanic and Atmospheric Administration (NOAA). www.weather.gov/ilx/swopwindscale. The Wind Scale (Force numbers 0-12) was abbreviated to begin with wind speeds that were relevant to tree damage. Beaufort # 6 7 8 9 10 11 12 Wind speed (mph) 25-31 32-38 39-46 47-54 55-63 64-73 74+ Wind force description Strong breeze Near gale force Gale force Strong gale force Storm Violent storm Hurricane growing season and parallel the paths of the predominant growing season winds. For example, trees lining east-west (approximately) streets in a community are more likely to suffer damage during wind loading events that move in approximately the same direction. In urbanized areas, especially older urbanized areas with mature trees and extensive infrastructure, tree damage follows the direct lines of the prevailing winds despite the friction offered by buildings. For instance, in at least one study, the extent of street trees damaged parallel to the wind direction was essentially twice the damage to street trees perpendicular to the prevailing winds (Lopes and Fragoso 2009). Site characteristics that can influence tree stability and resistance to failures include soil moisture, soil texture and structure, depth to water table, boulevard (the area between public sidewalks and street curbs) width and length, and friction (aka, frictional drag) (Wagar and Barker 1983; Mitchell 1995; Pokorny et al. 2003; Schindler et al. 2012). Friction or frictional drag is the effect that surface contact has on the speed and direction of (in this case) wind (WW2010 2010). The greater the surface areas that wind contacts, the greater the reduction of wind velocity. For instance, winds moving across a short-grass prairie or a parking lot experience some frictional drag, but not nearly as much as winds moving over and around large trees, for- ests, buildings or changes in topography. Acknowledging the impact of frictional drag, there remain inconsistencies connecting some of the other site factors to frequency of failures, especially whole tree fail- ures (partial or full wind throws). If soil conditions such as compaction, especially when combined with fine textures (e.g., clay) result in abnormally small root systems or poor fine root development (Rickman et al. 1965; Perry 1982; Costello et al. 1991; Nielson 2009), then it would follow that trees growing in those sites would be noticeably more vulnerable to full or partial windthrows (aka, tips). Dispro- portionate root systems resulting from varying degrees of ©2019 International Society of Arboriculture Tree impacts Larger tree branches moving Whole tree moves Twigs breaking off trees Slight structural damage occurs Tree broken up or small trees uprooted; considerable structural damage Moderate sized trees uprooted; large branches snapped off trees Large trees and branches downed soil compaction have been associated with higher frequen- cies of windthrows (Koisumi et al. 2007; Moore 2014), but to date, there is a dearth of research documenting the sig- nificance of compacted boulevard soil with the frequency of full or partial windthrows. Soil moisture content has been associated with whole tree failures (Mitchell 1995; Ray and Nicoll 1998; Kamimura et al. 2009; Schindler et al. 2012). Tree roots are opportu- nistic and proliferate in rhizospheres where both soil mois- ture and oxygen meet balanced, optimum levels (Perry 1982). Poorly drained soil reduces the friction between the soil and tree roots, hence making it more difficult for roots to “hold on” to the rhizosphere. Sites with high water tables, in particular water tables within 24 inches of the soil surface, result in atypically shallow root systems, hence less stability (Harris et al. 2004). Poorly drained to satu- rated sites are deficient in soil oxygen and therefore do not provide the minimum depth considered essential for stable tree root systems (Perry 1982; Mitchell 1995; Harris et al. 2004). Combining these two phenomena, uncharacteristi- cally shallow root systems with surface soils offering little friction to roots sliding through the soil, what results are trees that are more prone to whole tree failures. Boulevard width is the least quantified factor in terms of a direct linkage to the frequency of tree failures. Although it seems intuitive that trees disproportionate in size compared to their boulevard foot print would be more vulnerable to whole tree failures, little evidence exists to substantiate that, although it has been observed that as distance between the trees and the infrastructure (sidewalks) increases, whole tree failures decrease (Randrup et al. 2001). There are linkages between boulevard width, tree size (trunk diameter mea- sured at 4.5 feet [1.4 m] above ground, aka dbh), and whole tree failure rates; however that linkage combines two dis- tinct storm damage factors (site characteristic and tree characteristic) rather than distinguishing the impacts of a single factor (Wagar and Barker 1983; Lopes et al. 2008).
November 2019
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