Arboriculture & Urban Forestry 40(1): January 2014 1. Forestry – (trees in groups) economic damage on plantation grown trees, 2. Open-grown trees – (individual trees with branches, both excurrent and decurrent) tree stability and risk assessment, predominantly in urban areas, 3. Wind tunnels – small trees to measure drag coefficients in constant velocity winds, and 4. Modeling – (dynamic models of trees) com- puter studies, finite element methods and mathematical modeling. Forestry Forestry studies examine plantation trees (mainly conifers) and the economic losses caused by dam- aging winds (Moore and Maguire 2005; Peltola 2006), usually with the aim to determine thresh- old values of storm damage. Threshold values include wind speed, gustiness, duration of storm, terrain, soil type, soil moisture, stand characteristics (e.g., height, density, diameter at breast height, crown length), and the physical condition of a tree (Mayer 1987). The threshold value of wind speed at which damage to trees occurs, termed the critical wind speed, is an important variable for forest managers (Peltola 2006) and in forest modeling (Gardiner 1995; Moore and Maguire 2004; Frank and Ruck 2008). The factors of site, tree species, soil, wind climate, critical wind speed, and sil- vicultural treatments, such as thinning, are con- sidered together in order to calculate the risk of damage to a naturally regenerated forest or plan- tation. The forestry studies estimate the percent damage to a group average of trees, rather than explicitly predicting failure of any individual tree. To predict the percentage of trees in a forest stand likely to fail during a storm, mechanistic models have been developed (Gardiner et al. 2008; Frank and Ruck 2008; Schelhaas 2008; Wood et al. 2008). The models calculate the critical wind speed required to break or overturn trees, and then deter- mine the probability of damage at the geographic location of the trees, based on some assessment of local wind climatology and empirical relation- ships. Models have been shown to be valid in cer- tain circumstances (Gardiner et al. 2000), but their deterministic nature is sometimes at odds with field observations of wind throw. By defini- 7 tion, the models are restricted to excurrent trees in plantations and are really an application of stat- ics to a dynamic phenomenon, and so are not yet applicable to open-grown trees in urban areas. There has been considerable work using static pulling tests, mainly on forest conifers (Nicoll et al. 2006) and on small and young trees (Lund- ström et al. 2007) with a high slenderness ratio usu- ally above 50 and oſten over 100 (e.g., slenderness values [46-136] Hale et al. 2010; [58-94] Jonsson et al. 2006). Tree-pulling tests have had an impor- tant role in providing valuable information on mechanical stability of trees of varying size and tree species, and the information is useful in mecha- nistic modeling, but the simulation of static load- ing by tree pulling alone is not enough to explain the mechanical stability of trees (Peltola 2006). The critical wind speeds that cause failure depend on tree species, growth pattern, and location, and estimates vary. However, ultimately few tree species can survive violent storms with mean wind speeds over a period of 10 minutes, exceeding 30 m s-1 the top of the canopy without damage (Peltola 1996a). near Open-grown Trees The distinction between open-grown trees and forest trees is made in this review because of dif- ferences in the growth and form of the trees, par- ticularly with respect to their canopy architecture. Research on dynamic response of forest trees may not be applicable to open-grown trees that develop a complex distribution of branch masses. When applying dynamic methods to tree sway in winds, recent research has indicated that the branches and the form of the tree are important in understanding how trees respond in winds (James et al. 2006; Spatz et al. 2007; Rodriguez et al. 2008; Sellier and Four- caud 2009; Theckes et al. 2011; Ciſtci et al. 2013). Research on open-grown trees in winds aims to understand how individual trees respond in winds, and investigates aerodynamic properties (Baker and Bell 1992; Roodbaraky et al. 1994; Baker 1997; Ennos 1999), dynamic properties of frequency and drag (Kane and Smiley 2006; Kane and James 2011), effect of pruning dose on wind response (Gil- man et al. 2008a; Gilman et al. 2008b; Pavlis et al. 2008), and wind loads (James 2006; James 2010). There is very little data on the wind loading of open-grown trees during storms, and much of what ©2014 International Society of Arboriculture
January 2014
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