218 Miesbauer et al.: Effects of Tree Crown Structure on Dynamic Properties Arboriculture & Urban Forestry 2014. 40(4): 218–229 Effects of Tree Crown Structure on Dynamic Properties of Acer rubrum L. ‘Florida Flame’ Jason W. Miesbauer, Edward F. Gilman, and Mihai Giurcanu Abstract. Knowledge of tree dynamic properties is important to improve one’s ability to assess tree risk. Pull-and-release tests were per- formed on 16 Acer rubrum L. ‘Florida Flame’ trees in summer and winter over a two-year period, and natural frequency and damping ratios were calculated. One year prior to testing, trees were designated as either excurrent or decurrent and pruned to impose that form. During summer tests, trees were pruned to maintain designated form, and tests were performed before and aſter pruning. Trees were then systematically dissected to measure morphological and allometric characteristics. Excurrent trees had a higher natural frequency than decurrent trees in summer and winter, and pruning in summer increased the frequency of excurrent trees more than decurrent trees. Tree form and pruning had little effect on damping ratio. Decurrent trees had a larger percent of their branch mass in the top half of the crown than excurrent trees, which would subject them to larger wind-induced stress on their trunks and increase the risk of failure. Key Words. Acer rubrum; Damping; Decurrent; Dynamics; Excurrent; Natural Frequency; Pruning; Red Maple; Tree Biomechanics. Every year, thousands of trees are impacted by strong storms, causing severe property damage, electric outages (Simpson and Van Bossuyt 1996), human injuries, and fatalities (Schmidlin 2009). Factors such as tree form, size, condition, species, wind speed, pruning, and wood material properties impact tree resistance to windstorm damage (Putz et al. 1983; Francis 2000; Duryea et al. 2007).Trees growing in groups had a higher rate of survival than trees growing individually (Duryea et al. 2007), and removal of nearby trees predisposed surviving trees to failure by exposing them to wind forces to which they were not adapted (Gardiner et al. 1997). Knowledge of tree dynamic properties (i.e., natu- ral frequency and damping) is important to improve one’s ability to assess tree risk. Natural frequency (f) refers to the number of cycles a tree sways over a period of time and is expressed as a reciprocal of time. Damping is expressed as a ratio (ζ), measur- ing how well a tree is able to reduce the amplitude of its oscillation as it sways. A high damping ratio indicates that a tree is able to dissipate energy effi- ciently, which may reduce likelihood of uproot- ing or trunk failure in wind (Sellier and Fourcaud 2009). Previous studies showed linear (Mayhead ©2014 International Society of Arboriculture 1973; Moore and Maguire 2004; Jonsson et al. 2007; Kane and James 2011), or curvilinear (Sugden 1962; Kane and James 2011) relationships between f and the ratio of tree trunk diameter to height (DBH/Ht or DBH/Ht2 ζ than when they were leafless (Roodbaraky et al. 1994; Baker 1997; Kane and James 2011). Also, f was higher when temperatures were below freezing (Sug- den 1962; Granucci et al. 2013), likely due to stiffer wood at lower temperatures (Lieffers et al. 2001). Several studies have examined the impact of pruning on tree response to wind loads. Gilman et al. (2008a) reported that light crown thinning of young trees was less effective at reducing trunk movement than reducing, raising, lion-tailing, or structurally pruning, and there was no difference among the latter four pruning types at wind speed of 26.8 m/s. Upper trunk movement was less for young trees with crowns that were more heavily thinned or reduced than for those that were raised, when subjected to straight line winds (Gilman et al. 2008b). Other studies found that reduction pruning was more effective in reducing bending moment than thinning or raising (Kane and Smiley 2006). Reduction pruning increased f more than ). Trees in leaf had a lower f and higher
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