Arboriculture & Urban Forestry 34(2): March 2008 Arboriculture & Urban Forestry 2008. 34(2):101–109. 101 Tree Pulling Tests of Large Shade Trees in the Genus Acer Brian Kane and Peggi Clouston Abstract. Shade trees provide many benefits but can cause damage if they fail. Despite the potential for costly litigation that sometimes arises when damage occurs, there are no investigations of bending moments and stresses involved in failure of shade trees. Twenty-four shade trees of three species in the genus Acer were pulled to failure at a suburban property in Massachusetts, U.S. The maximum load and distance to failure were used to calculate maximum bending moment; stress at the point of failure was calculated from bending moment and stem cross-sectional dimensions. No trees uprooted, and failures were categorized as either stem at a lateral branch(es) or the attachment of codominant stems. Failures of codominant stems required one-half of the stress of stem failures. Similarly, failures of codominant stems occurred at only 45% of wood strength, whereas stem failures occurred at 79% of wood strength. Prediction of maximum bending moment from tree morphometric data was more reliable than prediction of maximum stress from tree morphometric data. Prediction of maximum bending moment and stress was more reliable for stem failures than codominant failures. Results are compared with similar tests on conifers. Implications of findings are discussed with respect to risk assessment of shade trees. Key Words. Codominant stems; tree failure; tree pulling; trunk stress. Shade trees are an important part of communities because they provide many diverse benefits (Nowak and Dwyer 2000). Large shade trees provide greater environmental benefits than smaller trees (Nowak et al. 2002), but large trees can also provide a greater degree of risk. As trees mature, structural defects often necessitate tree removal for safety reasons. Assessing the risk of failure of shade trees has been hampered by a lack of empiric data, especially with respect to assessment of structural defects. Existing studies entail observations of failed and standing trees after storms (Cutler et al. 1990; Gibbs and Greig 1990; Smiley and Fraedrich 1992; Duryea et al. 1996; Jim and Liu 1997; Francis 2000) Without empiric data, practitioners face the di- lemma of deciding whether to remove a tree, balancing the ben- efits provided with the liability sometimes associated with fail- ure (Mortimer and Kane 2004). There are many reports of applying a load with a winch to pull forest trees (almost exclusively conifers) until they uprooted or the stem failed (Fraser 1962; Fraser and Gardiner 1967; Somer- ville 1979; Smith et al. 1987; Fredericksen et al. 1993; Papesch et al. 1997; Moore 2000; Peltola et al. 2000). Such studies have explored many aspects of tree failure such as which tree mor- phometric data best predict the critical turning or bending mo- ment to cause failure and the effect of soil type and silvicultural practices on the probability of stem or root failure. Peltola (2006) reviewed the literature and described how data from the studies have been used to develop models to predict tree failure (e.g., Peltola et al. 1999; Gardiner et al. 2000; Ancelin et al. 2004). Although the basic mechanical principles (such as determination of drag, bending moment, and stress) should apply equally well to forest or shade trees, it is not clear that results from forest conifers can be extrapolated to shade trees. Shade trees display an obviously different structure from for- est trees. In particular, crowns tend to be taller and wider, trunks are less slender, and branches are larger relative to the size of the trunk. Shade trees often develop codominant stems, which de- velop when the trunk divides into two stems of approximately equal size. Often, the attachment between the stems makes a narrow angle, forming a “v-shape” in which bark occlusion oc- curs. Codominant stems are a common defect of shade trees (Gibbs and Greig 1990; Jim and Liu 1997), particularly among maples (Terho and Hallaksela 2005). Previous tree-pulling stud- ies have often excluded trees with defects (Smith et al. 1987; Fredericksen et al. 1993; Meunier et al. 2002), but for shade trees, defects are common and quite relevant when assessing the risk of failure (Gibbs and Greig 1990; Terho and Hallaksela 2005). The main objective of this study was to provide baseline data pertinent to failure of large shade trees. Of particular interest were: 1) which tree morphometric data best predict the maxi- mum stress and bending moment; 2) whether codominant stems predispose shade trees to fail; and 3) whether stress at the point of tree failure is related to the modulus of rupture (MOR) of wood samples taken from the tree. Because the methodology replicated previous studies of forest conifers, a secondary objec- tive was to compare results with those studies to determine whether mechanistic models of tree failure developed for forest trees could be applied to shade trees. METHODOLOGY Three common species were tested in this study, Norway (Acer platanoides L.), red (Acer rubrum L.), and sugar (Acer saccha- rum Marsh.) maples. Each has been planted extensively along streets and in residential yards in the Northeast and Mid-Atlantic states of the United States. In total, 30 trees were pulled to failure, but data from six trees were not collected properly as a result of a malfunction of the data logger. The trees were part of the landscaping of an institutional property in Belchertown, MA (USDA hardiness zone 5a). All trees had been planted as street trees, approximately 20 m (66 ft) apart from one another and 2 m (6.6 ft) from paved roads. This arrangement presumably re- stricted root growth under the road, but the remainder of the root system appeared to be unrestricted, although underground utili- ties were present in some cases. Because no trees failed by ©2008 International Society of Arboriculture
March 2008
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