284 Kane: Branch Strength of Bradford Pear al. 2000) applied the breaking load farther from the branch attachment point, it appears that all of these studies applied the load in such a way as to ensure that either the attachment or a section of the branch just beyond it would fail. Lateral branches of similar size to the parent branch presumably in- troduce the same problems of weak attachment that arise when a branch grows relatively large compared with the trunk (Shigo 1985). Thus, large lateral branches may serve as a point of weakness along the length of a parent branch. Although there is evidence to support (Putz et al. 1983; Jim and Liu 1997; Francis 2000) and reject (Hauer et al. 1993; Lilly and Sydnor 1995; Smiley et al. 2000; Farrell 2003) the notion that wood properties affect the likelihood of tree or branch failure, none of the branch breaking studies has mea- sured wood properties other than specific gravity from branches that were broken. Although specific gravity can predict wood strength, modulus of rupture (MOR) is still quite variable (USDA 1999). There is merit in measuring wood properties directly from the broken branches as op- posed to using an average test value from the Wood Hand- book (USDA 1999). The objectives of this study were to determine the breaking stress of branches of Bradford pears and to identify tree char- acteristics, including wood properties, that influence the strength of a branch attachment. A secondary objective was to determine the effect of assuming branch cross-sections as circular when calculating breaking stress of a tree branch. METHODS AND MATERIALS Bradford pear trees from two sites (Table 1) were tested: (1) the Virginia Tech campus in Blacksburg, Virginia, U.S., and (2) State Road 340 in Waynesboro, Virginia, U.S. Ten branches from three trees were tested from the site in Blacks- Table 1. Dendrometric information for trees used in the current study.z Site Tree no. LR 1 2 3 WB 1 Branches tested 1 3 6 2 22y 3 4 5 6 7 3 3 2 3 2 Tree dbh (cm) Range of branch diameter (cm) 26.7 13.6 32.0 7.1–11.6 35.6 7.8–13.6 30.7 14.6–17.8 29.1 16.6 26.7 10.6–15.4 32.8 11.3–13.7 32.3 9.7–12.1 29.1 9.7–13.7 31.9 9.2–13.3 Range of attachment angle (°) 47 26–36 15–26 38–58 15 37–40 30–40 30–61 13–50 33–53 zTrees came from two sites in Virginia, on the Virginia Tech campus (LR) and in Waynesboro (WB). yOnly one branch is included in the analysis because the second branch did not fail. ©2007 International Society of Arboriculture burg and 17 branches from seven trees were tested from the site in Waynesboro. One branch from the site in Waynesboro was too large and did not break, so it was not included in the analysis. Branches were selected on ease of testing and mea- suring. For example, at each site, space limitations restricted where the winch to pull branches could be located. Trees from the site in Blacksburg were spaced ≈4 m (13.2 ft) apart growing on the south side of and ≈5 m (16.5 ft) from a brick building. There were no obvious root obstructions at the site. Trees from the site in Waynesboro were growing ≈10 m (33 ft) apart in a boulevard median ≈5 m (16.5 ft) wide and 500 m (1650 ft) long. Trees from the site in Waynesboro had been watered and pruned as part of routine town maintenance; at the site in Blacksburg, trees had not received the same level of care. Trees at both sites had previously lost branches as a result of storms. A 2.5 cm (1 in) wide polyester webbing sling was girth- hitched to each branch to be tested. The sling was attached at least 1 m (3.3 ft) from the trunk and, in almost every case, distal of at least one lateral branch with a minimum diameter of one-third the diameter of the branch being pulled. A steel shackle connected the sling to a load cell (model L2356, 11,340 kg [24,948 lb] capacity; Futek Advanced Sensor Technology, Irvine, CA). The load cell was attached to an- other shackle that was connected to a 0.95 cm (0.38 in) steel cable. The cable was connected to a winch that applied the load at roughly 0.4 m/s (1.3 fps) (model # XD9000i; Warn Industries, Clackamas, OR). The winch was activated until the branch completely failed, usually within 5 to 10 seconds of applying the load. The load was always applied in a di- rection perpendicular to the branch bark ridge between the branch being tested and the trunk. The load cell measured tension in the cable two times per second; the data were collected by a data logger (ModuLogger™; Logic Beach, La Mesa, CA) and then downloaded into Microsoft Excel for processing. In addition to applied force (i.e., tension in the cable), the following measurements were recorded on each tree and branch: branch diameter at load point, branch diameter at trunk, trunk diameter above branch attachment, branch diam- eter at failure, inside bark branch depth (parallel to direction of applied load) and width (perpendicular to direction of ap- plied load) at the point of failure, angle of attachment be- tween the branch and the trunk, angle between the cable and the branch at failure, distance from applied load to the trunk, and distance from applied load to failure. Digital images were taken of the cross-sections of failed branches from the Waynesboro site but not the Virginia Tech site. Branch depth and width outside bark were measured from these images. Failure was categorized by type. If more than 50% of the failed fibers originated in the branch, it was categorized as a branch failure; if fewer than 50% of the failed fibers origi- nated in the branch, it was categorized as an attachment fail-
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