Arboriculture & Urban Forestry 34(5): September 2008 315 reported was considerably less than reported here. That disparity may be the result of Gilman (2003) testing much smaller branches, consisting predominantly of juvenile wood. There have been no previous studies of the strength of branch attach- ments of sawtooth oak. Pfisterer’s (2003) report of longer dura- tions of failure for narrow attachments compared with wide at- tachments of hazel (Corylus avellana L.) was not observed in the current study for any species. CONCLUSIONS Although the results from this and previous studies on predicting breaking stress largely agree on the importance of the ratio of branch to trunk diameter, the lack of strong coefficients of de- termination undermines one’s ability to predict failure with cer- tainty. Similarly, predicting breaking stress or the likelihood of failure from intuitively obvious factors such as inherent wood strength has also proved to be only somewhat reliable and spe- cies-dependent. In light of these ideas, arborists and urban for- esters should conservatively assess tree risk with respect to branch attachment strength. In the field, practitioners should consider the strength of codominant stems and attachments where branch diameter is roughly 70% of trunk diameter to be roughly one-half as strong as an attachment that includes a clearly subordinate branch. . Acknowledgements. We thank two anonymous reviewers for sug- -gesting helpful changes to earlier drafts of this manuscript. LITERATURE CITED Francis, J.K. 2000. Comparison of hurricane damage to several species of urban trees in San Juan, Puerto Rico. Journal of Arboriculture 26:189–197. Gilman, E.F. 2003. Branch-to-stem diameter ratio affects strength of attachment. Journal of Arboriculture 29:291–294. Green, D.W., J.E. Winandy, and D.E. Kretschmann. 1999. Mechanical properties of wood. In: Wood Handbook—Wood as an Engineering Material. Gen. Tech. Rep. FPL-GTR-113. USDA Forest Service For- est Products Laboratory, Madison, WI. 463 pp. Hauer, R.J., W. Wang, and J.O. Dawson. 1993. Ice storm damage to urban trees. Journal of Arboriculture 19:187–194. Jim, C.Y., and H.H.T. Liu. 1997. Storm damage on urban trees in Guangzhou, China. Landscape and Urban Planning 38:45–59. Kane, B. 2007. Branch strength of Bradford pear (Pyrus calleryana var. ‘Bradford’). Arboriculture and Urban Forestry 33:283–291. Kane, B., and P. Clouston. 2008. Tree pulling of large shade trees in the genus Acer. Arboriculture and Urban Forestry 34:101–109. Lilly, S., and T.D. Sydnor. 1995. Comparison of branch failure during static loading of silver and Norway maples. Journal of Arboriculture 21:302–305. MacDaniels, L.H. 1923. The Apple-Tree Crotch. Bulletin 419. Cornell University Agricultural Experiment Station, Ithaca, NY. 22 pp. ———. 1932. Factors affecting the breaking strength of apple tree crotches. Journal of the American Society for Horticultural Science 29:44. Miller, V.J. 1959. Crotch influence on strength and breaking point of apple tree branches. Journal of the American Society for Horticultural Science 73:27–32. Muller, U., W. Gindl, and G. Jeronimidis. 2006. Biomechanics of a branch–stem junction in softwood. Trees (Berlin) 20:643–648. ©2008 International Society of Arboriculture Pfisterer, J.A. 2003. Towards a better understanding of tree failure: Investigations into bending stresses of branch junction and stems of hazel (Corylus avellana L.) as a model organism. Mitteilungen- Biologischen Bundesanstalt fur Land und Forstwirtschaft 394: 272–279. Putz, F.E., P.D. Coley, K. Lu, A. Montalvo, and A. Aiello. 1983. Up- rooting and snapping of trees: Structural determinants and ecological consequences. Canadian Journal of Forest Research 13:1011–1020. Shigo, A.L. 1985. How branches are attached to tree trunks. Canadian Journal of Botany 63:1391–1401. Simpson, W., and A. TenWolde. 1999. Physical properties and moisture relations of wood. In: Wood Handbook—Wood as an Engineering Material. Gen. Tech. Rep. FPL-GTR-113. USDA Forest Service For- est Products Laboratory, Madison, WI. 463 pp. Smiley, E.T. 2003. Does included bark reduce the strength of codomi- nant stems? Journal of Arboriculture 29:104–106. Smiley, E.T., C.M. Greco, and J.G. Williams. 2000. Brace rods for codominant stems: Installation location and breaking strength. Jour- nal of Arboriculture 26:170–176. Brian Kane (corresponding author) University of Massechusettes-NRC 126 Holdsworth Hall Amherst, MA 01003, U.S.
[email protected] Robert Farrell Virginia Department of Forestry
[email protected] Shepard M. Zedaker Professor Department of Forestry Virginia Tech University Blacksburg, Virginia J.R. Loferski Professor Department of Wood Science and Forest Products Virginia Tech University Blacksburg, Virginia D.W. Smith Shelton H. Short Jr. Professor Emeritus Department of Forestry Virginia Tech University Blacksburg, Virginia
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