152 Dahle et al.: Branch Strength Loss Implications for Silver Maple judged that the differences were minimal and all cross- sections were considered circular. The control branches in this study had a failure stress value of 35.2 MPa (5,105 lb/in2), which is similar to the modulus of rupture, a measurement of stress, reported by the U.S. Forest Products Laboratory (1999) of 40 MPa (5,800 lb/in2) for green silver maple wood. This comparison suggests that branch wood has a strength property similar to trunk wood. Watersprouts were found to be 49% weaker than naturally occurring branches, confirming that silver maple water- sprouts are weaker than natural branches. Because stress is a measurement of force per unit area, it is not surprising that stress did not vary with diameter in the control branches. As the area of wood increased, the force required to break the branch increased proportionally. How- ever, for watersprouts, stress at failure decreased as diameter increased (Figure 4). This is most likely the result of a com- bination of the inherent weak nature of watersprouts and the greater likelihood of decay in larger (and thus older) water- sprouts (Figure 6). Although the amount of decay was ac- counted for when calculating stress (the cross-sectional area of decay and moment of inertia of decay were removed), this may not fully account for the presence of decay. A probe was used to identify the extent of decay and it may be that the amount of decay in the wood matrix was underestimated, further reducing the amount of sound wood present. Additionally, stress at failure decreased as the ratio of de- cayed wood:branch diameter increased in the watersprouts (Figure 5), suggesting that the presence of decay is correlated with strength loss. It may be that the nature of the failure changes as the size of the decay column increases. In a solid branch, the failure typically begins with outward compressive buckling on the lower side followed by tensile failure on the top. The possibility of compressive buckling becomes greater as the decay fungi invade the remaining wood matrix on the outer portions of the watersprout. Additional research is needed to determine the influence of decay on the strength property of the wood matrix. The presence of decay is often considered a likely conse- quence of heading cuts; indeed, Dahle et al. (2006) previously reported that 90% of silver maple leader with heading cuts contained decay. This study found internal decay in 52% of the watersprout failures and in none of the control branches. Internal decay occurred in 28% of the entire watersprout population. This contrasts with Gilman and Knox (2005) who did not see decay in crape myrtle (Lagerstroemia × ‘Natchez’) sprouts arising after heading cuts. Silver maple heartwood is considered a poor compartmentalizer (Giles 2001), whereas Gilman and Knox (2005) suggest that that ‘Natchez’ crape myrtle is good. It is likely that decay in these silver maples originated in the leader at or near the location of the heading cut, breached barrier walls, and moved into the watersprouts. If decay can move into rapidly growing water- ©2006 International Society of Arboriculture sprouts with large growth rings, it is likely to spread rapidly. Thus, less force will be required to induce failure in water- sprouts, further reducing their weak nature. Additional re- search is needed to determine the mode of action for decay movement from leaders into watersprouts. IMPLICATIONS The information presented here should aid utility arborists in making better informed decisions about the risk factors in the trees they manage. The conversion from round-over trimming to V-trimming does appear to predispose trees to failures. Watersprouts were found to be 49% weaker than naturally occurring silver maple branches and strength appeared to de- crease as the watersprouts grew in size. Utility arborists should examine their trees for the presence of decay when considering whether a tree is at risk of failure, especially when larger watersprouts are present. Managers may want to consider the long-term consequences of converting from round-over pruning to V-pruning and decide if other actions are appropriate, including the removal of trees subjected to multiple cycles of round-overs. This information is not limited to utility arboriculture. Many trees in urban areas suffer from topping. All arborists should be concerned with the negative aspects of topping. These data suggest that management alternatives might be limited after silver maples are topped, particularly when de- cay is present in the leaders. Acknowledgments. We thank Purdue University, the Department of Forestry and Natural Resources, and the TREE Fund for financial support. PSI Cinergy and Tipmont REMC donated tree crew time and The Care of Trees donated equipment used during this research. Joe Busch completed sketch portions of the drawings. Bruce Fraedrich and Tom Smiley, Bartlett Tree Research Laboratories, helped develop the initial goals of this research. Finally, Judy Santini provided statistical advice and Jason Grabosky reviewed the manuscript during preparation. LITERATURE CITED American National Standards Institute. 2001. American Na- tional Standards for Tree Care Operations—Tree, Shrub, and Other Woody Plant Maintenance—Standard Practices (Pruning) (A300, Part 1). Tree Care Industry Association, Inc., Manchester, NH. Dahle, G.A., H.A. Holt, W.R. Chaney, T.M. Whalen, D.L. Cassens, R. Gazo, and R.L. McKenzie. 2006. Implications of branch strength loss in silver maple trees converted from round-over to V-trims during electrical line clear- ance operations. Proceedings of the Environmental Con- cerns in Rights-of-Way Management Eighth International Symposium. (In press).
July 2006
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