148 Dahle et al.: Branch Strength Loss Implications for Silver Maple Branch Strength Loss Implications for Silver Maple (Acer saccharinum) Converted from Round-Over to V-Trim Gregory A. Dahle, Harvey H. Holt, William R. Chaney, Timothy M. Whalen, Daniel L. Cassens, Rado Gazo, and Rita L. McKenzie Abstract. Trees converted from round-over to V-trims could develop a canopy of potentially weak watersprouts growing on parent stems with a high probability of decay. This study was conducted to determine if the conversion from round-over to V-trimming predisposes silver maple (Acer saccharinum L.) trees to failure, and if decay is more likely to occur in watersprouts arising after heading cuts than in branches arising from lateral buds. Watersprouts were mechanically pulled from converted trees and found to be 49% weaker than normally occurring branches. Branch strength at failure (stress) decreased as watersprouts grew in size, and decay was present in over one-half of the watersprout at the point of failure. The conversion from round-over to V-trimming appears to predispose silver maple trees to failure. Key Words. Decay; line clearance; round-over; silver maple; strength loss; stress; topping; utility arboriculture; V- trimming; watersprout. Trees in utility rights-of-way require repeated cycles of trim- ming to maintain safe, reliable electric power. Tree crews are limited in the types of trimming that can be used to maintain conductor clearance for trees growing directly under the power lines. Traditionally, many utilities used round-over trimming to remove the canopy to a predetermined point leaving a symmetric crown below the conductors. Over the past few decades, directional pruning techniques have been adopted by many utilities, and V-trimming is used in trees below the conductors. V-trimming removes the interfering branches in the center of the tree and leaves the exterior portion (Johnstone 1983; Gilman 2002). Branches normally arise from lateral buds that are formed in the axil of the leaves along the parent stem (Kozlowski and Pallardy 1997). A newly forming lateral branch often forms a collar of overlapping branch and parent limb tissues (Shigo 1985; Neely 1991). Round-over trims are initiated by making heading cuts, which removes a branch at a point not associ- ated with a lateral branch or bud (American National Stan- dards Institute 2001; Gilman 2002; Harris et al. 2004). In response to the heading cut, many trees produce numerous shoots from dormant, or latent, buds with a bud trace to the parent stem’s pith (Maillette 1982; Kozlowski and Pallardy 1997) or from adventitious buds that are not connected to the parent stem’s pith (Kozlowski and Pallardy 1997). These vig- orously growing shoots are often called watersprouts (Gilman 2002; Harris et al. 2004). As the watersprout grows, initially there is little, if any, formation of a branch collar (Shigo 1989) and they are considered weak. Watersprouts arising after a heading cut are likely growing on parent stems (leaders) that, anecdotally, have a high prob- ©2006 International Society of Arboriculture ability of decay. If decay continues to spread, the parent stem may not be able to support the watersprouts as they increase in size. Indeed, internal decay columns were present in 90% of leaders in silver maple trees converted from round-over to V-trims (Dahle et al. 2006). Additionally, decay may breach barrier zones in the leader and move into the rapidly growing watersprout, thus escalating the potentially weak nature of the watersprout. Tree structure research has concentrated on normally de- veloped branches, particularly those arising from lateral buds. Lilly and Sydnor (1995) found branch strength to be the same in silver maples (Acer saccharinum L.) and Norway maple (A. platanoides L.). Furthermore, MacDaniels (1923 and 1932) and Miller (1958) with apple trees (Malus spp.) and Gilman (2003) with red maples (A. rubrum L.) showed that as the aspect ratio (branch diameter/trunk diameter) increased, the force needed to create a failure in a branch decreased. An investigation of codominant stems (high aspect ratio) in red maples determined that the presence of included bark reduced the strength of branches by 14% to 20% (Smiley 2003). Al- though previous studies have investigated the strength of nor- mally occurring branches, this study was designed to (1) de- termine if watersprouts arising after heading cuts are as strong as normally developing lateral branches, and (2) de- termine if decay is more likely to occur in watersprouts aris- ing after heading cuts than in normally developed branches. Silver maple was chosen as the test species because it is a frequently occurring urban tree in the U.S. Midwest, is sub- jected to heading cuts during utility-line clearance activities, and is often topped along roadsides or in private yards (Karlovich et al. 2000; Gartner et al. 2002). In addition, silver
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