Arboriculture & Urban Forestry 41(2): March 2015 Although not tested, a more aggressive program may be needed on trees with a larger aspect ratio. The decrease in aspect ratio over time and with increasing severity on pruned branches was less pronounced in the lower crown (branch 1) than upper crown (branch 3, Figure 1B, Figure 1C, Figure 1D; Equation 1). This difference became apparent the first year aſter pruning and persisted through year five, as indicated by the aspect ratio differences among pruning severities in the middle (branch 2) and upper crown (branch 3); there were no differ- ences among the 25%, 50%, and 75% TPS in the lower crown (branch 1). The pruning cuts on primary branches growing from the leader above branch 1 (the lowest measured branch) could explain the smaller change in aspect ratio in branch 1. Essen- tially, growth slowed on both sides of the union in response to removing branches from both. Smaller change in aspect ratio on pruned branches in the lower crown than in the upper crown suggests that low branches may require a higher pruning severity to effect that same change in aspect ratio as upper branches. The dramatic reduction in growth rate on branch 3 (upper crown) likely occurred because there were few or no pruning cuts (depending on the tree) made on branches originating from the leader above this point. This would largely be in accordance with others who showed a dramatic reduction in growth rate on the pruned stem when no branches were removed from the other side of the union (Downer et al. 1994; Gilman and Grabosky 2009). The dramatic reduction in growth rate on pruned branches in these studies clearly caused the reduc- tion in aspect ratio because there were no pruning cuts made on primary branches higher in the crown. Gilman (2003), Kane et al. (2008), and others showed that smaller branch:trunk diameter ratios are associated with greater branch union strength. Structural pruning strategies are designed to induce and maintain small ratios. Data from the current study showed that pruning to induce a smaller aspect ratio—as accomplished in other studies by pruning one branch (Gilman and Grabosky 2009)— can also be used on multiple branches of the same tree (Kristoffersen et al. 2010). The constriction that occurs within small aspect ratio unions (Eisner et al. 2002) should help trees resist decay organisms mov- ing from branches to trunk. This data suggests that branches in the lower crown should be pruned with 73 a greater severity than those in the upper crown to affect a similar reduction in aspect ratio (Fig- ure 1). Clearly this needs to be studied for a variety of tree types of different ages in different regions. LITERATURE CITED American National Standards Institute. 2008. American National Standard for tree care operations-Tree, Shrub, and Other Woody Pant Maintenance-Standards practices (Pruning). ANSI A300 (part 1). New York: American National Standards Institute. Bredenkamp, B.V., F.S. Malan, and W.E. Conradie. 1980. Some ef- fects of pruning on growth and timber quality of Eucalyptus grandis in Zululand. South African Forestry Journal 114:29–34. Clark, F.B. 1955. Black walnut responds to pruning. Journal of For- estry 53:362–365. Downer, A.J., M. Shaw, and D. Pittenger. 1994. The effect of pruning on branch growth in two oak species. Abstract #815. Hort- Science 29:550 Edberg, J., A.M. Berry, and L.R. Costello. 1994. Patterns of tree failure in Monterey pine. Journal of Arboriculture 20:297–304. Eisner, N.J., E.F. Gilman, J.C. Grabosky, and R.C. Beeson. 2002. Branch junction characteristics affect hydraulic segmentation in red maple. Journal of Arboriculture 28:245–251. Funk, D.T. 1979. Stem form response to repeated pruning of young black walnut trees. Canadian Journal of Forest Research 9:114–116. Gilman, E.F. 2003. Branch to stem ratio affects strength of attach- ment. Journal of Arboriculture 29:291–294. Gilman, E.F. 2014. Pruning Acer rubrum at planting impacts struc- ture and growth aſter three growth seasons. Arboriculture & Urban Forestry 41:11–17. Gilman, E.F. and S. Lilly. 2008. Best Management Practices Pruning. International Society of Arboriculture. Champaign, Illinois, U.S. Gilman, E.F., and J. Grabosky. 2009. Growth partitioning three years following structural pruning of Quercus virginiana. Arbo- riculture & Urban Forestry 35:281–286. Gilman, E.F., B. Kempf, N. Matheny, and J. Clark. 2013. Structural pruning: A guide to the green industry. Urban Tree Foundation, Visalia, California, U.S. Hanley, D.P., C.D. Oliver, D.A. Maguire, D.G. Briggs, and R.D. Fight. 1995. Forest pruning and wood quality of western North American conifers. College of Forest Resources, University of Washington, Seattle, Contribution, No. 77. Kane, B., and P. Clouston. 2008. Tree pulling tests of large shade trees in the genus Acer. Arboriculture & Urban Forestry 34:101–109. Kane, B., R. Farrell, S.M. Zedaker, J.R. Loferski, and D.W. Smith. 2008. Failure mode and prediction of the strength of branch attachments. Arboriculture & Urban Forestry 34:308–316. Kristoffersen, P., O. Bühler, S. Ugilt Larsen, and T.B. Randrup. 2010. Growth of newly established roadside trees in response to weed control and pruning. Arboriculture & Urban Forestry 36:35–40. Langstrom, B., and C. Hellqvist. 1991. Effects of different pruning regimes on growth and sapwood area of Scots pine. Forest Ecol- ogy and Management 44:239–254. MacDaniels, L.H. 1923. The apple-tree crotch. Cornell Univ., Agric. Exp. Sta. Bull. 419:1–22. ©2015 International Society of Arboriculture
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