Arboriculture & Urban Forestry 32(1): January 2006 33 THE EFFECTS OF PRUNING TYPE ON WIND LOADING OF ACER RUBRUM E. Thomas Smiley and Brian Kane Abstract. The wind load, bending moment, height, and weight were determined for 81 red maples (Acer rubrum) before and after pruning. Trees were thinned, reduced, lion tailed, or stripped of foliage. All three pruning treatments reduced wind load signifi- cantly compared to unpruned trees at all tested velocities (11, 16, and 20 m/sec [25, 35, and 45 mph]). Reduction in wind load increased with increasing velocity. Differences in wind-load reduction between reduction pruning and thinning were not significant at any velocity. The reduction in wind load was linearly related to the amount of weight removed by pruning treatments. Compared to the same trees prior to pruning, the center of pressure height was significantly lowered on thinned and reduced trees, while the center of pressure height did not change on lion-tailed trees. Key Words. Biomechanics; tree failure; tree pruning; wind resistance; windthrow. no longer recommended pruning practices (ANSI 2001; Lilly 2001; Gilman and Lilly 2002). Topping a healthy tree results either in a profusion of watersprouts near the cut or death of the remaining portion of the branch (Lilly 2001). Topping also facilitates the spread of decay from the point of the cut down into the branch (Shigo 1986). Lion tailing reduces a tree’s photosyn- thetic capability, especially on hot summer days: stomata of exterior foliage close and only the cooler interior foliage retains photosynthesis potential (Shigo 1986). Lion tailing also increases the potential for sunscald on thin-barked trees (Lilly 2001). While a substantial body of literature exists on the critical wind speeds to cause windthrow or stem breakage for commercially important coniferous timber species (e.g., Picea abies, P. sitchensis, P. radiata, and P. sylvestris), including effects of silvicultural treatments, stocking levels, and soil types (Cremer et al. 1982; Gardiner and Quine 2000), much less research has been conducted on wind loading of shade trees. Niklas and Spatz (2000) included a large, decurrent Prunus serotina in an examination of the uniform stress hypothesis. Others have investigated shade trees with regard to dynamic loading and natural frequencies (Baker and Bell 1992; Roodbaracky et al. 1994; Baker 1997). Pruning shade trees to reduce the risk of failure has been a tool that arborists and urban foresters have used since the early 20th century. Early arborists’ observations and intuition led to creating a smaller or more permeable canopy that they believed would reduce wind load and the risk of failure. One early pruning practice that intended to accomplish this was topping. With this type of pruning, the upper portions of branches were severed at a point on the branch where it was easy to cut or at a consistent height, usually an internode. Another type of pruning that began as a misapplication of thinning was lion tailing. With lion tailing, the lower foliage and branches on leaders and scaffold branches were removed, leaving a cluster of twigs and leaves on the distal portion of branches. Aesthetic concerns notwithstanding, both lion tailing and topping have adverse physiological consequences for trees and are The American National Standard Institute A300 pruning standard (ANSI 2001) recommends two pruning types, thinning and reduction, that, in theory, would reduce wind load on the tree while not having the negative aesthetic or physiological impact on the tree. Thinning is the selective removal of branches throughout the canopy to reduce canopy density. ANSI A300, §5.6.2, recommends removal of not more than 25% of live foliage during an annual growing season (ANSI 2001). Reduction, according to §5.6.4, is selective removal of branches to decrease canopy height and/or spread (ANSI 2001). With forest trees, branch removal from the side and top of the canopy provides the best protection against failure (Moore and Maguire 2001; Rowan et al. 2003), although many trees die as a result of such pruning. In a computer simulation, Hedden et al. (1995) showed that removal of 25% to 50% of canopy weight was the most effective way to reduce tree loss from windthrow or stem breakage under hurricane-force winds. It has been suggested that crown reduction is a better way than crown thinning to reduce the risk of shade tree failure because it reduces both the height and area of the canopy (Ball 2003). However, there do not appear to be any investigations of how pruning types affect wind loading of shade trees. The objective for this research was to determine the effect of different pruning types on wind loading of small shade trees. MATERIALS AND METHODS Red maples (Acer rubrum) grown in a field under nursery conditions at the Bartlett Tree Research Laboratory in Charlotte, North Carolina, U.S., with 3.7 × 4.6 m (12.2 × 15.2 ft) offset spacing were used for testing. Testing was ©2006 International Society of Arboriculture
January 2006
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