212 For shingle oak, raising removed more tree mass than reduction pruning and thinning, whereas reduction pruning reduced the center of pressure height, crown height, and crown width more than raising or thinning. DISCUSSION The difficulty in “eyeballing” pruning types was obvious be- cause neither raising nor reduction pruning changed crown height in accordance with our intended 25%, except for raising of Freeman maple. Because only entire branches were removed by raising, part of the disparity between intended and actual changes in crown height was the result of distal portions of remaining branches hanging below the height at which they were attached to the tree. Reduction of crown width was closer to the intended 25%, and the consistently smaller percent change may reflect the pruner’s conservative approach. By itself, drag is a less useful measurement than bending moment because bending moment determines mechanical stress in the trunk. When trunk stress exceeds wood strength, the stem will break, so reduction in bending moment is critical when assessing tree risk. Because bending moment is the product of drag and center of pressure height, reducing either will reduce bending moment. Reducing the center of pressure height would be somewhat less effective because the percent change in drag was greater than the percent change in the center of pressure height. Reducing center of pressure height would be more ef- fective for tall, narrow crowns because airflow may preferen- tially go around, as opposed to through, such crowns. This was evident for Freeman maples. For a tree with a short but dense crown, however, lowering the center of pressure would reduce bending moment less effectively. On larger trees, which have stiffer branches and would reconfigure less than small trees (Ber- tram 1989), drag may be proportional to the square of wind speed instead of the nearly linear relationship previously report- ed (Fraser 1962; Mayhead et al. 1975; Kane and Smiley 2006). In that case, reducing the center of pressure height would be less effective than reducing drag because of the linear relationship between center of pressure height and bending moment. Differences among pruning types with respect to the reduction of drag and bending moment can be attributed to several factors with one caveat. To a limited degree, differences among pruning types regarding drag reduction were an artifact of the way drag was calculated. Because center of pressure height was used to convert measured loads into drag (equation 2), changes in center of pressure height would have affected post-pruning drag values even if measured loads had not changed. For example, raising increased the center of pressure height, which reduced drag, in part, because drag was inversely proportional to center of pres- sure height (equation 2). The reciprocal was true of reduction pruning, which lowered the center of pressure height for each species. This artifact best explained the paradoxic finding for both species of oak that differences among pruning types for drag reduction did not match differences among pruning types for the reduction of bending moment. Considering that differences in drag reduction among pruning types were partially the result of changes in center of pressure height, and that, practically speaking, reduction in bending mo- ment is more important than drag reduction, differences among pruning types are discussed chiefly with respect to reduction in bending moment. ©2008 International Society of Arboriculture Pavlis et al.: Effects of Pruning on Drag and Bending Moment Reduction in bending moment was attributable in large part to reduction in tree mass, regardless of pruning type, as reported previously (Smiley and Kane 2006). Because crown shape var- ied by species, the effect of pruning varied in accordance with pruning-induced changes in mass and shape. Thus, it was not surprising that reduction pruning and raising most effectively reduced bending moment for Freeman maple and shingle oak, respectively, because those pruning types removed the most mass for each respective species. Differences among pruning types with respect to reduction of bending moment per kilogram of mass removed, however, indicated that additional factors were involved. Pruning-induced changes in crown shape and funda- mental mechanical principles (drag and bending moment) help explain the differences. Reduction pruning was the most effective pruning treatment per unit mass removed for several reasons. Reduction pruning removed almost exclusively high drag elements from the crown because only distal (and therefore mostly foliated) portions of branches were removed. Leaves contribute more drag to tree crowns than branches and twigs (Vogel 1994), which was obvi- ous when stripping red maples of foliage reduced drag and bend- ing moment better than any pruning type (Smiley and Kane 2006). Reduction pruning also lowered the center of pressure height of the tree, further reducing the bending moment. Unlike the other pruning types, reduction pruning shortened branches, which behave as individual drag elements in the crown (Voll- singer et al. 2005). Shortening branches reduced both branch slenderness and the distance from the branch attachment to the centroid of area of the branch, at which the resultant drag on the branch can be assumed to act. Consequently, reduced branches would deflect less, increasing crown porosity as foliage recon- figured. Foliage reconfiguration differs among species (Vogel 1989), however, so testing of additional species is warranted. Lastly, reduction pruning reduced the depth of the crown, per- haps further facilitating airflow through the crown. The fact that crown width was the second best predictor of postpruning bend- ing moment for reduction-pruned trees supported this notion. After trees were raised, drag reduction was probably achieved by air passing more easily under the crown. It was unlikely that reduction in drag was the result of changes in crown porosity, because raising did not remove any branches from the interior of the crown. However, the greater postpruning center of pressure height presumably offset the reduction of drag and caused the reduction in bending moment to be comparatively small. The finding that tree height and not crown height was the second best predictor of postpruning bending moment for raised trees sup- ported this idea because, although raising shortened crown length, it increased the center of pressure height. This was par- ticularly evident for shingle oak, for which raising removed the most tree mass, but on a per mass removed basis, it was the least effective pruning method for reducing bending moment. Raising did not affect drag coefficient (Mayhead et al. 1975) but im- peded crown reconfiguration (Fraser 1962) of a single Sitka spruce. In contrast with reduction pruning, raising removed a greater proportion of low drag elements from the crown because entire branches were removed. On a per mass removed basis, therefore, it was expected to be less effective at reducing bending moment. Crown reconfiguration was probably not affected by raising because individual branches were not modified like in reduction pruning.
July 2008
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