Arboriculture & Urban Forestry 34(4): July 2008 Of the three pruning types, thinning was the only one that did not change the shape of the crown. It was thus less likely that thinning would have facilitated airflow around the crown as may have been true for reduction pruning and raising. Vollsinger et al. (2005) observed that as wind speed increased, thinning did not noticeably change the pattern of crown area reduction (i.e., crown reconfiguration) of small deciduous trees in a wind tunnel. It thus makes sense that trunk diameter, a measure of tree size closely related to tree mass (Kane and Smiley 2006), was the second best predictor of postpruning bending moment for thinned trees as opposed to a measure of crown size as was true of reduction pruning and raising. The small increase in the center of pressure height did not appear to measurably offset the re- duction in drag as a result of removal of foliage. This idea was supported by the fact that thinning was generally better at re- ducing bending moment per unit mass removed than raising. Thinned crowns were more porous, but branch deflection may have reduced porosity as wind speed increased. This would vary by species because wood stiffness would influence the amount of deflection. Individual branches may have experienced greater drag because they were not shortened and may have been more exposed after pruning (Vollsinger et al. 2005), which helps to explain why thinning was generally not as effective as reduction pruning in terms of reduction in bending moment per unit mass removed. This reasoning contradicts Mayhead et al.’s (1975) report of a smaller postpruning drag coefficient for Sitka spruce, but the disparity may be the result of the fact that the reported reduction in drag coefficient was probably primarily the result of a reduction in crown frontal area through reconfiguration as wind speed increased (Rudnicki et al. 2004; Vollsinger et al. 2005). Although removing mass from the crown clearly reduces drag and bending moment, and this effect was most obvious when trees were stripped of all foliage (Smiley and Kane 2006), there are physiological limitations to this pruning approach. Removing too much foliage reduces photosynthesis and may cause undue physiological stress (Ennos 1997). Furthermore, removing fo- liage and twigs exclusively from the proximal portions of branches may reduce branch taper, which can increase the like- lihood of failure (Cremer et al. 1982; Putz et al. 1983; Petty and Swain 1985). Staebler (1963) demonstrated a reduction in trunk taper when more than two-thirds of the lower branches were removed. Thus, arborists need to be able to maximize the reduc- tion in bending moment while maintaining an appropriate amount of photosynthetic material in the crown. In this regard, reduction pruning was better than raising and, to a lesser extent, thinning. It was not entirely clear why crown area was such a poor predictor of post-pruning drag and bending moment, because previous studies have shown it to be a reasonably reliable pre- dictor of drag and bending moment of unpruned trees (Kane and Smiley 2006; Kane et al. in press). Crown area’s inability to predict post-pruning drag and bending moment was presumably related to the post-pruning increase in drag and bending moment per unit crown area for most pruning types and species. For all pruning types, the post-pruning percent reduction in crown area appeared to be unrealistically high for both species of oak. This was consistent with the prevalence of negative values for post- pruning reduction in drag and bending moment per square meter of crown area removed for those species. Inspection of the crown images confirmed that 50% crown area reduction was unrealis- 213 tic, so image analysis difficulties probably played a role. Com- pressing a three-dimensional crown into two dimensions may have clouded the analysis. Leeward branches may have been hidden by windward foliage in still air photographs, which would underestimate the actual surface area to which drag is related (equation 1). The likelihood of hiding leeward twigs would have been greater in wider crowns, and both oaks had wider crowns than Freeman maple. The results may not be en- tirely attributed to image analysis error, however, because Voll- singer et al. (2005) also observed an increase in post-pruning drag after removing one-third of branch mass on small deciduous trees. Crowns of their trees were small and appeared to be quite sparse, so image analysis was less likely to have been an issue. CONCLUSIONS Reduction of drag and bending moment were strongly correlated with the mass of foliage and twigs removed, whereas the effect of wind speed was essentially constant for the range of wind speeds tested. Future studies should attempt to investigate the effect of pruning at greater wind speeds, which are more likely to cause structurally sound trees to fail. Species must also be considered when selecting a pruning type because crown shape will influence how much biomass pruning removes. Wood prop- erties and the ability of foliage to reconfigure, which influence a crown’s porosity after pruning, are also related to species. A limitation of this and all other pruning studies to date is that comparatively small trees were tested. This is especially true with respect to the logarithmic increase in wind speed with height above the ground (Davenport 1968). The anemometers attached to the truck in the current study did not measure a change in wind speed between the 1.4 m (4.6 ft) that separated them, but for larger trees, the wind speed at the top of the crown may be substantially greater than at the base. Theoretically, rais- ing would even less effectively reduce drag than reduction prun- ing assuming such an increase in wind speed with height. Testing small trees is also problematic because they deflect and recon- figure more effectively than large trees. Although no trunks or branches failed during testing, it was not possible to determine whether this reflected the greater flexibility of small trees, the lack of structural defects (e.g., decay or weak branch attach- ments) on trees, or a combination of these factors. Trees were also exposed to a static force during testing, which does not reflect the gusty conditions that occur in reality. Prediction of tree failure based on static forces overestimates the wind speed at which failures have been documented (Oliver and Mayhead 1974). Pruning recommendations cannot be developed exclusively in light of mechanical considerations. Physiological considerations are also important as are the incidence of decay and regrowth after pruning. Further studies are needed to help determine prun- ing recommendations in terms of species, age, health, site con- ditions, tree risk, and aesthetics. The need to investigate dose is less important because much work, including the current study, supports the linear relationship between mass reduction and drag or bending moment reduction (Fraser 1962; Mayhead et al. 1975; Rudnicki et al. 2004; Vollsinger et al. 2005; Smiley and Kane 2006). Acknowledgments. This study was funded in part by a USDA FS grant through the National Urban and Community Forestry Advisory Council (NUCFAC). We acknowledge John Homyk, John James, and Nathan ©2008 International Society of Arboriculture
July 2008
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