Arboriculture & Urban Forestry 32(1): January 2006 35 and ImageJ software (Wayne Rasband, Research Services Branch, National Institute of Mental Health, Bethesda, MD) to determine the height of the center of pressure of the canopy. This is the distance from the buttress to the centroid of the frontal area of the tree. It is based on a still air image of the tree and will change to some degree as branches deflect and leaves reconfigure in the wind (Mayhead 1973). The centroid of any area is the point at which a disk of uniform thickness and constant density would balance and lay flat. The sum of infinitesimal wind loads on the entire canopy can be assumed to act at the center of pressure (Beer and Johnston 1988). Force measurements were multiplied by the ratio of dynamometer height to center of pressure height from the original ground level to convert force measurements into calculated wind load values. From pre- and post-pruning images of each canopy, the difference in center of pressure height was calculated. Bending moments were calculated by multiplying the calculated wind loads (the bending force) by the center of pressure height (the lever arm over which the bending force acts). To determine risk of tree failure, bending moment, not wind load alone, was used to calculate mechanical stress in the trunk. It is important to recognize that different amounts of foliage and twigs were removed for each pruning type. The objective was not to remove the same amount of foliage and twigs but rather to prune trees as an arborist would in the field. Although different amounts of foliage and twigs were removed, this is what would happen in practice because the A300 specifies in §5.6.2 and §5.6.4 the type and location of foliage and twigs to prune, not the amount (ANSI 2001). To account for varying amounts of foliage and twigs removed by different pruning types, we analyzed the wind-load reduction per unit of weight removed (i.e., amount of foliage and twigs pruned). In addition, examining reductions in both wind load and bending moment elucidated the influence of pruning type on the center of pressure height as well as the amount of foliage and twigs removed. This is important because bending moment, not wind load alone, determines the mechanical stress a tree endures. Statistical analysis was conducted using the SAS system, version 8 (SAS Institute Inc., Cary, NC). Pre- and post- pruning comparisons of tree weight, center of pressure height, wind load, and bending moment were analyzed as matched pairs (unpruned and pruned) of trees using the analysis of variance and Tukey’s Studentized Range test to separate means. Pairing pruned and unpruned data was necessary to correct for variation in tree size. Mean differ- ences among treatments refer to differences in reduction of wind load and bending moment from unpruned trees, not absolute values of wind load for each treatment. Regression analyses were also performed to investigate relationships between wind load and velocity and between reduction in wind load and the weight of foliage and twigs removed by pruning. Trees that were stripped of foliage were not included in the statistical analysis because of the small sample size. RESULTS AND DISCUSSION The mean tree diameter measured 15 cm (6 in) above grade was 7.54 cm (3 in). Mean tree weight was 16.1 kg (35.4 lb). Lion tailing removed an average of 3.32 kg (7.3 lb, 19%) of foliage and twigs per tree. This was significantly (P = 0.03) greater than reduction pruning or thinning, which removed an average of 2.43 kg (5.3 lb, 13%) and 1.93 kg (4.2 lb, 11%) of foliage and twigs, respectively. The intent of thinning and reduction pruning was to remove approximately equal leaf surface areas, but there was weak evidence to suggest that reduction pruning removed more biomass (P = 0.12). Although visually estimating how much foliage to remove represents the situation arborists face in the field, inherent variation in this method is shown by the coefficient of variation (CV) for the percentage of tree weight removed within a given pruning type (lion tailing CV = 23%, reduction CV = 36%, thinning CV = 25%). Considering this variation, combined with the complexity of measuring wind load on trees (Vogel 1994), higher P-values (less evidence to attribute differences to treatment rather than error) are presented throughout this section. This has been done to present potentially informative data for future investiga- tions, as suggested by Marini (1999). All three pruning treatments reduced wind load signifi- cantly (P < 0.01) compared to unpruned trees at all tested velocities (Table 1). Reduction in wind load increased with increasing velocity, presumably due to the curvilinear relationship between wind load and velocity. For unpruned trees, wind load was proportional to velocity raised to an exponent of 1.4 (Figure 2). This is less than the relationship between wind load and a rigid body, where wind load is proportional to velocity squared, but greater than what has been found for some conifers, where wind load was proportional to velocity (Fraser 1962; Mayhead et al. 1975). Differences among pruning types in the reduction in wind load were contingent on velocity (Table 2). At 11 m/sec (25 mph), there were no significant differences among any of the pruning types. As velocity increased, differences became evident (Table 2). This finding is probably due to the ability of tree canopies to reconfigure in the wind, which presents a smaller area upon which the wind acts. At some threshold velocity, reconfiguration is no longer effective and leaves begin to tear or break off (Vogel 1989). This finding would explain why pruning types start to show different abilities to reduce wind load only at higher wind velocities. It is not surprising that lion tailing reduced wind load more than reduction or thinning because it removed more weight—and wind load is related to tree weight (Mayhead et al. 1975; Rudnicki et al. 2004; Vollsinger et al. 2005). ©2006 International Society of Arboriculture
January 2006
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