Arboriculture & Urban Forestry 34(4): July 2008 209 wind speed increased. We did not account for this change di- rectly, but estimated it by winching over red maples in a separate experiment. We winched trees over and measured trunk deflec- tion per unit of trunk stress (see Kane et al. [in press] for a detailed description of this procedure). The estimated distal twig deflection would have increased drag values roughly 5% (both oaks) and 10% (Freeman maple). Presumably, twig deflection of reduction pruned trees would have been less because stiffness of distal twigs would increase after they were shortened. Measured loads (L) (minus the force resulting from accelera- tion) were also converted into drag-induced bending moments (M) at the base of the tree, M L*0.76 [4] where 0.76 is the distance from the base of the tree to the cable connecting the dynamometer to the tree. In addition to drag and bending moment, we measured tree mass, tree height, crown height, and crown width normal and parallel to wind direction before and after pruning. Figure 2. A tree secured to the sled in the bed of the pickup truck. The anemometers are visible in the front of the bed. The distance from the bottom to the top of the taller an- emometer represents 2.4 m (7.9 ft). that range to determine acceleration. Vehicular acceleration was multiplied by air density (1.226 kg/m3 [0.077 lb/ft3]) and esti- mated crown volume to determine the force resulting from ve- hicular acceleration. Crown volume (V) was estimated using measured still-air crown dimensions (height [h] and width nor- mal to vehicle direction [w]) and the formula for the volume of an egg (Narushin 2005), which visually approximated most crowns, V (0.6057 − 0.0018w)*hw2. [3] Because crowns are porous and reconfigure as wind speed in- creases, estimates of force resulting from acceleration were likely somewhat greater than the actual force. This bias may have varied among species as a result of differences in crown porosity and reconfiguration. The bias was small, however, be- cause the force resulting from acceleration was less than 2% of drag at any wind speed. We subtracted the force resulting from vehicular acceleration from measured loads of each tree. To use equation 2 to calculate drag from load measurements, it was necessary to determine the center of pressure height from a digital image of the crown. We removed any branches below the height of the cab of the truck and took high-resolution, still- air frontal images of tree crowns with an Olympus Camedia digital camera, model C-2500L (Olympus America, Center Val- ley, PA). Images were segmented in Adobe Photoshop v. 6.0 (Adobe Systems, Inc., San Jose, CA) and scaled according to crown dimensions. Area and centroid of area were determined for each crown using ImageJ software (Wayne Rasband, Re- search Services Branch, National Institute of Mental Health, Bethesda, MD). The distance from the top of the truck’s cab to the truck’s bed (0.97 m [3.2 ft]) was added to the centroid of crown area to determine the center of pressure height at which the resultant drag was assumed to act (see Figure 2 in Kane and Smiley [2006]). Because center of pressure height was deter- mined from a still-air frontal image of each tree crown, deflec- tion of distal twigs would lower the center of pressure height as Pruning Treatments Trees received one of three pruning treatments: raising, reduc- tion pruning, or thinning. Our intent was to mimic conditions faced by an arborist so all pruning was estimated visually by one person. For consistency, the same person pruned all trees in accordance with the ANSI A300 standard (ANSI 2001). For raised trees, all branches from the bottom 25% of the crown were removed. Maintaining crown shape, crown height and width, parallel and perpendicular to wind direction, were reduced by 25% for reduction-pruned trees. For thinned trees, approximately 25% of crown area was removed. For raising and reduction pruning, we chose 25% because it represented a meaningful but realistic change in crown height. For thinning, removal of 25% of crown area represented the maximum reduction in crown area suggested by the A300 Standard (ANSI 2001). Exact changes in crown area and tree mass were not known until images were analyzed and biomass removed was weighed, respectively. To account for variability in tree size before and after pruning, drag and bending moment were normalized by tree mass and crown area. Normalizing by mass, in particular, more accurately illus- trated the effect of different pruning types because pruning types removed different amounts of mass and mass is an excellent predictor of drag and bending moment (Mayhead et al. 1975; Rudnicki et al. 2004; Vollsinger et al. 2005; Kane and Smiley 2006). Data Analysis We used an analysis of variance to investigate the effects of pruning and wind speed on drag, bending moment, drag per unit area, drag per unit mass, bending moment per unit area, and bending moment per unit mass. We used the least significant difference method and Tukey’s honestly significant difference adjustment to separate means. We used regression analysis to determine which tree morphometric data best predicted post- pruning drag and bending moment for each pruning type at 22.4 m/s (50 mph). All analyses were performed using SAS v. 9 (SAS Institute, Cary, NC). RESULTS Pruning Treatments Reduction of drag and bending moment differed by pruning type within each species (Table 2). For Freeman maple, reduction ©2008 International Society of Arboriculture
July 2008
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