34 Smiley and Kane: Effects of Pruning Type on Wind Loading conducted between 9 September 2003 and 1 October 2003, while trees were in leaf. A 14 mm (9/16 inch) diameter hole was drilled through the trunk 10 to 15 cm (4 to 6 in) above the soil line. Trees were then cut 5 to 10 cm (2 to 4 in) above the soil line to avoid the root flare. Diameter at 15 cm (6 in) above the hole was measured, and pre- and post-pruning measurements of canopy height, total tree height, canopy spread, and tree weight were taken. Trees were fastened to a custom-made steel frame mounted in the bed of a pickup truck with a 12 mm (1/2 inch) diameter bolt (Figure 1). They were then hoisted into a vertical position and fastened to a Dillon model ED-2000 plus electronic dynamometer (Dillon, Fairmont, MN) using a 0.6 cm (1/4 inch) diameter extra-high-strength cable and a 2.5 cm (1 in) wide sling attached 76 cm (30.4 in) above the hole. The dynamometer cable remained parallel to the truck bed throughout the tests. After taking two force measurements with the canopy intact, the tree was pruned according to one of four types (listed below). Thinning and reduction were accomplished in the manner described in the ANSI A300 pruning standard (§5.6.2, §5.6.4) and ISA’s Best Management Practices: Tree Pruning (ANSI 2001; Gilman and Lilly 2002) for mature shade trees. To simulate field conditions, the percentage of foliage removed (described below) was based on visual estimation by an ISA Certified Arborist. For the first three trees, which were treated according to pruning types 1 through 3 (listed below), three Certified Arborists visually compared the amount of foliage and twigs removed to ensure that it represented the intended amount of foliage and twigs to be removed for the particular pruning type. The same Certified Arborist pruned all trees to maintain consistency in the amount and location of foliage and twigs removed for each pruning type. Figure 1. Truck setup used for testing wind loading. The truck and tree were driven with constant accelera- tion up to 20 m/sec (45 mph). Force measurements were recorded at 11, 16, and 20 m/sec (25, 35, and 45 mph). Before placing trees in the truck, an anemometer was placed where the tree would be, and wind speeds were checked against the speedometer. Above the cab height, anemometer readings did not vary more than 1.5 m/sec (3 mph). To compensate for ambient wind, two force measurements were recorded for each tree at each speed. This was accomplished by making two passes with the truck in directly opposite directions. The force values used for subsequent calculations of wind load were the average of these two runs. Typically, all testing was completed within 30 min of cutting down the tree, before there was visual evidence of foliar wilting. ©2006 International Society of Arboriculture Pre- and post-pruning measurements of tree weight tracked the amount of foliage and twigs removed. Because wind resistance is related to canopy weight (Mayhead 1973; Mayhead et al. 1975; Rudnicki et al. 2004; Vollsinger et al. 2005), we compared how effectively pruning treatments reduced wind load relative to the weight of foliage and twigs removed. This was important for two reasons. First, trees varied in size; therefore, removing a certain percentage of foliage and twigs did not remove the same amount of weight for each tree. Second, the pruning types did not necessarily remove the same amount of foliage and twigs. Treatments were as follows: 1. Thinning. Approximately 25% of the foliage was removed from the tree by removing small branches, 3 to 12 mm (1/8 to 1/2 in) diameter, throughout the canopy. Tree height and canopy width were not changed during thinning. This treatment was applied to 26 trees. 2. Reduction. Tree canopy height and width were reduced 25% by removing twigs from the perimeter of the canopy, primarily using reduction cuts. Typically, the sides of the tree were reduced 30 cm (12 in) and the top 60 cm (2 ft), although this varied due to differences in canopy dimensions. This treatment was applied to 25 trees. 3. Lion tailing. Foliage and small branches were removed from the bottom half of the canopy. Lion tailing was accomplished by twig pruning and hand removal of leaves attached to lower scaffold branches. Canopy height and width were not changed nor were scaffold or other branches greater than 1 cm (0.4 in) diameter removed. This treatment was applied to 23 trees. 4. Leaf stripping. All leaves were manually removed from the tree by hand stripping each branch. Twigs and branches were left intact. This treatment was applied to six trees. A digital image was taken of each tree before and after pruning. The images were analyzed using Adobe Photoshop
January 2006
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