Arboriculture & Urban Forestry 34(1): January 2008 size, shape, or density. Pruning dose was determined in the field as a visual estimate of live foliage removed. Winds were generated using a 1988 Chevrolet 5.7 L (1.48 gal) engine, a 2-1-power reduction unit, and a two-blade Sensenich 2 m (6.6 ft), left-hand rotation composite propeller (Sensenich Wood Propeller Co., Inc., Plant City, FL). The engine was mounted in an airboat secured onto two concrete piers so the propeller’s midpoint was at the estimated crown center of pres- sure on an average unpruned crown. This corresponded to one- third average total crown height, or an elevation of 3.1 m (10.23 ft) from the ground. This provided a vertical wind profile shown in Figure 1. Maximum winds were near the estimated crown center of pressure. Because the top third of the crown in these Highrise live oaks had very little foliage on a few thin stems, this positioned about 20% of crown foliage (estimated) outside the wind field shown in Figure 1 on the average tree. This may have biased results because the entire canopy was not in the main wind field on all trees; however, trees bend during testing, which brought more of the crown into the main wind field. Wind speed was measured 4 m (13.2 ft) from the propeller at the height equal to the estimated crown center of pressure (which we estimated for most trees as approximately one-third the way up the crown) with a Campbell anemometer (Met One 034B; Campbell Scientific, Inc., Logan, UT). It was calibrated by Campbell Scientific before purchase and we checked this cali- bration against the speedometer of a Jeep Grand Cherokee by extending it 48 cm (19.2 in) straight out the passenger window as it was driven from 4.5 m/s (10 mph) to 31.3 m/s (70 mph) in increments of 4.5 m/s (10 mph). Calibration with speedometer was within 3% of Campbell’s calibration for all wind speeds. Experimental Procedure The tree trunk was positioned 6 m (19.8 ft) from the propeller. The rootball [mass of tree in the rootball 288 kg (633.6 lb)] was secured in a 272 kg (598.4 lb) steel basket and cover plate fabricated to match the dimensions of the rootball. When secured with the solid steel cover plate fixed to the earth with four threaded rods sunken in concrete, the trunk remained stable in the rootball without rotating under load from the wind (Jones 2005). To account for any rotation of the trunk in the rootball during the 4-min blow period, we rezeroed the cable extension transducers (CETs) before blowing the tree again. Eight CETs 15 [63 cm (25.2 in; PT1A-UP-5K-M6-SG; Celesco Transducer Products Inc., Chatsworth, CA] were attached to the trunk 46 cm (18.4 in), 76 cm (30.4 in), 107 cm (42.8 in), and 137 cm (54.8 in) above the top-most root in the rootball. Because trunk movement responded similarly among treatments for all positions (Jones 2005), only data for the topmost CET is reported. The CETs were mounted to vertical posts positioned 4.6 m (15.18 ft) from the trunk so that cables approached the trunk parallel to the ground (Figure 2). Each was positioned so the cable from one CET approached the tree 90° from the other one centered on a straight line between the tree and the center of the propeller. Trigonometry was used to calculate the horizontal movement of the trunk from its starting point in the wind field. Motor revolutions per min (rpm) to wind speed correlations indicated that to achieve six targeted wind speeds, testing needed to be performed at 0 rpm, 1250, 2000, and 2750, back to 1250 rpm, and finally at 0 rpm. Data were collected at the second 1250 rpm and second 0 rpm to evaluate if the tree had shifted or lost foliage during the test. There was no statistical difference be- tween trunk deflection at the first 1250 rpm and the second 1250 rpm test; nor was there a difference in trunk deflection between 0 rpm before and after the test (Jones 2005). This meant that trees neither lost significant foliage nor shifted in the rootball during the test; however, foliage can dry during a wind test of more than approximately 10 min and this can influence crown reconfiguration during the test (Vogel 1989). Because all trees were blown similarly, the effect of foliage drying on compari- sons among pruning types is thought to be negligible. However, individual models for each tree may have over estimating trunk movement. Data were collected for 2 min at ambient conditions (0 rpm) and for 4 min at each rpm for a total of 20 min per tree per pruning dose. Trunk deflection was measured for 4 min at each targeted wind speed before pruning; then the tree was pruned at the 15% Figure 1. Vertical profile of average generated wind speeds measured on three different days (27 May 2004, 9 March 2005, and 15 March 2005). Profiles represent ambient, 1250, 2000, and 2750 (left to right) motor revolutions per min. Wind speeds were recorded at 0.5 Hz for 4 min at each elevation and averaged across days within an elevation. Figure 2. Schematic of tree (small open circle), metal basket (dotted circle) and tree-securing mechanism (square with two top supports), cable extension transducer (CETs), and anemometer positions. Numbers in parentheses indicate distance (in inches) from ground to position of CET on trunk. ©2008 International Society of Arboriculture
January 2008
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