Arboriculture & Urban Forestry 34(1): January 2008 less linearly with wind speed until the crown cannot reconfigure itself in the wind and then more exponentially at higher wind speeds. This debate will continue for a long time because it is difficult to measure forces and reconfiguration on large trees (Cullen 2005). Wind speed also increases with distance from the ground making taller trees more likely to receive damage than shorter trees (Niklas and Spatz 2000). These complex and poorly understood relationships make utilization of theoretical models predicting trunk movement and subsequent damage difficult to use in practice. The objective of this study was to determine how tree trunk movement was impacted by winds up to hurricane force after applying different pruning types to the crown. We measured movement with the assumption that more movement would equate to more damage to the tree, an assertion supported by others (James et al. 2006). MATERIALS AND METHODS In August 2001, we planted 80 cutting-propagated (clones) #1 liners of Quercus virginiana Cathedral Oak live oak in a field with sandy soil (Millhopper sand). Trees were located at the University of Florida Great Southern Tree Conference demon- stration site in Alachua County, Florida, U.S. (U.S. Department of Agriculture 1990 hardiness zone 8), spaced on 2.4 m (7.92 ft) centers in eight rows 3.6 m (11.88 ft) apart. All trees were drip-irrigated three times daily in the growing season and less often in the cooler months. All trees were pruned twice each year to one central leader through 2004. Trees were fertilized using 16:4:8 (N:P2O5:K2O; Parker’s Su- per Soilife; Chemsico Inc., Division of United Industries Co., St. Louis, MO). In January 2002, the amount applied was 65 g (2.28 oz); in May 2002, 210 g (7.35 oz); and July 2002, 300 g (10.5 oz). Thereafter, trees received 400 g (14 oz) three times per year, in February or March, May, or June. Every other tree was re- moved from the field November 2004; in June 2006, every other tree was again removed leaving 20 trees for this study spaced 9.7 m (32 ft) apart within rows and 7.3 m (24 ft) apart between rows. Trees averaged 12 cm (4.8 in) caliper and were 6 m (19.8 ft) tall with a 2.7 m (8.9 ft) wide canopy. Pruning types were defined in the American National Stan- dard for Tree Care Operations (American National Standards Institute 2001). Pruning treatments tested were crown raised, reduced, thinned, and nonpruned control trees. Trees were ran- domly assigned to a pruning type. Pruning types were blocked in time so that each block contained one raised, thinned, reduced and a nonpruned tree. One person executed all pruning to main- tain treatment consistency. Trees were pruned in the week before subjecting them to wind. Raising the crown removed branches back to the single central trunk beginning at the bottom of the crown and working upward; mean number of cuts was 11.2 with a diameter of 27.9 mm (1.1 in). Thinning removed branches back to the trunk throughout the entire crown in a visual attempt to evenly distribute remaining foliage throughout the crown; mean number of cuts was 12.2 with a diameter of 28.1 mm (1.1 in). Reduction made one cut through the trunk at the point that removed the targeted pruning dose from the top of the tree; mean cut diameter was 71 mm (2.8 in). Thirty-three percent of the foliage was the targeted pruning dose for all 15 pruned trees. The relationship between branch diameter at the trunk and foliage weight on those branches was used to estimate the 21 amount of foliage removed. This relationship was calculated by removing 15 branches from three extra trees removed before testing with a trunk caliper closest to the mean trunk diameter for test trees (12 cm [4.8 in]) for a total of 45 data points; five branches on each tree were in the top third of the crown, five were in the middle third, and five were in the bottom third of the crown. Branch diameter was measured where each branch met the trunk and all foliage was stripped from the branch and weighed fresh. A curve relating branch diameter to foliar fresh weight was calculated (Figure 1). Pulling all foliage from these three test trees gave the total foliar weight in the crowns from which an average was calculated. Once the average total foliar weight of these three trees was determined, the branch diameter squared corresponding to 33% of this number (Figure 1) was removed from each of the 15 pruned trees. Branches were pruned one by one and their squared diameters were summed. Pruning stopped on a tree once the sum of the squared diameters reached the point on the curve that was equal to 33% of average total canopy foliage weight. The soil was brought to field capacity within a 10 ft (3 m) diameter circle centered on the trunk so that soil conditions would be similar for each tree tested. This was done by calcu- lating the amount of water required to bring soil within the top 0.6 m (1.98 ft) to field capacity (1.4 m3 [49 ft3]) and then ap- plying 1.5 times that amount of water before testing each tree. Water was applied 6 hr before blowing trees. This allowed water to percolate into the soil and drain bringing it to field capacity. An airboat fan assembly created a large enough wind field to immerse the test trees. Power generation consisted of two stacked 496 in3 (8,267 mL) Chevrolet marine engines with Air- boat Drive Units CH3 belt-driven reduction assembly. The drive units spun two 2 m (6.6 ft) long counterrotating propellers, which produced approximately 45 m/s (110 mph) wind speeds at maximum throttle. Two portable walls each 3.7 × 3.7 m (12.21 × 12.21 ft) were positioned between the propellers and the tree 2.4m(7.92 ft) apart parallel to the wind field to confine the wind flow field and to improve flow uniformity. The walls began at the propellers and ended 1.5 m (4.95 ft) from the trunk. Behind the tree being tested was a 45° wind deflector, which protected the next tree in line to be tested. Control and data acquisition were integrated into one system built from PXI hardware, 16-bit Series Multifunction DAQ de- vices, and Labview software National Instruments (Austin, TX). At 20 ms intervals, serial port commands were sent to a Figure 1. Relationship between fresh foliage weight and di- ameter of the branch on which foliage was borne on 45 branches from three trees. ©2008 International Society of Arboriculture
January 2008
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