208 L.) maples, swamp white oak (Quercus bicolor Willd.), and shingle oak (Quercus imbricaria Michx.) (Table 1; Figure 1). Freeman maple has been planted as a street tree across the east- ern United States. Swamp white oak has been suggested as a good street tree because of its tolerance of poor soil (Dirr 1997). Shingle oak has been increasingly requested as a street tree ac- cording to urban foresters in Ohio (D’Amato et al. 2002). All trees were grown at the Urban Horticulture Research Center in Blacksburg, Virginia, U.S. (USDA hardiness zone 7a) and test- ing was conducted during July and August 2005. We cut trees just above the root flare and fastened the entire tree above the cut in a steel sled mounted in the rear of a pickup truck (Figure 2). The truck was then driven on a straight, nearly level course from 0 to 24.5 m/s (0 to 55 mph). Although wind flow was not laminar like in a wind tunnel, this method enabled us to test larger trees. To improve experimental rigor, we refined Smiley and Kane’s (2006) method as noted subsequently. Wind Measurements We measured wind speeds with two three-cup anemometers, one attached to either side of the cab of the pickup truck. Anemom- eters on the left and right sides of the truck were attached, respectively, 0.8 and 2.2 m (2.6 and 7.3 ft) above the cab (Figure 2). Wind speed data were collected at 1 Hz with Sensormetrics (Southbridge, MA) software. The anemometers were calibrated with the truck’s speedometer on each day of testing after the truck’s speedometer was calibrated with a radar gun before any testing. To reduce further the error resulting from ambient wind, we tested each tree twice, driving it in opposite directions on the test course, and we did not conduct tests when ambient wind speed exceeded 1.4 m/s. Because wind speeds measured with each anemometer did not differ beyond their inherent error (0.5 m/s), we averaged their values. Load Measurements We measured wind loads with a 9,800 N capacity Dillon EDxtreme dynamometer (Weigh-Tronix, Fairmont, MN; accu- rate to 9.8 N) and collected data at 2 Hz using Wedgelink software (MicroRidge Systems, Inc., Sunriver, OR). The dyna- mometer was attached to an anchor point on the sled by a steel cable and to the tree by a polyester webbing sling.Weused moment equilibrium to convert measured loads (L) into actual drag (D), D L*0.76/CPH, [2] where CPH is the center of pressure height, explained subse- quently (for details of this procedure, see Kane and Smiley [2006]). Because wind speeds were recorded one-half as frequently as load measurements, wind speeds at subsequent 1 sec intervals N Height (m) Crown height (m) Crown width (Y; m) Crown width (X; m) Pavlis et al.: Effects of Pruning on Drag and Bending Moment Figure 1. Typical forms for, clockwise from top left, Freeman maple, swamp white oak, and shingle oak tested in the cur- rent study. The distance from the bottom of the image to the anemometer on the right side of each image represents 2.2 m (7.3 ft). were averaged to create values for each half-second interval. For wind speeds between 13.4 and 24.5 m/s (30 and 55 mph), we plotted drag versus wind speed from both test runs of each tree and fit a straight line to the data. From the line, we predicted drag at 5 wind speeds (13.4, 15.6, 17.9, 20.1, 22.4 m/s [30, 35, 40, 45, 50 mph]) for each tree. If the coefficient of determination for the line was less than 0.90, we plotted each test run separately. In five cases (two Freeman maples, two swamp white oaks, and one shingle oak), doing so did not improve the coefficient of deter- mination, so those trees were removed from the analysis. Because we recorded instantaneous wind speeds, measured loads included a component of force resulting from vehicular acceleration. We determined vehicular acceleration by plotting wind speed against time for each tree; the slope of the best-fit line for the plot is the acceleration. Acceleration was nonlinear at speeds up to approximately 11.0 m/s but consistently linear from 13.4 to 24.5 m/s (30 and 55 mph), so we only plotted speeds in Table 1. Means (standard deviation in parentheses) for tree morphometric measures describing each species. Species CPH (m) Tree mass (kg) Diameterz (Y; mm) Diameterz (X; mm) Diametery (Y; mm) Diametery (X; mm) Freeman maple 16 4.83 (0.22) 3.67 (0.24) 2.40 (0.31) 2.10 (0.34) 2.19 (0.10) 20.3 (4.83) 79.2 (6.84) 81.0 (10.7) 66.2 (11.6) 67.5 (7.40) Swamp white oak 13 4.45 (0.19) 3.46 (0.23) 3.23 (0.58) 3.20 (0.53) 1.76 (0.07) 22.1 (4.75) 88.7 (8.67) 90.2 (8.93) 76.3 (8.28) 76.5 (7.05) Shingle oak 18 4.62 (0.46) 3.57 (0.46) 4.04 (0.57) 3.79 (0.63) 1.99 (0.22) 25.0 (8.04) 89.9 (9.55) 90.4 (10.5) 73.9 (10.3) 75.4 (9.61) Measured 0.76 m (2.5 ft) above the root flare. CPH center of pressure height; X direction perpendicular to the pickup truck’s motion; Y direction parallel to the truck’s motion. zMeasured 0.04 m (0.13 ft) above the root flare. y ©2008 International Society of Arboriculture
July 2008
| Title Name |
Pages |
Delete |
Url |
| Empty |
Search Text Block
Page #page_num
#doc_title
Hi $receivername|$receiveremail,
$sendername|$senderemail wrote these comments for you:
$message
$sendername|$senderemail would like for you to view the following digital edition.
Please click on the page below to be directed to the digital edition:
$thumbnail$pagenum
$link$pagenum
Your form submission was a success. You will be contacted by Washington Gas with follow-up information regarding your request.
This process might take longer please wait
