158 trees. Sitka spruce (Picea sitchensis Bong.) stabilized with a single stake had significantly less dry root mass than nonstabi- lized trees (Mayhead and Jenkins 1992). In addition, the area, depth, and volume of soil occupied by excavated roots ≥ 0.2 cm (0.08 in) in diameter were significantly less for staked trees. Given the potential disadvantages of TSS, there is a clear need to better understand TSS efficacy so that their use can be prop- erly justified and prescribed. Of particular interest is the ability of various TSS configurations/products to prevent tree destabi- lization, which has received limited attention from researchers. Whalley (1982) studied the stability of container-grown Leyland cypress (X Cupressocyparis leylandii Dallim. & A.B. Jackson) using three different staking materials. The trees were transplant- ed and then staked for three years. In the two years following stake removal, 92% of trees that had been staked with a rigid metal pole blew over, 10% of trees staked with a medium-weight bamboo stake blew over, 2% of trees staked with a lightweight cane stake blew over, and zero of the nonstaked trees blew over. Appleton and Beatty (2004) tested twelve different above and be- lowground TSS on field-grown, 5 cm (2 in) diameter Bradford pears (Pyrus calleryana Decne. ‘Bradford’) transplanted to an experimental field site. No trees, including nonstabilized trees, were leaning or damaged one year after installation, despite a category two hurricane passing through the area six months after planting. In contrast, they observed that all nonstabilized Chi- nese elm (Ulmus parvifolia Jacq.) evaluated on a nearby study site were significantly destabilized after the hurricane. Elms with either aboveground or belowground TSS had insignifi- cant leans both after the hurricane and one year after transplant. Limited inferences can be made from observational studies of TSS performance under prevailing weather conditions. Tree pull- ing tests are a commonly accepted approach to simulating wind forces (Peltola et al. 2000) and allow TSS to be evaluated under controlled experimental conditions. Eckstein and Gilman (2008) conducted pull tests on newly planted 7 cm (2.8 in) caliper, container-grown live oak (Quercus virginiana Mill.) to simulate wind loading effects on nine commonly used TSS. In a saturated sandy soil, six out of nine TSS required more force to destabi- lize than nonstabilized controls. The study did not calibrate de- stabilization forces with wind speeds; therefore, it was not pos- sible to relate TSS performance to landscape wind conditions. The current study evaluated the performance of three conventional TSS (staking, guying, and root ball anchor- ing) that are commonly used on recently transplanted land- scape trees. The research objectives were to investigate: 1. TSS effects on tree stability soon after planting and after one growing season 2. The relative strength of each TSS 3. TSS effects on short-term tree growth and development 4. TSS material costs and installation time Alvey et al.: Efficacy of Conventional Tree Stabilization Systems METHODS Study Site and Experimental Trees This research project had three components: a wind load experi- ment, a short-term TSS experiment, and a long-term TSS experi- ment. The experiments were conducted at the Virginia Tech Urban Horticulture Center in Blacksburg, VA, U.S. (USDA Hardiness Zone 6a) from April to December 2006. The soil at the Center was Groseclose silt loam (clayey, mixed, mesic Typic Hapludult). The average soil bulk density measured within the experimental plots prior to tree planting was 1.42 g/cm3 (SE = 0.01) at 5.0–7.5 cm (2–3 in) depth. During the May–December study period, maxi- mum daily wind speed at the site averaged 7 m/s (15 mph). In April 2006, 54 field grown, balled and burlapped white ash (Fraxinus americana L. ‘Autumn Purple’) were acquired from a local whole- sale nursery. Six trees were randomly selected from the group for the wind load experiment. The remaining 48 trees were assigned to the TSS experiments and their physical dimensions were mea- sured prior to planting (Table 1). Trunk taper (t) was calculated as [1] t = (DB − DT ) ÷ L, where DB is the trunk diameter measured 15 cm (6 in) above the root flare, DT is the trunk diameter measured 15 cm below the lowest scaffold branch, and L is the distance between them. TSS Experiment Installation The long-term and short-term TSS experiments were installed in May 2006. For each experiment, 24 trees were planted in 3 m (10 ft) wide rows in a staggered arrangement with 2.1 m (7 ft) between each tree. Planting holes were dug with a 61 cm (24 in) diameter tractor-mounted auger to a depth of 45 cm (18 in) and then wid- ened to 107 cm (42 in) using a shovel. Trees were placed in the center of the planting holes and their depth was adjusted such that the root flare was even with the soil grade. Wire baskets and burlap were removed from the top third of each root ball. Backfill soil was placed around each root ball, uniformly compacted, and watered thoroughly. Planting rows were mulched with fresh wood chips and irrigated when rainfall was less than 2.5 cm (1 in) per week. Immediately after planting, each tree was randomly assigned one of four stabilization treatments: staking, guying, root ball an- choring, or nonstabilized (control). There were six replications of each treatment in each experiment. All systems were constructed with untreated wooden components, and 1.3 cm (0.5 in) poly- propylene strapping (ArborTie®, Deep Root Partners L.P., San Francisco, CA) was used for guylines and stake straps (Figure 1). For the staking system, three 5 cm × 5 cm × 183 cm (2 in × 2 in × 72 in) stakes were driven 61 cm vertically into the soil 76 cm (30 in) equidistant from the trunk. A strap was attached to the top of each stake with a clove hitch backed by a half hitch and secured to the trunk with a bowline. For the guying system, Table 1. Physical dimensions of field-grown, balled and burlapped white ash (Fraxinus americana L. ‘Autumn Purple’) used in tree stabilization system experiments (n = 48). Measurements taken at planting. Trunk caliper (cm) 6.04 (0.04) Trunk Taper (mm/m) 12.40 (0.39) Tree height (m) 4.80 (0.06) Crown diameter (m) 1.74 (0.06) Root ball diameter (cm) 70.00 (0.54) Whole tree mass (kg) 187 (2) ©2009 International Society of Arboriculture
May 2009
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