162 tively rapid root regeneration in bare-root, one-year-old green ash (Fraxinus pennsylvanica Marsh.). In their study, intact roots resumed growth seven to 20 days after transplanting, fol- lowed by adventitious root development from callus collars of pruned roots 10 to 19 days later. Although no root regeneration measurements were taken in the study addressed in this paper, very few adventitious roots were observed on the surface of root balls as they were pulled from the soil. A more likely ex- planation is that the bending moment imposed on the trees was too small to cause destabilization. As mentioned, the canopies were relatively thin, presumably reducing bending moments ob- served in the wind load test, which provided the analytical ba- sis for the pull test. Had a species of similar nursery stock size, but denser canopy been evaluated, the results may have been much different. More research is needed to evaluate the stabil- ity of species differing in canopy density and crown architecture. Although 90% of all experimental ash trees (22 of 24) had trunk orientation change less than 10° after the 5-week pull test, it should be noted that two control trees were significantly destabilized (35° and 53°). Subsequent inspection of these trees revealed that roots within the root ball were very sparse and asymmetrically distrib- uted. This finding reiterates the importance of inspecting the root system of nursery stock to determine the need for stabilization. Based on the overall performance of the nonstabilized trees under both ambient and simulated wind conditions, stabilization may not be required for similar trees planted on moderately windy sites. TSS did not substantially inhibit ash tree growth or develop- ment during the first growing season after planting. After seven months, root growth into backfill soil was similar in stabilized and nonstabilized trees. Other researchers have found negative impacts of stabilization on root development (Jacobs 1954; Fayle 1976; Mayhead and Jenkins 1992), but trees in their studies were stabilized for much longer durations. There was a slight trend towards decreased trunk caliper growth and taper development in our staked ash trees relative to other treatment groups. Numer- ous past studies have observed negative effects of stabilization on trunk caliper growth and taper development (Harris and Hamilton 1969; Holbrook and Putz 1989; Svihra et al. 1999), but their sta- bilization systems were typically more rigid and installed longer than in the study detailed in this paper. The straps on staked trees were consistently tauter than those on guyed trees at the end of our study, which could have contributed to growth inhibition. An ex- periment of longer duration might have revealed more significant effects of staking on caliper growth and taper development. For landscape trees of similar size and structure, there should be little concern about detrimental effects of TSS on tree growth and de- velopment during the first growing season. Importantly, this is the recommended service life for a TSS in most landscape applications. When TSS components were removed at the end of the first growing season, stabilized and nonstabilized ash trees were equally resilient when pulled, which contradicts previous work (Wrigley and Smith 1978; Whalley 1982). Both of these studies, however, tested different degrees and durations of im- mobilization. Soil moisture levels and soil composition may also have affected results. Here, both nonstabilized and previ- ously stabilized trees remained upright until unrealistically large loads (more than twice the bending moment generated by a 25 m/s wind) were applied. This outcome was expected be- cause substantial root growth into backfill soil had occurred in both stabilized and nonstabilized trees during the first grow- ©2009 International Society of Arboriculture Alvey et al.: Efficacy of Conventional Tree Stabilization Systems ing season. Based on this study's results, TSS could be safely removed from small-caliper, field-grown ash trees after one growing season without concern for subsequent wind throw. In this study, TSS differed in the maximum force they endured before component failure. The guying system withstood forces 1.7 to 2.5 times greater than the root ball anchoring and staking systems, respectively. Eckstein and Gilman (2008) observed sim- ilar TSS performance patterns; guying and root ball anchoring systems were the top performers whereas trees with a 2-stake sys- tem were no more robust than nonstabilized trees. These findings were expected since the greater attachment height of the guylines should reduce the overturning moment imposed by the winch. Although the three systems here were equally effective in stabi- lizing ash trees against a moderate force, there may be circum- stances that require a more robust system. For example, vandals, who can exert more stress on small trees than even severe winds, often break landscape trees in urban areas. Where vandalism is a concern, guying systems, which were found to be the most robust, may be used. However, there is some concern that aboveground stabilization systems may increase the likelihood of vandalism and belowground systems may actually be a better choice. The results of this study and of Eckstein and Gilman (2008) suggest that a root ball anchoring system constructed of wooden stakes and cross braces performs very well in stabilizing both field and container grown trees. This belowground system may be preferred in locations where aboveground systems are undesirable due to space constraints or concerns for tripping hazards and aesthetics. It is important to note that the wind loads experienced by the trees driven in the pickup truck are not perfectly equivalent to the winch-induced loads applied to the experimental trees. The pulling tests applied a constant, unidirectional, static force, unlike the complex, dynamic forces induced by wind. There- fore, the results of pulling tests should be interpreted with cau- tion. TSS may perform differently under natural wind stress. However, the static forces of tree pulling are representative of the stress that vandals exert on a tree (Smiley et al. 2003). CONCLUSIONS This study evaluated three conventional tree stabilization systems under ambient and wind-simulating conditions. The systems per- formed equally well and even nonstabilized, medium-caliper, field-grown ash trees were tolerant of moderate to heavy wind loads. Similar trees should not require stabilization in the land- scape unless extreme wind or vandalism is an issue. However, more research is needed to evaluate the stability of trees differ- ing in nursery stock type, canopy density, and crown architec- ture before making unequivocal recommendations. The effect of soil moisture levels, soil composition, and fertilization on tree stability also needs to be evaluated. TSS components were very durable during the first growing season and did not substantially impact tree growth or cause trunk injuries. If TSS are removed after the first growing season, there should be limited concern for growth impacts or injuries. The guying system outperformed the other systems at extreme loads, indicating that this system may be preferable where vandalism or very strong winds are a concern. The root ball anchoring system should be a good alter- native when aboveground systems are undesirable due to space constraints or concerns for tripping hazards, vandalism, or aes- thetics. Although the three systems had similar installation times, the staking system components were twice as expensive and
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