Arboriculture & Urban Forestry 34(3): May 2008 187 Figure 4. Diagram showing how the liminal angle (LA) and individual root angle (IRA) were determined. LA, the angle from the vertical Z axis (plumb line through trunk origin) to each push pin (examples numbered 1 and 3 in the dia- gram), was calculated using X, Y, and Z coordinates pro- vided by the three-dimensional software (PhotoModeler). IRA was determined by calculating the angle, relative to the soil surface, of the root segment between adjacent push pins (example segments numbered 1–2, 3–4, and 5–6 in the dia- gram). Ten of the most vigorous, vegetatively propagated clones of each of Zelkova serrata genotypes (2Zelk4 and 2Zelk9) and Fraxinus uhdei genotypes (2Frax8, 2Frax9, 2Frax10, 3Frax13) were planted (in October 2000) 3.05 m (10.1 ft) apart in rows 3.05 m (10.1 ft) apart in the same plot of land used for the original seed-propagated trees. No cuttings of Pistacia rooted; therefore, this species was dropped from subsequent experimen- tation. The trees were irrigated weekly from early April to mid- November each year with a subsurface irrigation system using a 0.28 cm (0.7 in) OD soaker hose (Teknor Apex Co.). During each irrigation, the soil within approximately 1 m (3.3 ft) radius of the tree was brought to field capacity to a depth of approxi- mately 0.9 m (2.97 ft). Irrigation frequency and timing necessary to reach field capacity were determined early on in the project by taking soil cores down to an approximately 0.9 m (2.97 ft) depth followed by soil moisture content determinations. In February 2005, the root systems of the Fraxinus clones were excavated using the Air-Spade marked with brightly col- ored push pins and photographed to facilitate the creation of three-dimensional models using PhotoModeler software as be- fore. In February 2006, the same process was followed for the Zelkova clones. RESULTS AND DISCUSSION Survey The three tree species selected were of interest to the city ar- borists who were interviewed because of their positive attributes. However, these trees were rarely recommended or planted as a result of their reputations for causing sidewalk and curb damage. All three species have appeared on previously published lists of trees commonly thought to cause infrastructure damage (Ham- ilton 1984a; Wager and Barker 1985; Sommer and Cecchettini 1992; Sommer and Summit 1996; Costello and Jones 2003). D’Amato et al. (2002) suggested that Zelkova serrata was an “aggressive” species in terms of its root spread under sidewalks in a relatively short period of time compared with Gleditsia, Koelreuteria, and Quercus. Seed-Propagated Trees in the Field Many of the roots from the three species exposed with the Air- Spade were heart roots, originating from the buttress of the trunk, or lateral roots. There were very few tap, sinker, or striker roots (roots growing vertically downward from shallow lateral roots). Three-dimensional models of the experimental trees showed a distribution from very shallow-rooted to very deep- rooted (Figures 2 and 3). In the figures, a slope of –1.0 indicated a mean angle of –45° from the horizontal soil surface (see Fig- ures 2 and 3). The slope of the linear regressions provided some measure of the average rooting depth of each tree root system. The slopes ranged from –0.11 (shallow-rooted) to –1.7 (deep- rooted) for Fraxinus, from –0.54 to –2.47 for Pistacia, and from –0.51 to –2.07 for Zelkova. Linear equations were fitted to these data and the slopes used to compare root systems within each species. Deeper root systems had more negative slopes of the fitted lines. Pistacia chinensis trees had the most negative mean slope (deepest root systems) (Table 1) and also had the highest percentage of trees with slopes below –1.0 (data not shown). Fraxinus uhdei and Zelkova serrata were less negative and not significantly different from one another. Significant differences in LA where found among the three species. Pistacia chinensis had the lowest LA with Zelkova ser- rata intermediate and Fraxinus uhdei the highest (Table 1). Deeper root systems had lower mean LA values. There was a significant difference in mean IRA between Pistacia chinensis and Zelkova serrata (Table 1) with Fraxinus uhdei being inter- mediate. Deeper root systems had more negative mean IRA val- ues. Taken together, the slope, LA, and IRA values all indicated that Pistacia chinensis had, on average, the deepest roots sys- tems for the seed-propagated genotypes. Differences between Fraxinus uhdei and Zelkova serrata genotypes were not as clear, but both had root systems that were significantly shallower than Pistacia chinensis. All three methods (slope, LA, IRA) showed the same basic trends among the three tree species. Seed-propagated trees of Pistacia chinensis had the most negative slope, lowest LA, and more negative IRA values, all indicating deeper root systems for Pistacia. These results corroborate earlier work (Hamilton 1984a; Wagar and Barker 1985; Sommer and Cecchettini 1992; Sommer and Summit 1996; D’Amato et al. 2002). There are genetic differences in root architecture (e.g., depth) among trees species that may or may not be altered in response to soil con- ditions. Genotype Cutting Propagation The three tree species had different capacities for forming ad- ventitious roots in response to the treatments imposed. No roots were formed on any of the Pistacia chinensis cuttings; therefore, it was not possible to continue the research with that species. There were also dramatic differences in the abilities of the four genotypes of Zelkova serrata to form adventitious roots. No cuttings from genotypes 2Zelk2 and 2Zelk6 formed adventitious Table 1. Mean slope, liminal angle (LA), and individual root angle (IRA) of seed-propagated trees (22 months old) of three tree species. Genus Fraxinus uhdei Zelkova serrata Pistacia chinensis Slope −1.1 a −1.2 a −1.5 b LA (°)IRA (°) 47.8 a 39.4 b 33.7 c −39.5 ab −35.8 a −41.9 b Means followed by the same letter are not significantly different at P 0.01 using Scheffe’s mean separation test. ©2008 International Society of Arboriculture
May 2008
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