Arboriculture & Urban Forestry 34(3): May 2008 1997. The trees were planted in rows of 15 and spaced 3.05 m (10.1 ft) apart. There were three rows of each species making a total of 45 seed-propagated genotypes for each species. Each tree was given an identification that reflected its species, row num- ber, and tree number. For example, the fifth Fraxinus tree in row two would be 2Frax5. The field plot was chosen for its consistent soil texture (Yolo Loam with 43% sand, 32% silt, and 25% clay) and soil depth (more than 2 m [6.6 ft]). From mid-March to late November each year, all trees were irrigated weekly with a sub- surface irrigation system using a 0.28 cm (0.1 in) OD soaker hose (Teknor Apex Co., Pawtucket, RI). During each irrigation event, the soil within an approximately 1 m (3.3 ft) radius of the tree was brought to field capacity (approximately 22% moisture content) to a depth of at least 1 m (3.3 ft). Irrigation frequency and timing necessary to reach field capacity were determined early on in the project by taking soil cores down to an approxi- mately 0.9 m (2.97 ft) depth followed by soil moisture content determinations. Deep irrigation was practiced to reduce the po- tential influence of shallow irrigation on root growth and devel- opment. Periodic herbicide applications of XL 2G (Dow Agro- Sciences, Indianapolis, IN), glyphosate (Roundup; Monsanto, St. Louis, MO), and Rout (The Scotts Company, Marysville, OH) were used to control weeds. In December 1998, 15 trees of each species were randomly selected and excavated using a pneumatic technique (Air-Spade; Concept Engineering Group, Inc., Ve- rona, PA) (Nathenson and Jarabak 2001; Smiley 2001) that re- moved the soil without observable damage to roots larger than approximately 5 mm (approximately 0.2 in) in diameter. At this time, the selected trees ranged in height from 2.5 m (8.25 ft) to 3.5 m (11.55 ft) tall with trunk calipers at ground level of be- tween 5.1 cm (2.04 in) and 10.8 cm (4.32 in). After soil excavation, each root system was marked with brightly colored push pins as photograph reference markers. The push pins were inserted into major structural roots (larger than 2 cm [0.8 in]) at locations where the root changed direction or where there were large gaps (more than 20 cm [approximately 8 in]) between push pins. Root segments between push pins were deemed to be straight lines. Digital photographs were taken from four vantage points roughly 90° apart around the root system (Figure 1). Photograph files were imported into modeling soft- ware (PhotoModeler Pro 5.0; Eos Systems, Vancouver, BC, Canada) and marked to label specific root locations on each photograph using the colored push pins. The base of each push pin was used in this process to mark the uppermost surface of each root. Once marked, PhotoModeler software created three- dimensional models with accompanying X, Y, and Z coordinates for each push pin over the entire root system (Figure 2). X and Y coordinates from the three-dimensional models were used to calculate the linear distance (LD) of each push pin lo- cation from the origin in the X–Y plane (soil surface). Linear regressions of LD versus depth (Z coordinate) for each tree were conducted using the SAS PROC REG (SAS Institute, Inc., Cary, NC) (Figures 2 and 3). This was followed by an analysis of covariance using PROC GLM to determine significant differ- ences among the calculated slopes. We arbitrarily defined shal- low-rooted trees to have slopes between 0 and –1.0 and deep- rooted trees to have slopes less than –1.5. A tree that had a root distribution slope of –1.0 meant the average angle from the soil surface was –45° (see Figures 2 and 3). The X, Y, and Z coordinates were used to calculate the indi- vidual root angle (IRA) and the liminal angle (LA) (Johnson and 185 Figure 1. Example of one of four photographs taken of a root system that had been exposed using an Air-Spade. Push pin locations are shown on the root system. White, spherical ball (10.16 cm [4.1 in] in diameter) was used for dimensional calibration. Tibbitts 1968; Coutts and Nicoll 1991) (Figure 4). The IRA was defined as the angle (relative to the soil surface) for each root segment between adjacent push pins. A horizontal root segment had an IRA0 and a vertical root segment had an IRA–90. The angle from the vertical Z axis (plumb line through trunk origin) to each push pin was defined as the LA. The tree’s origin was defined as that point in the center of the main trunk where the X and Y coordinates were both 0; therefore, the origin was defined as 0, 0, 0. A vertical root had an LA0 and a horizontal root had an LA90. IRA and LA were calculated for each root segment (IRA) and for each push pin (LA) for 15 trees of each species. The IRA was determined by: 1) calculating the differ- ences (dx, dy, dz) between adjacent pushpin X, Y, and Z coor- dinates; 2) calculating the length of the root segment by taking the square root of the summed squares of the dx, dy, and dz values; and 3) taking the arcsine of dz divided by the length of the root segment. The LA for each push pin was determined by: 1) calculating the distance of each push pin from the X–Y origin (square root of x2 +y2); 2) calculating the distance of the pushpin from the X–Y–Z origin by taking the square root of (x2 +y2 + z2); and 3) taking the arcsine of (distance from X–Y origin/ distance from X–Y–Z origin). The end result of these calcula- tions provided angles (relative to the soil surface) of each root segment (IRA) and LAs of each push pin relative to the plumb line or Z axis through the trunk center (LA). These data were analyzed using PROC GLM (SAS Institute, Inc.). Vegetatively Propagated Clones Genotype Selection Based on the root architecture analysis from the seed-propagated trees, two shallow-rooted genotypes (i.e., those having a slope greater than –1.0) and two deep-rooted genotypes (i.e., those having a slope less than –1.5) from each species were selected for vegetative propagation. The shallow-rooted genotypes in- cluded 3Pist14 (slope–0.49) and 3Pist15 (slope–0.92) for Pistacia, 2Zelk4 (slope –0.57) and 2Zelk6 (slope –0.75) for Zelkova, and 2Frax9 (slope–0.61) and 2Frax10 (slope –0.82) for Fraxinus. The deep-rooted genotypes included 3Pist7 ©2008 International Society of Arboriculture
May 2008
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