Journal of Arboriculture 31(2): March 2005 85 leveled with elastic ties (to establish ground line) for measure- ment. Roots were found to be sufficiently rigid such that they maintained their position relative to one another when suspended. The diameter of all roots at 15, 60, and 122 cm (6, 24, and 48 in.) depths was measured, and their distance from the root system center point (a vertical axis extending down from the trunk) was recorded. This provided an assessment of the amount of root mass at each depth and lateral distance from the trunk. All diameter measurements at each of three depths (15, 60, and 122 cm [6, 24, and 48 in.]) and for four distance ranges (0 to 30, 31 to 61, 62 to 91, and 92 to 122 cm [0 to 12, 13 to 24, 25 to 36, and 37 to 48 in.]) were summed to give a cumulative root diameter (crd). The crd value was used to assess irrigation treatment effects on root distribution. Trunk diameter data were analyzed using a two-way ANOVA (randomized complete block design) with species and irrigation as factors, each at three levels. Means were separated using Fisher’s protected LSD test at P ≤ 0.05. For irrigation and root distribution, a repeated measures design was used with three treatment levels (irrigation volume) compared at four distances from the trunk and three levels of depth. For coast live oak (Q. agrifolia) root measurements, three trees were used for each irrigation treatment level. The data were analyzed using SAS Proc Mixed with fixed effects irrigation, depth and distance, and random effect tree. The analysis was carried out on the log transform of the original measurements. Residual plots were examined for any model violations. To determine the shoot:root ratio for coast live oak (Q. agrifolia), root systems and shoots (trunk, branches, and leaves) were air dried for 2 months and then weighed. Sample wood sections from roots and trunks were weighed fresh, oven dried, and weighed again to assess moisture content of air-dried samples. To establish an initial shoot:root ratio for container stock, six plants (not used in the field plot) were selected. After soil was washed from the root ball, roots and shoots were sectioned, dried, and weighed. RESULTS AND DISCUSSION All trees survived and grew for the duration of the study. Blue oaks were affected by powdery mildew during the summer months, but no trees were damaged severely. Irrigation treatment effects on trunk diameter are reported for all species, while shoot and root weights, shoot:root ratio, and root distribution are reported for coast live oak only. Trunk Diameter Trunk diameter growth was significantly greater for coast live oak than for valley oak or blue oak (Table 1). Valley oak growth was significantly greater than blue oak in all years except the first year after planting. Trunk diameter in 2001 was 11-fold greater than that in 1997 for coast live oak, 7.5- fold greater for valley oak, and 4.5-fold greater for blue oak. Mean annual increment of diameter growth was 33 mm/yr (1.3 in./yr) for coast live oak, 18.4 mm/yr (0.72 in./yr) for valley oak, and 9.3 mm/yr (0.37 in./yr) for blue oak. For all species, no significant differences in mean trunk diameter resulted from irrigation treatments for the 4-year duration of this study (Table 1). This finding is consistent with results reported by Gilman et al. (2002), indicating that irrigation volume had no effect on trunk caliper and tree height of southern live oak grown in a sandy soil over a 3-year period. In addition, a concurrent study evaluating irrigation effects on acorn stock of coast live oak, valley oak, and blue oak found no significant irrigation effects on trunk diameter growth during any year of the 4-year study (McCreary et al. 2002). Although our findings indicate that irrigation did not produce an increase in tree growth, it needs to be empha- sized that these results are linked to site and study condi- tions; therefore, similar results may not be found in soils that have low moisture status during the first year after planting or in soils that limit root development (chemically or physically). Table 1. Mean trunk diameter of coast live oak (Quercus agrifolia), valley oak (Q. lobata), and blue oak (Q. douglasii) from 1997 to 2001 for three irrigation treatments (0 ET0 Irrigation , 0.25 ET0 Species Coast live oak Valley oak Blue oak z treatmentz 0 0.25 0.5 0 0.25 0.5 0 0.25 0.5 1997y 13 ax 13 a 13 a 12 b 12 b 11 b 11 b 11 b 10 b 1998 40 a 46 a 45 a 28 b 24 b 27 b 18 c 22 c 17 c Irrigation treatments were initiated in 1998, one year after planting. yTrunk diameter at planting. xDiameter values with different letters within a column are significantly different (Fisher’s protected LSD test, P = 0.05). ©2005 International Society of Arboriculture 59 a 71 a 68 a 40 b 36 b 39 b 24 c 29 c 24 c , and 0.5 ET0 Diameter (mm) 1999 2000 105 a 121 a 116 a 67 b 62 b 66 b 37 c 45 c 37 c ), and mean height in 2001. Height 2001 139 a 152 a 140 a 88 b 83 b 85 b 45 c 54 c 44 c 2001 (cm) 341 325 351 326 388 378 216 219 214
March 2005
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