86 Costello et al.: Irrigation Effects on Growth of Newly Planted Oaks Tree Height After 4 years, average height of valley oak was 3.9 m (12 ft), while that of coast live oak was 3.6 m (11 ft), and blue oak was 2.3 m (7 ft) (Table 1). Because height is not a good measure of plant growth, however, treatment effects were not analyzed statistically. Shoot:Root Ratio Mean shoot:root ratio (across irrigation treatments) for coast live oak (calculated from dry weights) was 2.6:1, with a range from 2.3:1 to 3.1:1 (Table 2). Similarly, the shoot:root ratio for container plants measured prior to field planting was found to be 2.6:1 (Table 2). Although there was no significant difference in shoot:root ratio between 0 and 0.25 ET0 significantly higher ratio was found for the 0.5 ET0 treatments, a treatment. Table 2. Mean weight of shoots and roots (kg), and shoot:root ratios for coast live oak (Quercus agrifolia) for three irrigation treatments (0 ET0 0.5 ET0 Irrigation 0.25 0.5 Mean Pre-plant ) after 4 years and prior to planting (pre-plant). Shoot , 0.25 ET0 Root treatment weight (kg) 0 25.8 a* 25.0 a 38.7 b 29.8 0.107 weight (kg) 11.4 a 10.5 a 12.6 a 11.5 0.041 Shoot:root ratio 2.3 a 2.4 a 3.1 b 2.59 2.61 *Values in a column with different letters are significantly different (Fisher’s protected LSD test, P = 0.05). The mean shoot:root ratio for coast live oak indicates that this species partitions 2.6 times more biomass in the shoots than in the roots (on average). By comparison, Harris (1992), citing other reports, surmised that the shoot:root ratio for most trees under normal conditions ranged from 5:1 to 6:1. However, Burger et al. (1997) reported shoot:root ratios ranging from 0.4 to 4.1 for seven tree species grown in the field for 2 years: Lagerstroemia indica (0.4), Ginkgo biloba (0.68), Quercus lobata (2.03), Fraxinus latifolia (2.23), Sequoia sempervirens (2.78), Platanus racemosa (3.9), and Quercus agrifolia (4.1). Although other factors affect shoot:root ratios (e.g., soil physical and chemical properties), species is clearly an important determinant. Notably, the average shoot:root ratio did not change from the initial ratio measured for container stock (2.6:1). Considering the potential for restriction in root develop- ment in containers (largely due to container size), the shoot:root ratio may be expected to increase after field planting. However, this finding suggests that little or no change in biomass partitioning (between shoots and roots) occurred in the 4 years after planting. It is possible, how- ever, that root development initially was not limited by ©2005 International Society of Arboriculture , and container size in this study and, accordingly, growth relationships between shoots and roots were not impacted. Root Distribution Root distribution patterns were not significantly different for irrigation treatments. Significant interactions across irrigation treatments were found for cumulative root diameter (crd) at distance intervals from the trunk and at soil depths (Figure 2). At the distance interval closest to the trunk (0 to 30 cm [0 to 12]), crd was significantly greater at 15.2 cm (6 in.) than at 61 and 122 cm (24 and 48 in.). At the 31 to 61 cm (13 to 24 in.) distance interval, crd was significantly greater at the 61 cm (24 in.) depth than at 15.2 or 122 cm (6 or 48 in.). Moving farther from the trunk, crd continued to be significantly greater at 61 cm (24 in.) than at 15.2 or 122 cm (6 or 48 in.) depths. At the farthest distance interval (92 to 122 cm [37 to 48 in.]), crd was significantly greater at 61 cm (24 in.) than at 15.2 cm (6 in.), but the difference between 61 and 122 cm (24 and 48 in.) depths was not significant. At the 15.2 cm (6 in.) depth, very few roots were found in the distance intervals from 62 to 122 cm (25 to 48 in.). At 61 and 122 cm (24 and 48 in.) depths, however, roots were found at all distances from the trunk. Figure 2. Cumulative root diameter (crd) for coast live oak (Quercus agrifolia) at three depths and four distances from the trunk. Values are means for three irrigation treatments combined. At 15.2 cm (6 in.) depth, crd was greater closer to the trunk (0 to 30 cm [0 to 12 in.]) than at distances farther from the trunk (31 to 122 cm [13 to 48 in.]). At 61 cm [24 in.], crd was greatest at intermediate distances from the trunk (31 to 61 cm [13 to 24 in.] and 62 to 91 cm [25 to 36 in.]) . At 122 cm (48 in.) depth, increases in crd were found at distances farther from the trunk (62 to 91 cm [25 to 36 in.] and 92 to 122 cm [37 to 48 in.]) than closer to the trunk (0 to 30 cm [0 to 12 in.] and 31 to 61 cm [13 to 24 in.]).
March 2005
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