Journal of Arboriculture 31(4): July 2005 165 ) area around each tree was mulched to a depth of 0, 7.6, 15.2, or 22.9 cm (0, 3, 6, or 9 in.) with a mixed particle size, commercial, shredded pine bark mulch (Southwood Valley Turf, College Station, TX). Mulch treat- ments were separated between plants via two 0.61 m (2 ft) long, double-stacked, 10 cm (4 in.) tall CCA-treated land- scape timbers (Lowe’s, Bryan, TX). Mulch was replenished in the spring and fall of each year to maintain the desired treatment levels. Three sets of plants with one of each mulching thickness treatment were randomly chosen to monitor soil water potentials adjacent to the root ball. Soil water potentials in mulch treatments were monitored throughout the first growing season. Both species remained in the field under irrigation for 2 years after planting—a time frame in which the trees should have been well established in USDA plant hardiness zone 8b (Gilman 1997). After the second year, the K. bipinnata study was terminated due to low survival of some treatments. During the third growing season, irrigation was not provided to F. pennsylvanica to confirm that these plants were fully established. Height, trunk diameters at 15 cm (6 in.) above the soil surface, survival, and the percentage of the canopy exhibiting stress symptoms (chlorosis, marginal necrosis, and/ or premature leaf senescence) were measured at transplant to the field and at the end of each growing season. Each species was treated as a separate experiment. The statistical design was a randomized complete block design consisting of a factorial of three planting depths × four mulching thicknesses for each species. In the experiment with F. pennsylvanica, there were ten blocks containing a single plant replication of each treatment combination, whereas with K. bipinnata there were eight blocks. A square root transformation was performed on the percentage data to normalize variation prior to analysis. The results of this analysis were converted back to percentages for presenta- tion in tabular form. Data were analyzed using the general linear models procedures in the SAS System for Windows, Release 8.01 (SAS Institute, Inc., Cary, NC). 15.2 cm (6 in.) depth. Trees were irrigated daily for the first 4 weeks using drip tape (T-Tape®, T-Systems International Inc., San Diego, CA) at 10 psi to maintain moisture in the transplanted root balls and afterward when soil water potentials reached –1.5 kPa (–15 bars) in nonmulched control plots. The drip tape was located above the mulch. In factorial combinations with the three planting depths, four mulch thickness treatments were established. A 0.74 m2 (8 ft2 RESULTS AND DISCUSSION Green Ash Green ash did not exhibit any significant (P ≤ 0.05) interac- tions between planting depth and mulch thickness involving time after planting for height or trunk diameter (Table 1). Height growth responses for green ash were significant (P ≤ 0.05) only for main effects of time, mulch, and planting depth, with no two- or three-way interactions significant for this variable (Table 1*). Across planting depths and mulch treatments, mean green ash trunk diameter increased for all 3 years after transplanting, while height growth increased slowly during the first 2 years and more rapidly in the third (Table 1). The equal or greater increase in height and trunk diameter after cessation of irrigation in the third year suggests that the trees were fully established in the land- scape. This finding would be consistent with Gilman’s (1997) model of establishment in USDA plant hardiness zone 8b. With the exception of the 7.6 cm (3 in.) below-grade planting depth treatment, mulching had a consistent negative effect on green ash height growth when pine bark mulch depths were maintained at greater than a 7.6 cm (3 in.) depth (Table 1). The apparent inconsistency of no reduction in height or trunk diameter due to mulch thick- ness when green ash were planted below grade may have been an artifact of the much lower survival of trees that were planted below grade and then mulched. Perhaps only the more vigorous trees survived, biasing the growth response to this treatment because the smaller, less vigorous trees in the mulch treatments died in the second and third years of the study. Mean height after three growing seasons for green ash was reduced for trees transplanted 7.6 cm (3 in.) below grade (156 cm [61 in.]) compared to those planted at grade (166 cm [65 in.]), and planting 7.6 cm (3 in.) above grade slightly increased height growth (172 cm [68 in.]). Likewise, the proportion of the canopy exhibiting transplant stress symptoms during the first year following transplanting (Table 2) increased by 12% to 26% for plants with mulch depths of 7.6 cm (3 in.) or greater. Gilman and Grabosky (2004) reported that 7.6 cm (3 in.) thick hard- wood mulch treatments impeded light irrigation or rainfall events from penetrating to the planted root balls, but they found no effect of planting depth on container-grown live oak trunk diameter at 7 months after transplant. This apparent lack of response during the first growing season for live oaks is not inconsistent with the results of green ash in the current study because differential growth and survival responses to planting depth were minimal for green ash during the first growing season, but they were manifested more strongly the second and third years after transplant (Table 1). Significant (P ≤ 0.05) two-way interactions among planting depth and time after transplant, mulch thickness and time after transplant, and planting depth and mulch thickness were present for green ash survival (Table 1). Green ash survival declined much more rapidly over time after transplant for trees planted below grade than those at *Tables appear on pages 168–170. ©2005 International Society of Arboriculture
July 2005
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