Arboriculture & Urban Forestry 33(1): January 2007 67 Table 4. Main effect of planting depth on the adjusted relative growth of trunk cross-sectional area and trunk diameter of Fraxinus pensylvanica trees grown for 3 years in a field site in College Station, Texas, after transplant from 9.3 L (#3) black plastic containersz . Trunk cross-sectional area Planting depthy Year 2 (cm2 Below grade 190 bx At grade 395 a Above grade 350 a cm−2 ) Year 3 (cm2 258 b 484 a 414 a cm−2 ) Trunk diameter Year 2 (mm mm−2 −0.07 b 0.53 a 0.39 a zAdjusted relative growth of trunk cross-sectional area and trunk diameter refers to the incremental growth during the year relative to the initial value of the parameter. yRoot collar 7.6 cm (3 in) below grade, at grade, or 7.6 cm (3 in) above grade. xMeans followed by the same letter are not significantly different (P 0.05) using least squares means comparisons. Values represent the means of six plants per planting depth. canopy volume if planted below grade, whereas those planted above grade nearly doubled the canopy volume of those planted at grade (Table 3). Adjusted growth rates based on initial size for a given year differed among species (data not presented) but differed among planting depth treatments only for the second and third years’ growth for trunk cross- sectional area and second-year trunk diameter growth for green ash (Table 4). These data show similar patterns of responses as observed for F. pennsylvanica and Koelreuteria paniculata Laxmann in previous studies (Arnold et al. 2005), but also illustrate the wide magnitude of variation in species responses to planting depth and that the pattern of response varies depending on the measure of survival or growth process observed. Given the soil type and overall climatic conditions present in 6 years (two consecutive 3 year studies), growth and/or survival was reduced on all species tested when trees were planted as little as 7.6 cm (3 in) below grade. This is in contrast to the results of Gilman and Grabosky (2004) and some of the species tested by Browne and Tilt (1992), in which survival or growth of a few species did not appear to be impacted by planting below grade, in some cases with even deeper plant- ing than was tested in this study. Part of these differential results might be explained by the shorter duration of Gilman and Grabosky’s study (7 months). Differences in growth and survival in the current study were greatest after 3 years. Also, the differences might be the result of a substantially heavier soil on the Texas site than the sandy soils in Gilman and Grabosky’s Florida site. The soil type was not reported for Browne and Tilt’s work, which makes it difficult to identify if differences between the studies were soil-related or the result of species differences. Another issue is the size or age of the planting stock. Gilman and Grabosky’s trees were ) much larger and presumably older at transplant than the trees used in the current study. Although their larger sized stock may be more representative of commercial street tree instal- lations, the smaller sized container stock used here may be very representative of homeowner-planted trees from mass market garden centers or, in the case of oleander, represen- tative of commercial shrub installations. These smaller sized container stock represents millions of trees and shrubs planted annually in the United States. Short-term preliminary studies with Quercus virginiana Mill. on four contrasting soil types indicated that heavier soils and poor internal drainage tend to accentuate adverse responses in water relations, pho- tosynthetic gas exchange properties, and growth parameters to planting below grade (Bryan et al. 2005, 2006). Adverse responses to deep planting in those studies were less severe on better drained sandy soils. CONCLUSIONS Data presented in this article provide strong evidence for the adverse effects of even modest below-grade planting on con- tainer-grown species from five genetically diverse families. In addition, this study shows growth is adversely affected for both seed and cutting-propagated container-grown nursery stock. Additional work is needed to determine the effects of soil types, irrigation regimes, planting times, stock size, and other posttransplant cultural practices on remediation or ac- centuation of the severity of plant responses to deep planting. Likewise, the impacts of cultural practices during container nursery production on responses of trees to below-grade planting have not been documented. Acknowledgments. This project was funded in part by the Texas Agricultural Experiment Station and grants from the Morton Arboretum, National Urban and Community Forestry Advisory Council, TREE Fund, J. Frank Schmidt Family Charitable Foundation, and Texas Ornamental Enhancement En- dowment. Mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product by the authors, the Texas A&M University, or the Texas Agricultural Ex- periment Stationand does not imply its approval to the exclusion of other products and vendors that also may be suitable. LITERATURE CITED Arnold, M.A., G.V. McDonald, and D.L. Bryan. 2005. Plant- ing depth and mulch thickness affect establishment of green ash and bougainvillea goldenraintree. Journal of Arboriculture 31:163–170. Ball, J. 1999. Plant health care CEU series part VI: Tree planting—The foundation of PHC. Arborist News 8: 53–58. Broschat, T. 1995. Planting depth affects survival, root growth, and nutrient content of transplanted pygmy date palms. HortScience 30:1031–1032. ©2007 International Society of Arboriculture
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