178 Garcia-Chance et al.: Differential Environments Influence Initial Transplant Establishment sizes. Data indicate trees in larger container sizes, in some cases, decreased in height during the ini- tial growing season due to slight dieback. This can be seen in the #45 container-grown trees (Figure 4a) in College Station with an ending height of 244.3 cm, and in Starkville with 231.1 cm, versus the original 266.2 cm in the nursery (Table 2). This is indicative of greater transplant stress in the larger container sizes at both locations. Canopy width, rather than trunk diameter, was used as a growth measure for V. agnus-castus due to the variable trunk structure of V. agnus- castus. A curvilinear slope fit the effect of con- tainer sizes (R2 = 0.98) on canopy width (Figure 4c). The slope indicated a greater growth differ- ence among smaller size container-grown trees than the larger container-grown trees (#25 and #45). Similar to height, a slight decrease in width was measured in the Starkville #45 container- grown trees at 268.2 cm, versus the nursery 274.5 cm, indicating nominal dieback. Smaller con- tainer sizes produced a positive change in width during the first growing season post-transplant, whereas even with irrigation, larger container- grown V. agnus-castus growth languished. The lack of significance of locational effect for V. agnus-castus, is unique compared to the other two species (Table 4; Figure 2; Figure 3; Figure 4). However, this could potentially be explained by the documented wide environmen- tal adaptation of V. agnus-castus as evidenced by its resistance to heat, drought, soil variability, and pests (Gilman and Watson 1994). Although it is drought tolerant once established, it will grow faster with supplemental water, especially during initial transplant establishment (Welch 2008). Root growth for V. agnus-castus occurs in larger quantities immediately following trans- plant and extends its roots farther during the initial season post-transplant than A. rubrum or T. distichum (Garcia 2015). Perhaps with irrigation, V. agnus-castus received adequate water at both locations and had appropriate sunlight and temperatures for optimal growth, explaining the lack of significance for location. CONCLUSION Container sizes of transplanted stock had a pro- found effect in initial establishment of all three species during the initial growing season aſter transplant across growth measures and at both lo- cations (Figure 2; Figure 3; Figure 4). For A. rubrum and T. distichum, the best relative growth post- transplant was exhibited by #3 container-grown trees, whereas the best growth rates of V. agnus- castus were from #1 containers. Uniformly poor post-transplant growth was observed with #25 and #45 container-grown trees of all three species during initial post-transplant establishment (Fig- ure 2; Figure 3; Figure 4). This confirms reports from Struve (2009) and Gilman et al. (2010) that reported quicker establishment from smaller-sized planting stock. Where locational effects were ob- served, with the exception of percent change in height growth of A. rubrum, greater growth was found in Starkville, where temperatures and rain- fall were more moderate, suggesting that differen- tial responses associated with container size may be of greater importance in locations with more stress- ful growing conditions than in those with favorable climates. Additional work is needed to determine if these differences among container sizes persist into the future, at some point resulting in similarly sized landscape plants from a range of container sizes, and to determine if root growth or other drought mechanisms are differentially influenced. Documentation of the differences in container sizes by environment will prove useful as container- grown trees gain momentum in the industry. This research will allow homeowners, landscapers, and arborists to correctly select the container size that is best suited for transplant stress for their region, as well as to predict growth responses. Acknowledgments. The College Station portion of this study was included as part of a thesis written in partial fulfillment of the requirements for the M.S. degree by L.M. Garcia. This work was supported in part by funds from Texas A&M AgriL- ife Research and the Tree Research and Education Endowment (TREE) Fund. Mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product by the authors, Texas A&M University, or Texas A&M AgriLife Research, and does not imply its approval to the exclusion of other products or vendors that also may be suitable. ©2016 International Society of Arboriculture
May 2016
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