170 O'Connor et al.: Container Type Affects Root Development During Landscape Establishment cess. While researchers did not develop a rating system for root architecture, the measured percent distribution of new root growth for the three con- tainer types confirms that trees grown in the two fabric containers possess root systems with greater lateral growth and fewer roots concentrated within the circumference of the original planting root ball. The presence of SGRs has been suggested as a predictor of tree failure (Johnson and Johnson 1999). Other studies have suggested that SGRs can arise from container-related circling roots present at planting, increasing the potential for future tree failure (Meilleur 2009). Johnson and Hauer (2000) found that 73% of lindens that failed in storms in Minnesota, U.S., broke at the point where SGRs had constricted the stems. In a North Carolina, U.S., study, 75% of 400 air- spaded trees were found to have SGRs (Meilleur 2007). Prior to planting trees in the current study, the only corrective procedure used to eliminate circling roots on the container root ball was verti- cally slicing the root ball on the outside several times, using a box cutter, and hand-pruning to remove any visible circling roots. Gilman et al. (2010b) and Gilman and Wiese (2012) found that shaving the roots from the outer periphery (2.5 cm) of the root ball leads to reduced circling and girdling roots. Had root shaving been practiced when transplanting trees in the current study, researchers may have observed improved root systems with trees grown in BP containers, since root shaving has been found to increase straight, radial root production from the trunk (Gilman et al. 2010b). This suggests an area of future research—to compare post-transplant root growth and architecture of shaved root balls of BP-grown trees with that of non-shaved fabric containers. Aſter the third year in the landscape, the larg- est percentage (94%+) of excavated roots for all three container types were coarse roots (≥2.1 mm diameter). However, researchers found more fine roots aſter the first year in the landscape (12%– 13% of all roots for all container types) than in the second (approximately 3% for all container types) and third years (2%–5% for all contain- ers). While the importance of fine-root devel- opment on establishment success of landscape trees is unproven, it’s commonly stated that fine roots aid in tree establishment (Ham and Nelson ©2018 International Society of Arboriculture 1998) and that fine roots are important for the absorption of water and nutrients (Persson 1983). No container effects were found on height, cali- per, total dry leaf, and shoot weight and twig and leader growth of trees harvested in 2011, 2012, and 2013 (Table 2). While it is difficult to define when a transplanted tree has become fully established in the landscape, the absence of measurable container effects on aboveground growth could be explained by the findings of numerous studies suggesting that newly transplanted trees use carbohydrates to regenerate new roots, and that top growth may be significantly reduced until the new root system is sufficiently regenerated (Watson 1985; Lauderdale et al. 1995; Gilman et al. 1998; Marshall and Gil- man 1998; Ortega et al. 2006; Owen and Stoven 2008; Neal 2009). A study by Gilman (1997) found that in USDA zone 9 (Florida, U.S.), a 5.1 cm tree established in six months, while the same species in USDA zone 5 took 24 months to establish. Defin- ing when a tree has become established and mea- suring transplant success remains difficult (Watson 1985; Gilman 1990; Struve et al. 2000). However, the most limiting factor following tree establish- ment is irrigation, as found by Gilman et al. (1998). Since trees in this study were not drought-stressed, irrigation was not a treatment factor. The trees can be assumed to be fully established aſter one year. Researchers found that there were differences among the containers and their root systems in years two and three but not the first year aſter transplant. Researchers speculate that because neither soil moisture nor soil fertility were limiting factors in the study, they were not likely to see container effects on the height and caliper of trees follow- ing transplanting. Similarly, Gilman et al. (2010a) found few effects of container type on height and caliper when transplanted trees were given ade- quate irrigation and fertility, and Marshall and Gilman (1998) and Gilman et al. (2003) found no effects of container type on height and cali- per of red maple five months and five years aſter planting in the landscape, respectively. Gilman et al. (2003) concluded that irrigation frequency for the first 24 weeks following transplant in Florida was a more important factor than container type in influencing establishment and aboveground plant growth. Because successful establishment of transplanted trees in Colorado and the semi-arid
July 2018
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