Arboriculture & Urban Forestry 41(2): March 2015 compared to trees planted shallowly (Table 2). The difficulty and time-consuming nature of correc- tive measures (root remediation) on deeply planted container-grown trees suggests that it is vital for growers to produce root systems with straight roots and the root collar close to the substrate surface. The impact on anchorage from the factors of mulch placement, root remediation, and nursery planting depth interacted with each other, and varied with year aſter planting, making broad single-factor interpretation impractical. However, no combina- tion of factors (interactions) resulted in more than a 15% change in bending stress within an evaluated year (i.e., 3, 4, and 5 years aſter planting). Bending stress required to tilt trunks to various angles was most correlated with CSA associated with leeward and straight roots (elms) or windward and straight roots (maples, Table 7). Greater resistance to trunk tilting (bending stress) on trees planted from land- scape-sized containers has been associated with an abundance of CSA in windward roots on Quercus virginiana (Gilman and Wiese 2012), and large- diameter straight roots on Acer rubrum (Gilman et al. 2013) and Sweitenia mahogani L. (Gilman et al. 2014). Amount of deflected roots has also been associated with poor anchorage (Gilman and Wiese 2012; Gilman et al. 2013). The initial increase with time followed by a drop in bending stress required to tilt elm and maple trunks (Figure 3) suggests that trees could be more susceptible to tilting and perhaps uprooting in a storm as they became estab- lished beyond three or four years. It is not clear if this trend would continue beyond the fiſth year aſter planting or how this might apply to other species. In conclusion, trees of both taxa planted deeply in the nursery root ball had a greater amount and sever- ity of circling roots when planted into the landscape (Gilman et al. 2010b) and five years later (current study) compared to those planted shallowly in the nursery container. Circling roots present at the time of planting persisted five growing seasons aſter land- scape planting (Table 2), and new adventitious roots did not grow from the base of the trunk of either taxa. Circling root severity depended on root remediation at planting. Circling and other deflected roots were substantially pruned away at planting, which resulted in straighter roots, without impacting physiological stress (stem xylem potential), growth rate, or anchor- age up to five growing seasons aſter planting. By con- 85 trast, Watson and Clark (1993) found that new roots growing from the pruning cuts on remediated trees planted four to ten years earlier typically grew back into a position to girdle the trunk once again three years later. It was not clear, unlike the current study, if an effort was made in that study to cut roots in a man- ner that resulted in the face of the cut away from the trunk. Like Tate (1980) and Watson and Clark (1993) found, remediation was extremely time-consum- ing—up to three hours per tree—on Acer platanoi- des. Roots that occasionally grow over the root collar in natural forest systems are typically from nearby trees—not the tree being evaluated—and have not been associated with decline in health (Lyford and Wilson 1964). The abundance of roots in this posi- tion in nursery containers, caused by deflection from the container wall and/or deep planting, appears to increase the likelihood of one or more becoming large. This can cause instability (Gilman and Masters 2010), trunk injury (Table 4), and symptoms associ- ated with chronic stress (Arnold et al. 2007). How- ever, many trees with deformed root systems in the nursery and landscape appear to grow without stress symptoms (Byran et al. 2010; Harris and Day 2010; current study), indicating there is still more to learn about what constitutes a deformed root system. For now, following strategies that result in straight struc- tural roots radiating from the trunk appear to be rea- sonable, and there are methods of producing trees with this morphology (e.g., Gilman and Paz 2014). All studies to date, including the current one, were conducted on trees that met current nursery stan- dards for size (Anonymous 2004), and they show that roots deformed in nursery roots balls can persist for many years aſter landscape planting. Research should focus on trees larger than standard size because so many are planted, especially in slow markets (pers. obs.). Longer-term studies are also needed because some problems appear to manifest only decades aſter planting (Tate 1980). There appears to be agreement (Byran et al 2010; Harris and Day 2010; current study) that the root collar in nursery containers should be close to the substrate surface. Mulch placed over the root ball at planting did not improve growth or health on either taxa, and appeared to hinder trunk diam- eter growth on elms, which was demonstrated on other taxa in various climates (Arnold 2005; Singer and Martin 2009). Withholding mulch from the root ball surface dramatically reduced roots circling ©2015 International Society of Arboriculture
March 2015
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