42 Gilman et al.: Effect of Container Type and Root Pruning on Root Architecture a result of shaving (Table 4) shows the dramatic change in diameter of roots measured a few cen- timeters from the trunk. Instead of a few roots growing to a large diameter and deflecting in a circling, descending, or ascending manner at the #3 position, shaving nearly eliminated deflection (Figure 1; Figure 4). The increase in diameter of roots growing on the periphery of the finished #15 container (Table 4) showed that shaving increased the size of roots reaching the edge of the larger container. Although not directly mea- sured, shaving appeared to shift the root mass farther from the trunk, as in mahogany (Gil- man and Harchick 2014). Root deflection at the periphery was nearly eliminated by shaving the #15 when shifting into #45 containers (Figure 3). The impact on anchorage by changing the root system architecture from a few large deflected roots to a larger number of non-deflected, smaller, radially-oriented roots (in seven of eight #15 con- tainers tested, Figure 2) has not been extensively tested. Other researchers showed that young (four- to seven-year-old) Quercus virginiana Mill. trees with fewer deflected roots and more straight roots were either slightly more stable or equal in stability (depending on measurement date) to trees with many deflected roots inside the root ball (Gilman and Weise 2012). Moreover, reducing root deflec- tions on top of the root ball by root remediation when planting into the landscape reduced occur- rence of circling roots and roots growing over the root collar in Acer and Ulmus five years aſter plant- ing (Gilman et al. 2015a). Although not shown empirically, this may reduce negative impacts on health by reducing the likelihood of stem-girdling root formation. The one container type (JP) may not have responded to shaving because the num- ber of radial roots, even without shaving, was greater than all but one other type (SP, Figure 2). This desirable root attribute in the container with the smallest volume provides further evidence that the container wall configuration influenced root deflection more so than container volume. Root pruning by shaving the #3 and #15 root ball peripheries, as trees were shiſted, substan- tially reduced attributes associated with poor root systems in finished trees in all eight types of #45 containers (Table 7). This included a dramatic improvement in root system quality by reducing ©2016 International Society of Arboriculture the percentage of trees considered culls from 95% to 42% at the #3, and from 50% to 2.5% at the #15 positions. Moreover, there was a substantial reduc- tion in percent trunk circumference circled by roots at the #3 and #15 positions from root prun- ing (Table 8), and a reduced percentage of trees with roots growing over the root collar (Table 7). Root-ball shaving also increased the diameter of the five largest roots at the periphery of the #45 on the north and south sides of the ball, while decreasing the diameter of the five largest roots at the #15 position (Table 7). Like trees finished in #15 containers, this represented a shiſt in woody root mass away from the trunk. Shaving increased the number (except for RT, Figure 3) of radial roots in finished #45 containers compared to not shav- ing, similar to what occurred in #15 containers when #3 containers were shaved (Table 4). With more roots growing radially instead of downward or around, forming a cage or imprint of deflected roots, the likelihood of forming a natural root system (Lyford and Wilson 1964; Danjon et al. 2005), with some deep and some shallow roots more or less straight, may improve long-term root system quality and anchorage. The radially oriented root system at the #3 and #15 positions allowed more roots to reach the #45 container side wall (Figure 3), resulting in an increase in percent trunk circumference circled with roots at the #45 periph- ery for four of the eight container types, compared to not shaving (Table 8). However, these can be removed by shaving the root ball periphery when planting into the landscape (Gilman and Wiese 2012). However, it should be noted that shaving the periphery will not correct defects present on the root ball interior, which were severe unless trees were shaved during previous shiſts. Landscape contractors should check the interior of the root ball and correct these defects at planting because this taxon appears very prone to root defects. The long-term (more than about seven years) impact on health and anchorage of the root architecture imposed on trees during field and container nurs- ery practices has not been evaluated on any taxa. Fewer and smaller roots grew on the south side of all #15 and #45 containers (Table 5), as Gilman et al. (2010a) observed (not measured) on the same trees as the current study finished in #3 containers and measured in other studies
January 2016
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