40 Gilman et al.: Effect of Container Type and Root Pruning on Root Architecture Figure 4. A) Root system from #45 container not root pruned when shifting showing the prominent imprint from the #3 container (largest roots circling and descending center and top left) and a smaller imprint from the #15 container (circling roots bottom right). B) Root system from #45 container shaved when #3 and #15 were shifted to the larger container showing no #3 or #15 container imprint; there is a prominent imprint from the liner showing at least half the trunk circled with roots growing tight against the trunk. Liner root balls were not shaved when shifted into #3 containers. Note: Photographs were taken several months after trees were washed, which explains the cracked and dried appearance of trunk and root bark. DISCUSSION Differences in trunk diameter and tree height growth among container types were small, mea- sured over the course of the three-year study; this was typical of red maple and other taxa in previous studies (Marshall and Gilman 1998, in Florida, U.S.; Owen and Stoven 2008, in Oregon, U.S.; Neal 2009, in New Hampshire, U.S.; Amoroso et al. 2010, in Italy; Stoven O’Conner et al. 2013, in Colorado, U.S.). It appears fair to conclude, based on these studies, that trees grow about the same trunk di- ameter and height in most container types given appropriate water and nutrient management for the taxa and region, even with a 40% difference in substrate volume among types (Table 1). Although results could have been different under a different irrigation regime, the cited studies were performed under varying cultural regimes in different climates. When roots on this same set of trees as the cur- rent study were washed and measured from #3 containers, all container types showed less circling root length (by about one-third) compared to SS; however, circling root length still represented about half of the total root length (roots >2 mm diameter) growing on the root ball periphery, with the remain- ing growing in other directions (Gilman et al. 2010a). Similarly, trees in all #15 container types not shaved (except RT) had a smaller percentage of trunk circled ©2016 International Society of Arboriculture with roots at the #3 position than those in SS (Figure 1), indicating roots remained in the deflected posi- tion and grew larger, thus forming an imprint on the root system. In contrast, only trees in half the #45 containers (AP, FN, JP, and SP) had a smaller percentage of trunk circled than SS at the #3 posi- tion . It appears that roots smaller than the threshold measured (3 mm diameter) continued to grow in diameter in the circled fashion at the #3 position as trees were shiſted to larger containers, thus resulting in an increase in percentage of trunk circled with time for most container types (Table 8; Figure 4). It was difficult to isolate one container type that consistently generated roots in all three sizes tested [#3 (Gilman et al. 2010a), #15, and #45] that would be considered the most desirable, although there were trends. Finished trees in #15 FN and JP were always in the group of containers that had all of the following attributes: the least percentage of trees with roots >5 mm diameter growing over the root collar, the smallest diameter roots at the #3 posi- tion, and a relatively small number of ascending roots at the #3 position (Table 3). Trees in JP and SP #15 containers had more radial roots growing away from the trunk than all other containers not shaved (Figure 2); abundant radial roots have been associ- ated with good anchorage (Ortega et al. 2006). Less deflection may have allowed roots to grow freely
January 2016
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