80 Gilman et al.: Effect of Container Type and Root Pruning on Growth and Anchorage Aſter Planting formation of a more severe container imprint in propagation (Salonius et al. 2000; Gilman et al. 2012) and much larger (Gilman et al. 2014) con- tainers. Data from these studies combine to show that container type had little impact on short-term (five years) red maple shoot growth, root growth, or anchorage on trees planted from #45 containers, but could affect those planted from #3 and #15 con- tainers (Table 2) if retained in containers too long. It is possible that root attributes could vary—deeper in the soil—than measured in this study, although red maple roots typically grow from the top of the root ball (Gilman and Kane 1990; Gilman et al. 2003) and remain there (Lyford and Wilson 1964). Deeper roots are likely less able to cause health issues by girdling the trunk. What remains unan- swered is how long a retention time is too long, and what are the impacts on long-term health, growth, and stability. There is much to learn about the ulti- mate fate and impact of circling roots in containers. There was no correlation between imprint rating and top diameter of any size container (#3, P = 0.72; #15, P = 0.83; #45, P = 0.88), indicating differences in root response among #3 and #15 types were largely due to the nature of the container walls—not container dimensions—as found for this same set of finished red maple in #3 containers (Gilman et al. 2010a). Roots growing up (ascending) the liner container side wall and crossing over the root collar close to the trunk were not embedded into the trunk on the same set of trees finishing in #3 containers. These roots, sometimes as large as a finger, had embedded into the trunk by the time trees were five- years-old (March 2014), and although not quanti- fied, did not appear to be impacting growth. These roots were not graſted to the trunk as indicated by little or no white wood connecting one to the other, presence of swollen trunk tissue and bark cracking just above the root, and bark inclusions between the two tissues indicating poor connection. Occur- rence of these potential health issues can be reduced in this species (Gilman et al. 2012) and others by growing trees in propagation containers that pre- vent or reduce defects (Ortega et al. 2006), removing trees earlier (Harris et al. 1971a; Harris et al. 1971b; Salonius et al. 2000), or mechanical root prun- ing at planting (Balisky et al. 1995; Arnold 1996). There is evidence that tree orientation influ- enced red maple root growth in the nursery (Gil- ©2016 International Society of Arboriculture man et al. 2010a; Gilman et al. 2015), and that some of this carried over into the landscape. Increased root CSA growing to the north side compared to the south on landscape trees planted from #45 containers (Table 4) appears to have resulted from more root growth on that side in the nursery (Gil- man et al. 2015). High substrate temperatures are known to cause root death especially on the sun- nier, hotter container side (i.e., south and west side in the Northern Hemisphere, see Ruter 1993; Owen and Stoven 2008). This relationship suggests that some of the root growth variation among trees in a landscape and in research plots can be attributed to orientation in the nursery and how the tree might be ultimately oriented in the landscape. Some of this effect could also have been due to the more shaded and probably cooler container substrate and landscape field soil on the shaded side of the crown (north side in the Northern Hemisphere). In contrast to container type, root pruning by shaving while shiſting to larger nursery containers was consistently effective at dramatically reduc- ing the imprint (measured 40 (#15 containers) and 34 (#45 containers) months aſter field planting) imposed on the root system by all nursery contain- ers (Table 3). This is supported by others on a vari- ety of tree taxa (Weicherding et al. 2007). Unlike container type, which impacted anchorage (as measured by bending stress) only for trees planted from #45 containers (not those planted from #3 and #15) and then only when winched to four and five degrees, root pruning had a considerable impact on anchorage. Shaving trees when shiſting to larger containers resulted in better anchorage (smaller trunk rest angle following winching) to landscape soil 26 months aſter planting from both container sizes (#15 and #45) tested, compared to not shaving (Table 6). Reduced rest angle indi- cated less root ball overturning and hence stronger attachment to landscape soil. Straight roots have been associated with improved anchorage for Quercus virginiana (Gilman and Weise 2012), Acer rubrum (Gilman et al. 2014), and Swietenia mahagoni (Gilman and Harchick 2014) planted from containers; data from the current study sup- ports this. Deep roots under the trunk are also extremely important for anchorage on certain taxa and in certain soils, and they function structurally in combination with relatively straight roots close
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