Effect of Container Type and Root Pruning on Growth and Anchorage After Planting Acer rubrum L. into Landscape Soil

DISCUSSION

Trees remained well within ANSI Z60 (Anonymous 2014) and Florida Grades and Standards for Nursery Plants (Anonymous 2015) size requirements when finished in each container size and planted. Results could have been different if trees became larger by remaining in containers longer, which is commonly practiced (pers. obs.). For example, older roots of red maple (Gilman et al. 2012) and other taxa (Salonius et al. 2000; South and Mitchell 2005) can become suberized with increasing retention time in the container, making them resistant to producing new roots into landscape soil. Additionally, likelihood of developing a large imprint rating increases with retention time in container, and this has been associated with poor root growth into field soil (Salonius et al. 2000; Gilman et al. 2012). This can make trees less stable than those with roots growing straight from the trunk without deflection (Nichols and Alm 1983; Blanusa et al. 2007; Gilman et al. 2013; Gilman and Harchick 2014).

Excepting one year (2011, three years after planting #3 containers, data not shown), the lack of impact on trunk and height growth (Table 1) from installing trees from different nursery container types agrees with most other findings for other taxa planted from containers (Arnold 1996; Marshall and Gilman 1998; Gilman 2001). In contrast, there was a significant effect of container type on trunk cross-sectional area five years after planting red maple from seven container types, six of them different from the current study (Gilman et al. 2003). Despite being more stressed in that study in the weeks following landscape installation, trees planted from low profile containers with perforated side walls had larger trunks than those planted from four other types. Perhaps trees, such as red maple, which develop a shallow root system in soil regardless of soil type (Lyford and Wilson 1964; Gilman and Kane 1990), respond best when planted from a low profile (short and wider than most others) root ball because they become established quicker. Low profile containers position roots close to the soil surface so there are no deep roots that have to make their way to the landscape soil surface to proliferate. In contrast, roots at the bottom of a traditional shaped container (about as tall as they are wide) would have to grow up to near the soil surface before they proliferate; this could be the reason why trees from three of the five standard dimensioned containers grew slowest in that study (Gilman et al. 2003). There are other studies that show an impact of container type on shoot and root growth after planting (Arnold and Struve 1989). Lack of trunk diameter growth differences among container types in the current study could be due to the similarity of dimensions among the eight container types—none were considered low-profile.

There appeared to be no relationship between the amount of root circling or other root attributes at planting (Gilman et al. 2015) and container imprint rating following landscape planting when trees were five-years-old (Table 2). In other words, container types with the least circling roots at planting did not necessarily have the smallest imprint after growing in the landscape for several years. To support this finding, Gilman et al. (2003) found that despite significant differences in root weight and amount of deflected roots among seven different #15 container types when red maple trees were planted into the same field soil as the current study, all measured root attributes were identical five years after planting. There were some slight exceptions in the current study. For example, trees planted from #45 JP containers not shaved during nursery production had a lesser amount of circling roots at the #3 position at planting than all others (Gilman et al. 2010a), but three other containers joined JP as the group that produced the smallest imprint at the #3 position two years later (Table 2). This indicated that JP did not hold its position as the sole container with the least root deflections when evaluated 26 months after planting (Table 2). This could suggest that some roots circling while trees remained in the nursery container did not grow to become the largest roots that formed an imprint after planting into the landscape.

In apparent contrast, recent work on ‘Florida Flame’ maple showed that circling roots present at planting still remained in that position after five years in the landscape (Gilman et al. 2003) indicating the imprint formed early by deflections against container walls can remain for some time after planting. However, trees in that study were retained in #15 containers longer (14 months) than in the current study (12 months). Increased retention time in containers has been shown to encourage formation of a more severe container imprint in propagation (Salonius et al. 2000; Gilman et al. 2012) and much larger (Gilman et al. 2014) containers. 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 containers (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 unanswered 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 ultimate 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 quantified, did not appear to be impacting growth. These roots were not grafted 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. Occurrence of these potential health issues can be reduced in this species (Gilman et al. 2012) and others by growing trees in propagation containers that prevent 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 pruning at planting (Balisky et al. 1995; Arnold 1996).

There is evidence that tree orientation influenced red maple root growth in the nursery (Gilman 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 (Gilman et al. 2015). High substrate temperatures are known to cause root death especially on the sunnier, 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 shifting to larger nursery containers was consistently effective at dramatically reducing the imprint (measured 40 (#15 containers) and 34 (#45 containers) months after field planting) imposed on the root system by all nursery containers (Table 3). This is supported by others on a variety 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 shifting to larger containers resulted in better anchorage (smaller trunk rest angle following winching) to landscape soil 26 months after planting from both container sizes (#15 and #45) tested, compared to not shaving (Table 6). Reduced rest angle indicated 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 supports 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 to the surface (Danjon et al. 2005). Deep roots are rare in Acer rubrum (Lyford and Wilson 1964).

The data presented show that root deflections by container walls can influence root architecture at an early age, and some of these can remain with the tree for at least five years (Table 2). The container-induced imprint was significantly reduced (Table 3), and anchorage to landscape soil increased (Table 6), by shaving root balls during nursery production. Differences in imprint among container types (Table 2) were not related to anchorage, but deflected roots comprising the imprint could impact health, or anchorage, later. The long-term implications from differences in imprint rating among container types for trees planted from #3 and #15 containers (Table 2) remains unknown; longer-term studies will be needed to address this question. However, arborists report (pers. comm. and obs.) trees of the genus Acer are prone to developing roots that grow tangent to the trunk that can eventually form stem-girdling roots. Data presented in the current and past studies show that nursery production practices can influence formation of some of these roots.