Impact of Nursery Root Pruning and Tree Orientation at Planting on Growth and Anchorage

RESULTS AND DISCUSSION

Root pruning 12 L container root balls as they were shifted to 51 L containers in the nursery did not impact mean trunk diameter (37 mm, maple; 27 mm, oak) or tree height (3.4 m, maple; 2.4 m, oak) at landscape planting (Gilman et al. 2010b). This also held true after planting into the landscape (data not shown). For example, nursery root pruning had no effect on mean tree height increase (4.6 m, P = 0.69, maple; 5.0 m, P = 0.93, oak) through 2012 (four years after planting) or mean trunk diameter increase (121 mm, P = 0.27, maple; 132 mm, P = 0.73, oak) through 2013 (five years after planting). Changing tree orientation at planting—by rotating trees 180 degrees compared to their orientation in the nursery—also had no influence on tree height or trunk diameter increase for either taxon in any year after landscape planting (P > 0.73). There was no trunk bark damage evident on any trees at any time.

There was no impact (P > 0.42) of root pruning in nursery containers or tree orientation when planting into the landscape on root system imprint rating (from 12 L or 51 L containers) five years later on either taxon (data not shown). This occurred despite significant root architecture differences at planting—due to root pruning during nursery production (Gilman et al. 2010b)—and appears to suggest that deflections from container walls occurring five years previously did not persist and did not result in the potential, long-term health issues caused by roots circling the trunk. This might be attributed to the short duration in 12 L (14 months) and 51 L containers (seven months) prior to landscape planting; whereas trees are often retained for much longer by many growers (personal observation) even as they remain in conformance with industry standard sizes (American Association of Nurserymen 2014). A longer retention time for these and other taxa would likely have resulted in a greater container imprint on the root system as others found (Salonius et al. 2000; Gilman et al. 2012). Moreover, the type of 12 L container used in this study has been shown to reduce root system imprint and length of roots circling the container compared to smooth-sided containers (Gilman et al. 2010a). In addition, both taxa were propagated from cuttings, and there is evidence that each taxa continues forming new roots from the root collar when planted from 12 L into larger containers (Gilman and Harchick 2008, oaks; Gilman et al. 2013, maples). Continued new root production within the 51 L container may have allowed both to mostly outgrow the 12 L container imprint by generating new roots that became larger than those deflected by the smaller container. Results would likely be different for species that cease producing new roots from the root collar soon after seed germination, such as Tabebuia spp. (Gilman et al. 2010b).

Arborists and other practitioners report that stem-girdling roots in the landscape are a direct result of roots circling in nursery containers, and these can cause trees to become physiologically stressed or worse in the landscape (Giblin et al. 2005). Perhaps surprisingly, there is sparse documentation in the scientific literature of container-induced root deflections causing health issues in the landscape, although there is ample evidence that conifers planted from propagation containers in reforestation efforts can perform poorly when planted with root defects (Burdett 1978; Salonius et al. 2000). In related work, Tate (1980) and Giblin et al. (2005) showed that encircling roots in the landscape—typically attributed to planting too deeply—can cause a decline in health as long as three decades later. However, these roots could have been generated after planting and been the result of site conditions or other factors not inherent to the nursery stock. Arborists also report poor anchorage on trees with severe circling roots and other root defects from the nursery (Johnson and Hauer 2002); Gilman and Masters (2010) documented that some of the reported instability is directly attributed to root architecture created in the nursery and other factors.

Root pruning during production did not impact bending stress required to tilt maple trunks one to five degrees in either one (P > 0.19), two (P > 0.84), or three (P > 0.84) years after landscape planting (Table 1). Tree orientation at planting also had no impact on maple bending stress in the first three years after landscape planting—the only years tested (Table 1, P > 0.17). Lack of impact on post-planting maple anchorage from either main effect came despite significant pruning-induced changes in root architecture (i.e., smaller container imprint) within the root ball at planting five years earlier (Gilman et al. 2010b). No main effects (root pruning, tree orientation, side of tree) influenced maple root CSA; however, root pruning inexplicably reduced root CSA in the five largest roots when maple trees were rotated 180 degrees at planting but had no impact when trees were planted in their nursery orientation (Table 2).

Similarity in maple anchorage between treatments in the current study could have been due to the relatively small size of the trees (16 mm trunk diameter) in the 12 L containers prior to shifting into 51 L containers compared to a recent study on slightly older trees of the same maple (20 mm trunk diameter in 12 L container, Gilman et al. 2016) that showed better anchorage after planting of nursery stock that was root pruned when shifted to larger containers. Trees retained in containers for a longer period have more root defects with a greater root system imprint than those retained for a shorter period, leading to a smaller amount of roots growing into substrate or soil after planting (Salonius et al. 2000). Root pruning by shaving older propagation container root balls that were more imprinted had a greater impact on root architecture within 12 L root balls than shaving less pot-bound trees (Gilman et al. 2012).

Like maples, root pruning during nursery production did not impact bending stress required to tilt oak trunks one to three degrees at the end of the first or third year after planting (P > 0.75, Table 3). Unlike maples, at the end of the second growing season, there was a significant root pruning effect on bending stress required to winch oaks to all angles (P < 0.03; Table 3 and Figure 1). Specifically, shaving 12 L container root balls as they were shifted into 51 L containers resulted in about a 28% increase in bending stress required to tilt oaks five degrees (Figure 1); this indicated better anchorage. There are few other studies evaluating post-planting anchorage of trees root pruned during nursery production. Gilman et al. (2016) found that planting 57 L (similar size as current study) or 170 L red maple root balls that were shaved in the nursery improved anchorage in the landscape by 20%–25% compared to trees not shaved. Improved anchorage in that study was attributed to an abundance of straight roots in the root ball. This has been accomplished by shaving when shifting to a larger container size in the nursery, or by growing in certain porous-walled containers (Gilman and Paz 2014).

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Figure 1.

Effect of root pruning in nursery on trunk bending stress to winch oak two growing seasons (21 months, October 2010; P = 0.03) after planting into field soil; vertical lines represent SE.

In slight contrast to maples, oak tree orientation at planting affected (P = 0.06) bending stress required to tilt trunks at the end of the first year after planting, but only to four and five degrees (Table 3) and not in the second and third growing seasons. Specifically, rotating the north side of oaks 180 degrees to the south when planting into the landscape resulted in about a 15% decrease in anchorage when winched north to four and five degrees trunk tilt, compared to maintaining trees in the same orientation as in the nursery at planting (Table 4). Oak trees rotated at planting had nearly five times the root CSA (3,485 mm², averaged across root pruning) on the north side of the tree measured five growing seasons after planting than those not rotated at planting (776 mm²; Table 5). Although this indicated more growth from the hot (south) side of the container, trees not rotated at planting failed to show this response. This makes definitive conclusions on the impact of orientation in the nursery container on post-planting root growth difficult. There is evidence of reduced root growth on the periphery of the hot side of nursery containers (Ruter 1993), resulting in smaller diameter and fewer roots (Gilman et al. 2016) that can position more root tips within the root ball (Smith and McCubbin 1992) capable of growing out into landscape soil. An increase in root CSA outside the root ball could eventually contribute to anchorage by entrapping a larger volume—and therefore mass—of soil in the root plate to resist overturning (Fourcaud et al. 2007; Gilman and Masters 2010).

Stability in forest plantations has been associated with shallow roots in the windward direction (Stokes 1999), lateral roots positioned symmetrically around the trunk (Coutts et al. 1999), tap and sinker root mass (Danjon et al. 2005), leeward sinker root length (Hayfa et al. 2007), and combinations of these. There is far less known about the anchorage of trees planted from landscape-sized root balls, and results vary even for similar taxa. For example, root CSA measured toward the pulling winch three years after landscape planting of four-year-old Quercus virginiana Cathedral Oak from 170 L containers (Gilman and Masters 2010) has been associated with anchorage; whereas root CSA on the opposite side of the tree was most correlated with anchorage for a different cultivar of the same species (Q. virginiana Highrise®) planted from 57 L containers (Gilman and Weise 2012). Experimental design did not allow roots to be measured when trees were winched, making direct relationships between root attributes and anchorage difficult; however, there were some correlations. Maple and oak bending stress to tilt trunks three years after planting was positively correlated with root CSA in the largest five roots in the north and south sides of the tree measured five years after planting (Equation 2; Equation 3). Only about 20% of the variability in trunk tilt could be explained by root CSA. This significant but weak relationship suggests that although the CSA of the largest roots bridging the interface of container substrate and landscape soil play a role in anchoring trees in the first few years after planting, there are other factors involved. Future studies should excavate and characterize entire root systems—not just the top 12 cm, as in the current study—in an effort more comprehensively quantify anchorage. Results could have been different if trees were retained in containers longer, if porous-walled containers were used, or if trees were planted in a different season.

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