Arboriculture & Urban Forestry 42(4): July 2016 or bottom instead of growing away from the trunk in a radial manner, thus reducing the number of roots reaching the side of the current root ball. Cur- rent (Table 1; Table 4) and past (Gilman et al. 2013; Gilman et al. 2015) data suggest that trees remaining in a container for a longer period generate a greater portion of their root system in a small soil volume close to the trunk than those planted to field soil earlier, which can lead to less root growth into adja- cent landscape soil. This leads to less stable trees. Root remediation by slicing root balls and remov- ing potentially girdling roots on the top of the liner root ball had little influence on post-transplanting xylem potential, trunk diameter, or tree height; this was also found in other taxa planted from nursery containers (Gilman and Masters 2010; Harris and Day 2010). However, root pruning by slicing the root ball when planting containers into the field nursery increased root system quality in the current study and others (Gilman et al. 2009); despite this increase, 66% of trees at the end of the field produc- tion period graded as culls, and an average of 43% of trunk circumference remained circled on trees receiving the slicing treatment (Table 2). This data combined with others (Gilman et al. 1996; Weicherd- ing et al. 2007; Gilman and Masters 2010) calls into question the effectiveness of shallow root ball slic- ing. In a follow-up study with a different taxon, slic- ing much deeper (10–12 cm) into the side of a 57 L container root ball when field planting resulted in a significant increase in number of straight roots, and an increase in anchorage in one of the two post- planting years evaluated (Gilman and Wiese 2012). This was explained as a result of deep slicing, cutting through the previous container’s imprint, which not only severed roots growing on the outside of the current container, but cut some that had wrapped the smaller liner container. A more aggressive root pruning program (i.e., root ball shaving, Weicherd- ing 2007) is likely to improve root systems for trees that spend more time in containers prior to planting. Anchorage of freshly dug (the day prior to anchorage evaluation) trees from the nursery was reduced slightly (9%) by slicing container root balls at field planting (Table 2) two or three years earlier; however, there was no impact on anchorage 12 or 25 months aſter transplanting to the landscape. Another related study on Cathedral Oak trees showed that shallow slicing of 170 L root balls in several places 243 top to bottom—as in the current study—had little or no impact on anchorage measured 36 months aſter planting (Gilman and Masters 2010). There- fore, the impacts of slicing root balls to remediate root system defects appear to come with only small changes in anchorage, and these may not persist. Root pruning in the dormant season (Feb., Apr., Oct., and Dec. 2008, and Feb. and Apr. 2009) of Q. virginiana trees growing in the field nursery resulted in a small (9%) reduction in anchorage immediately following field digging (Table 4) compared to root pruning in the growing season (Apr., June., Aug., and Oct. 2008, and Apr. and Jun. 2009). This occurred despite an increase in number of roots at the edge of the dug root ball compared to growing season root pruning. Perhaps the slight (P = 0.10, data not shown) reduction in diameter of straight roots on trees pruned in the dormant season was responsible for the small change in anchorage. There are few pub- lished studies measuring root response of recently dug field trees to root pruning in different seasons. Those published on crown response showed that, like the current study, season of root pruning had little impact on trunk or shoot growth for several years following planting (Ferree 1992; Gilman 1992). CONCLUSION Despite sizeable differences in many attributes with- in the root ball imposed by container liner size, slic- ing the container liner root ball when field planting, and field nursery root pruning timing, there were few differences in trunk and tree height growth in the first three years aſter transplanting field- grown nursery stock to the landscape. This shows the adaptability of this taxon to cultural conditions created in the nursery and suggests that trees can be successfully established from a variety of nurs- ery production systems. The concentration of roots on the interior of the root ball resulting from es- tablishing field nursery trees from large (38 and 57 L) containers compared to the traditional small (11 L) containers did not reduce water stress (ex- cept for one measurement date) when trees were dug from the field nursery. Trees from large liners were slightly less stable when measured the day af- ter digging from the field nursery, but would likely be more stable if they were wrapped in burlap and wire as standard practice. Increased anchorage on trees planted into the field nursery from the smaller ©2016 International Society of Arboriculture
July 2016
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