164 Gilman et al.: Container Wall Porosity and Root Pruning Influence on Swietenia mahogani a smaller imprint, less circling, and greater num- ber of straight radial roots growing into landscape soil (Table 4). This study could not determine which, if any, of these explanations might apply. The deflected “cage” of roots (described in Gil- man and Masters 2010) imprinted on the periph- ery of the non-root-pruned planted 57 L root balls (shown by three times the root system imprint rating, 14 times the root circling, and 16 times the number of deflected roots compared to root pruned root balls; Table 5) likely pushed later- ally and downward against mineral landscape soil during winching. The stiff nature of this ligni- fied, imprinted “cage” of roots on trees not shaved apparently resisted deformation, and therefore overturning, as did the very different architec- ture of the many smaller diameter roots growing straight (horizontally) into landscape soil asso- ciated with shaved trees (Figure 1). Roots that grew horizontally across the root ball/landscape soil interface on root-pruned trees may not have been stiff enough to resist bending; although there were many more of them, representing more CSA (Table 5). Sinker roots from the abundant, radially oriented horizontal roots prevalent in root pruned trees (Table 5), along with stiffer horizontal roots, would be needed to maximize anchorage (Coutts et al. 1999). In addition, winching forced horizon- tal roots on the winch-ward side down into decom- posing nursery root ball substrate, which Gilman and Masters (2010) showed had less resistance to overturning than mineral soil. In a sense, the non- root-pruned trees lacked straight horizontal roots, whereas shaved trees lacked the stiff “cage” of ver- tical roots; either combination resulted in compa- rable anchorage in this test of very short duration. Others showed, on much older trees, that a few large roots provided better anchorage than many small- diameter roots (Coutts et al. 1999). This appears consistent with findings of the present study. There is evidence that shaving root balls at plant- ing enhances anchorage when evaluated one and two years aſter planting Quercus virginiana Mill. trees (Gilman and Wiese 2012; Gilman 2013). Moreover, bending stress required to tilt trees planted from containers increased with time in the first two years (Gilman and Wiese 2012), suggesting that only small treatment differences would be expected on trees planted just a few months earlier. Trees in the ©2015 International Society of Arboriculture current study may have been winched too soon aſter planting to measure an effect. Swietenia mahogani should be leſt in the ground for a longer period in order to more thoroughly evaluate anchorage. Similar to Swietenia mahogani planted into landscape soil from 9.5 L containers (Gilman and Harchick 2014), trunk tilt during winching to 4.1 MN/m2 bending stress in the current study was cor- related with several attributes of root architecture close to the trunk (Table 7). Many of these were correlated with each other, so a predictive model incorporating these would be skewed by autocor- relation. Increasing the deflected nature of the root system and reducing the number of straight roots lead to greater trunk tilting, which is sup- ported by Ortega et al. (2006) and others. This analysis also showed that visual estimates, such as root system imprint from the container and root ball symmetry, were as good at predicting anchor- age as more time-consuming measurements, such as percent trunk circled and occurrence of root systems graded as culls. These quick visual evalu- ations will make it relatively simple for growers, landscape architects, arborists and others to evalu- ate root system quality without time consuming measurements. However, low Pearson’s correlation coefficients (r = 0.32 to 0.39) showed that there was a substantial amount of unpredictability in bending stress, which displays a limited under- standing of all factors that govern tree anchorage. Vertical and circling roots caused by deflection in the propagation container can lead to poor root- ing out, resulting in instability (Lindström et al. 2005; Chapman and Colombo 2006) and growth reductions (Krasowski 2003), but researchers did not find this when planting from much larger 9.5 L containers (Gilman and Harchick 2014) or from 57 L containers (current study). Perhaps the process of growing trees in these larger nursery containers forced new roots from the root collar, resulting in more growing horizontally above roots deflected by propagation containers. This would have reduced impact of vertical and circular deformations imprinted by the propagation container. This phe- nomenon may have gone undocumented because previous studies have been performed on trees grown in propagation containers for reforestation, not on trees transferred to a larger nursery con- tainer as in the landscape horticulture profession.
May 2015
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