Effect of Tree Size, Root Pruning, and Production Method on Root Growth and Lateral Stability of Quercus virginiana

DISCUSSION

Slicing the outside periphery of 170 L container root balls in six places from top of root ball to bottom had no impact on tree stability three growing seasons after landscape planting, whether measuring bending moment (Table 3) or bending stress required to pull trees to increasing trunk tilts (Figure 4), or trunk resting angle immediately following release from a pull test (Table 5). This was not surprising since there was no impact on root growth into landscape soil from root ball slicing (Table 1). There is little published on container-grown trees of this size responding to mechanical root pruning (Gilman et al. 2010b) and no reports of stability measurements; however, others have shown on much smaller containers a redistribution (Gilman et al. 1996) or increase (Blanusa et al. 2007) in root number from mechanical slicing root balls at planting. Perhaps shaving off or removing the entire outer periphery of root ball at planting will increase root number and soil volume explored by roots as occurred when shifting trees from 11.3 L containers into 57 L containers (Gilman et al. 2010a).

Field-grown trees were more stable than comparably sized trees planted from 170 L containers requiring significantly more bending stress to reach 5, 10, 15 (Figure 4) or more (data not shown) degrees trunk tilt. Bending stress normalized bending moment to account for or remove the affect of a 13% difference in trunk caliper between these treatments when pulled (Table 1). In addition, trunk resting angle was less for trees transplanted from the field immediately following release from 15, 20, and 25 degree pulling tests than trees planted from 170 L containers (Figure 5). Reduced trunk resting angle 15 cm from ground indicated less permanent root ball overturning and hence stronger attachment to landscape soil. Using the calibration in Table 2, it is clear that trees transplanted from the field would have required much greater wind speed than 45 m·s⁻¹ to tilt lower trunk five degrees; whereas, trees planted from containers in 45 m·s⁻¹ wind tilted more than 10 degrees. Others also found that bending moment required to pull trunks to a 10 degree tilt was influenced by nursery production method for small seedlings planted as part of reforestation (Lindstrum and Rune 1999). Root systems on trees transplanted from the field were so stable that five of seven tree trunks broke as they were pulled past 15 degrees; whereas, no trees planted from 170 L containers broke because root balls overturned. The study authors could not estimate the wind speed required to tilt field-grown tree trunks to 15 degrees because this was well beyond our calibration; however, Table 2 shows that 45 m·s⁻¹ wind tilted trunks just three degrees.

Several factors could explain reduced stability of trees planted from 170 L containers compared to transplanted field-grown trees. Trees from 170 L containers had one-quarter the root CSA into landscape soil, one-third the root CSA/cm² of trunk CSA, one-half the number of roots, two-thirds the number of roots/cm² trunk CSA, and smaller diameter roots measured just outside the original root ball compared to field-grown trees (Table 1). The slightly (13%) smaller (Table 1) trunk diameter on 170 L container trees when pulled does not account for the dramatically less (one-fourth) root CSA area outside original root ball which Mickovski and Ennos (2002) showed scaled to the 1.7 power of trunk diameter. Root deflection downward or around container walls can also reduce stability (Lindstrom and Rune 1999, Ortega et al. 2006); in the present study, roots appeared to be deflected by each of three container sizes (3.7 L, 57 L, and 170 L). Despite trees of this size becoming established by about 18 months after planting in hardiness zone 8 (Gilman et al. 2010b) a substantial portion of large roots on 170 L container trees likely remained in the original root ball substrate volume three years after planting. For example, Gilman and Kane (1991) found that about 50% of root length was still inside original 11.3 L container root ball three years after planting Juniperus chinensis L. shrubs of four cultivars. In addition, trees in containers have greater root density inside the root ball due to deflected roots compared to trees grown in a field nursery (Harris and Gilman 1993; Figure 6; Figure 7).

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Figure 6.

Root system configuration of 170 L container-grown trees when planted into landscape. Note roots were deflected down and around at the position of the 57 L container. No large roots extend from the trunk to edge of 170 L root ball.

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Figure 7.

Root system of field-grown tree at planting into landscape showing many roots growing directly from trunk to edge of root ball.

In contrast, roots were fairly straight to root ball edge on fieldgrown trees in the current study (Figure 7) because <25 mm trunk caliper Cathedral Oak generate new roots from the root flare (Gilman and Harchick 2008); when 3.7 L liners of Cathedral Oak were planted into the field nursery in the current study they were approximately 13 mm caliper indicating retained capacity to generate new roots from the root flare. Larger diameter (Table 1) straight roots (Figure 7) on trees from the field likely contributed to (Mickovski and Ennos 2002) increased bending stress required to pull trees (Figure 4), and reduced resting angle following pulling (Figure 5). Roots on trees transplanted from the field were immediately placed in tension, as Stokes (1999) showed when trees were subjected to lateral load from winching. The main roots in the current study did not form large branch roots and tapered little to the edge of the root ball (Figure 1; Figure 7); branching or significant bending results in a hinge point or fulcrum on the lee side when loaded laterally (Coutts et al. 1999). Trunk bending stress on trees pulled from 170 L containers was transferred into a more complex array of deflected roots (Figure 6), and then into fewer and smaller (compared to trees transplanted from field; Table 1) diameter roots just outside the original root ball. In addition, some roots grew along the periphery of 170 L root ball tangent to trunk for some distance prior to growing into landscape soil (Figure 2). Young roots tangent to trunk likely provide less stability than radially oriented straight roots when loaded in tension (Coutts 1983; Harrington and Howell 1998). In addition, secondary roots originating from main roots growing along container walls also deflect and grow along the wall. Main roots may produce only one set of secondary roots unless injured later. The suberization of these main roots (South and Mitchell 2005) may explain why trees from 170 L containers produced only one-quarter the root CSA into landscape soil as trees from the field nursery. Perhaps trees retained in a container for a shorter period could produce more roots quickly into landscape soil.

The natural condition of tree roots appears to be fairly straight for the first half meter or more from the trunk (Stout 1956; Lyford and Wilson 1964; Gilman et al. 1987). There was good correlation (r = 0.85 to 0.93, depending on trunk tilt) between trunk bending moment and root CSA for field-transplanted trees (Table 5). Lack of straight roots could explain why there was poor correlation between moment or stress and root CSA on trees planted from 57 L and 170 L container trees. As stress was transferred from the trunk base into roots on field-grown trees, they resisted stress and held the trunk in place with little overturning because there were many roots on the lee side pushing against mineral soil in the root ball and in the landscape (Figure 4). Roots on container trees were bent around and/or down (Figure 2; Figure 6), as others have shown on large containers (Gilman et al. 2010c), which has been associated on smaller containers with reduced stability (Ortega et al. 2006). Field trees produced four times the root CSA as trees from 170 L containers just outside original root ball edge. The likely result was more root mass closer to, and directly under, the trunk on container-grown trees (although mass was not measured) because roots were deflected down and around instead of growing radially out as in trees from the field.

Soil and root mass on the windward side of trees contributes to overturning resistance (Fourcaud et al. 2007). Coutts (1986) found 13% to 45% of anchorage was attributed to soil and root mass in the root-soil plate. In organic substrate there was less resistance to overturning due to lower soil density (Fraser and Gardiner 1967). Less substrate mass in a container root ball than in a field root ball of mineral soil would reduce overturning resistance. In some sense, the stiff complex of enlarging deflected roots rotated inside low density organic container substrate as the trunk was pulled or blown laterally. Therefore, it may be reasonable to encourage rapid growth in roots pointed away from the container substrate perhaps by discouraging development (Gilman et al. 2010a) of this deflected root complex so main roots grow quickly into mineral landscape soil. This would allow roots on the lee side close to the trunk to push against stiffer mineral soil in wind events; this may increase stability.

Roots on leeward side are pushed down into soil when wind loaded (Stokes 1999). Large woody roots on trees planted from 170 L containers pushed against decomposing soilless organic substrate in container root balls when trunks were pulled. Roots close to the trunk first sank into the decomposing root ball on leeward side followed by lifting on windward side. The observed hinge point on the lee side was positioned at either the 3.7 L or more typically 57 L container wall depending on the tree. This may have contributed to reduced pulling stress required to tilt trees planted from 170 L containers compared to trees transplanted from the field (Figure 4). More subsidence in trees planted from containers (Table 1) than trees from the field provides evidence that organic material in container root balls decomposed significantly over the three growing seasons following planting. The study was not designed to decipher the contribution to reduced stability from root deflection and decomposing substrate.

In contrast, roots close to the trunk on trees from the field pushed against stiffer (compared to organic soils, Fraser and Gardiner 1967) mineral soil that comprised the field root ball. Field trees did not appear to sink much on the lee side (this was not measured); instead windward side lifted with leeward hinge point farther away from trunk. Ennos (1995) and Coutts et al. (1999) suggested that more overturning moment is required to tip a tree if the hinge point is farther from the trunk. Perhaps as trees grow larger and woody roots become stiffer, stability on container-grown trees will increase as roots compensate for deformities by growing larger. Past work supports this on trees planted from much smaller containers. For example, Nichols and Alm (1983) found that conifers planted from 5 cm diameter containers were less stable 7–9 years after planting than naturally regenerated trees, but by 18 years after planting there was no difference. Others have similarly determined that conifers from small containers eventually gained stability comparable to naturally generated trees (Nielson 1998).

Stability has been associated with shallow roots in the windward direction (Stokes 1995), symmetrical lateral roots (Coutts et al. 1999), tap roots (Danjon et al. 2005), leeward sinker root length (Hayfa et al. 2007), and combinations of these and other attributes on various soil types. There appears to be no consensus on optimum root structure for tree stability. Mickovski and Ennos (2002) showed that root CSA and trunk DBH together accounted for 52% of overturning moment. Hayfa et al. (2007) found that combinations of root number, root depth, and trunk taper explained up to 80% or more of variability in trunk bending stress to pull trees to a set angle. In the current study, it was found that 97% of overturning moment was accounted for by trunk diameter and total root CSA measured just outside original root ball in upper 25 cm of soil (Equation 3). Root CSA and diameter of largest roots, especially in the windward direction, correlated best with trunk bending stress required to pull trees to 10 degrees tilt from unloaded start position (Table 4). Root size (diameter or CSA) on windward and leeward sides was most correlated with maximum bending stress encountered while pulling to 25 degrees (Table 4); however, wind required to push trees to this extent would exceed 45 m·s⁻¹. Maximizing root CSA and diameter of roots in the upper 25 cm soil profile outside of original root ball appears crucial for encouraging stable live oak in this soil type.

The overturning bending moment generated by winching to simulate wind load to uproot trees increases proportionately with 1.6 the power of trunk diameter to as high as 2.9 the power of trunk diameter (Crook and Ennos 1998; Stokes 1999; Mickovski and Ennos 2003; Tanaka and Yagisawa 2009). Overturning bending moment for live oak in the current study (Equation 3) scaled to the 3.4 power of trunk diameter (caliper) but the trunk diameter lower (0.15 m) on the trunk was measured, which could have contributed to the larger exponent. In contrast, trunk strength scaled to the third power of trunk diameter (Crook and Ennos 1996; Tanaka and Yagisawa 2009). This indicates the trunk on many trees become stronger than the root system as trees age commonly resulting in root failures instead of trunk failures on mature trees.

Roots on trees planted from 57 L containers responded like trees planted from 170 L containers except 57 L trees had larger root diameter/cm² trunk CSA (Table 1). Relatively large diameter roots may explain the similarity of 57L trees to trees transplanted from the field in bending stress required to tilt trunks to 15 degrees (Figure 4) or more (data not shown). Evidence of this is the importance of large root diameter in resisting maximum overturning (Table 4). Pulling force normalized for trunk CSA (bending stress; Figure 4) required to tilt trees planted from 57 L containers was similar to trees from 170 L containers indicating similar stability three growing seasons after planting. There should be tests for more variation in container size, different species, and allow more time after planting before evaluating stability.

This project is limited because it only compared one species grown under one set of conditions in one soil type in one organic substrate in one container type. Production practices including type of container, time in container, irrigation, fertilization, root pruning, and other practices influence root morphology inside the root ball, and these need to be studied in order to better understand attributes of stable trees in urban environments.