Root System Morphology Influences Lateral Stability of Swietenia mahagoni

MATERIALS AND METHODS

February 2009 seeds of Swietenia mahagoni were germinated in two propagation containers with different wall attributes (EP = Elle pots or SM = smooth-sided) and retained for 12 months (as described in Gilman and Paz 2014). Root balls on 80 trees from each propagation container were root pruned (shaved) as they were shifted in February 2010 into 3.8 L containers with different wall attributes (SC = smooth-sided black plastic, 19 cm top diameter × 19 cm tall, model PF400, Nursery Supplies, Chambersburg, Pennsylvania, 40 trees; and PC = black plastic container with porous sides and bottom, Pioneer pot^®, 19 cm top diameter × 17 cm tall, Pioneer Farms, Visalia, California, U.S., 40 trees) by removing the outer 5 mm of the liner root ball sides and bottom with sharp scissors (Fiskars, #FSK01004342). Roots were pruned by one person to standardize procedure. Remaining 80 trees from each were not root pruned when shifted into the SC (40 trees) and PC (40 trees) containers. In August 2010, trees in SC and PC were shifted into larger-sized 9.5 L containers of the same type (SC, model PF1200, 27 cm top diameter × 24 cm deep; PC, 28 cm top diameter × 17 cm deep) and placed on ground cloth randomly in rows.

In April 2011, 10 randomly chosen trees of each treatment combination (two liners × two root pruning treatments × two 3.8 L then 9.5 L containers × 10 trees = 80 trees) were planted into field soil [Millhopper fine sand (loamy, silicaceous, hyperthermic Grossarenic Paleudults)] with less than 2% organic matter) in Gainesville, Florida. Trunk diameter (mean = 14 mm) and tree height (mean = 111 cm) were recorded at planting. Top of the root ball was positioned even with surrounding soil, and trees were placed 0.9 m apart within three rows spaced 2 m apart. Some main roots emerged from the trunk base within 1 cm of the substrate surface. No root manipulation was performed at planting. One tree from each treatment combination was randomly assigned to a block of eight trees for a total of 10 blocks. A 0.5 m diameter circular soil area around each tree was irrigated with 8 L through a Roberts spray stake (Model SS-AG160BLK-100), which was divided into three daily applications to encourage rapid growth. Fertilizer (20 N, 0 P₂O₅, 8 K₂O; 65 g in April, 120 g in June and August 2011) was applied to a 50 cm diameter circular area around each tree. Shoots were not pruned at or after planting. A wood chip (utility pruning waste) mulch layer 10 cm thick was placed across the whole plot, almost up to the trunk, and glyphosate was applied periodically for weed control.

In November 2011, all 80 trees were winched with a hand crank that was crafted of a bent steel rod to evaluate lateral stability. No rain occurred during the three days required to pull trees. A force transducer (Model SSM-BYJ-50, 22.7 Kg, Interface Scottsdale, Arizona, U.S., non-repeatability—±0.02% RO) was placed in line with a non-stretch string secured around the tree with a tightened zip tie at 20 cm from the ground. Trees were pulled at a rate of approximately 10 mm∙sec⁻¹ once in the NE, NW, SE, and SW compass directions to a bending stress (σ) of 4.1 MN/m² calculated individually for each tree from trunk diameter measured 10 cm from the ground using Equation 1. This slow, winching rate allowed researchers to pull at the targeted bending stress. This bending stress was chosen so that the trunk nearly returned to the pre-pulling start angle following practice winching, on extra trees from the same group planted nearby, indicating slight root or soil failure. During winching tests, load was sampled at 2 Hz using a 16-bit data acquisition system (National Instruments Corporation, Austin, Texas, U.S.) and displayed and archived in real-time on the laptop running Lab-View software (v: 7.0; National Instruments, Austin, Texas, U.S.). Trunk angle was recorded just prior to each winching by placing a digital level (18 cm long, M-D SmartTool Angle Sensor Module 92346) accurate to the tenth of a degree on the bottom 18 cm of trunk on the side opposite the hand crank (windward). A fifth and final winching to the SW applied a bending stress of 13.8 MN/m². With the tree held in position after each of the five winchings, the angle under tension and the rest angle following release of the winching string were recorded. The pre-winching trunk angle was subtracted from these angles to calculate change in angle as a result of winching.

1

where σ = bending stress; F = pulling force; d = distance from pulling point to inclinometer; and R = trunk radius (calculated as halving diameter measured with a diameter tape).

All 80 trees were dug up in December 2011 following winching, using a square-tipped shovel forming a circular root ball 40 cm across and 40 cm deep shaped in a cone typical of a tree dug from a field nursery. This shape and volume was large enough to harvest the planted 9.5 L container root ball intact. Soil and container substrate were washed from roots. Roots were measured for many attributes described in the appropriate tables. Root diameter was measured to 0.1 mm; root depth was measured to the nearest mm.

RESULTS AND DISCUSSION

Root pruning liners, when shifting into 3.8 L containers, did not impact aboveground growth after landscape planting; however, two measured root attributes were affected. Shaving liners increased cross-sectional area (CSA, calculated from diameter measured just beyond trunk) of straight roots equally for both propagation containers (Table 1). Shaving liners grown in EP had no influence on CSA of deflected roots because there were few deflected roots to remove (Gilman and Paz 2014); however, shaving SM liners dramatically reduced deflected root CSA compared to not shaving (Table 1) as with red maple (Acer rubrum L., Gilman et al. 2012). Reported effects on crown growth from manual root pruning of trees planted from propagation containers into field soil vary in the literature. Some authors found reduced crown growth (Arnold and Young 1991), no effect (Persson 1978; Gilman and Wiese 2012), or greater growth (Krasowski and Owens 2000) following root pruning at planting. Although trunk angle during winching was not impacted by root pruning the liner in the current study (data not shown), anchorage on other species has been compromised when roots deformed from growing in propagation containers were not corrected at planting (Sibley and Seaby 1982; Balisky et al. 1995). Deflections down or around the propagation container in the current study may not have been severe enough to influence stability because trees were retained in propagation containers for only 12 months. Some nurseries retain trees longer.

Although trunk diameter increased in the landscape most for trees propagated in EP and then shifted to PC, the 3–4 mm difference might be imperceptible (Table 2). There was also no tree height response due to liner or larger container wall attributes (data not shown), indicating little impact on growth for the first seven months after landscape planting from root form imposed by containers.

Trunk angle during winching in the initial direction (NE) at 4.1 MN/m² was not different among treatments; however, trunk rest angle following winching was greatest (least stable, 0.7 degrees) for the treatment combination with the most root CSA (653 mm², which was about 2 to 15 times greater than other treatments) deflected by the 9.5 L container (i.e., those grown in EP liners then shifted to SC containers, Table 2). This was likely due to roots dislodging permanently within the root ball, and could be explained by EP containers sending roots more laterally (44 degrees from horizontal, Table 3) than downward (52 degrees for SM), forcing more to grow into the 9.5 L sidewall. This was also described for red maple (Gilman et al. 2012), implying that a liner with a desirable root system shifted to a larger container with smooth walls resulted in a poor 9.5 L root system. In contrast, the large imprint resulting from deflection by the SM liner (3.2, Table 3) appeared to have a lesser impact on anchorage because root CSA deflected by liner was about 5 times greater (487 mm², or 30% of the CSA in the five largest roots, Table 2) in the SM/SC treatment combination than all others (which had ≤7% deflected) with no difference in winching-trunk angle. The new roots initiated near the flare after liners were shifted to larger containers (Gilman and Paz 2014; Figure 1) helped trees overcome the potential instability associated with the large deflections that occurred within the liner.

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Figure 1.

Four root systems from mahogany grown in the field for seven months after planting from a PC (A, B) or SC (C, D) container. Note the prominent imprint on the root system and vertical orientation of structural roots associated with poor anchorage (SC container, bottom) and little imprint and horizontal orientation associated with better anchorage (PC container, top).

Roots grew into landscape soil primarily from existing root tips in all treatments. Roots occasionally grew from segments behind the root tip in seemingly unpredictable places. Roots rarely grew after field planting from the oldest portions of the root system, including the basal 30 mm of the largest diameter roots deflected by container walls—as judged by a consistent color of these new roots (not measured)—which explains the long, curved segments of circling roots lacking lateral roots (Figure 1). Rarely produced were roots that grew from bends caused by a large root deflecting at the container wall, and when produced they were much smaller than the parent root. It was not clear if these small roots would become major roots on the tree.

Propagating in EP and then shifting to either of the larger containers resulted in increased CSA of straight roots (Table 3), similar to red maple in containers (Gilman et al. 2012). Roots on trees in SM liners angled downward 52 degrees (Table 3) before redirecting sharply upward as they grew out from the liner as shown by the negative end-minus-start angle values of the five largest roots (Table 2). This form is unlike the natural habit of tree roots (Lyford and Wilson 1964). Compared to trees in SC, trees in PC had shallower (Table 4) and straighter (Table 5) roots in both the trees grown on containers for this study (Gilman and Paz 2014), and at the edge of the dug root system seven months after landscape planting. This indicated that the more horizontal root system generated in the original planted 9.5 L container root ball continued developing in this fashion once in the landscape. In addition to aiding container growers, roots growing at a shallow angle to the soil surface that were fairly straight could help growers that propagate by direct seeding into field soil reach their goal of producing roots closer to the soil surface (Hewitt and Watson 2009). Under current practices many species can develop a deeper root system with fewer shallow roots than desired.

Trees planted into field soil from PC propagated in either liner tilted at a lesser angle under both bending stresses and immediately following pulling (rest angle) than trees from SC containers (Table 4). Trees that tilted less had a smaller root ball circumference lacking large roots (80 degrees), seven times the CSA in straight roots, one-third the change in root azimuth direction from trunk origin to root ball edge, more root CSA, and shallower roots (Table 4) than the less stable SC trees. Also associated with the less stable trees was greater visible root system imprint from the containers resulting in an eight-fold increase in cull root systems (Anonymous 1998) and a 2.5-fold increase in percent trunk circled by roots. Number of straight roots was dramatically increased (eight-fold) as was root ball symmetry (2.5-fold increase) in trees more resistant to overturning (PC) and there were fewer roots growing tangent to the trunk (Table 5). Neither container eliminated circling roots or those growing tangent to the trunk.

Roots of other species that deflected downward in containers often grew out the bottom after planting into soil, resulting in artificially deep and deformed roots under the trunk (Sibley and Seaby 1982; Balisky et al. 1995; Salonius et al. 2000). Deflection of main roots downward forces them to grow parallel to one another and touch directly under the trunk (as in SC in the current study, Table 6) causing constrictions and inclusions that restrict passage of substances through vascular tissue (Lindström and Rune 1999). Distributing root tips in the lateral (horizontal) position throughout the root ball profile on young mahogany instead of at the bottom (Gilman and Paz 2014) allowed lateral roots to grow in a more natural straight position (Figure 1; Table 6). Lindström and Rune (1999) and others also showed that trees became more stable after planting when some roots were able to grow in a more horizontal orientation instead of being deflected downward or around.

Attributes of the five largest roots are important for stability because Coutts (1986), Gilman and Grabosky (2011), and others showed that the largest several roots can comprise 70% or more of the root system CSA. Despite a 15% (P = 0.03) smaller CSA in the five largest roots (Table 6), trees from PC were better anchored than trees from SC. This demonstrates the large role played by root morphology. Improved anchorage was likely due to a small percentage (23%) of roots deflected, little root contact (13%) with adjacent roots, and five times the number of straight or branched roots on trees from PC than from SC (Table 6). Other studies appear to support these findings. For example, large, shallow, straight roots on red maple planted from 11 L and 95 L containers were at least partially responsible for better stability compared to trees planted from much larger containers (230 L and 983 L, Gilman et al. 2013). Although others (Fourcaud et al. 2008; Gilman and Wiese 2012) also showed that shallow lateral roots with developing sinkers on young trees contribute substantially to overturning resistance, this is by no means a universal theorem, as both Khuder et al. (2007) and Mickovski and Ennos (2002) found deeper rooting associated with better anchorage, especially on deep soils. Symmetrical root systems may be most important in shallow soils where trees form a plate-like root system (Coutts 1983; Danjon et al. 2005).

Other techniques have been employed to develop lateral roots close to the surface. For example, trees produced in a nearly wall-less container (Jiffy pellets Jiffy^®, Jiffy Products of America, Inc., 600 Industrial Pkwy., Norwalk, Ohio, U.S.) or in copper-treated containers (Chapman and Colombo 2006) had more root tips and straighter lateral roots growing close to the substrate surface compared to containers that deflect roots down. Although this can enhance stability in the early years after planting, plantation trees installed from propagation containers can become better anchored with time as roots develop compensatory growth in response to mechanical stress (Lindström and Rune 1999). Furthermore, a symmetrical root system may not be as important on soils that promote deep rooting (Coutts et al. 1999).

Trunk tilt during winching to 4.1 MN/m² bending stress was correlated with several attributes of root morphology inside the root ball (Table 7). Increasing the deflected nature of the root system and reducing the number of straight roots lead to greater trunk tilting. This analysis also showed that visual estimates, such as imprint from the container and root ball symmetry, were nearly as good at predicting anchorage as more time-consuming measurements, such as percent trunk circled and azimuth difference between the root origin and root end at the edge of the root ball. These quick visual evaluations will make it relatively simple for growers and others to gauge root system quality without time-consuming measurements.

Lateral stresses applied to the trunk base in this study may not exactly represent those experienced by trees of similar size subjected to loads from ice, snow, or wind. Researchers pulled close to the ground to ensure that rotational bending stress was applied to the root system. The trunk and roots on larger trees in nature would be stiffer and the center of pressure in wind higher in the crown, resulting in different forces than those applied in this study. Although this study was not designed to assign percentage contribution from each measured root attribute to overturning, the data suggest a direction of future work. Cultural practices that increase cross-sectional area of roots growing straight from the root ball into the landscape soil without deflection, and/or entrain a large soil mass close to the trunk, should enhance anchorage by resisting overturning.

CONCLUSIONS

Although root forms often associated with health problems and trunk wood defects—such as circling and descending roots—can be caused by the propagation container, anchorage in the current study was more influenced by root morphology imparted by the larger container than by the propagation container. This occurred because root initiation from the flare continued after shifting liners into the 3.8 L containers, especially when vertical root growth was discouraged by air-pruning at the bottom. Trees with root balls containing horizontal-oriented straight roots, little imprint from the container, and few deflected roots were most stable after landscape planting. Growing in a production system that 1) discourages vertical root growth very early in production, 2) extends the period of root initiation at the root flare, and 3) minimizes root deflection provides for better anchored trees seven months after planting into the landscape than trees grown with large descending and circling roots.