28 Gilman and Harchick: Root System Morphology Influences Lateral Stability Reduced anchorage of trees from nursery contain- ers compared to trees transplanted from a field nurs- ery is attributable to a combination of root deflection in the containers, less root ball mass, and low root ball soil strength due to voids created by decompo- sition of the organic substrate typical of container- grown trees (Gilman and Masters 2010; Gilman et al. 2013). Although relative contribution was not assigned to each of these three attributes, previous models developed in forest stands suggest that root- soil plate depth, shape, and mass were responsible for a significant (13%–45%) portion of overturning resistance (Coutts 1983; Ennos 1995). Roots grow- ing in the windward direction (Stokes 1999) and the location of the rotation axis (hinge point), among other factors, also contribute (Fourcaud et al. 2008). Though advances have been made in describ- ing anchorage mechanics on trees planted from small propagation-sized root balls typical of plan- tations, few studies have been performed on trees planted from the much larger root balls typical in urban landscapes. The strategy of growing roots radially away from the base of the trunk, instead of deflecting down, up, or around, appears well- suited for binding together a large mass of soil and roots into a root-soil plate that resists over- turning (Gilman et al. 2010b; Gilman et al. 2013). Root diameter decreases suddenly at the transi- tion point of one container to the next larger size because roots that grow beyond the deflection point are typically much smaller in diameter than the deflected root (Gilman and Paz 2013). Sudden reductions in root diameter from root deflections (Gilman and Masters 2010) and at branch points (Coutts 1983) are a source of leeward hinge points in root-soil plates when trunks are subject to lat- eral forces such as winching or wind. Stability may be improved by reducing these deflections inside the root ball, and planting root balls containing a large number of roots that either stop elongat- ing or branch once they meet the container wall. Because straight lateral roots appear to be associ- ated with well-anchored trees planted from propa- gation containers (Salonius et al. 2000), the main goal of the present study was to determine the influ- ence of root form in a container root ball on root attributes and anchorage one growing season aſter planting into landscape soil. Swietenia mahagoni (L.) Jacq. (mahogany) was chosen due to its popu- ©2014 International Society of Arboriculture larity as urban landscape trees in tropical and sub- tropical regions of Florida, U.S., and the Caribbean. MATERIALS AND METHODS February 2009 seeds of Swietenia mahagoni were ger- minated in two propagation containers with differ- ent 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 shiſted in February 2010 into 3.8 L contain- ers with different wall attributes (SC = smooth-sided black plastic, 19 cm top diameter × 19 cm tall, model PF400, Nursery Supplies, Chambersburg, Pennsyl- vania, 40 trees; and PC = black plastic container with porous sides and bottom, Pioneer pot® , 19 cm top di- ameter × 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 shiſted into the SC (40 trees) and PC (40 trees) containers. In August 2010, trees in SC and PC were shiſted into larger-sized 9.5 L contain- ers of the same type (SC, model PF1200, 27 cm top di- ameter × 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 in June and August 2011) was applied to a 50 cm O5 , 8 K2 treatment combination (two liners × two root prun- ing treatments × two 3.8 L then 9.5 L containers × 10 trees = 80 trees) were planted into field soil [Mill- hopper 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 manip- ulation 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 P2 O; 65 g in April, 120 g
January 2014
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