282 Gilman and Masters: Root Growth and Lateral Stability of Quercus virginiana Reported effects of mechanical root pruning in containers on root growth and morphology vary. One study showed that light cutting of circling roots of shrubs enhanced amount of roots growing into container substrate outside original root ball (Bla- nusa et al. 2007). In contrast, Gilman et al. (1996) showed that slicing 11.3 L (22 cm tall x 25 cm top diameter) container root balls top-to-bottom on Burford holly (Ilex cornuta ‘Burfordii’) at planting into field soil resulted in a redistribution of roots, not an increase in roots compared with nonpruned controls. Krasowski and Owens (2000) found root systems of mechanically pruned Picea glauca (Moench) Voss produced greater root growth than control or chemically root pruned treatments despite a smaller root ball at planting. Burdett (1978) and Dunn at al. (1997) showed a reduction in root circling and root deflection downward in propagation container trays treated with copper hydroxide compared to untreated trays. Treated trees produced root systems post-planting similar to naturally regenerated trees resulting in enhanced stability compared to not pruning; there was identical stability between copper treated and naturally regenerated trees. There are few studies on mechanical or chemical root prun- ing in large landscape-sized containers on stability follow- ing planting into landscape soil. There are no reported studies on impact of root ball composition on post-planting stability, and none have attempted to calibrate bending stress or pulling angle to wind speed. Objectives of the present study were to evaluate impact of slicing into the periphery of container root balls, initial tree size at planting, and root ball composition on post-planting tree stability in a simulated wind and rain storm. MATERIALS AND METHODS Planting One-hundred-twenty Quercus virginiana Mill. ‘SNDL’, PP#12015, Cathedral Oak trees propagated from cuttings were planted April 2005 into a field with Millhopper fine sand (loamy, silicacous, hyperthermic Grossarenic Paleudults) with less than 2% organic matter according to Gilman et al. (2010b). Thirty of each of the following four planting treatments were in- stalled: 57 L (41 cm tall x 43 cm top diameter, Nursery Supplies Inc., Fairless Hills, PA, U.S.) smooth-sided containers, 170 L (47 cm tall x 75 cm top diameter) smooth-sided containers, 170 L smooth-sided containers sliced (3 to 5 cm deep) down the sides in six equidistant positions at planting, or transplanted from an adja- cent field. Trees at planting were within standard nursery industry guidelines for root ball size (Anonymous 2004), and grew for three growing seasons after landscape planting prior to testing. One [(4 cm x 4 cm) x (45 cm)] long wooden stake was driven into soil 60 cm east and west of trunk to monitor tree subsid- ence following planting. This position was just outside the pe- riphery of the root balls. A string was stretched from top of east stake to top of west stake so it rested against tree trunk. A vis- ible line was drawn on the trunk to mark string position at plant- ing March 2005 and three growing seasons later October 2007. Evaluating Stability The Alachua County, Florida, soil survey was used to determine amount of water to add (757 L) and amount of time to wait (6 hours) to bring a 2.4 m x 2.4 m plot, 1.2 m deep, around each ©2010 International Society of Arboriculture tree to field capacity. The actual amount of water added was 1.5 times the amount needed (757 L x 1.5 = 1135.5 L), help- ing ensure soil saturation consistency. Water was applied in October 2007 thru PVC and four low-profile sprinkler heads, controlled by battery-operated timers. Each tree was pulled 6–6.5 hours after irrigation ceased. This allowed water to per- colate into soil and drain, bringing soil to field capacity prior to evaluating tree stability. Added water simulated a large volume rain event often associated with hurricanes and other storms, and standardized soil moisture conditions among replicates. The seven trees with a trunk diameter 15 cm from ground (caliper) closest to mean caliper for each planting treatment were pulled with a steel cable and electric winch (Model 40764; Chicago Electric Power Tools, Inc., Camarillo, CA, U.S.) to evaluate lateral tree stability in a strong storm. Four or five, depending on planting treatment, of the seven trees for each treatment were pulled in the 110 Azimuth (from north) direction; others were pulled in the 20 to 50 Azimuth direction. There was no prevailing wind direction at the site. An electronic inclinometer (model N4; Rieker Inc., As- ton, PA, U.S.) was mounted to a fabricated steel plate (5.1 cm x 7.6 cm). The plate was secured with zip ties to trunk base 15 cm from soil surface which was located immedi- ately above the swollen flare on the largest of the 28 trees pulled. A 3,629 kg capacity load cell (SSM-AF-8000; Inter- face Inc., Scottsdale, AZ, U.S.) was placed in-line with the steel pulling cable attached to trunk at estimated crown center of gravity. Trees were pulled so cable was parallel to ground. The center of gravity was estimated on each tree by to- taling cross-sectional area (CSA, calculated from diameter measured with a diameter tape) of all branches at the point where they emerged from the central trunk. The cable was at- tached to the central trunk at the centroid of the CSA, such that one-half of total branch CSA was below and above pull- ing point. The cable was pulled at a rate of 2 cm·s-1 until trunk base tilted 5 degrees relative to its nondeformed (unloaded) shape, and then cable was let slack. Trunk angle was re- corded during the pull and immediately after the cable went slack; angle immediately following the pull was referred to as resting angle. One minute later, tree was pulled slowly until trunk tilted 10 degrees, let slack, and resting angle record- ed. Tree was pulled to 15, 20, and 25 degrees following the same procedure. Pulling was concluded if the trunk cracked. During pulling tests, load cell and inclinometer measure- ments were sampled at 2 Hz using a 16-bit data acquisition system (National Instruments Corporation, Austin, TX, U.S.) and displayed and archived in real-time on a laptop running LabView software (v: 7.0; National Instruments, Austin, TX, U.S.). The trunk bending stress at position of inclinometer at each 5 degree increment was calculated as: (pulling force × distance from pulling point to inclinometer × trunk radius at inclinometer) ÷ (0.25π × trunk radius4 ). Trunk radius was cal- culated by halving diameter measured with a diameter tape. Two randomly chosen field-grown trees and two 170 L con- tainer trees from blocks not pulled were subjected to a wind field generated by the machine described below to calibrate trunk tilt (15 cm from ground) resulting from pulling with tilt resulting from wind. The wind field was placed four meters from the north edge of foliage crown. Following soil saturation as described above, the wind speed was increased from ambient to 45 m·s-1 over 80
November 2010
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