158 Gilman et al.: Container Wall Porosity and Root Pruning Influence on Swietenia mahogani lar area centered on the trunk starting at 6:00 pm in the evening prior to pulling. No rain occurred during the two days required to pull trees. A force transducer (Model SSM-BYJ-50, 22.7 Kg, Inter- face, Scottsdale, Arizona, U.S., non-repeatability —±0.02% RO) was placed in line with a non-stretch cable that was secured around the tree with a tight- ened Zip tie at 20 cm from the ground. Trees were pulled at a rate of approximately 10 mm∙sec-1 once due azimuth north 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 stop pulling at the targeted bending stress. This bending stress was chosen so that the trunk nearly returned to the pre-pulling start an- gle following practice winching—indicating slight root or soil failure—on extra trees from the same group planted nearby. 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 realtime on the laptop running LabView soſtware (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 crank (windward). A second winching applied a bending stress of 8.2 MN/m2 . With the tree held in position by the winching cable, the angle under tension and the rest angle following release of the winching cable were recorded. The pre-winch- ing trunk angle was subtracted from these angles to calculate change in angle as a result of winching. [Equation 1] 𝜎𝜎 = 𝐹𝐹 ∙ 𝑑𝑑 ∙ 𝑅𝑅 𝜋𝜋 4 ∙ 𝑅𝑅4 where, σ = bending stress F = pulling force d = distance from pulling point to inclinometer R = trunk radius (calculated as halving diameter mea- sured with a diameter tape) All 40 trees were excavated in December 2012 following winching, using a square-tipped shovel, forming a circular root ball 60 cm across and 60 cm ©2015 International Society of Arboriculture 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 57 L container root ball intact. Soil and container substrate were washed from the whole root ball. Roots were measured for many attributes described in the appropriate tables. Root diameter was measured to 0.1 mm. Statistical Analysis Data were analyzed in a randomized complete block design with one tree from each propagation container type (2) × nursery container type (2) × root pruning at landscape planting (2) combination in each of five blocks totaling 40 trees. Three-way analysis of variance in the GLM procedure within SAS (SAS Institute, Cary, North Carolina, U.S.) was used to evaluate impact of main effects and inter- actions on measured parameters. The three main effects were propagation container type, nursery container type, and root pruning at landscape plant- ing. Means of main effects were separated with Duncan’s multiple range test; interaction means were separated with LSD. Pearson’s correlation coefficient was used to make correlations between root attributes and bending stress. Significant results were reported at P < 0.05 unless indicated. RESULTS AND DISCUSSION Trunk diameter and tree height growth in the nurs- ery and aſter planting to the landscape were not affected by propagation or nursery container type or root pruning at landscape planting, and there were no interactions (data not shown). Nursery container main effect was highly significant for many measured root attributes of finished trees in 57 L containers (Table 1). As found on Swietenia mahogani grown in 9.5 L containers (Gilman and Paz 2014), trees in PC developed a higher-quality root system (as defined in Anonymous 2009 and Anonymous 2015) than trees in SC. Specifically, root system imprint leſt by the propagation con- tainer (liner) and the 3.8 and 9.5 L containers, percent root ball circumference without roots, oc- currence of culls due to circling roots, and percent trunk circled inside the 57 L container periphery were all smaller in PC containers by about half or more than trees in SC. Attributes associated with high quality had larger values for trees grown in PC than in SC. These values included radial root sym-
May 2015
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