Arboriculture & Urban Forestry 34(4): July 2008 Trees were pulled by hand cranking the winch (1 revolution/ sec) until the inclinometer on the top of the root ball measured 20° or the tree broke. Maximum force measured by the load cell up to 20° from horizontal was used for comparison among treat- ments. Data were analyzed with a two-way analysis of variance, including Duncan’s multiple range test, and Tukey-Kramer ad- justments for multiple comparisons (P 0.05). RESULTS AND DISCUSSION The force to failure differed among tree stabilization systems (Table 1, P0.0001); however, direction was not significant for any individual stabilization system tested (Table 2). Therefore, stabilization systems were compared averaged over both direc- tions (Table 1). The Terra Toggle™, Brooks Tree Brace, and 2 × 2’s withstood the largest forces. There was no difference in force to failure between the Terra Toggle™ and Brooks Tree Brace (Table 1); and these two systems had the highest mean force to failure. The amount of force the 2 × 2’s withstood (181.4 kg [399.1 lb]) was statistically similar to Brooks Tree Brace (213.1 kg [468.8 lb]) but less than Terra Toggle™ (234.2 kg [515.2 lb]). Mean force to failure for the Tree Staple™ (67.2 kg [147.8 lb]), dowels (61.5 kg [135.3 lb]), and T-stakes (50.5 kg [111.1 lb]) were no greater than controls (29.5 kg [64.9 lb]). The Tree Staple™ and dowels were statistically similar to ArborBrace (99.7 kg [219.3 lb]), but not the T-stakes. Of the three guying systems, rebar and ArborTie (143.9 kg [316.6 lb]) and Duck- bill (130.1 kg [286.2]) withstood the most amount of force, and there was no statistical difference between the two. Force to failure on the third guying system, ArborBrace, was statisti- cally similar to the Duckbill but lower than rebar and ArborTie. All three guying systems (rebar and ArborTie, Duckbill, and ArborBrace), as well as Brooks Tree Brace and the T- stakes, secured the trunk two-thirds of the total height of the tree or lower (i.e., over the first major limb), consistent with the critical height of staking published by Leiser and Kemper (1968). Brooks Tree Brace (Figure 2C) and rebar and ArborTie (Figure 2F) both had anchors driven straight down into the soil; this angle was shown by Smiley et al. (2003) to require twice as much extraction force as anchors driven at an angle toward or away from the tree. The Duckbill stabilization system failed on five trees in di- rection 1 (Figure 1) because the wire–cable snapped between the Table 1. Force to failure for each tree stabilization system. Stabilization system Terra Toggle™ Mean forcez (kg [lb]) 234.2 (515.3) ay Brooks Tree Brace 2 × 2’s Rebar and ArborTie Duckbill ArborBrace Tree Staple™ Wood dowels T-stakes Control Means with the same letter are not significantly different (P 0.05, Duncan’s multiple range test). zAverage of two pulling directions (n 10). y 213.1 (468.9) ab 181.4 (399.0) bc 143.9 (316.7) cd 130.1 (286.2) de 99.7 (219.3) ef 67.2 (147.8) fg 61.5 (135.4) fg 50.5 (111.0) g 29.5 (65.0) g Table 2. Force to failure by direction for each tree stabilization system. Stabilization system (direction) Mean forcez (kg [lb]) CVy (/) Brooks Tree Brace (2) Terra Toggle™ (1) Terra Toggle™ (2) 2 × 2’s (1) Rebar and ArborTie (2) Brooks Tree Brace (1) Duckbill (2) 2 × 2’s (2) Duckbill (1) ArborBrace (1) ArborBrace (2) Rebar and ArborTie (1) Tree Staple™ (2) Wood dowels (1) Wood dowels (2) T-stakes (2) T-stakes (1) Tree Staple™ (1) Control Means with the same letter are not significantly different (P 0.05, Duncan’s multiple range test). x U-bolt cable clamp and the soil surface. In direction 2, the an- chors came out of the ground on three trees and the U-bolt cable clamps failed on the remaining two. The U-bolt cable clamps that came with the Duckbill failed to secure the cable under high forces when the tree had the support of two guylines (di- rection 2), allowing the cable to slip periodically despite being tightened adequately. In direction 1, the tree only had the support of one guyline, which snapped on all five trees tested in that direction. The ArborBrace guying system was similar to the Duckbill conceptually. However, ArborBrace’s ArborAnchors™ never came out of the ground like three trees staked with Duckbill’s anchors, and ArborBrace’s polypropylene guylines never snapped the way Duckbill’s cable guylines snapped. The ArborBrace failed when the guylines stretched and cut through the soil, allowing the tree to bend more and the root ball to rotate. ArborBrace’s metal tension buckle securely fastened the guy- line and no slipping occurred. The difference between the Duck- bill and ArborBrace was that the amount of force it took to stretch ArborBrace’s polypropylene guylines was less than the breaking strength of Duckbill’s wire cables. Therefore, as the tree was pulled, ArborBrace’s polypropylene guylines stretched, allowing the root ball to rotate. Meanwhile, the wire cables of Duckbill had little or no stretch but suddenly broke or the anchor was pulled out of the ground. The third guyline-type stabilization system tested was rebar and ArborTie. Rebar pulled out of the ground and/or bent as the tree was pulled during each repetition, but the ArborTie never snapped. Rebar and ArborTie in direction 2 broke one tree (299.9 kg [659.8 lb]) at the tie-in point on the trunk. Perhaps larger-diameter rebar would have been more difficult to pull from the ground or bend. The Terra Toggle™did not break any trees in half but cracked the trunk at the base on the compression side (facing direction of ©2008 International Society of Arboriculture 261.5 (575.3) ax 247.5 (544.5) ab 225.4 (495.9) ab 212.6 (467.8) ab 193.7 (426.2) abc 164.7 (362.4) abcd 159.0 (349.8) abcd 150.1 (330.2) bcde 101.2 (222.6) cdef 99.9 (219.8) cdef 99.5 (218.9) cdef 94.2 (207.2) cdef 86.3 (189.9) def 62.0 (136.3) def 61.1 (134.4) ef 50.5 (111.1) f 50.4 (110.9) f 48.1 (105.9) f 29.6 (65.1) f 0.28 0.15 0.24 0.27 0.43 0.12 0.22 0.52 0.20 0.18 0.48 0.14 0.28 0.12 0.34 0.18 0.19 0.13 0.22 zAverage of one pulling direction (n 5), except the control (n 10). yCoefficient of variation: standard deviation/mean. 219
July 2008
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