52 Dahle et al.: Does Modulus of Elasticity Vary Due to Dormancy and Temperature? Table 1. Mean (±SE) diameter, age, moisture content (MC), specific gravity (SG, based on oven-dried and green volume), and flexural modulus of elasticity (E). Sprouts were harvested before leaf drop (pre-dormant) or after leaf drop (dormant) and conditioned at 21.1°C (warm) or -6.7°C (cold). Means with the same letter were not significantly different with a Tukey HSD comparison (alpha = 0.05). Treatment Pre-dormant warm Pre-dormant cold Dormant warm Dormant cold P-value Diameter (cm) 3.6 ± 0.06a 3.6 ± 0.06a 3.3 ± 0.05b 3.3 ± 0.05b <0.0001 Age 3.5 ± 0.10a 3.7 ± 0.08a 2.9 ± 0.05b 3.0 ± 0.08b <0.0001 either similar or slightly lower than the mature wood reported in Wood Handbook. Having a higher average SG in the dormant samples could help explain the increase in average E. However, the SG does not appear to be a driving factor in the varia- tion in E, as it was insignificant in a SLR model between SG and E (P = 0.2997) in the juvenile sprouts. While there is a positive correlation between SG and E for wood materials, the non-significant relationship between SG and E in this study may be due to the relatively small variation within the material sampled and the small sample size. Sprout age (P < 0.0001, Table 1) was greater for pre-dormant sprouts than the dormant sprouts, yet E did not vary with age (P = 0.6662) in a SLR. As the sprouts were most likely completely composed of juvenile wood and it is possible that annual variations in material properties are influencing the results. As diameter was identified as the only significant factor in the MLR model, researchers ran an ANCOVA (P < 0.0001, N = 118) to determine if diameter could be a covariate with the two treatments. However, the only significant variable was diameter (P < 0.0001), while the following variables and covariates were MC (%) 51.0 ± 0.35b 50.0 ± 0.35b 57.6 ± 1.16a 55.8 ± 1.25a <0.0001 SG 0.55 ± 0.01c 0.60 ± 0.01b 0.64 ± 0.01a 0.64 ± 0.01a <0.0001 E (MPa) 1015.6 ± 49.2b 1142.5 ± 49.7ab 1214.3 ± 49.0a 1290.0 ± 57.2a 0.0028 N 28 30 30 30 not significant: temperature (P = -0.5898), season (P = 0.7288), temperature*diameter (P = 0.6811), and season*diameter (P = 0.7780). Interestingly, the relationship between E and diameter was nega- tive (Figure 1). It is unclear why E decreases with diameter. It is possible that other factors, such as weather, influenced wood formation that altered E, as two growing seasons occurred between the dormant sampling and pre-dormant sampling. While the overall values of E did not vary greatly, researchers may wish to investigate the influence of weather on the material properties of watersprouts. CONCLUSIONS Modulus of elasticity did not vary with temperature in the watersprouts, suggesting that the likelihood of watersprouts bending into energized powerlines does not change with temperature. While dor- mant sprouts (warm and cold) had higher aver- age flexural elasticity than the warm pre-dormant sprouts, they were not statistically different than the cold pre-dormant sprouts. As such, it does not appear that the flexibility of the watersprouts dif- fers as trees enter dormancy. Surely, the presence of leaves will increase the bending moment, which may lead to an increase deflection of the water- sprouts. Yet, in order to ascertain if the difference in E amounts to an appreciable change in failure likelihood, further research is needed to evaluate modulus of rupture and the difference in the inter- ception of loading due to the presence of leaves and the relation to strain concentration in the wood. Acknowledgments. We would like to thank the U.S. Forest Ser- vice for funding this research through a McIntire-Stennis grant (WVA00108) and the Division of Forestry and Natural Resources at West Virginia University. We also thank the anonymous reviewers who provided valuable insight that have improved this manuscript. Figure 1. Ordinary least-squared regression between modu- lus of elasticity (E, MPa) and watersprout diameter. ©2017 International Society of Arboriculture
March 2017
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