50 Dahle et al.: Does Modulus of Elasticity Vary Due to Dormancy and Temperature? tion when a bending type force is applied, within the linear portion of the stress-strain diagram. Generally, lower values for E equate to high flex- ibility (Hibbeler 2005), and higher values lead to stiffer wood that can withstand higher loading events (Dahle and Grabosky 2010). E is lower in green wood (moisture content, MC > 30%–34%) compared to dried wood that is used as a con- struction material (Kretschmann 2010). While the relationship between E and moisture and tempera- ture has been well established in wood materials that are below the fiber saturation point (Lavers 1983; Kretschmann 2010; Spatz and Pfiesterer 2013), little is known about how or if E varies in standing trees as they move into dormancy, and if changes in ambient temperature affect E. While research has shown MC tends to increase aſter leaf drop in diffuse hardwood species (Clark and Gibbs 1957), it is unclear if this seasonal shiſt in MC will result in an increase in E in watersprouts. Additionally, knowledge on how ambient tem- perature affects the material properties of wood is based, for the most part, on construction-grade lumber and commercially important tree species. Temperature and availability of precipitation are the main factors that hinder plant performance (Harris et al. 2004). It is understood that material properties of wood increase as temperature and moisture content decrease (below fiber saturation point) (Gerhards 1982), but it is not well under- stood how temperature impacts material prop- erties of green wood (moisture content >30%). Understanding how material properties vary in trees will add to the collective knowledge of how trees survive or fail during loading events, such as ice accumulation and snowfall. Models that predict branch failures could help the arboricultural com- munity understand which branches are more prone to failures. Such models would need to include various inputs, including branch allometry, axial and radial variations in material properties, varia- tion in material properties’ due to age (maturation), and differences in static and/or dynamic loading due to the presence/absence of leaves. While there is a growing body of knowledge in many of these areas, gaps remain, such as the amount of loading that occurs during storm events; how loads move from branches, down the stem, and into the roots; and how variation in morphology and material ©2017 International Society of Arboriculture properties allow trees to resist failure. The goal of this research was to determine if the modulus of elasticity of juvenile wood varies with temperature (frozen versus warm) and seasonality (pre-dormant versus dormant). This knowledge can help the util- ity sector better understand if temperature or sea- sonality leads to watersprouts, and are more likely to undergo larger deflections that could contact energized power lines due to snow or ice accumula- tion before the leaves have senesced in the autumn. METHODS AND MATERIALS Samples were taken from trees growing at West Virginia University’s Research Forest, located in Monongalia County, West Virginia, U.S. The site chosen was a 29.5-hectacre, completed three-stage shelterwood cut. The regrowth trees were all naturally regenerated stump sprouts that can be considered similar to regrowth oc- curring aſter storm damage or heading cuts. A total of 120 northern red oak (Quercus rubra L.) trees were sampled, 60 during the dormant stage, and 60 during the pre-dormant stage. Dor- mant samples were taken from 01 February through 08 February 2013, while pre-dormant sampling was conducted throughout September 2014. As two growing seasons had elapsed, the pre-dormant sampling targeted sprouts of the same size as the previous dormant sprouts. Only one sprout was harvested from each stump. For each sampling season, 60 sprouts were randomly separated into two equal-sized groups. Thirty of the sprouts were placed at room temperature, estimated at 21.1°C (warm), the other half in a CSZ-H/AC environ- mental unit at -6.7°C (frozen), for five days, respec- tively. Two pre-dormant sprouts were damaged during handling and were subsequently not tested. The sprouts were subjected to a three-point bend- ing test with a span of 44.45 cm using a universal test machine (UTM) (Instron® model MTS 810) at a rate of 0.16 cm per minute. The warm samples were tested at room temperature (21.1°C), and the frozen samples were taken from the environmental unit and immediately tested at -6.7°C. The samples were not taken to failure during the three-point tests due to high flexibility. The span:depth ratio was selected in accordance to a 14:1 cm length to diameter ratio. Force versus deformation (i.e., slope) was obtained from the load cell and cross-
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