The effect of temperature on mechanical properties of standing lodgepole pine trees

Abstract

To estimate strength parameters of living lodgepole pine stems over a range of temperatures (-16 to +17°C), trees were winched near or past the point of breakage, during which the applied force and deflection of the stem were measured. Trees were 43 years old, 10 m tall, and since the experiments were conducted in the late winter and early spring, when the soil was frozen and the roots were held rigid, the resistance of the stem to deflection could be isolated from the resistances of the root and soil. Static flexure theory for cantilever beams was used to estimate stress, strain, Young's modulus (E), and modulus of rupture (MOR) of the stem. Trees were stiffer and stronger in the winter when wood was frozen, with a nearly 50% increase in E and MOR compared with the spring, when wood was thawed. In winter stems failed on the tension side, while in spring stems buckled on the compression side. Compared with strength estimations reported in the literature from small samples of clear green wood at standard temperatures, modulus of elasticity (MOE) estimates of the whole stem were 35% lower in spring, and in winter MOR exceeded published values by 53%. This suggests that the sway behavior of trees is probably temperature dependent in northern forests and whole-tree strength characteristics should be considered in wind sway models used in these regions.

Effects of cold temperatures on breakage of lodgepole pine and white spruce twigs

“…Given similar dimensions and MOR, branches with high MOE would be stiffer and more brittle. Silins et al (2000) observed increases in both MOE and MOR of the main stem in standing lodgepole pine trees as air temperatures declined below 0°C. Schmidt and Pomeroy (1990) observed a similar increase in stiffness of live subalpine fir (Abies lasiocarpa (Hook.)…”

“…Consistent with our findings, Schmidt and Pomeroy (1990) observed a linear relationship between force to produce a certain deflection and temperatures less than 0°C on lodgepole pine branches. Silins et al (2000) suggested increased tree or branch stiffness at sub-freezing temperatures may be caused by both a change in the MOE of the wood fibres and changing MOE of ice crystals within the frozen sapwood lumens.…”

“…Although the moisture content varied between twigs of lodgepole pine and white spruce (Table 1), moisture content in excess of fibre saturation point (as in these twigs) has little influence on wood strength (Richardson 1976). However, temperature dependence of MOE and MOR in living trees (i.e., above fibre saturaton point) has been suggested to result from both changes in mechanical properties of wood and that of ice in the lumens with changes in temperature below 0°C (Silins et al 2000).…”

Tree characteristics related to stem breakage of Picea glehnii and Abies sachalinensis

“…Soil is frozen up to around 50-80 cm below the surface in winter, which may help the roots remain rigid and resistant. Although low temperatures increase the stiffness and strength of trees (Silins et al, 2000), stem breakage was a more significant cause of tree death than was uprooting in forests subjected to strong prevailing winds and heavy snowfall in this area.…”

Effect of cold temperatures on the shear behavior of glued laminated beams

“…Silins et al [9] investigated the effects of temperature on standing lodgepole pines and found that the modulus of elasticity and tensile strength of the trees increased with decreasing temperatures. Umbanhowar [10] and Schmidt and Pomeroy [11] focused there work on the effects of cold on the branches of trees, with similar results.…”