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

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.