Spiral grain—An adaptation of trees to withstand stem breakage caused by wind-induced torsion

Expressions are derived for the internal mechanical stress components in trees when exposed to bending and torsion by wind forces. The crown is modeled as rectangular, and the stem taper so that the maximum bending stress is equal at all heights. Trees with asymmetric crowns will undergo torsion as well as bending, and the shear stresses associated with torsion are determined and compared to the bending stresses. The results show that shear failure is likely to occur relatively high up on the stem when the crown asymmetry is a few decimeters. The fact that torsion might be critical to the strength of trees towards wind forces, introduces an explanation to spiral grain growth. It is shown that spiral grain in the direction of the wind-induced torque improves the bending strength of the tree. Spiral grain is therefore beneficial for trees with a prevailing external torque.