The mechanical role of bark

The ability of stem bark to resist bending forces was examined by testing i n bending segments of Acer saccharium, Fraxinus americana, and Quercus robu r branches with and without their bark. For each species, the bark contribu ted significantly to the ability of stem segments differing in age to resis t bending forces, but its contribution was age-dependent and differed among the three species. The importance of the mechanical role of the bark decre ased basipetally with increasing age of F. americana and Q. robur stem segm ents and was superceded by that of the wood for segments greater than or eq ual to 6 yr Old. A. saccharum bark was as mechanically important as the woo d for stem segments 7 yr old but was not a significant stiffening agent for younger or older portions of stems. On average, the stiffness of the bark from all three species was 50% that of the wood. However, the geometric con tribution to the flexural rigidity of stems made by the bark (i.e., the bar k's second moment of area) was sufficiently large to offset its lower stiff ness (Young's modulus) relative to that of the wood. A simple model is pres ented that shows that the bark must be as mechanically important as the woo d when its radial thickness equals 32% that of the wood and its stiffness i s 50% that of the wood. Based on this model, which is shown to comply with the data from three species purported to have stiff woods, it is evident th at the role of the bark cannot be neglected when considering the mechanical behavior of juvenile woody stems subjected to externally applied bending f orces.