Critical wind speed at which trees break

Synopsis

Cyclone Klaus struck a large swath of Western Europe in 2009, damaging millions of trees of all types and sizes, from towering oaks to bushy pines. Observations of the aftermath revealed the greatest damage occurred in regions where the winds topped 90 miles per hour (40 m/s). A tall thick tree should be just as strong as a short thin tree. Many scientists, including Leonardo da Vinci and Galileo, have shown this to be true for wooden beams, but they haven’t agreed on how the force needed to snap a beam scales with its length and diameter. To better characterize wood strength, Emmanuel Virot of the École Polytechnique, France, and his colleagues performed simple experiments on horizontally held rods madefrom beech. Fixing one end, the researchers applied increasing weight at the other, recording thecurvature of the rod as it bent and eventually snapped. The data showed that wood breaks at a criticalcurvature radius, which depends on the diameter and length of the rod. Using this relation and a model of wind force, the researchers arrived at a scaling law for the wind speed at which a tree breaks. The speed does not depend on the elastic modulus of the wood, which is consistent with data showing that hardwoods (e.g., oaks) are just as wind susceptible as softwoods (e.g., pines).