Résumé
There are two main strategies for trees to resist wind: the “stress-resistance” strategy, based on the ability to withstand wind forces, and the “stress-avoidance” strategy, based on the ability to reduce wind forces by reconfiguration of the stem and/or the crown. As illustrated by Esope’s fable “the reed and the oak”, it is commonly admitted that stress-avoidance is associated with small sized plants and stress-resistance large sized plants. In trees, a transition in strategies between small and large stems is reflected by the juvenile wood transition: small axes produce a deformable wood that easily bends under wind loads, while bigger axes produce stronger wood adapted to withstand the wind forces.Here we revisit this question from a theoretical perspective. First, we question the commonly admitted idea that wind-resistance is associated to large size and wind-avoidance to small size. We show that a size effect occurs only if an external allometric constraint is considered: a small tree is more adapted to reconfiguration not because it is small, but because it is more slender. Then, we analyse the functional space of wood properties based on a micromechanical approach. We identify the structural parameters underlying the trade-off in properties between strong wood, adapted to stress-resistance, and deformable wood, adapted to stress-avoidance. Using a model taking into account constraints on stem dimensions and wood properties, we compute how the CWS (critical wind speed at which the tree is predicted to break) varies with tree size and material properties. The model predicts a critical diameter (typically between 10 and 20 cm) below which CWS is optimised with deformable material, and above which CWS is optimised with strong material. These results are strongly supporting the functional interpretation of juvenile wood transition as a transition from stress-avoidant to stress-resistant strategies.