Mechanical properties of “flexure wood”: compressive stresses in living trees improve the mechanical resilience of wood and its resistance to damage

Abstract

Key message: Mechanical acclimation of young poplars (Populus tremula × Populus alba, INRA 717-1B4) submitted to periodic stem bending is mainly driven by compressive strains. Flexure wood and compressive flexure wood exhibit higher mechanical resilience and lower mechanical damage. Context: It is well known that thigmomorphogenesis modulates tree growth and the anatomical structure of wood. However, nothing is known about the mechanical behaviour of the tissues of fresh wood formed under mechanical stimulation. Aims: We investigated the elastic and plastic properties of the fresh wood of young poplar trees (Populus tremula × Populus alba, INRA 717-1B4) submitted to periodic controlled stem bending that mimics the mechanical effect of wind. Methods: For a set of trees, we applied symmetrical bending treatments, which led to the formation of “flexure wood”. For another set of trees, asymmetrical bending treatments, including compression (or tension) only, were applied and generated specific wood formation: “compressive flexure wood” and “tensile flexure wood”. We investigated the elastic and plastic properties of these woods at the stem and at the local tissue levels. Results: The results revealed that fresh wood formed under compressive treatments is more resistant to damage (damage reduced by 44%) and a higher mechanical resilience (+ 33%), suggesting that this tissue is able to withstand higher mechanical strains than “normal wood”. This improvement could explain the higher mechanical strength of the stem to bending (+ 42%). Conclusion: When trees experience repetitive mechanical stimulations, they adjust the plastic plastic behaviour of their wood in a way that improves the mechanical safety. This demonstrates the adaptive benefit of the mechanical acclimation of trees.