Team : MECA

PhD student : NIEZ Benjamin

Thesis supervisors :  BADEL Eric ,  MOULIA Bruno , Jana Dlouha

Dates : October 2015 - December 2018

Description

Trees, rooted in the ground, adapt their development to their fluctuating environment and in particular to the mechanical conditions permanently imposed by the wind. The storms of recent decades have highlighted the major role, for the long-term survival of trees, of the process of acclimatization to mechanical stresses due to the wind. At the first order, the wind mainly exerts repeated bending forces on the branches and stems of trees which tend to oscillate during windy episodes. These bendings lead to a modification of the growth of trees in height, diameter or at the level of the root system but also the formation of wood with specific anatomy and properties; which is referred to as "bending wood". During this thesis work, we developed original experimental devices allowing us, on the one hand, to apply repeated unidirectional bending treatments on young poplar stems, by controlling the amplitude of the deformations applied to the stem, and on the other hand, to control different levels of water stress. Growth monitoring during a complete growing season has shown that the mechanical acclimatization of trees is a process which, although very costly in terms of biomass construction, proves to be essential and takes place even under conditions of severe water stress. In addition, we were able to demonstrate that the increase in biomass linked to this acclimatization occurs mainly in the areas where tissue deformations are the strongest; thus leading to particular section geometries which considerably increase the bending rigidity of the stems. Finite element mechanical modeling also revealed that these configurations resulting from acclimatization lead to a better distribution of mechanical stresses, in particular by lowering the intensity of the maximum compressive stresses experienced by the wood. In order to descend into spatial scales, we developed original characterization tools and methods that made it possible to measure, at the tissue level, the impact of the different types of stresses (repeated compression and/or traction), generated during stem bending, on the hydraulic and mechanical properties of green wood; both from the point of view of usual properties (elastic behavior, hydraulic conduction) and from the point of view of safety functions (fracture, sensitivity to cavitation, etc.). We were then able to highlight the unique behavior of wood formed under repeated compression stresses, which in particular shows a clear increase in its capacity to undergo significant deformations with very little damage. All the experimental and modeling results at the tissue and whole organ scales indicate that the acclimatization of secondary growth and the intrinsic properties of the wood material provide a mechanical benefit for the tree's longevity in its fluctuating windy environment.

Link

https://hal.science/tel-02060329v1