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Acclimatation de l'arbre aux flexions répétées et conséquences sur le comportement mécanique et les propriétés hydrauliques du bois vert : Biologie végétale

Abstract : Trees, anchored in the ground, adjust their development to their fluctuating environment and particularly to the mechanical conditions daily imposed by wind. Storms of last decades enlightened that acclimation of trees to mechanical stresses due to wind is a vital requirement for their long-term survival. Wind mainly leads to repeated bending of the branches and stems of trees that swing during windy events. These bending cause a modulation of trees growth in height, in diameter or at the level of the root system but also imply the formation of a wood with specific anatomy and properties; this wood was defined as “Flexure wood”. In our work, we developed original experimental setups in order to apply a treatment of repeated unidirectional bending to the stem of young poplars, while controlling the magnitude of the strains applied to the stem. The setup controlled different levels of hydric stress too. The monitoring of the growth during a growing season showed that the mechanical acclimation is a process that, although costly in terms of biomass construction, turns out to be essential and takes place even under severe hydric stress. Besides, we demonstrated that the increase of biomass, linked to this acclimation, is mainly provided in the areas where tissue strains are the highest; leading to specific cross-section geometries that considerably increase the stem bending rigidity. A mechanical modelling using Finite Elements also enlightens that these configurations, due to acclimation, lead to a better distribution of mechanical stresses, especially by decreasing the intensity of maximal compressive stresses endured by wood. In order to investigate the microscopic scale, original experimental devices were developed at the tissue scale to measure the effect of different mechanical stresses (repeated compression and/or tension), applied by stem bending, on the hydraulic and mechanical properties of green wood; as much in terms of usual properties (elastic behaviour, hydraulic conduction) as in terms of security functions (rupture, sensibility to cavitation, …). Thus, we enlighten the specific behaviour of wood formed under repeated compressive stresses that shows a significant increase of its ability to withstand high levels of strains with a very reduced damaging. The whole experimental and modelling results, as much at the tissue scale as at the tree scale, points out that the acclimations of secondary growth and wood properties provide a mechanical benefit for tree sustainability in its fluctuating windy environment.
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https://tel.archives-ouvertes.fr/tel-02060329
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Submitted on : Thursday, March 7, 2019 - 12:34:06 PM
Last modification on : Wednesday, September 2, 2020 - 3:22:43 AM
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2018CLFAC046_NIEZ.pdf
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  • HAL Id : tel-02060329, version 1

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Benjamin Niez. Acclimatation de l'arbre aux flexions répétées et conséquences sur le comportement mécanique et les propriétés hydrauliques du bois vert : Biologie végétale. Biologie végétale. Université Clermont Auvergne, 2018. Français. ⟨NNT : 2018CLFAC046⟩. ⟨tel-02060329⟩

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