Résumé
The building of a tree is the result of wood growth through successive division, expansion and maturation of living cells at the periphery of the trunk and branches. During this process, diameter growth is combined with sapwood pre-stress to allow posture control by the generation of growth forces in the living wood cells. These mechanical aspects of tree building can be characterised at each peripheral position by parameters describing the amount of material produced, wood rigidity and the strain induced by the maturation process. In-situ assessment of maturation strains at the trunk periphery of beech trees, combined with laboratory measurements of ring width (RW), wood density (D) and wood specific modulus (SM), was used to examine biomechanical aspects of juvenility corresponding to young stages of the tree, as well as the correlation or trade-off between sapwood pre-stressing and the generation of forces in the living wood layer used to control tree posture. The radial variations of RW, D and SM, averaged over 86 trees, were close to the “typical radial pattern” of juvenile wood for softwood plantation trees: decrease in RW and increase in D and SM from pith to bark in the juvenile phase. But D only increased in the very first rings, then remained more or less constant. Furthermore, for all three parameters there were many discrepancies in the pattern of variation between trees and even between plots. This is a good indication that the mechanical juvenility of the wood was more related to the biomechanical conditions experienced by the trees in the young ages than to the age of the tree as such (which is the case for fibre length). The level of pre-stress and posture control forces were strongly dependent on the maturation strain as the first explanatory factor. But pre-stress is independent of RW, whereas posture control force is strongly dependent on this growth parameter. This opens the way to trade-offs between these two biomechanical functions of wood fibres.