Résumé
The postural control of trees is ensured by the generation of asymmetric longitudinal stresses around the circumference in the newly formed wood layer during the maturation process. In normal and tension wood, these longitudinal (L) tensile stresses are associated with tangential (T) compressive stresses. This study aims at assessing the relationship between L and T maturation stresses, in order to discriminate between hypothetic micromechanical models of maturation stress generation at cell-wall level. In this study, the strain during the release of maturation stress along L and T directions (RLMS and RTMS) were measured at breast height on 5 positions around the circumference of 30 Eucalyptus nitens trees. Experiments were performed in a 17 years-old plantation on trees having DBH within a range of 18-28 cm. Up-right trees as well as tilted trees were selected in order to include various biomechanical situations. These measurements also enable evaluation of the Poisson's ratio of wood. Results show that all trees produce asymmetric RLMS around the periphery but with a high between-trees dispersion of the mean RLMS level: the maximal RLMS of some trees was in several cases found lower than the minimum of some other trees. Analysis of the relation with tree diameter indicates that the higher the growth rate, the lower the mean RLMS in the trees. Moreover, the difference between the maximum and the minimum RLMS within a tree decreased when growth rate increased. Measurements also indicate a clear relationship between the magnitude RLMS and RTMS, even after accounting for the Poisson effect: the higher the tensile longitudinal stress, the higher the tangential compressive stress. This relationship is strong at the between-tree level and could be detected at the within-tree level. These results will be analysed in relation with wood microstructure and the consequences for tree biomechanics will be discussed.