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Trees generate mechanical stresses at periphery of stem and branches to improve their strength and to control the orientation of their axes. This key factor in the biomechanical design of trees, named “maturation stress”, occurs in wood fibres during cellular maturation when their secondary cell wall thickens. In this study, the spatial and temporal stiffening kinetics of the different cell wall layers were recorded during fibre maturation on a sample of poplar tension wood using atomic force microscopy. The thickening of the different layers was also recorded. The stiffening of the CML, S1 and S2-layers was initially synchronous with the thickening of the S2-layer and continued a little after the S2-layer reached its final thickness as the G-layer began to develop. In contrast, the global stiffness of the G-layer, which initially increased with its thickening, was close to stable long before it reached its final maximum thickness. A limited radial gradient of stiffness was observed in the G-layer, but it decreased sharply on the lumen side, where the new sub-layers are deposited during cell wall thickening. Although very similar at the ultrastructural and biochemical levels, the stiffening kinetics of the poplar G-layer appears to be very different from that described in maturing bast fibres.

Recent studies have shown that the inner bark is implicated in the postural control of inclined tree stems through the interaction between wood radial growth and tangential expansion of a trellis fiber network in bark. Assessing the taxonomic extent of this mechanism requires a screening of the diversity in bark anatomy and mechanical stress. The mechanical state of bark was measured in 15 tropical tree species from various botanical families on vertical mature trees, and related to the anatomical structure of the bark. Significant tensile or compressive longitudinal stresses were observed in the stems of most species. Tensile longitudinal stress was observed in various botanical families and was always associated with fibers arranged in a trellis-like structure and strong dilatation of rays. The highest tensile stress was recorded in species with gelatinous fibers forming a treillis. Compressive stress was typically associated with a large amount of sclereids in the bark, supporting the differentiation of sclereids as a potential origin of the generation of longitudinal compressive stresses in bark. In species exhibiting both a fibrous trellis structure and a significant amount of sclereids, the sign of longitudinal stress may depend on the balance between these two mechanisms. Key words: bark anatomical structure, mechanical stress, sclereids, secondary phloem, tree biomechanics, tropical species. De recentes etudes ont mis en evidence que l'ecorce interne est impliquee dans le maintien de la posture des tiges inclinees grace a l'interaction entre la croissance radiale du bois et l'extension tangentielle d'un reseau de fibres en treillis dans l'ecorce. L'evaluation de l'etendue taxonomique de ce mecanisme requiert une analyse plus importante de la diversite des structures anatomiques et des etats de contrainte de l'ecorce. L'etat mecanique de l'ecorce a ete mesure chez 15 especes tropicales de diverses familles botaniques sur des arbres adultes verticaux, et mis en relation avec sa structure anatomique. Des contraintes longitudinales significatives ont ete mises en evidence chez la plupart des especes. Selon l'espece, ces contraintes consistaient soit en une tension soit en une compression. La contrainte de tension longitudinale, observee ici sur diverses familles botaniques, etait toujours associee a des fibres arrangees en treillis et une forte dilatation des rayons. La tension la plus forte a ete mesuree chez les especes cumulant la structure de fibres en treillis et la presence de couches gelatineuses. La contrainte de compression est typiquement associee a une grande quantite de sclerites, suggerant que la differenciation de sclerites est potentiellement impliquee dans la genese de contraintes longitudinales de compression dans l'ecorce. Chez les especes, presentant a la fois une structure de fibres en treillis et une quantite significative de sclerites, le signe de la contrainte longitudinale pourrait dependre de la balance entre ces deux mecanismes. [Traduit par la Redaction] Mots-cles : anatomie de l'ecorce, biomecanique de l'arbre, contraintes mecaniques, especes tropicales, sclerites, phloeme secondaire., Introduction To maintain the vertical posture during growth, self-supporting woody plants developed mechanisms to generate the mechanical forces that offset the effect of gravity (Almeras and Fournier 2009). The understanding [...]

One foundational assumption of trait-based ecology is that traits can predict species demography. However, the links between traits and demographic rates are, in general, not as strong as expected. These weak associations may be due to the use of traits that are distantly related to performance, and/or the lack of consideration of size-related variations in both traits and demographic rates. Here, we examined how wood traits were related to demographic rates in 19 tree species from a lowland forest in eastern Amazonia. We measured 11 wood traits (i.e. structural, anatomical and chemical traits) in sapling, juvenile and adult wood; and related them to growth and mortality rates (MR) at different ontogenetic stages. The links between wood traits and demographic rates changed during tree development. At the sapling stage, relative growth rates (RGR) were negatively related to wood specific gravity (WSG) and total parenchyma fractions, while MR decreased with radial parenchyma fractions, but increased with vessel lumen area (V A). Juvenile RGR were unrelated to wood traits, whereas juvenile MR were negatively related to WSG and axial parenchyma fractions. At the adult stage, RGR scaled with V A and wood potassium concentrations. Adult MR were not predicted by any trait. Overall, the strength of the trait-demography associations decreased at later ontogenetic stages. Our results indicate that the associations between traits and demographic rates can change as trees age. Also, wood chemical or anatomical traits may be better predictors of growth and MR than WSG. Our findings are important to expand our knowledge on tree life-history variations and community dynamics in tropical forests, by broadening our understanding on the links between wood traits and demography during tree development. [ABSTRACT FROM AUTHOR]

The different hypotheses about buttress function and formation mainly involve mechanical theory. Forces were applied to two trees of Sloanea spp., a tropical genus that develops typical thin buttresses, and the three-dimensional strains were measured at different parts of the trunk base. Risks of failure were greater on the buttress sides, where shear and tangential stresses are greater, not on the ridges, in spite of high longitudinal (parallel to the grain) stresses. A simple beam model, computed from the second moment of area of digitized cross sections, is consistent with longitudinal strain variations but cannot predict accurately variations with height. Patterns of longitudinal strain variation along ridges are very different in the two individuals, owing to a pronounced lateral curvature in one specimen. The constant stress hypothesis is discussed based on these results. Without chronological data during the development of the tree, it cannot be proved that buttress formation is activated by stress or strain. Key words: biomechanics; buttress; Eleaocarpaceae; French Guiana; Sloanea spp.; tropical trees; wood.