Critical review on the mechanisms of maturation stress generation in trees

International audienceTrees control their posture by generating asymmetric mechanical stress around the periphery of the trunk or branches. This stress is produced in wood during the maturation of the cell wall. When the need for reaction is high, it is accompanied by strong changes in cell organization and composition called reaction wood, namely compression wood in gymnosperms and tension wood in angiosperms. The process by which stress is generated in the cell wall during its formation is not yet known, and various hypothetical mechanisms have been proposed in the literature. Here we aim at discriminating between these models. First, we summarize current knowledge about reaction wood structure, state and behaviour relevant to the understanding of maturation stress generation. Then, the mechanisms proposed in the literature are listed and discussed in order to identify which can be rejected based on their inconsistency with current knowledge at the frontier between plant science and mechanical engineering

Functional diversity in gravitropic reaction among tropical seedlings in relation to ecological and developmental traits

International audienceGravitropism is necessary for plants to control the orientation of their axes while they grow in height. In woody plants, stem re-orientations are costly because they are achieved through diameter growth. The functional diversity of gravitropism was studied to check if the mechanisms involved and their efficiency may contribute to the differentiation of height growth strategies between forest tree species at the seedling stage. Seedlings of eight tropical species were grown tilted in a greenhouse, and their up-righting movement and diameter growth were measured during three months. Morphological, anatomical and biomechanical traits were measured at the end of the survey. Curvature analysis was used to analyse the up-righting response along the stems. Variations in stem curvature depend on diameter growth, size effects, the increase in self-weight, and the efficiency of the gravitropic reaction. A biomechanical model was used to separate these contributions. Results showed that (1) gravitropic movements were based on a common mechanism associated to similar dynamic patterns, (2) clear differences in efficiency (defined as the change in curvature achieved during an elementary diameter increment for a given stem diameter) existed between species, (3) the equilibrium angle of the stem and the anatomical characters associated to the efficiency of the reaction also differed between species, (4) the differences in gravitropic reaction were related to the light requirements: heliophilic species, compared to more shade-tolerant species, had a larger efficiency and an equilibrium angle closer to vertical. This suggests that traits determining the gravitropic reaction are related to the strategy of light interception and may contribute to the differentiation of ecological strategies promoting the maintenance of biodiversity in tropical rain forests

Transverse shrinkage in G-fibers as a function of cell wall layering and growth strain

International audienceTransverse drying shrinkage was measured at microscopic and mesoscopic levels in poplar wood characterised by an increasing growth strain (GS), from normal to tension wood. Results show that: (a) The drying shrinkage, measured as a relative thickness decrease, was significantly higher for G-layer (GL) than the other layers (OL), GL shrinkage was not significantly correlated with GS and OL shrinkage was negatively correlated with GS. (b) In gelatinous fibre (G-fibre), lumen size increased during drying and this increase was positively related with GS, but in normal wood fibre lumen size decreased during drying. These findings suggest that GL shrank outwards, so that its shrinkage feebly affected the total cell shrinkage and the mesoscopic shrinkage was controlled by the OL shrinkage which shrank inwards. (c) Measurements done on 7×7 mm² thin sections evidenced a negative correlation between transverse shrinkage and GS, significant in T direction but weak in R direction. These observations at both levels allow to discuss the contribution of GL to the mesoscopic shrinkage of tension wood

Compression stress in opposite wood of angiosperms: observations in chestnut, mani and poplar

In order to face environmental constraints, trees are able to re-orient their axes by controlling the stress level in the newly formed wood layers. Angiosperms and gymnosperms evolved into two distinct mechanisms: the former produce a wood with large tension pre-stress on the upper side of the tilted axis, while the latter produce a wood with large compression pre-stress on the lower side. In both cases, the difference between this stress level and that of the opposite side, in light tension, generates the bending of the axis. However, light values of compression were sometimes measured in the opposite side of angiosperms. By analysing old data on chestnut and mani and new data on poplar, this study shows that these values were not measurement artefacts. This reveals that generating light compression stress in opposite wood contributes to improve the performance of the re-orientation mechanism

Modélisation biomécanique de la genèse des précontraintes dans le bois

La production de bois précontraint est un élément-clé du design biomécanique des arbres, mais le mécanisme de mise en place de ces précontraintes reste inconnu. Nous explorons différentes hypothèses sur ce mécanisme en relation avec les paramètres de la microstructure du bois (cellulaire, polylamellé, composite à fibre) et le comportement de ses constituants durant sa formation

Mesures de déformations macroscopiques et cristallines dans le bois et implications pour une modélisation multi-échelle

Le bois est un matériau au comportement fortement hygroscope et viscoélastique. Sa microstructure complexe implique de nombreux niveaux d’organisation : un échantillon millimétrique est constitué de plusieurs types de cellules (vaisseaux, rayons et fibres) orientées dans différentes directions ; les fibres, constituant la fraction majoritaire et déterminante pour les propriétés mécaniques, sont des cellules allongées d’une longueur de l’ordre du millimètre et d’une largeur de quelques dizaines de microns ; leur paroi est un multicouche épais de quelques microns ; chaque couche est un matériau composite constitué pour moitié d’une matrice polymérique amorphe, et pour moitié de microfibrilles rigides ayant une orientation spécifique ; les microfibrilles, organisées en agrégats épais de quelques dizaines de nanomètres, sont constituées de cellulose aux deux-tiers cristalline ; les cristaux de cellulose sont larges de quelques nanomètres et faits de longues chaines de cellulose dont le motif élémentaire (lattice) mesure quelques angströms ; la longueur des zones cristallines, l’agencement entre cellulose amorphe et cristalline, et la structuration en agrégats sont mal connus. Les modélisations multi-échelles du comportement mécanique du bois se basent sur l’emboitement de méthodes d’homogénéisation représentant chacun de ces niveaux d’organisation. L’objectif du travail présenté est de vérifier expérimentalement les limites de ces modèles en comparant la déformation macroscopique du bois et la déformation à la plus petite échelle (cristal de cellulose, mesuré par diffraction de rayons X), et ceci pour différents types de comportement (élastique, post-élastique et viscoélastique) et à différentes humidités. Les résultats montrent que les déformations cristallines mesurées sont toujours significativement inférieures à celle prévues sous les hypothèses sous-jacentes aux modèles multi-échelles, notamment quand le bois est humide. Cette différence est interprétée comme résultant de légères approximations (rectitude et longueur infinie des éléments constitutifs) faites par les modèles à chaque niveau d’organisation, dont l’erreur se cumule lors de l’intégration multi-échelle

Radial Variations of Vibrational Properties of Three Tropical Woods

International audienceRadial trends of vibrational properties, including the specific dynamic modulus (E'/r) and damping coefficient (tand), were investigated for 3 tropical rainforest hardwood species, Simarouba amara, Carapa procera and Symphonia globulifera by free-free flexural vibration test. The microfibril angle (MFA) was estimated through X-Ray diffraction. Consistent patterns of radial variations were observed for all studied properties. E'/r was found to decrease from pith to bark, which is strongly related to the increasing pith-bark trend of MFA. The variation of tand along the radius can be partly explained by MFA, and partly by the gradient of extractives due to heartwood formation. The coupling effect of MFA and extractives could be separated through the analysis of log(tand) - log(E'/r) diagram. For the studied species, the extractive content putatively associated to heartwood formation generally tends to decrease the wood damping coefficient. However, this weakening effect of extractives was not observed for inner part of the heartwood, suggesting the mechanical action of extractives was reduced during their chemical ageing

Caractérisation nanomécanique des parois cellulaires du bois à différents stades de leur différenciation

L’explication de la réorientation des arbres repose sur la compréhension de la différenciation cellulaire lors de la croissance secondaire au niveau du cambium au cours de laquelle la paroi cellulaire va s’épaissir et se rigidifier occasionnant des précontraintes de croissance. Cette étude a pour objectif de mesurer les propriétés mécaniques de la paroi cellulaire par microscopie à force atomique