Active mechanisms of re-orientation are necessary to maintain the verticality of tree stems. They are achieved through the production of reaction wood, associated with circumferential variations of three factors related to cambial activity: maturation strain, longitudinal modulus of elasticity (MOE) and eccentric growth. These factors were measured on 17 mature trees from different botanical families and geographical locations. Various patterns of circumferential variation of these factors were identified. A biomechanical analysis based on beam theory was performed to quantify the individual impact of each factor. The main factor of re-orientation is the circumferential variation of maturation strains. However, this factor alone explains only 57% of the re-orientations. Other factors also have an effect through their interaction with maturation strains. Eccentric growth is generally associated with heterogeneity of maturation strains, and has an important complementary role, by increasing the width of wood with high maturation strain. Without this factor, the efficiency of re-orientations would be reduced by 31% for angiosperms and 26% for gymnosperms. In the case of angiosperms, MOE is often larger in tension wood than in normal wood. Without these variations, the efficiency of re-orientations would be reduced by 13%. In the case of gymnosperm trees, MOE of compression wood is lower than that of normal wood, so that re-orientation efficiency would be increased by 24% without this factor of variations.

Variation in xylem density within individual trees can be substantial, and the relationship between branch xylem and trunk xylem densities varies considerably among families and sites. As such, whole-tree biomass estimates based on nondestructive branch sampling should correct for both taxonomic and environmental factors. Furthermore, detailed estimates of the vertical distribution of wood density within individual trees are needed to determine the extent to which relying solely upon measures of trunk wood density may cause carbon stocks in tropical forests to be overestimated.

-On 10 trees from 10 species of French Guyana tropical rainforest in a clear active process of restoring verticality growth strains were measured in situ in order to determine the occurrence of tension wood within samples. Wood specimens were cut in the vicinity of the growth strains measurements in order to measure some mechanical and physical properties. As suspected, tensile growth strains was very much higher in tension wood zone, because longitudinal modulus of elasticity was slightly higher. Longitudinal shrinkage was also much higher in tension wood than in opposite wood.tension wood / opposite wood / tropical rainforest / physical and mechanical properties Résumé -Comparaison du bois de tension et du bois opposé de dix arbres provenant d'espèces différentes de forêt tropicale humide. Des mesures de contraintes de croissance ont été réalisées sur 10 arbres en cours de redressement actif appartenant à 10 espèces de la forêt tropicale humide de Guyane Française afin de s'assurer de la présence de bois de tension. Des échantillons de bois, prélevés au voisinage des mesures de contraintes de croissance, ont permis de mesurer un certains nombres de propriétés physiques et mécaniques. Comme présumé les contraintes de croissance sont beaucoup plus élevées au niveau du secteur de bois de tension, car le module d'élasticité est légèrement plus élevé. Le retrait longitudinal est aussi plus élevé dans le bois de tension que dans le bois opposé.bois de tension / bois opposé / forêt tropicale humide / propriétés physiques et mécaniques

This study aims at better understanding the respective influences of specific gravity (1), microfibril angle (MFA), and cell-wall matrix polymers on viscoelastic vibrational properties of wood in axial direction. The wide variations of properties between normal wood (NW) and compression wood (CW) are in focus. Three young bent trees (Picea abies, Pinus sylvestris and Pinus pinaster) that recovered verticality were sampled. Several observed differences between NW and CW were highly significant in terms of anatomical, physical (1, shrinkage, CIELab colorimetry), mechanical (compressive strength), and vibrational properties. Specific dynamic modulus of elasticity (E'/1) decreases with increasing MFA, and Young's modulus (E') can be satisfactorily explained by 1 and MFA. Apparently, the type of the cell wall polymer matrix is not influential in this regard. The damping coefficient (tan2) does not depend solely on MFA of NW and CW. The tan2 -E'/1 relationship evidences that, at equivalent E'/1, the tan2 of CW is approx. 34% lower than that of NW. This observation is ascribed to the more condensed nature of CW lignins, and this is discussed in the context of previous findings in other hygrothermal and time/frequency domains. It is proposed that the lignin structure and the amount and type of extractives, which are both different in various species, are partly responsible for taxonomy-related damping characteristics.

-Colour parameters of European beech were measured using CIELab system. 103 logs from 87 trees in 9 sites were cut into boards to study the radial variations of wood colour parameters. Both site and tree effects on colour were observed. Patterns of red heartwood occurrence were defined. When excepting red heartwood there was still a highly significant effect of site and tree. Axial and radial variations were small, except very near the pith or in red heartwood, suggesting possible early selection at periphery under colour criteria. Red heartwood is darker, redder and more yellow than normal peripheral wood. Fagus sylvatica L. / CIELab colour system / solid wood / red heartwoodRésumé -Études sur le Hêtre (Fagus sylvatica L.). 1. Variations des paramètres de couleur du bois. Les paramètres de couleur du hêtre européen ont été mesurés à l'aide du système CIELab. Cent trois grumes obtenues à partir de 87 arbres abattus dans 9 sites ont été débitées en quartiers afin de déterminer les variations radiales des paramètres de couleur. Des effets site et arbre sur la couleur ont été observés. Des types avec ou sans coeur rouge ont été définis. En exceptant le coeur rouge on conserve un effet hautement significatif du site et de l'arbre. Les variations radiales et axiales étaient faibles, excepté très près de la moelle ou dans le coeur rouge, suggérant la possibilité d'une sélection précoce sur des critères de couleur en périphérie. Le coeur rouge est plus foncé, plus rouge et plus jaune que le bois normal périphérique.Fagus sylvatica L. / système de couleur CIELab / bois matériau / coeur rouge