Description du sujet de thèse

Tropical forest trees exhibit a diversity of ecological temperaments (light affinity, growth, size at maturity). The relationship between density and wood mechanical properties on the one hand, and temperament on the other, make density a functional trait used to describe the mechanical strategy of a species. However, the mechanical performance of a tree depends on both its wood properties and its morphology (height, diameter). Based on a database on the mechanical properties of wood from French Guiana, field measurements and a modeling approach coupling tree morphology and variability of wood mechanical properties, this project aims to (i) identify a panel of tree biomechanical strategies and refine ecological temperaments, (ii) propose relevant functional traits to describe the mechanical strategy of species and (iii) improve the prediction of wood quality with respect to species temperament and tree size. Keywords: Tree biomechanics, wood properties, ecological temperament, tropical forest

Contexte de travail

It is now estimated that more than 1,800 tree species are present in French Guiana (Molino et al., 2022). In recent decades, several attempts have been made to describe the main strategies of plants. Today, species are grouped around models of ecological temperaments mainly defined by criteria related to growth rate, light requirements and dimensions at maturity (Collinet, 1997; Favrichon, 1994; Gourlet-Fleury, 1997; Oldeman et al., 1991) reflecting a compromise between performance and safety (Jaouen, 2007). Tree temperaments have a direct impact on the variability of their wood: a multifunctional construction material, wood ensures the mechanical, hydraulic and storage needs of the tree throughout its development. Thus, from the heartwood (produced at the young stage) to the wood under bark (produced at a more mature stage), the properties of the wood are adjusted according to the needs of the tree and the constraints of the environment. We observe, particularly in tropical environments, strong gradients of properties in the tree (Bossu et al., 2018; Morel et al., 2018). The relationships between ecological strategies of species and variations in density in the tree are now well known (Lachenbruch et al., 2011) and have been described in tropical species (Hietz ​​et al., 2013; Lehnebach et al., 2019). They are well explained by rules of biomass allocation linked to the growth rate of the species. Today, the mechanical performances of trees are often read through density (Chave et al., 2009; van Gelder et al., 2006). But beyond density, the specific modulus (intrinsic mechanical property of the material and independent of density) is a major source of variation in the mechanical performance of wood with ontogeny. In the young stages, during the installation phase of the tree within the forest structure, the tree rapidly adjusts the mechanical properties of its wood. However, the ecological strategies of tropical tree species have never been explained from the intrinsic mechanical properties of their wood (specific modulus, microfibril angle, grain angle). In tropical rainforests, where we have access to the widest diversity of tree species in the world, the last decades of research have allowed the accumulation of data that highlight different patterns of variation in mechanical properties, from the heart to the bark. By studying the gradients of density and specific modulus of wood in the tree, we can retrospectively analyze the performance of the material produced at each stage of the tree's life. The mechanical performance of a tree depends both on these properties and on the morphology of the trees (height, diameter). This is why, in the absence of precise data on the evolution of the morphology of the trees during the installation phase, it is difficult to characterize the biomechanical strategy of the tree and to link it to the ecological temperament of the species.