Team : MECA

PhD student : HEIM Lucie

Thesis supervisors : Louis Denaux, BADEL Eric , Kévin Candelier, Remy Marchal

Dates : November 2019 - June 2023

Description

With the aim of combining agroecology and increased biomass production in response to ecological transition, agroforestry has been experiencing a resurgence in temperate zones over the past twenty years. However, agroforestry trees are still considered to be auxiliary to crop or livestock production, and are rarely the subject of studies aimed at characterising their wood in order to better exploit its value. Growing in spaces that are far removed from those of more conventional forestry, particularly due to lower tree densities per hectare, agroforestry trees are subject to increased exposure to wind and light, as well as strong interactions with annual crops and numerous human interventions at the canopy and root system levels. The growth dynamics of three temperate hardwood species (hybrid walnut, poplar and black locust) from agroforestry (AF) systems and control forest plots (FC) were compared, species by species. The secondary growth of agroforestry trees was found to be faster than that of forest controls, reflecting the presence of a more significant thigmomorphogenesis phenomenon linked to the wind regime experienced by agroforestry trees. The influence of growth conditions on the physicochemical properties of walnut and poplar trees (AF and FC) was then analyzed. While the extractable wood content of walnut trees did not differ depending on the silvicultural treatment, analyses carried out by NIR hyperspectral imaging highlighted differences in chemical composition between agroforestry and forest walnut trees, particularly in terms of extractables and lignin content. In poplar, a lower lignin content was observed in wood from the agroforestry plot compared to that of the forest control wood, which is similar to a greater presence of bending wood in agroforestry poplars. Subsequently, comparative analyses of certain physical properties between AF poplars and FC poplars showed that agroforestry poplars have lower average humidity levels and a lower proportion of sapwood and false heartwood than control poplars. Furthermore, the average density of the wood does not differ depending on the silvicultural treatment. Finally, the quality of agroforestry poplar wood with respect to peeling and its recovery in the technical wood products sector was evaluated. AF and FC poplars were peeled. The resulting veneers were characterized and compared according to the wood origin. For each origin, laminated veneer lumber panels were then designed and mechanically characterized by 4-point bending tests. The results obtained, compared with certain physical properties of the wood such as the angle of the microfibrils of the cell walls and the wood density, showed that AF poplars have superior mechanical properties than FC poplars. As AF poplars adapted to wind-induced bending during their growth, they produced larger diameter wood with higher mechanical properties than wood from the control forest system, making it a good resource for recovery in the form of reconstituted materials following peeling. Thus, this doctoral work provides new knowledge, contributing to a better understanding of the development and growth of trees in an agroforestry environment and the properties of their wood. The results obtained are promising and allow a first approach in terms of a way of valorizing agroforestry trees within the wood industry.

Link

https://hal.science/tel-04472702v1