Résumé
During tree growth, mechanical stresses accumulate in the trunk and branches due to the increasingweight and wood maturation. The progressive increase of the weight of the tree induces self-supportstress whose distribution is very different from a homogeneous beam with load uniformly distributedover its section. Because the vertical position is challenging gravity, increase in tree weight duringgrowth may disturb the mechanical equilibrium of the stems or branches. During the formation of anew wood layer at the periphery of the stem or branch, physico-chemical transformations occurringwith the thickening of the cell wall generate mechanical stresses at periphery, called maturationstress, and used by the tree to control posture and preserve shape in a standing position. Thecombination of self-support stress and maturation stress is called growth stresses.Measuring the corresponding released strain at the periphery of stems may evidence maturationstress: isolating a piece of stressed wood from surrounding tissues leads to shrinkage if the stress istensile, swelling if the stress is compressive. These measurements may be achieved by differentexperimental means, using simple strain gauges for example. The occurrence of radial cracks at a logend, or lumber crooking, may also evidence growth stresses. We will present methods to evaluatethese growth strains.Patterns of growth stress always exhibit very non-linear variations across the diameter: although thestress increment associated to a radius increment has linear variations within the section, theintegration of this stress over growth is non-linear. We will present the modelling of some growthscenarios leading to very different growth patterns, and discuss the consequences of the state of prestressingon stem strength: What effect of pre-stressing on tree buckling? What happens in case ofwind loading? May a branch break due to its self-loading?