Modelling the growth stress in tree branches: eccentric growth vs. reaction wood

This work aims to model the mechanical processes used by tree branches to control their posture despite their increasing weight loading. The two known options for a branch to maintain its orientation are the asymmetry of maturation stress, including reaction wood formation, and eccentric radial growth. Both options can be observed in nature and influence the stress distribution developed in the branch each year. This so-called "growth stress" reflects the mechanical state of the branch. In this work, a growth stress model was developed at the cross-section level in order to quantify and study the biomechanical impact of each process. For illustration, this model was applied to branches of two 50-year-old trees, one softwood Pinus pinaster, and one hardwood Prunus avium (wild cherry tree), both simulated with the AmapSim discrete element software. For the wild cherry tree, the computed outputs highlighted that the eccentricity of radial growth seems to be as efficient as the formation of reaction wood to maintain postural control despite the increasing gravity. For the pine tree, eccentric radial growth appears to be less efficient than the formation of reaction wood. But although it does not necessarily act as a relevant lever for postural control, it greatly modifies the profile pattern of mechanical stress and could provide mechanical safety of the branch. This work opens experimental perspectives to understand the biomechanical processes involved in the formation of branches and their mechanical safety.