Team : MECA

PhD student : VAN ROOIJ Arnoul

Thesis supervisors :  GRIL Joseph ,  BADEL Eric , Gael Godi

Dates : November 2019 - July 2023

Description

The study of tree branches has often been neglected in favor of that of trunks. Yet, their simple posture constitutes an impressive defiance of gravity. Although they appear to be mechanically oversized, these branches sometimes break under the pressure of a fluctuating environment. While the causes are identified in most cases, there are still situations where the mystery remains intact; this is evidenced by the phenomenon of "summer breaks": branches falling at the end of hot, windless days. These branches do not initially show any particular signs of weakness, and post-mortem analysis by experts does not allow us to understand the phenomenon underlying these sudden breaks. In a context of urban warming, policies for developing wooded areas to mitigate urban heat islands are developing, and this problem of sudden falls is becoming an important issue. To address this complex question, we proposed a multidisciplinary approach. Surveys were conducted through the press as well as among managers and urban tree experts, in order to collect information mainly concentrated among these practitioners. They made it possible to highlight some avenues of research. From a theoretical point of view, branch breakage could not be studied without addressing the aspect of their mechanical condition. A numerical model was therefore developed to simulate the implementation of longitudinal constraints in a branch during its growth; which inexorably leads to an increase in its dimensions and mass. Applied to virtual branch models, this model made it possible to evaluate the respective mechanical performances and roles of the motors of postural control of a branch by distinguishing the strategies of the deciduous and coniferous groups. Finally, different experimental components aimed to feed and/or test this model. This work also highlighted intrinsic markers of the branch or environmental conditions (the season) which seem to affect the oversizing criterion of the branches and therefore, a priori, their resistance to breaking. All of this work opens up numerous experimental and modeling perspectives, in order to confirm and extend these initial results to a wider range of species. Beyond the initial question of summer breakage, this project is part of a broader theme of understanding the biomechanical functioning of branches, which allows them to ensure their sustainability throughout their development despite their increasing dimensions and weight, and the fluctuating environment in which they evolve.

Link

https://hal.science/tel-04281436v