A root-to-foliage tree dynamic model for gusty winds during windstorm conditions

Abstract

Improving forest wind risk models relies on a better understanding of the tree response to damaging gusts. However, our empirical knowledge of the tree dynamic response to wind is limited by the feasibility of field experiments at recording the dynamics of both, the tree aerial structure and the tree root-system, under windstorm conditions. Here, we present the first complete root-to-foliage tree dynamic model simulating the tree aerial structure and root-soil system responses to turbulent winds, including the possibility of tree breakage and tree uprooting. This model allowed us to investigate the impact of both (1) various anchorage conditions, related to the soil humidity or root rigidity, and (2) the history of gusts experienced by the tree, on the dynamics and windthrow vulnerability of a Pinus pinaster tree. The model suggests a negligible sensitivity of the upper tree swaying dynamics to the anchorage condition as opposed to the stem base movement. The low-frequency motions and natural vibration modes of the tree aerial structure in response to the wind are felt by the roots, with a propagation intensity depending on the root-soil system properties. Only the extremities of the transverse roots appear weakly impacted by the tree aerial motion. A study of the tree uprooting to the wind load demonstrates the importance of the gustiness history experienced by the tree in weakening the tree anchorage, and thus in progressively enhancing the tree vulnerability to uprooting. This result suggests that the critical overturning bending moment predicted by static tree-pulling experiments may overestimate the ultimate critical bending moment in situations where trees are damaged progressively by a succession of wind gusts. This novel model represents a promising tool for better understanding the tree response to gusty winds for various plant anchorage conditions.