The bark side of the force
What forces enable trees to stand upright? To grow straight, plants need a motor system that controls their posture by generating forces to offset gravity. Scientists have long thought that this motor force was controlled only by the internal forces induced in wood. In a study published on 4 August 2018 in New Phytologist, researchers from the CNRS and Cirad show that bark is also involved in the generation of mechanical stresses in several tree species.
As the vertical position is unstable—for both trees and humans—mechanisms are needed to offset the effect of gravity. To ensure that they grow vertically, trees need both a "skeletal" system, which is achieved through the stiffness and strength of the trunk material, and a "motor" system to control their posture by generating forces that offset gravity. Although the skeletal function of bark has already been demonstrated, its motor function (as the tree's "muscle") has generally been overlooked in biomechanical and ecological studies, as wood was considered to be the only tree tissue with active mechanical functions.
To understand the role of bark, researchers from the Ecofog laboratory (CNRS/AgroParisTech/Cirad/Inra/Université de Guyane/Université des Antilles) and the LMGC laboratory (CNRS/Université de Montpellier) grew tropical species at a tilted angle. The forces that developed in the stems of the young staked trees produced a stem curvature as soon as the stakes were removed. In certain species, when the bark of the stem was removed, this curvature was lost, which demonstrates that the forces responsible for stem up-righting are located in the bark.
The mechanism generating these forces is linked to the specific structure of the bark, where the fibers are organized in the form of a trellis. When the stem grows, the development of the layers of wood increases the circumference of the bark. The trellis structure of the fibers in the bark is such that this stress generates forces along the stem. If the stem is titled, the growth is more rapid on the upper side of the stem, leading to the generation of asymmetric forces. It is this asymmetry that allows the stem to curve upwards. In five of the nine tree species studied, the generation of forces enabling the tree to offset gravity is thus not simply due to the maturing of the walls of the wood cells, but also to the wood growth in this smartly organized trellis of fibers in the bark.
Nature has solved an infinite number of engineering problems. The need for trees to grow vertically by optimizing the organization of their tissues is today a potential source of remarkable inspiration for materials science.