A bent tree can produce wood on the opposite side to gradually regain a straight shape.
Researchers observed this mechanism in young poplars. Placed horizontally, the trees began to curve their stems upward. The plant reacts in particular to gravity to reorient its growth. The poplar then produces a particular type of wood on the upper side of its stem. Called tension wood, it exerts force as it contracts. This traction acts on the shape of the stem and gradually pulls it back upward.
Diagram of a young poplar forming tension wood, with microscope views of the cell walls of tension wood and opposite wood.
Credit: Normand et al., Scientific Reports, CC BY 4.0.
After about ten days, the poplars were placed on a rotating device. This disrupted the direction of gravity. Light coming from all directions also removed any luminous reference points.
The tree was then left with just one piece of information: the curved shape of its own stem. This ability to perceive the position or shape of its body is called proprioception. Earlier studies had already established that plants possess this ability.
The poplars then began to straighten up. The tension wood that had previously formed on the upper side stopped appearing. The researchers found the same type of wood on the opposite side. The force exerted therefore changed sides.
The experimental setup
In concrete terms, tension wood can produce two opposite movements depending on where the tree forms it. The tree can first bend its stem to regain the vertical position, then trigger an opposing force to avoid overshooting that position.
This process helps explain how a tree controls its posture despite wind, a slope, or deformation. It combines several pieces of information, including gravity, light, and its own curvature. Poor coordination can also create internal stresses and alter the quality of the wood.
The researchers now want to use this knowledge to better study tree shape and wood quality. This mechanism could also be of interest in selecting cultivated plants capable of remaining upright when their stems risk falling to the ground.