Trees Can Tell When They’re Crooked and Grow Special Wood to Straighten Themselves

New experiments reveal a sophisticated posture-control system hidden inside living wood.

Trees know when they’re crooked.

When a trunk bends, a tree can sense that change in its own shape and grow specialized wood on the convex side of the curve to pull itself straight again. New experiments in poplar trees reveal a surprisingly sophisticated posture-control system.

In experiments with young poplar trees, researchers isolated this ability from two of the plants’ most familiar guides: gravity and directional light. The curved trees still straightened. More surprisingly, microscopic examination revealed that they did so by switching the side of the stem on which they produced tension wood, a specialized tissue capable of generating powerful pulling forces.

Tension wood was thought to be something that forms on the upper side of a leaning hardwood stem to pull it upward. Instead, the researchers found that its position can flip as a tree continuously balances two kinds of information: which way gravity points and how curved its own stem has become.

Which Way Is Up?

Scientists have suspected for years that plants possess a form of proprioception, or the ability to sense the configuration of their own bodies.

A 2012 study in the Proceedings of the National Academy of Sciences developed a mathematical model showing that gravity sensing alone could not explain how shoots smoothly straighten without repeatedly overshooting their target. Studies that came later increasingly treated plant posture as a feedback system in which gravity sensing and curvature sensing operate together. A 2019 review of plant posture control described this continuous correction as fundamental to how upright plants grow.

But how does this feedback drive the woody parts of a tree?

To find out, Alexandre Caulus, Félix Hartmann and their colleagues at INRAE and Université Clermont Auvergne first placed young hybrid poplars—European aspen (Populus tremula) and white poplar (Populus alba) cross—horizontally. The trees responded conventionally, bending upward over roughly 10 days.

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Then came the clever part.

The researchers placed the curved trees inside spherical chambers illuminated evenly from every direction. This way, there was no directional light cue. They also continuously rotated the plants around a horizontal axis on a clinostat. The rotation continuously changes the direction of gravity relative to the plant, effectively suppressing its ability to use gravity as a stable directional signal.

What remained relevant to the straightening response was the stem’s own curvature.

Over the following weeks, the bent stems straightened.

Switching Sides

During the original upward bend, tension wood formed on the upper side of the stem. But after the researchers suppressed gravity sensing, production there stopped. Within days, new tension wood began appearing on the opposite, convex side, creating a force that progressively reduced the curve.

Tension wood is not simply ordinary wood growing in an unusual place. Its fibers often develop a thick, cellulose-rich gelatinous layer (G-layer). The cellulose strands inside this layer run almost parallel to the length of the fiber. As the tissue matures, it generates tensile stress, allowing the wood to pull on the surrounding stem.

The new tissue formed during straightening had the same anatomical signatures as conventional tension wood. Compared with normal wood, it contained roughly three times as many fibers relative to vessels. Its G-layer was also about 37% thicker than the tension wood produced during the initial upward bend.

This effectively gives a tree two opposing pulling systems. Tension wood on one side can create a bend; tension wood on the other can later counteract it.

The response is not instantaneous. The study found a delay of about two days between changing the sensory conditions and the tree beginning to reverse its bend. Cambial cells need time to divide and develop into new wood, while formation and mechanical activation of the G-layer take additional time.

Fix Your Posture!

The broader implication is that tension wood does not simply appear on the upper side of a leaning tree, as it is often described in textbooks.

Instead, its position depends on competing sensory information. Gravity tells a tree how it is oriented, while proprioception tells it how curved its stem has become. When gravity sensing dominates, tension wood forms where it can pull the stem upward. When curvature sensing takes over, the tissue can shift sides and straighten the stem instead.

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“What we have uncovered is a genuine sensorimotor loop operating in the woody parts of trees!” Bruno Moulia, an INRAE research director and senior author of the study, said in a statement.

You could say that, in a sense, trees develop “muscle” to fix their posture, but the comparison is useful only up to a point. Trees have neither muscles nor a nervous system, and the researchers have not yet identified the full molecular pathway linking the detection of curvature to the production of tension wood. What the experiments establish is the feedback system itself: the tree senses its changing shape and alters where it builds force-generating wood in response.

Studies in Arabidopsis had already shown that gravity-driven bending and active straightening work together to control plant posture. The new experiments show how trees can accomplish the same kind of correction using newly formed wood.

Trees repeatedly face winds, snow loads, uneven growth, and disturbances that knock stems away from their preferred position. Staying upright is therefore not a one-time developmental achievement. It is something a tree continually corrects.

The study was published in the journal New Phytologist.