Trees Straighten Their Own Trunks With Wood That Pulls Like a Muscle

Trees can tell what shape they are in, and they can do something about it. A young tree whose trunk has been bent can grow itself back toward straight, using a special kind of wood that pulls the way a muscle does. Researchers already knew that plants can sense their own posture. What they did not know was how that sense turns into movement. A new experiment with young poplar trees has now shown the trick: the tree builds pulling wood on one side of its stem, then builds the same pulling wood on the other side, and the two forces straighten it out.

A tree on a slowly turning platform

To study this sense on its own, the researchers had to switch off the other signals a plant uses to decide which way is up. Plants normally respond to gravity and to the direction of light. So the team built an unusual setup: young trees on a horizontal platform that rotated around its axis, inside a sphere lit equally from every direction. The rotation meant gravity never gave the trees a steady sense of down, and the even light meant there was no brighter side to lean toward.

The trees began the experiment lying horizontally. As they grew, their stems curved upward, lifting their tops back toward vertical. After about ten days, when the curve was clear, the trees were moved into the rotating sphere. Over the following weeks the stems slowly straightened until they were nearly straight again.

The wood that pulls in both directions

The straightening depends on tension wood. Ordinary wood is mostly a support. Tension wood does something extra: as it develops it generates a pulling force, so when it forms along the upper side of a stem it acts like a contracting muscle and hauls the stem upward.

When the team looked at the wood laid down during the straightening, they found a change. The original tension wood, on the upper side, stopped developing once the trees were in the apparatus. At the same time, identical tension wood started forming on the opposite side, pulling the other way. With the two forces working against each other, the curved stem came back to straight. The tree's own sense of its shape appears to decide where that wood is laid down.

That was a surprise. Tension wood had been thought to form only on the upper side of a stem, something you can see near the bases of trees on mountain slopes, where trunks curve to stay upright. The new work shows it can appear on opposing sides and pull in different directions, much like the opposing muscle groups animals use to hold a posture. Scientists call this body-position sense proprioception, and in 2012 researchers working with INRAE showed that plants have it too.

Straighter trees, sturdier crops

Being able to correct your own shape matters most when something knocks you over. Storms and landslides bend and displace trees, and the ability to regain a useful posture helps them survive. That skill may matter more as climate change brings forests more extreme conditions.

The finding also points to practical uses. If researchers can identify plants with a particularly good sense of their own shape, they may be able to select varieties that grow straighter, hold their posture better and produce higher-quality wood. In agriculture, the same knowledge could help develop cereal crops that are less likely to fall over.