Trees have neither voice nor ears, yet they spend their lives exchanging messages: alert scents, electrical impulses, signals relayed by roots and vast networks of fungi. Here’s how this discreet language works, and what it changes for both the walker and the gardener.
If we stick to the dictionary, language would be the exclusive domain of humans, the only ones capable of expressing themselves and exchanging with their peers. Trees, on the other hand, do not speak, and the cracking of branches on a windy day or the rustling of leaves are not due to their will: these are sounds endured, not messages.
However, trees have a very effective means of attracting attention, and that means is odors. We know from our own experience how much a scent can attract or, conversely, repel us (just think of the success of perfumes and deodorants).
Today, we know that trees exchange information in at least three ways: through odors, visual signals, and electrical signals, which travel through kinds of nerve cells located at the tips of their roots. A fourth avenue, that of sounds, is just beginning to be explored by scientists.
It all begins in the 1970s, in the African savanna, where researchers are interested in a species of acacia that giraffes willingly graze on its leaves. When a giraffe starts to eat, the tree reacts in just a few minutes: it increases the toxic substance content of its leaves, enough to cut off its visitor's appetite.
The giraffes then move away, but not towards the neighboring tree: they avoid all the acacias in the vicinity and only resume eating about a hundred meters away. The explanation lies in one word: ethylene. The attacked acacia releases this gas, which warns other acacias that danger is approaching, and these trees in turn load their leaves with toxic substances without waiting to be grazed.
The giraffes have perfectly understood the trick. They seek out the trees that have not received the message, or they move against the wind: since the alert travels through the air, the acacia located upstream has not yet been warned, and the meal can continue without unpleasant surprises.
Our temperate forests are the setting for comparable scenes, as beeches, oaks, and firs also know how to defend themselves when an insect attacks. When a caterpillar bites into a leaf, the tissues of the leaf immediately transform around the wound.
At the same time, the leaf sends out electrical signals, somewhat like our body does when we get hurt. The difference lies in the speed: in humans, the signal travels in a few milliseconds, while in trees it progresses at a rate of one centimeter per minute. It takes an additional hour for the tree to produce the defensive substances that will spoil the caterpillar's meal.
It's slow, and the tree cannot go any faster, even in emergencies. But no part of the tree works in isolation: if an insect attacks the roots, the information spreads throughout the entire tree and can, if necessary, trigger the release of odors by the leaves.
These scents do not disperse randomly. The tree composes tailor-made substances according to the desired purpose, which generally helps it fend off an attack within a few days.
However, it must know who it is dealing with. Each species of insect has its own unique saliva: by identifying it, the tree can surely pinpoint the culprit among the many insects it recognizes. This means that it possesses, in its own way, a true sense of taste.
Once the insect is unmasked, the tree can emit substances that attract the predators of that insect. Elms and pines, for example, call forth small wasps that lay their eggs inside the caterpillars. The larvae hatch safely and grow by devouring the caterpillar from the inside: an unenviable end, but the tree, rid of its parasites, can grow again.
Moreover, the call does not always need to be targeted. Animals perceive all the chemical signals emitted by trees, so they know that an attack is underway and which species is responsible. All those who relish these little insects then come rushing in on their own.
Trees also know how to manage without external help. The oak circulates tannins, bitter and toxic substances, in its bark and leaves. While they may not kill all insects, they transform appetizing foliage into inedible greenery.
The willow achieves the same result thanks to salicin, which is equally formidable for insects. For humans, however, this is good news: a tea made from willow bark, the ancestor of aspirin, soothes headaches and fever.
These defenses do have a weak point, though: they take time to establish. Hence the importance of an early warning system capable of alerting neighboring trees before insects reach them.
Smells have a significant drawback: they dissipate very quickly in the air, so their range often remains less than a hundred meters.
However, they provide a double service to the tree. Inside the tree, the alert signal travels very slowly, while a smell can cover several meters in no time: using the air allows the tree to warn its more distant branches earlier, in addition to neighboring trees.
That said, this aerial route offers no guarantees, as depending on the wind, some neighbors may not be alerted (giraffes have understood this well). That is why trees do not rely solely on the air.
To secure their backs, trees also send their messages through their roots, which connect the trees to each other and function well in both rain and strong winds.
In the roots, information circulates chemically, but also electrically, with the signal traveling at a rate of one centimeter per second. Compared to the speed of our nervous system, this speed seems quite slow, but some animals, like jellyfish or worms, do not move much faster. As soon as the news reaches them, all the oaks in the vicinity circulate large quantities of tannins through their vessels.
This network owes much to the size of the root system: the roots of a tree cover an area more than twice the span of its crown. Underground, the roots of different trees intertwine and multiply contact points, which are opportunities to exchange information.
Not all roots touch, and each forest is home to a few solitary trees that are not inclined to cooperate. Should we fear that a handful of individualistic trees will block the flow of alerts? No, because it is often the fungi that ensure the continuity of the network.
Fungi function somewhat like the Internet and its fiber optics. Their filaments, called hyphae, are invisible to the naked eye but have a density that is hard to imagine: a single teaspoon of forest soil contains several kilometers of them. Over the centuries, a single fungus can spread over several square kilometers and connect entire forests.
This network relays signals from tree to tree and circulates information about insects, soil drought, or any other danger. Scientists have even given it a name, the "Wood-Wide-Web," in other words, the web of the forest.
It should be noted that research on the nature and quantity of the information exchanged is still in its early stages. It is not excluded that different species, despite being competitors, also come into contact, as fungi follow their own strategy and make excellent intermediaries.
A weakened tree sees its defenses dull, and perhaps also its ability to communicate, which would explain why insects specifically target fragile trees. It is possible that insects are on the lookout for warning signals, then bite the bark or a leaf to see if the silent trees react.
This silence may stem from a serious illness, but it can also result from a break in the connection with the fungal network. Deprived of any information, the tree is unaware of the danger, allowing caterpillars and beetles to feed at will.
The solitary trees we mentioned earlier are in the same situation. Despite appearing perfectly healthy, their isolation leaves them in ignorance, and thus at the mercy of insects. In the forest, staying connected to other trees is a complete form of protection.
Not all exchanges between trees and insects revolve around attacks and diseases. Some messages are much more pleasant, and you have surely already sensed them: they are the scents of flowers.
These odors are not a matter of chance, and they do not seek to please us. Willows, chestnut trees, and fruit trees emit them to attract bees and invite them to fill up on nectar. This sweet liquid is the reward for pollination, which bees ensure without even knowing it.
The colors and shapes of flowers play the same role. They make the flowers stand out amidst the greenery of the foliage, much like a sign that signals from afar the entrance of a restaurant.
Trees were thought to make no sound, but it may not be that simple. Researchers from the University of Western Australia, led by Monica Gagliano, had the idea to listen to the soil. Since trees are impractical to study in a laboratory, the researchers preferred to work with cereal seedlings, which are much easier to manipulate.
Their devices quickly recorded a slight crackling of the roots at a frequency of 220 hertz. The most astonishing finding was yet to come: when exposed to this same sound, the roots of other seedlings, which were not the source of the noise, turned their tips in the direction of the sound. In other words, these grasses perceive this frequency and thus "hear" in their own way.
Do plants also exchange information through sound? The question remains open, as this field of research is still in its early stages.
Other scientists from the University of Bern, the Swiss Federal Institute of Technology in Zurich, and a Swiss federal institute dedicated to forests, snow, and landscapes have been listening to trees, particularly at night. The trees they were examining emitted a faint whisper.
At night, photosynthesis pauses, and the tree's crown hardly transpires. The tree takes this opportunity to absorb water, causing its trunk to gain a little diameter, while the water remains almost motionless in the vessels. So where do these sounds come from?
According to the researchers' hypothesis, the sounds arise from tiny bubbles of CO2 that form in the fine conduits filled with water. There is no evidence that this whisper serves to communicate, but it reminds us that a tree, seen from the inside, is far from silent.
For the walker, these discoveries change the way to explore a forest. Here are some tips to perceive at your scale what the trees are communicating:
Feel: the scent of willows, chestnut trees, or flowering fruit trees is a call to bees, so take the time to watch the bees foraging.Look: the bright colors of flowers and their shapes also serve as signals. On an oak or a beech, a leaf nibbled by a caterpillar has already triggered the alert.Listen: if you hear faint crackling in the forest, the wind may not be the only culprit.Examine: in spring, just before the buds open, water flows so strongly in the trunk that a stethoscope placed on the bark allows you to hear it.
Note: maple water producers use this listening method. Highly sought after, maple water is collected only during the snowmelt, then it becomes syrup through evaporation.
In the forest, exchanges are not limited to trees: bushes and grasses also share information, as probably do all the plants living in the forest. The contrast is striking as soon as one enters a cultivated area, where vegetation becomes very silent.
Under human influence, cultivated plants have lost much of their ability to communicate, both through the air and the soil. Almost mute and deaf, these plants become easy prey for insects, which partly explains the massive use of pesticides in modern agriculture.
A path emerges: to draw inspiration from the forest and allow a bit of nature to reclaim its place in cereal or potato fields, so that plants can regain their voice.
Trees do not speak, but they are far from silent. Here are their main means of communication:
Alert scents, like the ethylene from acacia, carried by the air often over less than a hundred meters.Electrical signals, one centimeter per minute in a leaf and one centimeter per second in the roots.A network of intertwined roots, relayed by the filaments of fungi.Fragrances, shapes, and colors to attract pollinators.Perhaps sounds, a still very new avenue.
During your next walk in the forest, take the time to smell, observe, and listen: beneath your feet as well as above your head, a discreet conversation is taking place.
Author Audrey on 26 May 2023
Home and Garden : DIY
Author Loïc on 11 August 2026
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