Excerpt: How trees talk to each other

Excerpted from The Hidden Life of Trees: What They Feel, How They Communicate – Discoveries from a Secret World by Peter Wohlleben:

According to the dictionary definition, language is what we use when we talk to each other. Looked at this way, we are the only beings who can talk, because the concept is limited to our species. But wouldn’t it be interesting to know whether trees can also talk to each other? But how? They definitely don’t produce sounds, so there’s nothing we can hear. Branches creak as they rub against one another and leaves rustle, but these sounds are caused by the wind, and the tree has no control over them. Trees, it turns out, have a completely different way of communicating: they use scent.

Scent as a means of communication? The concept is not totally unfamiliar to us. Why else do we use deodorants and perfumes? And even when we’re not using these products, our own smell says something to other people, both consciously and subconsciously. There are some people who seem to have no smell at all; we are strongly attracted to others because of their aroma. Scientists believe pheromones in sweat are a decisive factor when we choose our partners—in other words, when we choose who we want to make babies with. So it seems fair to say that we possess a secret language of scent, and trees have demonstrated that they do, as well.

For example, four decades ago, scientists noticed something in the African savannah. The giraffes there were eating umbrella thorn acacias, and the trees didn’t like this one bit. It took the acacias mere minutes to start pumping toxic substances into their leaves to rid themselves of the large plant eaters. The giraffes got the message and moved on to other trees in the vicinity. But did they move on to trees close by? No, for the time being, they walked right by nearby trees and resumed their meal only when they had moved about one hundred meters farther on.

The reason for this behavior is astonishing. The acacia trees that were being eaten gave off a warning gas (in this case, ethylene) that signaled to neighboring trees of the same species that a crisis was at hand. Right away, all the forewarned trees also pumped toxins into their leaves to prepare themselves. The giraffes were wise to this game and therefore moved farther away to a part of the savannah where they could find trees that were oblivious to what was going on. Or else they moved upwind. For the scent messages were carried to nearby trees on the breeze, and if the animals walked upwind, they could find acacias close by that had no idea the giraffes were there.

Similar processes are at work in our forests. Beeches, spruce, and oak all register distress as soon as some creature starts nibbling on them. When a caterpillar takes a hearty bite out of a leaf, the tissue around the site of the damage changes. In addition, the leaf tissue gives off electrical signals just as human tissue does when it is hurt. However, the signal is not transmitted in milliseconds, as human signals are, instead the plant signal travels at the slow speed of one centimeter per minute. Accordingly, it takes an hour or so before defensive compounds reach the leaves to spoil the pest’s meal.

Trees live their lives in the really slow lane, and even when they are in danger, this seems to be their top speed. But this slow tempo doesn’t mean that a tree is not on top of what is happening in different parts of its structure. If the roots find themselves in trouble, this information is broadcast throughout the tree, and this can result in the leaves giving off scent compounds. And not just any old scent compounds, but compounds that are specially formulated for the task at hand.

Let’s look at a weapon in the trees’ scent arsenal that can help them fend off attack for a while. When it comes to some species of insects, trees can accurately identify which bad guys they are up against. They can do this because the saliva of each species is unique, and the tree can match the saliva to the insect. Indeed, the match can be so precise that the tree can release pheromones to summon specific beneficial predators. The beneficial predators help the tree by eagerly devouring the insects that are bothering them. For example, elms and pines call on small parasitic wasps that lay their eggs inside leaf-eating caterpillars. As the wasp larvae develop, they devour the caterpillars bit by bit from the inside out. Not a nice way to die. The result, however, is that the trees are saved from bothersome pests and can keep growing with no further damage. The fact trees can recognize saliva is, incidentally, evidence for yet another skill they must have. For if they can identify saliva, then they must also have a sense of taste.

A drawback for scent compounds is that they disperse quickly in the air. Often they can only be detected within a range of about one hundred meters. Quick dispersal, however, also has its advantages. As the transmission of signals inside the tree is very slow, a tree can cover long distances much more quickly through the air if it wants to warn distant parts of its own structure. Often, however, a specialized distress call isn’t necessary when a tree needs to mount a defense against insects. The animal world simply registers the tree’s basic chemical call for help. It then knows some kind of attack is taking place and predatory species should mobilize. Whoever is hungry for the kinds of critters that attack trees just can’t stay away.

Trees can also mount their own defense. Oaks, for example, carry bitter, toxic tannins in their bark and leaves. These either kill chewing insects outright or at least affect the leaves’ taste to such an extent that they change from a delicious crunchy mouthful to a bite of something biliously bitter. Willows produce salicylic acid as a defensive compound, which works in much the same way as tannin. Salicylic acid has a different effect on people, however. It is a precursor of aspirin, and tea made from willow bark can relieve headaches and bring down fevers.

Related: For Peter Wohlleben, trees and humans aren’t so different

Such defense mechanisms, of course, take time. Therefore a combined approach is crucially important for arboreal early-warning systems. Trees don’t rely exclusively on dispersal in the air, for if they did, some neighbors would not get wind of the danger. Accordingly, they also send messages via their roots. Each tree is attached to the root network and it operates no matter what the weather. News bulletins are sent via the roots not only by means of chemicals but also by means of electrical impulses that travel at the speed of one centimeter per second. That’s faster than the leaves’ signals, but in comparison with our bodies it is, admittedly, still extremely slow. However, there are species in the animal kingdom, such as jellyfish or worms, whose nervous systems conduct impulses at a similar speed, and once the latest news has been broadcast, all oaks in the area promptly pump tannins through their veins.

Tree roots extend a long way, more than twice the spread of the crown. And so the root systems of neighboring trees inevitably cross and grow into one another—although, there are always some exceptions. Even in a forest or wood, there are loners, misanthropes who want little to do with others. The question is, can such anti-social individuals block alarm calls simply by not participating? Luckily, the answer is no. For in most cases, fungi act as intermediaries to guarantee quick dissemination of news.

These fungi operate like fiber optic Internet cables. Their thin filaments penetrate the ground, weaving through it in almost unbelievable density. One teaspoon of forest soil contains many kilometers of these “hyphae.”4 Over centuries, a single fungus can extend over many square kilometers and network an entire forest. The connections transmit signals from one tree to the next, helping the trees exchange news about insects, drought, and other dangers. Science now even talks about a “Wood Wide Web” pervading our forests. What and how much information is exchanged are subjects we have only just begun to research. It is possible that different tree species are in contact with one another even if they regard each other as competitors. And the fungi are pursuing their own agendas and could be very much in favor of conciliation and equitable distribution of information.

If trees are weakened, it could be they lose their conversational skills along with their ability to defend themselves. Otherwise it’s almost impossible to explain why insect pests specifically seek out trees whose health is already compromised. It’s conceivable that in order to do this, insects listen to trees’

urgent chemical warnings, and then test individual trees that don’t pass the message on by taking a bite out of their leaves or bark. A tree’s silence could be due to a serious illness or, perhaps, to the loss of its fungal network, which would leave the tree completely cut off from the latest news. The tree no longer registers approaching disaster, and the doors are open for the caterpillar and beetle buffet. The loners I just mentioned are similarly susceptible—they might look healthy but they are blissfully unaware of what is going on around them.

In the symbiotic community of the forest, not only trees, but also shrubs and grasses—and possibly all plant species—exchange information this way. However, when we step into farm fields, the vegetation becomes very quiet. Thanks to selective breeding, our cultivated plants have, for the most part, lost their ability to communicate above or below ground—you could say they are deaf and dumb—and therefore they are easy prey for insect pests.5 That is one reason why modern agriculture uses so many pesticides. Perhaps farmers can learn from the forests and breed a little more wildness back into their grain and potatoes so they’ll be more talkative in the future.

Communication between trees and insects doesn’t have to be all about defense and illness. Thanks to your sense of smell, you’ve probably picked up on many feel-good messages exchanged between these distinctly different lifeforms. I am referring to the pleasantly perfumed invitations sent out by tree blossoms. Blossoms do not release their scent at random or to please us. Fruit trees, willows, and chestnuts use their olfactory missives to draw the attention of passing bees and invite them in to sate themselves. Sweet nectar, concentrated sugar juice, is the reward for the dusting the insects receive while they visit. The form and color of blossoms is another signal, somewhat like a billboard that clearly stands out from the general green of the tree canopy and points the way to a snack.

So, trees communicate by means of olfactory, visual, and electrical signals. (The electrical signals travel via a form of nerve cell at the tips of the roots.) What about sounds? Let’s get back to hearing and speech. When I said at the beginning of this chapter that trees are definitely silent, the latest scientific research casts doubt even on this statement. Along with colleagues from Bristol and Florence, Monica Gagliano from the University of Western Australia has, quite literally, had her ear to the ground.6 It’s not practical to study trees in the laboratory, therefore researchers substitute grain seedlings because they are easier to handle. They started listening, and it didn’t take them long to discover that their measuring apparatus was registering roots crackling quietly at a frequency of 220 hertz. Crackling roots? That doesn’t necessarily mean anything. After all, even dead wood crackles when it’s burned in a stove. But the noises discovered in the laboratory caused the researchers to sit up and pay attention. For the roots of seedlings not directly involved in the experiment reacted. Whenever the seedlings’ roots were  exposed to a crackling at 220 hertz, they oriented their tips in that direction. That means the grasses registered this frequency, so it makes sense to say they “heard” it.

Plant communicating by means of sound waves? That makes me curious to know more, because people are also set up to communicate using sound. Might this be a key to getting to know trees better? To say nothing of what it would mean if we could hear whether all was well with beeches, oaks, and pines, or whether something was up. Unfortunately, we are not that far advanced, and research in this field is just beginning. But if you hear a light crackling sound the next time you go for a walk in the woods, perhaps it won’t be just the wind …

From the book The Hidden Life of Trees: What They Feel, How They Communicate – Discoveries from a Secret World, by Peter Wohlleben. English translation by Jane Billinghurst. Published in 2016 by Greystone Books and David Suzuki Institute. Reprinted with permission of the publisher.