Trunk call: the trees that talk

Richard Grant reveals how trees can chat – and even warn each other of danger – via the ‘wood-wide web’

I’m walking in the Eifel mountains in western Germany, through cathedral-like groves of oak and beech, and there’s a strange feeling of entering a fairy tale. The trees have become vibrantly alive and charged with wonder. They’re communicating with one another, involved in tremendous struggles and death-defying dramas. To reach enormousness, they depend on a complicated web of relationships, alliances and kinship networks.

Wise old mother trees feed their saplings with liquid sugar and warn the neighbours when danger approaches. Youngsters take foolhardy risks with leaf-shedding, light-chasing and excessive drinking, and usually pay with their lives. Crown princes wait for old monarchs to fall, so they can take their place in the full glory of sunlight. It’s all happening in the ultraslow motion that is tree time, so what we see is a freeze-frame of the action.

My guide here is a kind of tree whisperer. Peter Wohlleben, a German forester and author, has a rare understanding of the inner life of trees, and is able to describe it in accessible, evocative language. He stands very tall and straight, like the trees he most admires, and on this cold, clear morning, the blue of his eyes precisely matches the blue of the sky. Wohlleben has devoted his life to the study and care of trees. He manages this forest as a nature reserve, and lives with his wife, Miriam, in a rustic cabin near the remote village of Hümmel.

Now, at the age of 53, he has become an unlikely publishing sensation. His book The Hidden Life of Trees: What They Feel, How They Communicate, written at his wife’s insistence, has sold nearly a million copies in Germany since its original publication, in 2015, and has hit the bestseller lists in 11 other countries.

A revolution has been taking place in the scientific understanding of trees, and Wohlleben is the first writer to convey its amazing results to a general audience. The latest studies, conducted at respected universities in Germany and around the world, confirm what he has long suspected from close observation in this forest: trees are far more alert, social, sophisticated – and even intelligent – than we thought.

With his big green boots crunching through fresh snow, Wohlleben takes me to two massive beeches growing next to each other. He points up at their skeletal winter crowns, which appear careful not to encroach on each other’s space. ‘These two are old friends,’ he says. ‘They are considerate in sharing sunlight, and their root systems are closely connected. In cases like this, when one dies, the other usually dies soon afterwards, because they are dependent on each other.’

Since Darwin, we have generally thought of trees as disconnected loners, competing for water, nutrients and sunlight, with the winners shading out the losers and sucking them dry. The timber industry in particular sees forests as wood-producing systems and battlegrounds for survival of the fittest.

There is now a substantial body of scientific evidence showing instead that trees of the same species are communal, and will even form alliances with trees of other species.

Forest trees have evolved to live in cooperative, interdependent relationships, maintained by communication and a collective intelligence similar to an insect colony. These soaring columns of living wood draw the eye upwards to their outspreading crowns, but the real action is taking place underground, just a few inches below our feet.

‘Some call it the “wood-wide web”,’ says Wohlleben in German-accented English. ‘All the trees here, and in every forest that is not too damaged, are connected to each other through underground fungal networks. Trees share water and nutrients through the networks, and also use them to communicate. They send distress signals about drought and disease, for example, or insect attacks, and other trees alter their behaviour when they receive these messages.’

Scientists call these mycorrhizal networks. The fine, hairlike root tips of trees join together with microscopic fungal filaments to form the links of the network, which appears to operate as a symbiotic relationship between trees and fungi, or perhaps an economic exchange. As a kind of fee for services, the fungi consume about 30 per cent of the sugar that trees photosynthesise from sunlight. The sugar is what fuels the fungi, as they scavenge the soil for nitrogen, phosphorus and other mineral nutrients, which are then absorbed and consumed by the trees.

For young saplings in a deeply shaded part of the forest, the network is literally a lifeline. Lacking sunlight, they survive because big trees, including their parents, pump sugar into their roots through the network. Wohlleben likes to say that mother trees ‘suckle their young’, which stretches a metaphor but gets the point across vividly.

Once, he came across a gigantic beech stump in this forest, four or five feet across. The tree had been felled 400 to 500 years ago, but scraping away the surface with his penknife, Wohlleben found something astonishing: the stump was still green with chlorophyll. There was only one explanation. The surrounding beeches were keeping it alive, by pumping sugar to it through the network. ‘When beeches do this, they remind me of elephants,’ he says. ‘They are reluctant to abandon their dead, especially when it’s a big, old, revered matriarch.’

To communicate through the network, trees send chemical, hormonal and slow-pulsing electrical signals, which scientists are just beginning to decipher. Edward Farmer at the University of Lausanne in Switzerland has identified a voltage-based signalling system that appears strikingly similar to animal nervous systems (although he is not suggesting that plants have neurons or brains). Alarm and distress appear to be the main topics of tree conversation, although Wohlleben wonders if that’s all they talk about. ‘What do trees say when there is no danger and they feel content? This I would love to know.’

Trees also communicate through the air, using pheromones and other scent signals. Wohlleben’s favourite example occurs on the hot, dusty savannahs of sub-saharan Africa, where the wide-crowned umbrella thorn acacia is the emblematic tree. When a giraffe starts chewing acacia leaves, the tree notices the injury and emits a distress signal in the form of ethylene gas. Upon detecting this gas, neighbouring acacias start pumping tannins into their own leaves. In large enough quantities these compounds can sicken or even kill large herbivores.

Giraffes are aware of this, however, having evolved with acacias, and browse into the wind, so the warning gas doesn’t reach the trees ahead of them. If there’s no wind, a giraffe will typically walk 100 yards – farther than ethylene can travel in still air – before feeding on the next acacia. Giraffes, you might say, know the trees talk to one another.

‘They call me a “tree-hugger”, but I don’t believe trees respond to hugs’

Trees can detect scents through their leaves, which, for Wohlleben, qualifies as a sense of smell. They also have a sense of taste. When elms and pines come under attack by leaf-eating caterpillars, for example, they detect the caterpillar saliva, and release pheromones that attract parasitic wasps. The wasps lay their eggs inside the caterpillars, and the wasp larvae eat the caterpillars from the inside out. ‘Very unpleasant for the caterpillars,’ says Wohlleben. ‘Very clever of the trees.’

A recent study from Leipzig University and the German Centre for Integrative Biodiversity Research shows that trees know the taste of deer saliva. ‘When a deer is biting a branch, the tree brings defending chemicals to make the leaves taste bad,’ he says. ‘When a human breaks the branch with his hands, the tree knows the difference, and brings in substances to heal the wound.’

Our boots crunch on through the glittering snow. From time to time, I think of objections to Wohlleben’s anthropomorphic metaphors, but more often I sense my ignorance falling away. I have taken trees for granted, in a way that will never be possible again.

We reach an area that he calls ‘the classroom’. Young beech trees, in their individual ways, are tackling the fundamental challenge of their existence. Like any tree, they crave sunlight, but down here below the canopy, only three per cent of the light is available. One tree is the ‘class clown’. Its trunk contorts itself into bends and curves, ‘making nonsense’ to try to reach more light, instead of growing straight and true like its more patient classmates. ‘It doesn’t matter that his mother is feeding him, this clown will die,’ says Wohlleben.

Another tree is growing two absurdly long lateral branches to reach some light coming through a small gap in the canopy. Wohlleben dismisses this as ‘foolish and desperate’, certain to lead to imbalance and fatal collapse. He makes these blunders sound like conscious, sentient decisions, when they’re really variations in the way natural selection has arranged the tree’s unthinking hormonal command system. Wohlleben knows this, of course, but his main purpose is to get people interested in the lives of trees, in the hope that they will defend forests from destructive logging and other threats.

Wohlleben used to be a cold-hearted butcher of trees and forests. His training dictated it. In forestry school, he was taught that trees need to be thinned, that helicopter-spraying of pesticides and herbicides is essential, and that heavy machinery is the best logging equipment, even though it tears up soil and rips apart the mycorrhizae. For more than 20 years, he worked like this, in the belief that it was best for the forests he had loved since childhood.

He began to question the orthodoxies of his profession after visiting a few privately managed forests in Germany, which were not thinned, sprayed or logged by machine. ‘The trees were so much bigger and more plentiful,’ he says. ‘Very few trees needed to be felled to make a handsome profit, and it was done using horses to minimise the impact.’

At the same time, he was reading early research about mycorrhizae and mother trees, and studies about tree communication coming out of China, Australia, the United States, Britain and South Africa. When he was ordered to clear-cut the forest near his home village of Hümmel – the fairytale forest we’ve been walking through all morning – he invented excuses and prevaricated for several years. Then, in 2002, he went to the villagers and performed a mighty feat of persuasion.

After hearing his arguments, they agreed to give up their income from timber sales, turn the forest into a nature reserve, and allow it to return to its primeval splendour. In 2006, Wohlleben resigned from his state forestry job to become manager of the old beech forest for the town. Both Wohlleben and the villagers, perhaps, were tapping into the old German romanticism about the purity of forests.

To generate income, he created a wildwood cemetery, where nature lovers pay for their cremated remains to be

buried in simple urns. ‘The trees are sold as living headstones,’ he says. There is some light horse-logging, and visitors pay to take tours of the forest. For many years, Wohlleben led these tours himself, using vivid, emotional phrasing to dramatise the largely inscrutable life of trees. People enjoyed it so much that Wohlleben’s wife urged him to write a book.

He has been taken to task by some scientists, but his strongest denouncers are commercial German foresters, whose methods he calls into question. ‘They don’t challenge my facts because I cite all my scientific sources,’ he says. ‘Instead, they say I’m “esoteric”, which is a very bad word in their culture. And they call me a ‘tree-hugger’, which is not true. I don’t believe that trees respond to hugs.’

Five-thousand miles away, at the University of British Columbia in Vancouver, Suzanne Simard and her grad students are making astonishing new discoveries about the sensitivity and interconnectedness of trees in the Pacific temperate rainforests of western North America. In the view of Simard, a professor of forest ecology, their research is exposing the limitations of the Western scientific method itself.

Simard is a warm, friendly, outdoorsy type with straight blonde hair and a Canadian accent. In the scientific community, she’s best known for her extensive research into mycorrhizal networks, and her identification of hyperlinked ‘hub trees’, as she calls them in scientific papers, or ‘mother trees’, as she prefers in conversation. Peter Wohlleben refers extensively to her research in his book.

Mother trees are the biggest, oldest trees in the forest with the most fungal connections. They’re not necessarily female, but Simard sees them in a nurturing, supportive, maternal role. With their deep roots, they draw up water and make it available to shallow-rooted seedlings. They help neighbouring trees by sending them nutrients, and when the neighbours are struggling, mother trees detect their distress signals and increase the flow of nutrients accordingly.

Over a sandwich lunch on campus, Simard explains her frustrations with Western science. ‘We don’t ask good questions about the interconnectedness of the forest, because we’re all trained as reductionists. We pick it apart and study one process at a time, even though we know these processes don’t happen in isolation. When I walk into a forest, I feel the spirit of the whole thing, everything working together in harmony, but we don’t have a way to map or measure that. We can’t even map the mycorrhizal networks. One teaspoon of forest soil contains several miles of fungal filaments.’

She takes me to a magnificent old grove of western red

Alarm and distress appear to be the main topics of tree conversation

cedars, bigleaf maples, hemlocks and Douglas firs. In the forest, Simard’s face brightens and her nostrils flare as she breathes in the cool, damp, fragrant air. She points to a massive, cloud-piercing giant with long, loose strips of greyish bark. ‘That red cedar is probably 1,000 years old,’ she says.

‘It’s mother tree to the other cedars here, and it’s linked to the maples, too. Cedar and maple are on one network, hemlock and Douglas fir on another.’

Why do trees share resources and form alliances with trees of other species? Doesn’t the law of natural selection suggest they should be competing? ‘Actually, it doesn’t make evolutionary sense for trees to behave like resource-grabbing individualists. They live longest and reproduce most often in a healthy, stable forest. That’s why they’ve evolved to help their neighbours.’

If neighbouring trees keep dying, gaps open up in the protective forest canopy. With increased sunlight, the trees left standing can photosynthesise more sugar, and grow faster, but, Simard says, they’re also more vulnerable and short-lived. The mycorrhizal support system weakens. In summer, more hot sunshine reaches the delicate forest floor, drying out the cool, damp, evenly regulated microclimate that such forest trees prefer. Damaging winds can penetrate the forest more easily, and without neighbouring tree crowns, the chance of being uprooted increases.

Looking up at these ancient giants with their joined-together crowns, it’s extraordinary to contemplate everything that they must have endured and survived together over the centuries. Lethal threats arrive in many forms: wind storms, ice storms, lightning strikes, wildfires, droughts, floods, a host of constantly evolving diseases, swarms of voracious insects.

Tender young seedlings are easily consumed by browsing mammals. Hostile fungi are a constant menace, waiting to exploit a wound, or a weakness, and begin devouring a tree’s flesh. Simard’s research indicates that mother trees are a vital defence against many of these threats; when the biggest, oldest trees are cut down in a forest, the survival rate of younger trees is substantially diminished.

Not all scientists are on board with the new claims being made about trees. Where Simard sees collaboration and sharing, her critics see selfish, random and opportunistic exchanges. Lincoln Taiz, a retired professor of plant biology at the University of California, Santa Cruz, and the co-editor of the textbook Plant Physiology and Development, finds Simard’s research ‘fascinating’ and ‘outstanding’, but sees no evidence that the interactions between trees are ‘intentionally or purposefully carried out’. Nor would that be necessary. ‘Each individual root and each fungal filament is genetically programmed by natural selection to do its job automatically,’ he says. Taiz thinks that human beings are fatally susceptible to the mythology of thinking, feeling, speaking trees. In ancient Greece, trees delivered prophesies. In medieval Ireland, they whispered unreliable clues to finding leprechaun gold. Talking trees have starred in any number of fantasy books and Hollywood films, from The Wizard of Oz to The Lord of the Rings to Avatar. Taiz sees the same old mythological impulse underlying some of the new claims about tree communication and intelligence, and the success of Wohlleben’s book and Simard’s Ted talk How Trees Talk to Each Other, which garnered well over two million views online.

In 2007, Taiz and 32 other plant scientists published an attack on the emerging idea that plants and trees possess intelligence. He is willing to ‘be liberal and go along with the idea’ that trees exhibit a ‘swarm intelligence’, but thinks it leads us down an erroneous path toward tree consciousness and intentionality. ‘The appearance of purposefulness is an illusion. Natural selection can explain everything we know about plant behaviour.’

From his house in Henley-on-thames, the eminent scientist Richard Fortey expresses similar criticisms. Now semi-retired, he was a palaeontologist at the Natural History Museum in London, and visiting professor of palaeobiology at Oxford. He has recently published The Wood for the Trees,

about four acres of woodland that he owns in the Chiltern Hills. It is a magisterial work, and rigorously pruned of all sentiment and emotion.

‘The mother tree protecting its little ones?’ he says with gentle scorn. ‘It’s so anthropomorphised that it’s really not helpful. The case is overstated and suffused with vitalism. Trees do not have will or intention. They solve problems, but it’s all under hormonal control, and it all evolved through natural selection.’

When informed that Simard also detects a spiritual aspect in forests, Fortey sounds appalled. ‘Spiritual?’ he says, as if the word were a cockroach on his tongue. ‘Oh dear, oh dear, well there’s nothing to be said about that. Look, trees are networkers. They do communicate in their own way. What worries me is that people find this so appealing that they leap to faulty conclusions. Namely that trees are sentient beings like us.’

A notable offender in this regard, says Fortey, is Peter Wohlleben. ‘There’s a lot of good new science in his book, and I sympathise with his concerns, but he describes trees as if they possess consciousness and emotions. His trees are like the Ents in The Lord of the Rings.’

When told about this criticism, Wohlleben smiles. ‘Scientists insist on language that is purged of all emotion,’ he says. ‘To me, this is inhuman, because we are emotional beings, and for most people, scientific language is extremely boring to read. The wonderful research about giraffes and acacia trees, for example, was done many years ago, but it was written in such dry, technical language that most people never heard about it.’

Wohlleben’s first priority is to not be boring, so he uses emotional storytelling techniques. His trees cry out with thirst, they panic and gamble and mourn. They talk, suckle and make mischief. If these words were framed in quotation marks, to indicate a stretched metaphorical meaning, he would probably escape most of the criticism. But Wohlleben doesn’t bother with quotation marks, because that would break the spell of his prose.

Does he think that trees possess a form of consciousness? ‘I don’t think trees have a conscious life, but we don’t know,’ he says. ‘We must at least talk about the rights of trees. We must manage our forests sustainably and respectfully, and allow some trees to grow old with dignity, and to die a natural death.’

In rejecting the confines of the careful, technical language of science, he has succeeded more than anyone in conveying the lives of these mysterious, gigantic beings, and in becoming their spokesman.

The Hidden Life of Trees, by Peter Wohlleben, is published by William Collins (£9.99). To order your copy for £8.99 plus p&p, call 0844-871 1514 or visit books.telegraph.co.uk

‘It worries me that people think that trees are sentient beings like us’

Umbrella thorn acacias alert others to giraffe attack so they can take preemptive action

Peter Wohlleben in the beech woods he manages in Hümmel, western Germany