smithsonianmag.com/science-nature Wohlleben takes me to two massive beech trees growing next to each other. He points up at their skeletal winter crowns, which appear careful not to encroach into each other’s space. “These two are old friends,” he says. “They are very considerate in sharing the sunlight, and their root systems are closely connected. In cases like this, when one dies, the other usually dies soon afterward, because they are dependent on each other.”
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, sold more than 800,000 copies in Germany, and has now hit the best-seller lists in 11 other countries, including the United States and Canada.
A revolution has been taking place in the scientific understanding of trees, and Wohlleben is the first writer to convey its amazements to a general audience. The latest scientific studies, conducted at well-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. There is now a substantial body of scientific evidence that refutes that idea. It shows instead that trees of the same species are communal, and will often 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 upward to their outspreading crowns, but the real action is taking place underground, just a few inches below our feet.
Some are calling it the wood-wide web – “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 behavior 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 basic 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 percent of the sugar that trees photosynthesize 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 the sunlight to photosynthesize, 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 both stretches a metaphor and gets the point across vividly.
Once, he came across a gigantic beech stump in this forest, four or five feet across. The tree was felled 400 or 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 been studying the electrical pulses, and he has identified a voltage-based signaling system that appears strikingly similar to animal nervous systems (although he does not suggest 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.” Monica Gagliano at the University of Western Australia has gathered evidence that some plants may also emit and detect sounds, and in particular, a crackling noise in the roots at a frequency of 220 hertz, inaudible to humans. Trees also communicate through the air, using pheromones and other scent signals. Wohlleben’s favorite example occurs on the hot, dusty savannas 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, neighboring acacias start pumping tannins into their leaves. In large enough quantities these compounds can sicken or even kill large herbivores.
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.”
Wohlleben used to be a coldhearted butcher of trees and forests. His training dictated it. In forestry school, he was taught that trees needed to be thinned, that helicopter-spraying of pesticides and herbicides was essential, and that heavy machinery was 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 minimize 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, the United Kingdom and South Africa. When he was ordered to clear-cut the forest near his home village of Hümmel—the fairy tale 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 slowly return to its primeval splendor. In 2006, Wohlleben resigned 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 also pay to take tours of the forest. For many years, Wohlleben led these tours himself, using lively, vivid, emotional phrasing to dramatize the largely inscrutable, ultra-slow-motion life of trees. People enjoyed it so much that Wohlleben’s wife urged him to write a book along the same lines.
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. “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.”
Simard’s research indicates that mother trees are a vital defense 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.
When told about criticism, that he describes trees as if they possess consciousness and emotions, 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 stretchy 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.