Hug That Tree – It May Be Crying: New Evidence Shakes Up Science

Some foresters are calling it the ‘wood-wide web...’

Tue 04 Apr 23


“I talk to the trees, but they don’t listen to me” begins the song by American lyricists Lerner and Loewe.

But maybe they do. Or at least they talk to each other.

Controversial German forester Peter Wohlleben says “yes, they do.”

His ideas have shaken up the scientific world. He says he has a rare understanding of the inner life of trees, able to describe it in accessible, evocative language. 

Now, at the age of 59, he has become an unlikely publishing sensation. His book The Hidden Life of Trees; What they Feel has sold more than 800,000 copies in Germany, and has hit the best-seller lists in 11 other countries, including the US and Canada.

Peter Wohlleben has a rare understanding of the inner life of trees.
Peter Wohlleben has a rare understanding of the inner life of trees.

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.

Since evolutionist Charles Darwin, we have generally thought of trees as striving, 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 that refutes that idea, says the Smithsonian magazine. 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 millimetres below our feet.

“Some are calling it the ‘wood-wide web,’” says Wohlleben, standing in a German forest.

“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; they 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.”

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 signalling 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 we would all love to know.

Meantime, science has now unearthed studies that when plants and trees are stressed they ‘cry’.

The research shows water-stressed or injured plants and trees emit high-pitched sounds that could have implications for silviculture and horticultural monitoring.

Plants do not suffer in silence, it seems. Instead, when thirsty or stressed, plants make “airborne sounds”, according to a study published in the Cell1 journal.

Plants that need water or have recently had their stems cut produce up to roughly 35 sounds an hour, the authors found. But well-hydrated and uncut plants are much quieter, making only about one sound an hour.

The reason you have probably never heard a thirsty plant make noise is that the sounds are ultrasonic – about 20–100 kilohertz. That means they are so high-pitched that very few humans can hear them. Some animals, however, probably can.

Bats, mice and moths could potentially live in a world filled with the sounds of plants, and previous work by the same team has found that plants respond to sounds made by animals, too.

Studies by Prof. Lilach Hadany (right) and Prof. Yossi Yovel of Tel Aviv Aviv University show for the first time that airborne sounds from stressed plants can be recorded at a distance and classified. (Photo credit: Tel Aviv Aviv University)
Studies by Prof. Lilach Hadany (right) and Prof. Yossi Yovel of Tel Aviv Aviv University show for the first time that airborne sounds from stressed plants can be recorded at a distance and classified. (Photo credit: Tel Aviv Aviv University)

To eavesdrop on plants, Lilach Hadany at Tel-Aviv University in Israel and her colleagues placed tobacco (Nicotiana tabacum) and tomato (Solanum lycopersicum) plants in small boxes kitted out with microphones. The microphones pick up any noises made by the plants, even if the researchers can’t hear them.

The noises were particularly obvious for plants that were stressed by a lack of water or by recent cutting. If the sounds are pitched down and sped up, “it is a bit like popcorn – very short clicks”, Hadany says. “It is not singing.”

These plant sounds have been processed to make them audible to the human ear.

Plants do not have vocal cords or lungs. Hadany says the current theory for how plants make noises centres on their xylem, the tubes that transport water and nutrients from their roots to their stems and leaves. Water in the xylem is held together by surface tension, just like water sucked through a drinking straw. When an air bubble forms or breaks in the xylem, it might make a little popping noise, and bubble formation is more likely during drought stress.

But the exact mechanism requires further study, says Hadany in an interview with Nature Magazine.

The team produced a machine-learning model to deduce whether a plant had been cut or was water stressed from the sounds it made, with about 70% accuracy. This result suggests a possible role for the audio monitoring of plants in forestry, farming and horticulture.

To test the practicality of this approach, the team tried recording plants in a greenhouse. With the aid of a computer program trained to filter out background noise from wind and air-conditioning units, the plants could still be heard.

Pilot studies by the authors suggest that tomato and tobacco plants are not outliers. Wheat (Triticum aestivum), maize (Zea mays) and wine grapes (Vitis vinifera) also make noises when they are thirsty.

Previously, Hadany’s team has also studied whether plants can ‘hear’ sounds, and found that beach evening primroses (Oenothera drummondii) release sweeter nectar when exposed to the sound of a flying bee.

So are plant noises an important feature of ecosystems, influencing the behaviour of plants and animals alike? The evidence isn’t yet clear, according to Graham Pyke, a retired biologist at Macquarie University in Sydney, who specialiaes in environmental science.

He’s sceptical that animals listen to the moans of stressed plants.

“It is unlikely that these animals are really able to hear the sound at such distances,” he says. He thinks the sounds would be too faint.

Further research should shed more light on the matter. But Pyke says he’s perfectly willing to accept that plants ‘squeal’ when stressed.


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