The world ran away with a story about empathic trees that communicate with each other. It turned out to be a fairy tale

If trees talk, they lie. This is the conclusion of scientists who are researching communication and assistance by plants via fungal networks. In doing so, they are putting a theoretical axe to the roots of the often idyllically discussed 'wood wide web', or the helpful internet of nature. There has also been strong criticism of the often cited evidence.

How it all started: In 1997, Canadian forest ecologist Suzanne Simard and five co-authors described in Nature how trees exchanged sugars, apparently via fungi. Simard went on to argue that trees could also send distress signals and targeted aids to distant neighbours in need. Her message and she herself traveled the world, met a mostly willing press and inspired many researchers to publish their own work. The forest-wide web was born. Plants were said to be connected by their roots to a vast underground fungal network through which they send messages and aids. A bestseller by German forester Peter Wohlleben, The Hidden Life of Trees (2015), really got the discourse going. Trees were not inert objects but intelligent persons with thoughts and desires, and they converse via fungi that connect their roots “like fibre-optic internet cables”.

Trees communicating and helping each other over a considerable distance, in the original image without regard to species or race, brought a welcome message. The world ran away with it, from children's books to environmental manifestos. The image of tree networks was unprecedentedly sunny, also in cultural sections of newspapers. In a need for comforting stories, perhaps due to secularization, a hype grew. Because of the large-scale tree altruism and the 'intelligence' of trees, biology would have to be rewritten and even the nature of man redefined. After all, it was part of a more beautiful whole than ever thought.

But the criticism is now hitting hard. Less and less of the original image remains.

Porcini mushrooms

The basis remains, and has been known for a long time. Fungal networks are connected to trees, shrubs and smaller plants. Mycorrhizal fungi, such as those of edible mushrooms – truffles, chanterelles and porcini mushrooms – spread through the forest floor with their filaments. They connect to plants via their roots. They have a symbiotic relationship in this way; everyone benefits. The plants receive hard-to-reach nutrients from the soil and the fungi receive carbon compounds – thanks to the photosynthesis of the plants. At least temporarily, the roots of neighbouring trees are also connected to each other in this way. Such a common network is called CMN, common mycorrhizal network . But is it also a kind of neural network, albeit without neurons?

Take a plant that is suddenly attacked. Previously, it seemed that being attacked by a herbivore or pathogen caused other plants to scale up their defense mechanisms. Quite a few researchers assumed that this was based on active signaling by the victim, via their network. That evidence is now controversial and much discussed. That is why the step of the new research is useful. Here too, we will skip the evidence for an underground communication system for the time being. What happens if it were there? A team of researchers from the University of Oxford and the Vrije Universiteit Amsterdam used computer and calculation models to investigate all kinds of hypothetical circumstances. And they did not exactly find selfless honesty. Oxford biologist Thomas Scott when asked: "If you are vulnerable at the moment, you do not gain much if others can increase their defenses in time thanks to you. Moreover, those others are already competing with you for sunlight and nutrients. It is extremely difficult to find situations in which plants are evolutionarily selected to warn their fellow species of an impending attack."

Scott makes it even worse, for sunny-minded nature observers. “In many scenarios, plants actually prefer to signal dishonestly. To lie. For example, they can indicate that there is feeding when there is no herbivore present. Then they mislead local competitors so that they invest in expensive defense mechanisms. Indeed – harming your neighbors instead of being altruistic. Honest and active signaling would only be a stable strategy under very restrictive conditions. At most towards your pollinator or seed disperser, if you are not in direct competition with them.”

But is there perhaps an exception? What about direct relatives, such as the brother of a willow, the sister of a seedling? After all, they share genes. And wasn't kin selection and altruism towards conspecifics considered to be family an important phenomenon in animals? Scott points to a special characteristic of trees. "Kinship competition is the opposite. A certain altruism works when your offspring or brothers and sisters spread out to compete with others elsewhere. Plants are quite immobile, and so through selection will tend to compete with neighbouring relatives."

Looking for alternatives to 'good stories' is very important

Unintentionally honest, plants may be. Scott: “Suppose that neighboring plants do indeed scale up their defense mechanisms when one is attacked. You can also explain that in other ways. Perhaps plants cannot suppress the release of an unintended clue during an attack that serves as a signal to others. The second possibility is that of the helping fungus. It depends on the plants in its network for carbohydrates, so keeping them in good condition can count. Perhaps it detects when one has been attacked and warns the others to prepare themselves. That is also a purely theoretical possibility.”

In all cases, trees listen well at most, without transmitting complex information. The intelligence and empathy that trees were suddenly attributed with, is then not really necessary.

From Canada, researcher Justine Karst reports that she appreciates the approach of Scott and others. "The strength is that they consider theoretical alternatives to the idea that signaling would have evolved. Looking for alternatives to 'good stories' is very important," she responds when asked. As a forest ecologist, Karst is one of the top specialists. "But I am cautious about two assumptions that they follow for the discussion. Such as: that the flow of resources between plants and fungi is a balanced 'trade'. This biological market view is quite popular. But we should be open to other possibilities."

Karst herself has ample experience in debunking a story that may be too good to be true. With two colleagues, she recently gave the web story a new twist, with a critical look at scientific history. “All three of us have studied forest fungi our entire careers, and even we were surprised by the extraordinary claims that surfaced in the media. Had we missed something? That is why we thoroughly re-examined frequently cited field studies, including some of our own – and also how they were cited.”

That brought quite confronting news. There was not only the popular-scientific and cultural triumph of the forest-wide web. Science itself was also quite carried away. There were enormous gaps, contradictions and incorrect representations in studies, but especially: in the citing of them. Karst, summarizing: “To say yourself that seedlings generally grow better or survive better when they are connected to each other via CMNs is a generalization that is simply not supported. Other commonly reported claims – that trees use CMNs to signal danger, to recognize offspring or to share nutrients with other trees – are based on equally thin or misinterpreted evidence.”

The excitement about a new idea led to an over-eagerness to confirm it, whether consciously or unconsciously wishful thinking. “This is known as confirmation bias, and it is easy to fall into that trap. We saw this in our own work. Selective citing of studies, eliminating alternative explanations, but also re-citing them from one presentation to the next, drove the discourse further and further away from reality. An example? A 2009 study that used genetic techniques to map the distribution of mycorrhizal fungi is now often cited as evidence that trees transfer nutrients to each other via CMNs – even though that study did not actually investigate nutrient transfer.”

Where Nature previously gave Suzanne Simard’s work a big shout-out, it now published the findings of Karst and others. The general reaction among peers? Karst: “One of relief, I think. The spell has been broken, so to speak. This is good news for the field, because it opens up a lot of fresh ideas and alternative ways of thinking about CMNs.”

The excitement about the idea was so great that people wanted to see it confirmed

The criticism was certainly not only aimed at Simard, but he did take it to heart. She recently came up with a not very convincing rebuttal. The visions and imagery that she presented in popular and autobiographical work, she had not used exactly the same in her scientific work. And the public simply needed tasty and easily digestible wording. But of course you can ask yourself whether, if that public is so taken with it, you should not put on the brakes for the sake of balance. Simard has now expanded her idyll with 'The Mother Tree' - which must be sought in every area. A bit older, wise and generously protective, that tree takes a lot in her environment under her holistic care. Incidentally, she can be hermaphroditic, that's true.

Karst: “According to Simard, we are now ‘reductionists’, and therefore cannot understand how forests function because they are too complex. And we simply have something against anthropomorphism. She and her fellow publicists see reflecting on our previous research and ideas as contradictory – as a weakness. Of course, it is difficult to examine our own research as well. But if we cannot change our minds in the light of new evidence or challenges to old ideas, then we are not scientists, we are ideologues.”

It seems premature to claim that trees recognize relatives, communicate, or send resources. The communication architecture of humans has failed to measure that of trees. “It has been fascinating to watch the spread of the forest-wide web in various media. What is worrying is how rarely this idea has been challenged across the board. So yes, this is ultimately about more than just science itself. We show how easily confirmation bias, unverified assertions, and credulous reporting can distort research results beyond recognition over time. That should serve as a cautionary tale for scientists and journalists alike.”