‘Talking trees’
flow via fungi between lab-grown plants.
Years later, Suzanne Simard, then an ecologist with the British Columbia Ministry of Forests, demonstrated two-way carbon transfer in a forest between young Douglas fir and paper birch trees. When Prof Simard and her colleagues shaded Douglas firs to reduce how much they photosynthesised, the trees’ absorption of radioactive carbon spiked, suggesting that underground carbon flow could boost young trees’ growth in the shady understory.
Prof Simard and colleagues published their results in 1997 in the journal Nature, which splashed it on the cover and christened the discovery the “wood-wide web”. Soon after, a group of senior researchers criticised the study, saying it had methodological flaws that confounded the results. Prof Simard responded to the critiques, and she and her colleagues designed additional studies to address them.
Over time, the criticisms faded, and the wood-wide web gained adherents. Prof Simard’s 1997 paper has garnered almost 1,000 citations and her 2016 TED Talk, “How trees talk to each other,” has been viewed more than 5 million times.
In his book The Hidden Life of Trees, which has sold more than 2 million copies, Peter Wohlleben, a German forester, cited Prof Simard when describing forests as social networks and mycorrhizal fungi as “fibre-optic internet cables” that help trees inform each other about dangers such as insects and drought.
Subterranean forest research has continued to grow, too. In 2016, Tamir Klein, a plant ecophysiologist then at the University of Basel and now at the Weizmann Institute of Science in Israel, extended Prof Simard’s research into a mature Swiss forest of spruce, pine, larch and beech trees.
His team tracked carbon isotopes from one tree to the roots of other nearby trees, including different species, in an experimental forest plot. The researchers attributed most of the carbon movement to mycorrhizal fungi but acknowledged they had not proven it.
Prof Simard, who has been at the University of British Columbia since 2002, has led further studies showing that large, old “mother” trees are hubs of forest networks and can send carbon underground to younger seedlings.
She has argued in favour of the view that trees communicate via mycorrhizal networks and against a long-held idea that competition between trees is the dominant force shaping forests. In her TED Talk, she called trees “super-cooperators”.
But as the wood-wide web’s popularity has soared both inside and outside scientific circles, a sceptical reaction has evolved. Last year, Kathryn Flinn, an ecologist at Baldwin Wallace University in Ohio, argued in Scientific American that Prof Simard and others had exaggerated the degree of cooperation among trees in forests.
Most experts, Prof Flinn wrote, believe that groups of organisms whose members sacrifice their own interests on behalf of the community rarely evolve, a result of the powerful force of natural selection among competing individuals.
Instead, she suspects, fungi most likely distribute carbon according to their own interests, not those of trees. “That, to me, seems like the simplest explanation,” she said in an interview.
Even some who once promoted the idea of shared fungal networks are rethinking the hypothesis. Prof Jones, one of Prof Simard’s coauthors in 1997, says she regrets that she and her colleagues wrote in the paper that they had evidence for fungal connections between trees. In fact, Prof Jones says, they did not examine whether fungi mediated the carbon flows.
For their recent literature review, Prof Karst, Prof Hoeksema and Prof Jones rounded up all the studies they could find that made claims about either the structure or the function of such underground fungal networks. The researchers focused on field studies in forests, not lab or greenhouse experiments.
In an August presentation based on the review at the International Mycorrhiza Society conference in Beijing, Prof Karst argued that much of the evidence used to support the wood-wide web hypothesis could have other explanations.
For example, in many papers, scientists assumed that if they found a particular fungus on multiple tree roots or that resources moved between trees the trees must be directly linked. But few studies ruled out alternate possibilities, for instance, that resources could travel part of the way through the soil.
Some experimenters, including Prof Karst and her colleagues, have installed fine meshes and have sometimes added trenches or air gaps between seedlings to disrupt hypothesised fungal networks and then tested whether those changes altered growth.
But those tactics also reduce how much soil a seedling can directly gather nutrients or water from, or they alter the mix of fungi growing inside the meshes, making it difficult to isolate the effect of a fungal network, Prof Karst said.
The researchers also found a growing number of unsupported statements in the scientific literature about fungal networks connecting and helping trees. Frequently, papers such as Dr Klein’s are cited by others as providing proof of networks in forests, Prof Karst and colleagues found, with caveats that appeared in the original work left out of the newer studies.
“Scientists,” Prof Karst concluded in her presentation, “have become vectors for unsubstantiated claims”. Several recent papers, she notes, have called for changes in how forests are managed, based on the wood-wide web concept.
Prof Karst said “it’s highly likely” that shared fungal networks do exist in forests. In a 2012 study, Prof Simard’s team found identical fungal DNA on the roots of nearby Douglas fir trees.
The researchers then sampled soil between the trees in thin slices and found the same repeating DNA segments known as “microsatellites” in each slice, confirming that the fungus bridged the gap between the roots. But that study did not examine what resources, if any, were flowing through the network, and few other scientists have mapped fungal networks with such rigour.
Even if intertree fungal networks exist, however, Prof Karst and her colleagues say common claims about those networks don’t hold up. For example, in many studies, the putative networks appeared to either hinder tree growth or to have no effect.
No one has demonstrated that fungi distribute meaningful amounts of resources among trees in ways that increase the fitness of the receiving trees, Prof Hoeksema said. Yet nearly all discussions of the wood-wide web, scientific or popular, have described it as benefiting trees.
Others, however, remain convinced that time will vindicate the wood-wide web.
While how ubiquitous shared fungal networks are and how important they are to tree growth remain open questions, Prof Averill of ETH Zurich said the title of Prof Karst’s presentation — “The decay of the wood-wide web?” — incorrectly suggests that the very concept is faulty.
Instead, he hopes scientists will build on the tantalizing clues gathered so far by looking for networks in more forests. Indeed, members of Prof Karst’s team have generated what Prof Averill considers some of the most compelling evidence for the wood-wide web.
“It’s very clear that in some forests in some places, different trees are absolutely connected by fungi,” he said.
Dr Klein of the Weizmann Institute said his team has placed its speculation about a network on firmer ground by using DNA sequences to map fungi in a 2020 follow-up study of the same Swiss forest and a 2022 lab study using forest soil. (Prof Karst and her colleagues said that in their view, even those studies did not truly map fungal networks in a forest.)
And while Dr Klein agrees that scientists still need to improve their understanding of why trees and fungi are moving all that carbon around, he is more optimistic than the Karst team that some of the bolder claims will be borne out.
“If you ask me if in the future, we will be showing that trees actually can communicate, I would not be surprised,” he said.
Prof Simard agreed that few real-world fungal networks have been mapped using DNA microsatellites because of the difficulty in doing such studies. Kevin Beiler, the graduate student who led the fieldwork for the 2012 study with Prof Simard, “spent five years of his life mapping out these networks”, Prof Simard said. “It’s very time-consuming.”
Despite those challenges, she said, studies published on other forests using other methods have convinced her that shared fungal networks are common.
“The field of mycorrhizal networks has been sort of plagued by having to keep going back and redoing these experiments,” Prof Simard said.
The forest ‘is still a very mysterious and wonderful place.’
PROF JUSTINE KARST
A MYCOLOGIST AT THE UNIVERSITY OF ALBERTA
“At some point, you have to move to the next step.”
Comprehensive field studies of the type Prof Hoeksema seeks would be a heavy lift for most university scientists working on typical grant timelines, Prof Simard said.
“None of these studies can do everything all at once, especially when you’re working with graduate students,” she said. “You have to piece it together.”
And while Prof Simard has for years called for forest managers to consider her findings, she said she was not aware of any forest being managed solely on behalf of fungal networks.
The new critique is the latest flare-up in a decades-old debate about the role of fungi in forest ecosystems, said Merlin Sheldrake, an independent mycologist whose book Entangled Life was referenced in the Ted Lasso episode that alarmed Prof Hoeksema. Scientists have long struggled to interpret intriguing but fragmentary shreds of evidence from the invisible underground realm.
Since Prof Karst gave her talk, she, Prof Hoeksema and Prof Jones have submitted a paper to a peer-reviewed journal.
And lest you worry that a less webby woods could feel a tad drab, the researchers maintain that there’s plenty of intrigue even if it turns out that trees aren’t whispering secrets to one another via subterranean fungal channels.
“The true story is very interesting without this narrative put on it,” Prof Karst said. The forest “is still a very mysterious and wonderful place,” she added.