In recent years, naturalists all around the world have fallen in love with the “wood wide web” — a vast network of mycorrhizal fungi that attach to the roots of trees.
Proponents of the hypothesis believe these mycorrhizal fungi networks facilitate the exchange of nutrients and water between multiple trees — sometimes across species — in a forest ecosystem. Basically: trees talk to each other — sort of. It’s an idea that’s gained traction in pop culture due to movies and shows like Avatar and The Last of Us, as well as the books of forest ecologist Suzanne Simard.
“The idea of a mutualistic, sharing network of fungi linking trees ... has caught the public imagination and appeals to our sense of nature as a harmonious web of interconnected species,” Dan Bebber, an ecology professor and member of the British Mycological Society, tells Inverse.
But in a recent paper published in the journal Nature Ecology & Evolution, researchers shatter the most popular claims underpinning the wood wide web, saying they are “largely disconnected from evidence.” Further, the researchers believe existing scientific literature is biased toward citing the positive effects of such mycorrhizal networks.
“We are very happy that the public is so enthusiastic about forest fungi. However, some of the claims were extraordinary and that made us wonder about the evidence to support them,” study authors Justine Karst, Melanie Jones, and Jason Hoeksema told Inverse in a joint response.
Challenging the Wood Wide Web
After reviewing the existing scientific literature, the Nature study challenges three of the most popular claims surrounding the wood wide web.
Claim 1: Common mycorrhizal networks — the basis for the wood wide web — are widespread throughout forests.
These networks occur when fungi connect the roots of multiple plants underground. The researchers don’t dispute they exist, but they say the scientific evidence doesn’t support them being as widespread or permanent in forests as popular beliefs suggest.
“With current technology, it is difficult to confirm that continuous, non-transient mycelial connections exist between trees in the field,” the researchers write.
Claim 2: These mycorrhizal networks transfer resources — like nutrients — and boost the growth of tree seedlings
While there are studies touting the positive benefits of these common mycorrhizal networkers, the researchers in the Nature study say these benefits can be explained through alternate causes. That doesn’t mean common mycorrhizal networks don’t occur — just that they haven’t been validated through field research.
Put another way: the field experiments have not “conclusively demonstrated” interplant transfer occurs through the wood wide web, and, therefore, the debate on the function of these mycorrhizal networks remains “unresolved” according to the Nature paper.
“Most of the studies that indicate the presence of nutrient sharing via fungi could have alternate explanations such as flow of nutrients through soil,” Bebber, who was not affiliated with the paper, tells Inverse.
Claim 3: Mature trees communicate with their offspring through fungal networks
“The claim that mature trees preferentially send resources and defense signals to offspring through CMNs has no peer-reviewed, published evidence,” write the researchers.
In other words: science has not yet proven a core tenet of the wood wide web hypothesis.
Not a single field study in forests supports the claim that mature trees communicate with their young saplings through common mycorrhizal networks.
In fact, the only peer-reviewed study that provides any such evidence was a study of cultivated trees in a greenhouse. But the tree signals in that study were negated when researchers allowed neighboring seedlings to interact with each other, proving that the mycorrhizal networks weren’t the reason these trees signaled each other.
Offering a Scientific Mea Culpa
Finally, the researchers say the number of unsupported claims regarding mycorrhizal networks has doubled in the past 25 years in the scientific literature — and that this bias toward reporting the positive effects of the networks could obscure our scientific understanding of their role in forests.
Some of those unsupported citations came from the researchers’ own prior publications. Reporting this finding was “difficult” but necessary, the study authors say. Prior research on mycorrhizal networks has inspired forestry management and ecological practices, but those practices may not be supported by scientific evidence.
“After conducting our review and reflecting on its results, some of our previous ideas on [common mycorrhizal networks] in forests were challenged. It is important for the public to understand that this is how science is supposed to work,” the study authors tell Inverse.
The study authors say that their findings show how the public is ahead of the science when it comes to the wood wide web.
“Perhaps without intention, scientists have contributed to the mischaracterization of CMNs. So, we also need to work on science communication,” the researchers add.
The Scientific Community’s Response
Suzanne Simard — the world’s leading proponent of the wood wide web hypothesis — was blunt.
“I stand by my research,” she said in a statement made to Inverse through her publicity assistant.
Simard added: “Applying reductionist science to complex systems accelerates the exploitation and degradation of forests worldwide.”
But other ecologists were pleasantly surprised by the findings of the Nature research, which they thought presented a strong argument. Bebber calls the study “extremely important.”
“The authors of this new paper have done an excellent job of reviewing the evidence for the hypothesis of a ‘wood wide web’ — whereby adult trees share resources with their offspring via mycorrhizal links — finding that this hypothesis has no substantial scientific support,” Bebber says.
“I’m not ready to completely dismiss the idea of the common mycorrhizal network, however, like the authors, I want to see stronger evidence,” Jeremiah Henning tells Inverse. Henning is an assistant professor of biology at the University of South Alabama.
Henning says the paper can help the scientific community “re-evaluate our ideas and thoughts” on the prevalence of the common mycorrhizal network and “demand stronger evidence and conduct better experiments.”
For the general public and media, Henning discusses how the paper challenges our popular — and sometimes problematic — understanding of nature. He says media and scientists alike are enthralled with the idea of a common mycorrhizal network as a “higher being” that connects an ecosystem.
“I think we want to look at nature in an altruistic, supportive, nurturing way; however, that really isn’t how nature functions and authors are explicit in the role that we anthropomorphize nature,” Henning adds.
Where Do Fungal Networks Go From Here?
Part of the challenge underpinning the lack of scientific evidence for the wood wide web: it’s pretty hard to study underground mycorrhizal networks in forests without destroying them. The evidence may be underneath our feet, but it might as well be in outer space.
“Working in below-ground environments is really really challenging because we cannot see the organisms, “Henning says, adding “when we sample root tips or mycorrhizal communities, we have to destructively sample the soil to do that.”
While the scientific evidence behind the “wood wide web” may be in dispute, that doesn’t mean mycorrhizal fungi are unimportant, experts stress.
“There are so many interesting questions to explore about [common mycorrhizal networks] in forests,” the study authors tell Inverse.
Bebber says that scientists have found mycorrhizal relationships in plant fossils dating to 400 million years ago, making it an extremely long-lasting evolutionary connection.
“Maintenance of mycorrhizal networks, particularly in highly disturbed soils as we find in agriculture, is an important area of research,” Bebber adds.
To improve research on mycorrhizal networks, the researchers offer recommendations to help scientists design better experiments. Recommendations range from using dye tracers to test water flows between plant species via mycorrhizal networks and mapping the genotypes of trees and mycorrhizal fungi in “a wide range of forests worldwide.”
Without better evidence, the researchers conclude that “we risk turning the wood-wide web into a fantasy beneath our feet.”
