The “wood-wide web” concept, which has taken root in popular media, challenged by researchers
The idea that forest trees can “talk” to each other through a connective underground web of delicate fungal filaments tickles the imagination.
Dubbed the “wood-wide web,” the idea that underground fungi allow trees to share resources with their seedlings – and even protect them – definitely puts the “fun” in fungus.
But the science behind those ideas is unproven, cautions University of Alberta expert Justine Karst.
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In a perspective published in Nature Ecology & Evolution, Karst and two colleagues contest three popular claims about the capabilities of underground fungi known as common mycorrhizal networks, or CMNs, that connect roots of multiple plants underground. Fungi are living organisms such as moulds, yeast and mushrooms.
“It’s great that CMN research has sparked interest in forest fungi, but it’s important for the public to understand that many popular ideas are ahead of the science,” says Karst, associate professor in the U of A’s Faculty of Agricultural, Life & Environmental Sciences.
While CMNs have been scientifically proven to exist, there is no strong evidence that they offer benefits to trees and their seedlings, the researchers suggest.
To evaluate the popular claims, Karst and co-authors Melanie Jones of the University of British Columbia Okanagan and Jason Hoeksema of the University of Mississippi reviewed evidence from existing field studies.
They found that one of the claims, that CMNs are widespread in forests, isn’t supported by enough scientific evidence. Not enough is known about CMN structure and its function in the field, “with too few forests mapped,” and only two studies showing common fungal links among trees. Whether those links persist long enough to benefit the trees isn’t known, Karst and her co-authors note.
The second claim, that adult trees transfer resources such as nutrients to seedlings through CMNs and that they boost survival and growth, was also found to be questionable.
A review of 26 studies, including one in which Karst is a co-author, established that while resources can be transferred underground by trees, CMNs don’t necessarily bring about that flow, and seedlings typically don’t benefit from CMN access. Overall, their review revealed roughly equal evidence that connecting to a CMN would improve or hamper seedlings, with neutral effects most commonly reported.
As well, the differing common research methods used to explore CMN function each had their limitations, leaving room for the possibility that resources could be transferred in other ways, such as through soil, and that other factors may affect seedling growth and survival.
The third claim that adult trees preferentially send resources or “warning signals” of insect damage to young trees through CMNs is not backed up by a single peer-reviewed, published field study, Karst and her co-authors note.
The single published study they found that explored tree signalling through CMNs in response to insect attack was an experiment on potted seedlings in a greenhouse. The experiment showed that the role CMNs played in signalling was cancelled when roots of neighbouring seedlings could interact, and “CMNs in forests would not be present in the absence of roots,” Karst notes.
Another published study testing kin effects between tree seedlings showed that carbon was transferred through a soil solution, not a CMN. And two other studies related to resource transfer or signalling from mature trees also proved inconclusive for various reasons.
Coupled with the issue of sparse evidence is how the results of the existing scientific studies tend to be cited, or referenced, in the scientific literature, Karst and her co-authors note.
The research trio evaluated statements from more than 1,600 published papers on how results were cited from 18 influential field studies on CMNs. They then rated the citations as either “supported” if there was strong evidence in the original study or “unsupported” if evidence was weak or non-existent.
They found that the rate of unsupported citations – some of which came from their own published studies – has almost doubled in the last 25 years.
They noted that the unsupported statements tended to “overstate results and disregard confounding effects” of the research in a way that promotes the positive effects of CMNs in forests.
Though unintentional, that overblown information then shapes – and can distort – the public narrative about CMNs, says Karst. That could, in turn, affect how forests are managed.
“Distorting science on CMNs in forests is a problem because sound science is critical for making decisions on how forests are managed. It’s premature to base forest practices and policies on CMNs per se without further evidence. And failing to identify misinformation can erode public trust in science.”
Karst and her colleagues recommend some approaches to researchers for designing future CMN field studies that can address alternative explanations and ensure accurate interpretations of results.
There is a great deal that scientists actually do know about the value of mycorrhizal fungi in forests, Karst adds.
The fungi draw nutrients and water from the soil and pass those to the trees, and also protect the roots from pathogens. And by using and storing carbon from the trees, the fungi also benefit the forest.
“Mycorrhizal fungi are essential for the growth and survival of trees and have an important role in forest management and conservation practices,” says Karst, “even if trees are not talking to each other through CMNs.”
| By Bev Betkowski
Bev Betkowski is a reporter with the University of Alberta’s Folio online magazine. The University of Alberta is a Troy Media Editorial Content Provider Partner.
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