You’ve probably heard the stories: that through an intricate network of underground fungi, trees send nutrients and warning signals back and forth to one another. In Pulitzer Prize-wining novels, New York Times feature articles, PBS documentaries, and TED talks, there have been ample mentions in recent years about the “woodwide web,” or the fungus-mediated connections that supposedly help forests thrive. But that concept may not be all it’s cracked up to be.
Every so often in science, a revision is in order. A prevailing idea inflates to inaccurate proportions. A set of experiments is taken out of context. Uncertainty gets ignored in favor of the most interesting explanation. Something snowballs from one small over-emphasis to the whole story. Through these multiple avenues of misinterpretation and more, existing research might not actually support the importance of fungal connections between trees for forest health, according to a new analysis. In other words, the “woodwide web” could be canceled.
A sweeping review study, published this month in Nature Ecology and Evolution, presents a counter-narrative to the popular ideas that have come to define our understanding of underground fungi in forests. In the review, three ecologists looked back at all of the published studies they could find on these fungal webs, called common mycorrhizal networks (‘mycorrhizal’ means fungal root).
They searched for evidence to back up three common stories, repeated across media: that common mycorrhizal networks are widespread, that they result in shared tree resources and improve seedling performance, and that trees communicate defense signals through the underground web of fungi. Yet in lieu of convincing evidence, what the biologists found was a compounding series of thinly supported claims and experiments that might not show what scientists previously thought. This forest internet system, built by fungus and establishing a direct line from tree-to-tree, could be no more than a myth.
Through their analysis of hundreds of previously published papers, the ecologists noted that there’s very little evidence to suggest that common mycorrhizal networks are widely occurring. In fact, there are only five total maps of these fungal webs in just two different forest types—not nearly enough to support the assumption that these webs of connections between trees, mediated by fungus, are present in forests around the world.
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The scientists also found that, in studies of resource sharing and tree growth, possible alternate explanations make it impossible to say if trees are passing nutrients to one another via fungi. Carbon and minerals could simply be traveling through soil, not along fungal filaments between trees. Even if mycorrhizal networks are helping trees send support to one another, there’s no evidence that it happens in large enough quantities to boost forest health. The handful of lab experiments have yielded a confusing mix of results, wherein sometimes trees seem to benefit from access to underground fungal connections and sometimes they fare worse.
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Additionally, the researchers noted that there hasn’t been a single peer-reviewed study demonstrating that trees share defense signals via common mycorrhizal networks in an actual forest setting. In one of the most referenced lab-based experiments, the presence of nearby roots seemed to cancel out the benefit of mycorrhizal connections, a set-up that would never occur in a natural forest.
Finally, the ecologists looked beyond the individual studies to the connections between them. There, they noted it’s not just in popular media that unsupported ideas spread. Scientific research of common mycorrhizal networks has selectively and repeatedly cited a small number of studies that show positive correlations between tree health and mycorrhizal connections, while largely ignoring the rest. Whether inadvertent or intentional, scientists have demonstrated similar biases as the media and the public, according to the researchers.
It seems that we all wanted to believe in the “woodwide web.” To find out why we should be more skeptical, I spoke with the three authors of the new review: University of Alberta ecologist Justine Karst, University of British Columbia biologist Melanie Jones, and University of Mississippi biologist Jason Hoeksema. Below is our conversation, lightly edited for clarity and length.
Lauren Leffer, Gizmodo: Can you explain what you actually found in your review, in the simplest terms possible?
Justine Karst: Well, I guess I would say in very plain words: The [media] story is ahead of the science. That’s, I think, the basic message. But it involves scientists as well: That, perhaps without intention, scientists have contributed to this mis-characterization of common mycorrhizal network function in forests.
Gizmodo: So you’re saying that underground mycorrhizal networks aren’t important for forest or tree health?
JK: No, no, we would not say that. So, we have to back up. We do not dispute the importance of mycorrhizal fungi in forests and ecosystems—forming soils, all of that. There is a lot of evidence to support their importance in forests and other ecosystems as well. What we focused on were these connections: Fungi physically connecting roots of two different trees or seedlings. We were really poking around to find out, ‘what do we know about those connections?’ ‘What are their ecological relevance?’ ‘How do they influence forests?’
And one of the things that came out of this is we actually don’t really know much about the role of the fungi in this story. But I wouldn’t go as far to say that common mycorrhizal networks are not important in forests. I would say that these popular claims are not supported by evidence.
Jason Hoesksema: I think the jury is still out on how important common mycorrhizal networks are in forests, per se. To reiterate, that doesn’t mean that mycorrhizal fungi are not important. We know a great deal about the importance of mycorrhizal symbiosis for trees. But these physical connections between trees—it really still remains to be seen if they’re important in forests.
Gizmodo: What prompted this review?
JK: I felt there were a lot of extraordinary claims about mycorrhizal networks and forests out there. And to the point that I didn’t necessarily remember some of the studies, or I thought maybe I wasn’t on top of the literature.
I thought maybe I was missing some papers, so I wanted to go back and check, well, what is the support for these claims? Am I missing something? And that’s when I contacted Melanie, my former PhD advisor, because I thought, ‘well, Melanie must know all these papers that are coming out.’ And so we chatted about it. And then it kind of goes from there.
Melanie Jones: I’ve been aware for a while that some of the papers, some of the early papers, weren’t always getting cited correctly. Maybe in the excitement of the early days, things were twisted slightly to put a more positive spin on the results. Then of course, that can get amplified over time and get locked into a story that there were only positive results, or that the results were strong. The questions that are raised in the papers, or the alternative explanations drop away. I was aware of that for the early papers, but I hadn’t done a deep dive into some of the more recent ones. So this was a really good excuse to do that.
JH: For me, it was a very similar process. I was hearing these extraordinary claims in the media and wondered whether I was missing something. And I think there were a couple of instances in particular where I was pushed over the top, to dive back into it. Some of my graduate students and I started reading some of these recent papers more closely. We found that the evidence didn’t match up with some of the claims.
Gizmodo: When you all started this, what were you expecting to find?
JK: There were so many surprises on the way. I was not expecting to see the citation bias. That was a huge surprise for me. I was not expecting to find that, for example, there’s no evidence or testing of the claim that big old trees are sending defense signals to kin in a forest.
JH: I expected to find a lot of variability in results, a lot of variability and outcomes. I already had a feel for that. And I knew that the outcomes of these experiments were highly variable in what I had read before. But what I did not expect was, the deeper we dug, how the results were sometimes contradictory to the current narratives. And also some of the real limitations of the experiments that came out in our group discussions, that was a surprise to me as well.
Gizmodo: Do you have any idea why those three ideas you examined in your review seem to have taken off in such a big way?
MJ: I think especially the ideas of sharing or moving material between trees, and especially among related individuals—it’s a very heartwarming story. And these days, I think people are looking for heartwarming stories. There’s enough bad negative stuff, people in-fighting, all that kind of thing out in the world. People want to hear something happy.
And, you know, I just want to say that, in this system, we are talking about a symbiotic system, where fungi and plants are growing together. And although they’re under their own natural selection and they’re functioning as individuals. Source: Gizmodo