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Hey, it's Noam. For the past couple months, our reporter Manding Nguyen has been curious about how plants interact with the world. She did an episode about whether plants can see. Definitely check that one out if you missed it. And it was eye-opening to me because it reminded me just how little we really know about what plants are capable of. This week, she heads out into the forest in search of something even weirder. Growing up in Northern California, I got really familiar with redwood trees.
I'd go hiking all the time, and I'd love looking at the ferns, the huckleberry bushes, and the trees. But what I was looking at was just a small part of the forest. There was a hidden world underground.
Fungi, they really underlie life on Earth as we know it. This is evolutionary biologist Toby Kears, and she was telling me all about a special relationship that trees have with underground fungi, or fungi. There's lots of different ways to pronounce that word. Fungi are incredibly important in these ecosystems.
90% of all plant species on Earth form an association with these mycorrhizal fungi. Mycorrhizal fungi aren't the fungi you might be most familiar with, like mushrooms with caps. Most of these fungi are made up of delicate, tiny threads that stretch underground, kind of like a subway system.
So if you were to take a handful of soil and see some of these fine threads that are white in the soil, they could be all kinds of fungi. And the threads of these fungi penetrate or entangle with the roots of trees and plants, sometimes with multiple trees at the same time. These organisms, they've been under natural selection for 450 million years to work with plants. These trees and fungi have a symbiotic relationship with each other.
Each partner has something that the other doesn't. Plants take up carbon, they make really tasty sugars and fats, and then they feed it to mycorrhizal fungi. Now in return, the mycorrhizal fungi, they forage out into the soil and they collect resources. They collect nutrients like phosphorus and nitrogen. And these phosphorus and nitrogen comes into their networks and then they move those nutrients to the plant root where they trade.
Scientists have known about the symbiotic relationship for decades. But recently, the story has gotten weirder. Some call it the wood wide web. The idea is that trees communicate with each other. Somehow, they use fungi like telephone wires, linking up the forest in a big social network.
It's kind of a beautiful idea, like the sort of thing you'd hear in a popular TED talk. Underground there is this other world, a world of infinite biological pathways that connect trees and allow them to communicate.
and allow the forest to behave as though it's a single organism. Trees could be warning each other of predators gnawing on their leaves. Much like animals on land or in the sea, trees cooperate through complex systems of communication. Or they might be sending carbon to young saplings to help them grow. And ultimately helping each other survive.
or sharing nutrients to trees that are being shaded out or getting sick. Enabling them to help feed and protect each other. But alongside all this excitement, there's been growing pushback. There are scientists who are saying, "This might be a little too good to be true. There might be something else going on." It's a welcome debate. We need to lean in and figure out what is actually happening. The wood wide web is a captivating idea. But is it real? Are trees actually talking?
This week on Unexplainable, we take a closer look at the forest. If it's true that trees are talking, it's a big deal. It would upend a lot of assumptions scientists have had about trees, like the idea that trees and all organisms are locked in a Darwinian struggle with winners and losers. Individual trees compete for light, water, nutrients in the soil, and the ones that survive reproduce. We're talking survival of the fittest.
But recently, the Wood Wide Web has shaken up that narrative. The first moment I ever came across it was maybe almost 10 years ago. This is Gabriel Popkin. He's a journalist who's done a ton of reporting on the Wood Wide Web. So it started out in lab experiments. I believe it was a scientist in the UK. A scientist named David Reed. He
He did a study in 1984 showing that carbon could move between plants through a fungal network that connected their roots. So what scientists can do is they can sort of label molecules of carbon dioxide, and that allows you to trace your particular carbon from one place to another. It's kind of like keeping track of your solo cup at a party by writing your name on it. So that's how I think at a basic level,
These scientists were able to show that you feed carbon dioxide to one plant and sometime later it shows up in another plant. But just because you see something in the lab doesn't mean that it actually happens in the real world. So about a decade later, another scientist named Suzanne Simard picked up the torch. Dr. Simard basically tried to do this experiment that had been done in a lab in a forest. She planted a bunch of seedlings and paid attention to two species in particular.
paper birch and Douglas fir trees. She noticed that the two trees seemed to be connected by the same network of fungi, and she wanted to know if the trees could share nutrients.
So she fed each of the seedlings the special labeled forms of carbon. And she was able to trace carbon going from one tree to the other. In fact, she was able to show that it could go both ways from the Douglas fir to the birch and the birch to the Douglas fir. Suzanne's team showed that carbon didn't seem to be moving randomly between the trees either. It was going to the tree that needed more support.
If the fir tree was in the shade and getting less sunlight, carbon would flow that way. If the birch was losing its leaves in the fall, it'd flow the other way. They claimed that the carbon was moving from one tree to the other through a fungal network. This was the first experiment that showed trees could swap carbon in an actual forest, not just in a lab. And this was really a big deal at the time. It was published in Nature, which is one of the top scientific journals. And splashed on the cover of the journal, the Wood Wide Web.
It's become one of those papers that people cite over and over again. I think it's gotten around a thousand citations, which is very, very few scientific papers ever get to that level. If trees of totally different species could be sharing resources, it means that trees might be more cooperative than scientists originally thought. And I think that perspective has really resonated with a lot of people. And it's really changed, I think, the way that much of the public views forests as well.
So Gabriel, this experiment was done back in 1997. What else have we learned since then? Yeah, so Suzanne Simard herself has done a lot of follow-up. Some scientists had shown that, you know, you could sample some fungi at the roots of one tree and then find the same species of fungus on the roots of a nearby tree. And then they would say, well, hey, that suggests that these two trees are connected. But I think a lot of people felt like
Well, you could just have two different individuals of the same species growing, but they might not be connected to each other. So what Dr. Samar did is she and her graduate student did the sort of painstaking work of tracing one individual fungus from tree to tree using DNA to scientists who
are maybe more skeptical about some of the claims around the wood wide web. I think that that paper, that study, at least demonstrated that fungi do link trees in forests.
So they were able to actually trace this one individual fungus that connected all these trees in a forest. Yeah, so I was just going to say that that's a complicated and expensive experiment to do, and it hasn't been replicated many times. A lot of Dr. Samard's work has been done either with young trees in a forest, like what you might call saplings,
or in the lab or greenhouse. And obviously forests are full of old trees as well. And so there was a scientist in Switzerland who was able to kind of take advantage of an experiment that was already being done in a much older, kind of mature forest of several different types of trees.
They were already feeding trees with carbon dioxide. And what he was able to do is show that carbon dioxide would be fed to one type of tree. And he was able to show that it later showed up in other species of tree. This isn't just something that happens in the lab and among saplings, but it also can happen among big trees.
So it sounds like, you know, there have been some pretty compelling and interesting experiments that have showed like these networks exist. They link trees. There's somehow a way for chemicals to go from one tree to another.
I guess I'm curious, like what the implications are if it's proven true that trees are sharing resources through these fungal networks. Yeah, well, I think it could have a lot of implications for how we should manage forests. And there are a lot of people who think that these fungal networks are really important for young trees as they're getting started in life. Maybe their root systems aren't that well developed and they can really benefit from having the help of a sort of pre-existing fungal network
And if we know that, for example, large trees, which Dr. Simard has coined this term, mother trees, are sending carbon to younger trees, then that would mean that we probably would not want to just clear cut an entire forest because then you have no mother trees left to provide that carbon to the next generation of trees. And so there have been arguments that we should take a lighter touch in the forest.
In the last decade, the idea of the wood wide web has taken off in a way that science about this stuff rarely does. It's an idea that's shown up in novels, movies, and TV shows, like even in Ted Lasso. We used to believe that trees competed with each other for light. Suzanne Simard's fieldwork challenged that perception that we now realize. The forest is a socialist community. Trees work in harmony to share the sunlight.
But the story of the wood wide web may have gotten ahead of the science. More and more scientists felt like the public conversation around trees and forests and the wood wide web and fungi had sort of taken on a life of its own. And maybe lots of people potentially are sort of getting this somewhat of a fantasy about how forests work rather than the true picture. Up next, the pushback. Support for Unexplainable comes from Greenlight.
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The tree is talking. I am no tree. As the idea of the wood wide web became more popular, some scientists started to speak up against it. I also realized how widespread these claims were. This is an interview with Justine Karst, a scientist who recently published a critique of this idea.
She and her team wrote a paper called The Decay of the Wood Wide Web. We reviewed a lot of popular media to extract what we thought, these are the claims we see over and over and over again. Gabriel reported on this pushback for the New York Times. And he told me about some of the issues scientists have had with talking trees. So one of them is just this idea that fungal networks link trees in forests throughout the world. They pointed out that
You know, this has been shown in this one study in British Columbia, and I think they found one other study in Japan. And other than that, they felt like this really rigorous mapping of fungal networks in forests had not really been done anywhere else. So they said, you really can't make a universal claim just based on two studies. So that was one thing. Oh, wow. I think possibly even more important, they...
really argued against claims that trees can share resources with each other in a way that one tree can sort of help another tree. And almost no experiments has anybody actually traced anything going from one tree through a fungal network to another tree. So even in that original study with Suzanne Simard, they found carbon in both of the trees was
but they never actually traced the exact path of the carbon? Like it could have gotten there in a different way? Yeah, so they claim that the carbon was moving from one tree to the other through a fungal network. One of the scientists who was a co-author on that paper, Dr. Melanie Jones, she told me that if she were to write the paper again today, she would argue to take out that statement because she felt like they really hadn't
actually proven that the carbon traveled all the way from the roots of one tree to the roots of the other through a fungal network. So that just, I think to me, that really illustrates how hard it is to make claims and to really know what's happening underground because you can't see it. You're kind of always reliant on these sort of indirect forms of evidence. And there's a lot of room for different interpretations. And, you know, some people, you
may feel like, well, it's the most obvious explanation is that a fungal network was involved, but others might feel like they want more evidence or that other mechanisms could be involved. Like what other kind of things? I mean, in fact, it's known that nutrients, water can diffuse through the soil without any
sort of organism directing it. Now, that diffusion is going to be kind of slow and inefficient. And some of the sort of wood wide web believers will argue that there's no way that diffusion alone could account for the amount of carbon or nutrients they sometimes see moving from one tree to another. So I think that it's certainly not like proven that diffusion is what's happening. But if diffusion is playing a role, that really
means that this idea of trees helping each other, trees communicating with each other, it just makes it a little less plausible because it sort of makes it feel like all these compounds moving from tree to tree is less directed, less intentional, more sort of accidental. Like no one has shown that trees are actually doing this on purpose. Could the fungi be the ones doing this on purpose? I think the fungi have much more agency and power
in this relationship than they've typically been given credit for. It may just be that tree one exchanges some sugar with a fungus in exchange for some nutrients, and then the fungus may say, "Hey, I've got this sugar. Now I'm going to go take it over here and give it to this other tree because I know that if I sort of strengthen my relationship with the second tree, then this tree will provide me sugar in the future."
Okay, so the paper is basically saying like, hey, fungal networks might not be as widespread as we think. And even if they are, trees might not actually be sharing resources through them, at least not on purpose. Yeah, I think so. You know, I think the claim that trees are communicating and talking to each other is sort of the boldest
claim. And as I believe it was Carl Sagan who said, extraordinary claims require extraordinary evidence. Yeah, I guess like this narrative, it does feel like we're anthropomorphizing trees, which I have a sense that scientists are not a fan of, like this idea of like communicating and cooperating. Is that kind of a fair criticism? Yeah, I think it's really important to recognize that trees are really different from us. They live in one place. They don't move.
The brain is sort of like this centralized control and command center for humans and most animals. Trees don't have that. We really do want to be careful about anthropomorphizing trees because trees are very, very different from us in a lot of really important ways. It doesn't mean that there isn't complicated, profound, nuanced stuff going on in the forest. It just means that we can't sort of map
our own concepts onto that. We need to really try to understand trees and forests on their own terms. Yeah. I'm curious where after all this, like where this leaves you, like, do you want the wood wide web to exist? Are you just kind of... Yeah, it's kind of funny. Like I've never felt like I needed it to exist. You know, I often just like go for a hike in the forest like anybody else. And I don't really stop and look at things. But if
When I do sort of slow down and just like look at a tree for a while, like there's just so much going on. I don't feel like I need the wood wide web to exist because I feel like forests are already extremely interesting and complex and mysterious. But, you know, if it does exist, that just adds another layer.
The big question that people have been so focused on is, do they help trees talk underground? And I think it's really time that we reframe the question. When I was speaking to Toby Kears, the evolutionary biologist up top, she told me she wasn't interested in this whole wood wide web thing. She was actually interested in something more fundamental. What are all the things that these fungi do? And then all of a sudden the world just opens up.
For Toby, the idea of the wood wide web might be distracting us from the more interesting thing that's going on here, which has to do with the fungi. You know, when we talk about things like the wood wide web, it makes it sound like it's just part of the infrastructure of forests and that it's not a living organism. There's this mystery that's being excluded by a metaphor. Fungi are not passive accessories. It's not just the trees that hold a deep mystery, but all the life that teems underground.
I really think the underground represents the ultimate unknown. And it's shocking to me how little we know about something so vast and important. Science benefits from debate, right? We need people to engage and be curious to move science forward in this field. And I think the worst thing in science is if people actually feel lulled into thinking they found the answer.
These days, when I go hiking out in Northern California, I don't just look at the trees. I've actually been foraging for mushrooms recently. So I spend a lot of time with my hands in the dirt and my face really close to the ground. It feels kind of like a treasure hunt. And when I do find mushrooms, I know that there are threads of fungi that are stretching beyond what I can see and doing things that I don't know. But it also feels weirdly comforting being that close to the ground, just knowing that there's so much hidden life down there.
and that it might make everything I see up here possible. This episode was produced by me, Manning Nguyen. There was editing from Brian Resnick and Catherine Wells, mixing and sound design from Christian Ayala, fact-checking from Zoe Mullick, guidance from Meredith Hodnot, and music from Noam Hassenfeld. Bird Pinkerton followed the tiny doctor down a long, winding hallway. "'This is just one of the eight tentacle hallways at the hospital,' he said. "'This way to the head.'"
If you have any thoughts about this episode or ideas for the show, please email us. We're at unexplainable at vox.com. And we'd also love it if you left us a review or a rating. Unexplainable is part of the Vox Media Podcast Network, and we'll be back next week.