Trees Talk To Each Other. 'Mother Tree' Ecologist Hears Lessons For People, Too

May 4, 2021
Originally published on May 4, 2021 9:17 pm

Trees are "social creatures" that communicate with each other in cooperative ways that hold lessons for humans, too, ecologist Suzanne Simard says.

Simard grew up in Canadian forests as a descendant of loggers before becoming a forestry ecologist. She's now a professor of forest ecology at the University of British Columbia.

Trees are linked to neighboring trees by an underground network of fungi that resembles the neural networks in the brain, she explains. In one study, Simard watched as a Douglas fir that had been injured by insects appeared to send chemical warning signals to a ponderosa pine growing nearby. The pine tree then produced defense enzymes to protect against the insect.

"This was a breakthrough," Simard says. The trees were sharing "information that actually is important to the health of the whole forest."

In addition to warning each other of danger, Simard says that trees have been known to share nutrients at critical times to keep each other healthy. She says the trees in a forest are often linked to each other via an older tree she calls a "mother" or "hub" tree.

"In connecting with all the trees of different ages, [the mother trees] can actually facilitate the growth of these understory seedlings," she says. "The seedlings will link into the network of the old trees and benefit from that huge uptake resource capacity. And the old trees would also pass a little bit of carbon and nutrients and water to the little seedlings, at crucial times in their lives, that actually help them survive."

The study of trees took on a new resonance for Simard when she was diagnosed with breast cancer. During the course of her treatment, she learned that one of the chemotherapy medicines she relied on was actually derived from a substance some trees make for their own mutual defense. She explains her research on cooperation and symbiosis in the forest, and shares her personal story in the new memoir Finding the Mother Tree: Discovering the Wisdom of the Forest.


Interview Highlights

Finding the Mother Tree: Discovering the Wisdom of the Forest by Suzanne Simard
Penguin Random House

On working for a logging company in British Columbia in her 20s

It was the late 1970s when I started; they were clear-cutting and just starting to plant trees. And so, of course, this was completely different than what I saw my grandfather do and my dad and uncles. They just took out the odd tree here and there. But this was wholesale taking out all the trees, the big ones and the little ones. And that was my first job in the forest industry, which to me was quite shocking. But it was also extremely exciting because it was so dangerous. And I was also one of the first girls to be in the industry.

On being a young forester and realizing that fungus was key to the health of a forest

In the forest floor ... there's all kinds of bugs, but there's also lots of fungi. And the fungi are so colorful. There's yellow ones and purple ones and white ones and ... they grow right through the forest floor to the point where it kind of looks like gauze, almost. And so I was finding this yellow fungus. And yet, when I pulled up the seedlings that were not doing so well — they were yellow and dying — I realized that their roots were kind of black and straight. ... And so I wondered, what were they missing? Were they missing this fungus? Was this fungus ... a pathogen or was it a helper fungus?

And eventually I learned that these were a special kind of helper fungus called a mycorrhizal fungus — which just means that the fungus is the type that grows through the soil and picks up nutrients and water and brings it back to the seedling. ... So eventually, I was able to put together that these little seedlings that were not doing so well were missing their mycorrhizal fungi.

On the critical relationship between trees and fungi

Keep in mind that all trees and all plants — except for a very small handful of plant families — have obligate relationships with these fungi. That means that they need them in order to survive and grow and produce cones and have fitness — in other words, to carry their genes to the next generations. And the fungi are dependent on the plant or the trees ... because they don't have leaves themselves [for photosynthesis]. And so they enter into this symbiosis in that they live together in the root, and they exchange these essential resources: carbohydrates from the plant for nutrients from the fungus, in this two-way exchange which is very tight, almost like a market exchange. If you give me five bucks, I'll give you five bucks back. It's very, very tightly regulated between those two partners in the symbiosis. But, yes, all trees and all plants in all of our forests around the world are dependent on this relationship.

On how trees can help each other by sharing nutrients

[At the time] birches were considered weeds. There was a huge program to spray and herbicide these trees to get rid of them because the foresters viewed the birches as competing with Douglas fir, competing for light especially. I was observing in these plantations, though, that when they weeded out the birches, when they sprayed them or cut them, that there was a disease in the forests that would just start spreading like a fire. It was called Armillaria root disease. I really thought, we're doing something wrong here. And so I wanted to know whether the birches were somehow protecting the firs against this disease and that when we cut them out it actually made it way worse.

I had learned about these mycorrhizal fungi and how they could actually protect trees against diseases. And I'd also heard about David Reed's work in the U.K., where he had shown that in the laboratory that trees could be linked together by mycorrhizal fungi and pass carbon between them. So I tested this between birch and fir in my sick plantations.

I planted birch and fir and cedar together in little triplets. ... And I traced how those carbon molecules went back and forth between the birch and fir and they didn't actually end up in the cedars. Because the cedars, they form a different kind of mycorrhizal fungus that doesn't associate with either birch or fir. So [the cedar] wasn't actually in the network with birch and fir, and it picked up hardly any of this isotope.

I knew that birch and fir were sharing carbon below ground — much against the prevailing wisdom that they only compete for light and also that the more that birch shaded Douglas fir, the more carbon was sent over to Douglas fir. So there was a net transfer from birch to fir that was sort of mitigating its shading effect.

In this way the ecosystem was maintaining its balance — the birch and fir could coexist because of this collaborative behavior that was sort of offsetting some of the competition that was going on.

On the ways her own breast cancer diagnosis shaped her research

It definitely had a big influence on me, and my life has changed as a result, but it changed my research, too. That was when I started working with kin recognition, seeing whether or not these old trees, especially when they were dying, could recognize and help their kin. And I had graduates come on to actually ask those questions. You know, if a tree is dying, do they send more [nutrients and other signals] to their kin? And we found that they do.

Then I also started some research — one of the main chemotherapy medicines that was administered to me was paclitaxel [also called Taxol]. Paclitaxel is a defense agent — actually a defense chemical — that is produced by the Pacific yew tree, or all yews around the world, actually. It was essential to my recovery — this compound that trees produce to defend themselves against diseases.

And so I thought, you know what, I want to find out more about this. I started a study with a new graduate student, Eva, and she is looking at the neighborhood of yews — whether they're associated with old cedars and maples, and how their neighbors might influence their ability to produce high-quality Taxol to increase their defense.

We just found out that these trees are all connected together by this muscular mycorrhizal network, which provides the avenues for them to communicate this information. So, yeah, we're embarking on that work. I'm hopeful that it will help us to, for one thing, conserve these trees for their medicinal qualities — because they are ingenious in what they've done. They've evolved these, what we call medicines, but they're for themselves to defend themselves against illness as well. The cancer treatment is what drove me to do this study. And I'm so excited to find out what we learn.

On why it's important to let an old tree go through the long dying process on its own

[Trees] get old. They do eventually decline. And dying is a process, and it takes a long, long time. It can take decades for a tree to die. In the process of dying, there's a lot of things that go on. And one of the things that I studied was where does their energy — where does the carbon that is stored in their tissues — where does it go? And so we label some trees with carbon dioxide — with C13, which is a stable isotope — and we watched as we actually cause these trees to die. We stress them out by pulling their needles off and attacking them with budworms and so on. And then we watched what happened to their carbon.

And we found that about 40% of the carbon was transmitted through networks into their neighboring trees. The rest of the carbon would have just dispersed through natural decomposition processes ... but some of it is directed right into the neighbors. And in this way, these old trees are actually having a very direct effect on the regenerative capacity of the new forest going forward.

This is a completely different way of understanding how old trees contribute to the next generations — that they have agency in the next generations. And our practices of salvage logging to get rid of dying trees, or trees that have just died or have been burned in wildfires — if we go in and cut them right away, we're actually short-circuiting that natural process.

Our studies suggest it would have knock-on effects to the regeneration coming up. They're not going to be as well prepared for their lives coming forward. So I've been trying to tell people: Let's hold back on this salvage logging until trees have had the chance to pass on this energy and information to the new seedlings coming up.

Sam Briger and Thea Chaloner produced and edited this interview for broadcast. Bridget Bentz, Molly Seavy-Nesper and Deborah Franklin adapted it for the web.

Copyright 2021 Fresh Air. To see more, visit Fresh Air.

DAVE DAVIES, HOST:

This is FRESH AIR. I'm Dave Davies, in today for Terry Gross. Do you remember this scene from "The Wizard Of Oz" when Dorothy and the Scarecrow happen upon an apple orchard and she picks an apple from a tree?

(SOUNDBITE OF FILM, "THE WIZARD OF OZ")

JUDY GARLAND: (As Dorothy) Ouch.

CANDY CANDIDO: (As Angry Apple Tree) What do you think you're doing?

GARLAND: (As Dorothy) We've been walking a long ways. And I was hungry. And - did you say something?

CANDIDO: (As Angry Apple Tree) She was hungry.

ABE DINOVITCH: (As Apple Tree) She was hungry.

CANDIDO: (As Angry Apple Tree) Well, how would you like to have someone come along and pick something off of you?

GARLAND: (As Dorothy) Oh, dear. I keep forgetting I'm not in Kansas.

DAVIES: Our guest today, Suzanne Simard, has spent decades studying trees. And while they don't talk to humans, she's shown, through some groundbreaking research, that they do communicate with each other in some pretty astonishing ways - sharing nutrients, warning of danger and helping their own offspring get off to a good start in life. Simard grew up in the forests of Canada and has worked in the logging industry, the Canadian Ministry of Natural Resources and Forestry and in academia, where she's published over 200 studies about the complex relationships that exist among trees and plants in forests.

Her ideas were dismissed, even mocked, by forestry officials and some scientists at first, but not anymore. Simard is now a professor of forest ecology at the University of British Columbia. She has a new memoir which explains some of her research and remarkable findings and shares her personal story, which includes her treatment for breast cancer. The book is "Finding The Mother Tree: Discovering The Wisdom Of The Forest." She joins me from her home in Nelson, British Columbia. Suzanne Simard, welcome to FRESH AIR.

SUZANNE SIMARD: Thank you. It's great to be here.

DAVIES: Your interest in this subject was spawned by, well, a life of deep connections to the forest that goes back generations. Tell us just a bit about your family and its involvement in the forests.

SIMARD: Well, yeah. Actually, both sides of my family, my mother's and father's side, have had deep relationships with the forest. But I'll talk mostly about my dad's side. And so the Simard family actually emigrated from France to Quebec and moved across Canada in the early 1900s to settle in the inland rainforests of British Columbia. And a funny story about this is that, you know, they actually thought they were going to California but ended up in these rainforests. And they decided to stay because they were so beautiful. And they were horse loggers. And so they settled around a lake called Mabel Lake and, you know, spent their livelihoods over multiple generations logging with horses. And that's what I grew up around in those forests and watching this kind of old-fashioned way of harvesting forests.

DAVIES: Right. So they cut down trees. Horses pulled them to, I guess, some - to a river, right? And then they would float downstream (laughter) to the sawmills?

SIMARD: They actually hauled them to a flume, which then shot them down into Mabel Lake. And my grandfather and my great-grandfather built that flume. And they also built a water wheel, which provided electricity to the houseboats that the loggers stayed on, which we used to also live in when we went to Mabel Lake. And - yeah. And so then those logs would be boomed together and then sent down what was called, well, the Shuswap River, which had chucks in them, the Skookumchuck Narrows. And it was very, very dangerous, exciting work.

DAVIES: So you grew up with a connection to the forest. I have to ask you this, you ate dirt as a kid?

SIMARD: Oh, yeah. I loved eating dirt (laughter).

DAVIES: Like chewing up and swallowing (laughter)?

SIMARD: And swallowing and eating the worms and the bugs and - yeah. And my mom used to have to deworm me all the time because I was always, you know, kind of full of it (laughter).

DAVIES: How does a mom deworm the kid who's gotten worms from eating dirt?

SIMARD: Well, she had her special medicine that I'd had to drink about once every two months to get rid of the worms.

DAVIES: So as a young woman, you get a job with a logging company - right? - the people that clear-cut areas. Tell us what that is and what your job was, what you were doing for this logging company.

SIMARD: Yeah. So when I was about 20 years old, I was an undergraduate student in the UBC Faculty of Forestry. And we all got summer jobs in those times. And that's still normal for students. But I got a job with a logging company in the Lillooet Mountains, which is just on the east side of the Coast Mountains in British Columbia. And it was for, you know, one of those early logging companies that was the beginning of sort of industrial logging. It was the beginning of clear-cutting. So it was the late 1970s, early 1980s. And back then, you know - until then, logging had been sort of, a little bit regulated, but not a whole lot. And there wasn't a lot of reforestation going on.

But when I started, yeah, they were clear-cutting and just starting to plant trees. And so of course this was completely different than what I saw my grandfather do and my dad and uncles. You know, they just took out the odd tree here and there. But this was, like, wholesale, taking out all the trees, the big ones and the little ones. And that was my first job in the forest industry, which, to me, was quite shocking. But it was also extremely exciting because it was so dangerous (laughter). And I was also one of the first girls to be in the industry.

DAVIES: And your first job was to look at what - they called it a plantation when they had replanted trees in an area that had been clear-cut. And in the first chapter, you talk about going on to check on - I forget what kinds of trees they were. Spruce...

SIMARD: Spruce trees.

DAVIES: Right - to see how they're doing, right? I mean, they're just coming up. And, you know, as you pursue this work in your career, a lot of these experiments involve really sophisticated equipment and carefully planned things. But what struck me about this was how much you learned from close observation of the earth itself. I mean, this is kind of amazing. So maybe you can just explain what you saw when you went out to look at these new spruce trees and how they were doing.

SIMARD: Yeah. Well, those new spruce trees were so different than what I had grown up looking at, right? I grew up in wild primary rainforests. They were old-growth forests. And these spruce seedlings were being planted into were, you know, forests that had been cut down or clear-cut, as we talked about, and then planted little seedlings that had been grown in nurseries. So you know, unlike when my grandfather was logging, when the seeds just came in naturally and regenerated naturally, these were artificially planted in rows and all of the same species. So it was a monoculture of spruce. And so the forest looked completely different.

It was - instead of this complex, diverse cathedral, they were more like corn plantations, except that, you know, back then, in the early '80s and late '70s, the other plants were also there as well. At least when I started working for the forest industry, they would plant the trees and hope they lived and - you know, and then let the brush grow up alongside it. So it was kind of messy. But it was just one species of tree that we were putting back, which, to me, was a big concern because, you know, these forests were multi-species forests. They were diverse.

DAVIES: And you had pulled up some other - I think it was - was it a pine sapling or something. And you looked carefully at the roots beneath. And you saw a yellow color, which you didn't find in these newly planted spruce trees that weren't doing so well. This turns out to be really significant, right?

SIMARD: Yeah. Yes. Exactly. So in the forest floor, you know, I mentioned there's all kinds of bugs. But there's also lots of fungi. And the fungi are so colorful. Like, there's yellow ones and purple ones and white ones. And they infiltrate or they grow right through the forest floor to the point where it kind of looks like gauze, almost. You know, like, it can be so thick, especially in the high-elevation forest I was working in.

And so I was finding this yellow fungus. And yet when I pulled up my - the seedlings that were not doing so well, they were, you know, yellow and dying. And I realized that their roots were - you know, they were kind of black and straight and hadn't grown out of their plug, the plugs. We call those plugs that you grow - that you plant into the soil.

And so I wondered, you know, what were they missing? Were they missing this fungus, or was this fungus - you know, was it a pathogen, or was it a helper fungus? And eventually, I learned that these were a special kind of helper fungus called a mycorrhizal fungus, which just means that the fungus is the type that grows through the soil and picks up nutrients and water and brings it back to the seedling and exchanges it for photosynthate. So eventually, yeah, I was able to put together that these little seedlings that were not doing so well were missing their mycorrhizal fungi.

DAVIES: Right. What's kind of critical about this is that these fungi, the little - tiny, little fibers that come out at the end of them that connect with plant roots actually connect with them and exchange nutrients with the roots of trees and bushes, right? This is pretty amazing.

SIMARD: It is. I mean, you know, keep in mind that all trees and all plants, except for a very small handful of plant families, have obligate relationships with these fungi. That means that they need them in order to survive and grow and produce cones and have fitness - in other words, to carry their genes to the next generations. And the fungi are dependent on the plant or the trees for photosynthate because they don't have leaves themselves. And so they enter into this symbiosis in that they live together in the root, and they exchange these essential resources - carbohydrates from the plant for nutrients from the fungus - in this two-way exchange, which is, you know, very tight, almost like a market exchange. You know, if you give me some - five bucks, I'll give you five bucks back. You know, it's very, very tightly regulated between those two partners in the symbiosis. But, yes, all trees and all plants in all of our forests around the world are dependent on this relationship.

DAVIES: And, of course, the critical thing that you end up pursuing is that the exchange isn't just between a particular fungi - am I saying this right? - and a tree, but the fungi can be connected to more than one tree, and the nutrients can move from one tree to another, right?

SIMARD: Yeah, exactly. So, you know, keeping in mind, like, there are about 55,000 species of fungi in the world, and a group of those are the mycorrhizas, of which there are thousands of those, too. And a single tree can actually associate with hundreds of different species of mycorrhizal fungi. So a Douglas fir, for example, in a forest can have, you know, 10 or 20 or 100 species associated with it.

Some of those species of fungi are what we call generalist fungi, and they can link with many other species of plants and trees. And so they form this network that joins the trees even of different species together. And then there are some fungi that are specific to tree species, and they can form their own inter - or intratree species networks that are, you know, exclusive, say, to Douglas fir and don't join in, for example, birches and alders and so on.

DAVIES: So there you are, 20 years old, kind of putting this together. And the logging company wanted to know, hey; we've planted all these spruce trees we want to grow up big and strong. You had to report back. How much of this did you tell them? How much of this theory about what might be wrong did you share with them?

SIMARD: Well, I didn't say a word (laughter) because I was so - you know, my job was to go and evaluate plantations and report back, you know, how they were doing and what should we do. And I was just formulating my ideas as a brand-new young female forester. And I really didn't have much of a voice. But I started gathering my observations like - you know, observation as a forester or a scientist is, like, absolutely essential. That's where you really formulate your best ideas, along with, you know, reading as well.

But I didn't really know much. I was so young. And so I was kind of afraid to say anything because, you know, you get laughed at. And I didn't really - you know, I didn't have a lot of understanding. And so I just started doing my own research on my own and actually started, you know, looking at these roots in my basement with - I bought a little microscope. And eventually, I sort of taught myself about mycorrhizal fungi. And I realized, you know, this was part of the solution. But I didn't - I still couldn't put it together. I didn't have enough. So I had to actually go back to school and learn more about them.

DAVIES: That's just so funny to think of you, this 20-year-old rookie forester, coming on what would eventually be a pretty revolutionary idea, and you're just putting it together. Wow.

SIMARD: Yeah. Well, you know, and the foresters and forestry at that time was really focused on, you know, managing how trees compete with each other. And so what I was looking at was a mutualism between a fungus and a tree. And mutualisms weren't really part of the parlance of foresters back then. They were very much about, you know, trying to manage so that trees weren't outcompeted by neighbors or weren't competing with each other. And so we were very much focused on how fast they grew or how far apart they were or what their neighbors were and managing that part of it. But the collaborative part was just kind of - sissy I think is what people would've thought of it. So I just kept my mouth shut, actually.

DAVIES: We need to take a break here. Let me reintroduce you. We are speaking with Suzanne Simard. She's a professor of forest ecology at the University of British Columbia. Her new book is "Finding The Mother Tree: Discovering The Wisdom Of The Forest." We'll continue our conversation in just a moment. This is FRESH AIR.

(SOUNDBITE OF ALLISON MILLER'S "SHIMMER")

DAVIES: This is FRESH AIR, and we're speaking with Suzanne Simard. She's a professor of forest ecology at the University of British Columbia. Her new book about how trees cooperate in the forest is called "Finding The Mother Tree: Discovering The Wisdom Of The Forest."

So you eventually leave the logging company and get more education, and you end up at the forestry ministry. I guess it's technically the Ministry of Natural Resources and Forestry. And you have the chance there to do some experiments to test some of these ideas. You know, one of your most important experiments involved putting some birch trees and pine trees together in various circumstances to see if they might be transferring nutrients from one to another. What's significant - these are two different species of trees. And the idea was these fungi, these mycorrhizal fungi, might be actually facilitating a transfer of nutrients between two different species of trees. How did you do this?

SIMARD: Well, you know, I picked paper birch, and it was actually Douglas fir, not pine. But I picked paper birch and Douglas fir to study because these were the trees that grew up in the forest I grew up in. And they were early successional species that - so the forest companies were planting a lot of Douglas fir to replace the old Douglas firs that they cut down because they got, you know, a lot of money from. The paper birch they didn't care about because it wasn't a marketable species at that time. And so they viewed it as a weed.

And keeping in mind they viewed a lot of different trees as weeds back then - not just the birches, but also pines were considered weeds back then. Now they're not. But the birches were considered weeds, and they were - there was a huge program to spray and herbicide these trees to get rid of them because they, the foresters, viewed the birches as competing with Douglas fir, competing for light, especially.

And I just - you know, I was observing in these plantations, though, that when they weeded out the birches when they sprayed them or cut them that there was a disease in the forests that would just, like, start spreading like a fire. It was called Armillaria root disease. I really thought, we're doing something wrong here that - you know, and so I wanted to know whether or not - whether the birches were somehow protecting the firs against this disease and that when we cut them out, that it actually made it way worse.

And so I had learned about, you know, these mycorrhizal fungi and how they could actually protect trees against diseases. And I'd also heard about David Read's work in the U.K., where he had shown that, you know, in the laboratory that trees could be linked together by mycorrhizal fungi and pass carbon between them.

And so I tested this between birch and fir, you know, in my sick plantations. And I - so I planted birch and fir and cedar, actually, together in little triplets. And I labeled the birch and fir with two separate isotopes. One was carbon-14 and one was carbon-13. And I traced how those carbon molecules went back-and-forth between the birch and fir. And they didn't actually end up in the cedars because the cedars - they form a different kind of mycorrhizal fungus that doesn't associate with either birch or fir. So it wasn't actually in the network with birch and fir, and it picked up hardly any of this isotope.

So I knew that birch and fir were sharing carbon below ground, much, you know, against the prevailing wisdom that they only compete for light, and also that the more that birch shaded Douglas fir, the more carbon it actually sent over to Douglas fir. So there was a net transfer from birch to fir that was sort of mitigating its shading effect. And so in this way, the ecosystem was maintaining its balance in that the birch and fir could co-exist because of this collaborative behavior that was sort of offsetting some of the competition that was going on.

DAVIES: So instead of getting rid of the birch, who were seen as competition for the fir trees, when they stayed, the firs were healthier.

SIMARD: Yes. Yes, they were. They were able to - actually, the disease was reduced, and the firs grew better and they survived better.

DAVIES: Right. And this gets just a little technical, but you managed to use these two different isotopes - one you put in the fir trees with a little canopy, and then the other's in the birch trees - and you were able to eventually show that molecules that - I may get this wrong, but were photosynthesized in the fir tree had eventually made their way down through the fungi, up to the birch trees and vice versa. And so this took a while. You use some pretty sophisticated stuff. You finally look at this in the lab. And this is quite a moment, isn't it?

SIMARD: It was quite a moment. You know, I was - had all my data that I brought back from Canada. I was in Oregon State doing my Ph.D. with Dave Perry. I actually had this little office that was a bug-rearing room. It was covered with tiles. And so I would always sit in that room with my door closed with the light on with my little computer. I had a little laptop. And I'd just tap, tap, tap away with my data.

And then, you know, one day I'm just, like, getting to the bottom of the day, like, I'm doing my final analysis, and I did this analysis of variance on my data, and the patterns popped out, you know, that carbon was moving back-and-forth. That was exciting in itself. But the fact that as fir became more and more shaded, it was getting more and more carbon, I just about jumped out of my chair. And I ran over to Dave Perry's office, which was just around the corner. I'm going, Dave, Dave, you know, the more that birch shades Douglas fir, the more carbon it gets. And we're both, you know, so excited. And it was quite a thrilling moment.

DAVIES: So the birch is actually, like, tending to the fir tree. Hey, buddy. You need some help. You're shaded. I'm going to make sure you get some more carbon, done through these fungi underneath - pretty amazing.

SIMARD: Yeah.

DAVIES: We're going to take another break here. Let me reintroduce you once again. We are speaking with Suzanne Simard. She's a professor of forest ecology at the University of British Columbia. Her new book is "Finding The Mother Tree: Discovering The Wisdom Of The Forest." She'll be back to talk more after this short break. This is FRESH AIR.

(SOUNDBITE OF CHICK COREA AND GARY BURTON SONG, "WHAT GAME SHALL WE PLAY TODAY")

DAVIES: This is FRESH AIR. I'm Dave Davies, in today for Terry Gross. We're speaking with Suzanne Simard, professor of forest ecology at the University of British Columbia. Her groundbreaking research has shown that trees in forests communicate and cooperate with each other in some remarkable ways. Simard's new book is "Finding The Mother Tree: Discovering The Wisdom Of The Forest."

So let's talk about what else you've learned and we've learned about ways that trees, essentially, communicate with each other. I mean, you did the experiment when you found that you had these birch and fir trees and that if the fir tree was getting too much shade, the birch tree would actually send it, you know, nutrients through the fungi underneath the ground to help the fir tree. What other kinds of communication have you observed?

SIMARD: Yeah. So as I mentioned, you know, there was other work going on at the time. And that's where we really got to understand these other things that we're communicating between the trees. And so there was some work in the U.K. and there was some work in China right around the same time. And I'll explain the work in China because that researcher, her name is Yuan Yuan Song, actually came over and worked with me as a postdoc in our forests. But she was working with tomato plants. And they form a network with what are called arbuscular mycorrhizal fungi.

And she would - what she did in her experiment is she would injure one of the plants - tomato plants that were in a network with other tomato plants. And she would injure it with a pathogen. Or she also did this with insects. And that donor plant or that injured plant would then send signals through the mycorrhizal network to neighboring tomato plants, which would then upregulate their genetic code for making defense enzymes and produce more defense enzymes. And then she would challenge those tomato plants with those insects and pathogens again. And they were more resistant.

And so what she discovered was that these tomato plants were communicating about their health status and what was the herbivores that were actually attacking them and conveying that information to their neighbors so that the neighbors could upregulate their own defense and survive. And so Yuan Yuan came over. And I contacted her. And I said, can we try this in our forests? Maybe this is happening in forest, too. And so she came over. And we did experiments with Douglas fir and Ponderosa pine. And we found, basically, the same thing was happening. So Douglas fir and Ponderosa pine are linked together in an active mycorrhizal network. And we would injure the Douglas fir with one of the insects that's, you know, a big herbivore in our forests right now, western spruce budworm.

And when the Douglas fir was injured, it was - send signals - the same thing - signals to the Ponderosa pine, so a different species. And again, it would upregulate its RNA, produce these defense enzymes, of which there were many, and increase the resistance of the pine against the injury. And so we discovered that these very same processes were happening in these - in the forest as were happening in the agricultural fields that Yuan Yuan was studying. So this was a breakthrough, that it's more than just resources moving through the networks. It's actually information that will - actually, is important to the health of the whole forest.

DAVIES: So one plant, when it senses a danger, can actually warn the other plants? Get ready. Trouble is coming.

SIMARD: Yes. That's right. I'll add in here that we discovered that this was happening through belowground mycorrhizal networks. But other researchers in the U.S. actually had been studying this in aboveground. So the trees that are injured could actually produce volatile organic compounds that are emitted into the atmosphere. And their neighbors pick up these VOCs. And then they can upregulate their own defense artillery. So it's actually multiple ways that these plants are actually communicating their health status and what's attacking them.

DAVIES: Wow. So there is this connective tissue between so many of these plants in the forest through these fungi underneath the ground. And you've noted that when you map this stuff, this network of connections kind of resembles the neural networks in the brain. I mean, how much integrated communication are we talking about (laughter)?

SIMARD: Well, I'll describe how we discovered that. And then, hopefully, it'll reveal, you know, what this means. So I got a graduate student in the late 2000s. His name is Kevin Beiler. And, you know, I asked him to map what the network looked like in the forest - and keeping in mind that, you know, we were still mired in this controversy of whether networks even existed. And what did they look like? And we were fortunate that it was at that time when there had been some previous work done where, you know, there were certain molecular tools called microsatellites that had been developed by the geneticists, including some fungal geneticists at Oregon State, which allowed us to identify individual fungi in the soil, as well as individual trees.

And by being able to identify and map these individual fungi, we could tell which fungi were linking which trees together. And so Kevin made this map of what that network looked like in a Douglas fir forest. And it was an uneven-aged forest. These are the kinds of forests that would grow on the east side of the Cascade mountains. My neck of the woods in Canada, we call them the interior dry-belt forests. So they have old trees, and young trees regenerate under their canopy. And so what was revealed in the map was that the old trees were connected to almost every other tree in the forest. So they were the hubs of the network. And they were linked to all the little, smaller trees and saplings and intermediates. And the reason that they were the most highly connected is because they had massive root systems with lots of growing points and lots of fungi on them.

And so these old trees were the hubs of the network. And when you start doing, you know, analytical work on networks like that using graph theory, you know, there are patterns that emerge. And what emerged out of the pattern is that this was - what we call a complex network, with, you know, a few large nodes, which are the old trees, and lots of small, connected nodes. So those hubs, we started to do some experiments around them and realized that these old trees were actually facilitating the growth of the seedlings that were growing up underneath them. And so that network pattern of the old - big old trees connected to smaller nodes was the same - it was a neural network. It's what's called a biological neural network. And those patterns are - it's a very similar pattern to our own biological neural networks in our brains. And, in fact, these kinds of complex networks are repeated in many, many systems, right? They are very efficient systems. They're good at transmitting information. In our brains, it would be thought patterns. And they're very resilient.

DAVIES: So as you've looked at this, you've discovered that the older trees are wired into, connected to many, many other trees and plants in the forest. You call these hub trees or mother trees, right? What's their role in the forest?

SIMARD: Yeah. So these hub trees are connected to most of the other trees. And because they have these big root systems, they're able to make these connections. And in connecting with all the trees of different ages, they can actually facilitate the growth of these understory seedlings. And so they do this by - you know, the seedlings will link into the network of the old trees and benefit from that huge uptake resource capacity. And the old trees would also pass a little bit of carbon and nutrients and water to the little seedlings at crucial times in their lives that actually help them survive.

DAVIES: Wow. They're like the big mom or dad of the forest.

SIMARD: Yeah. I mean, they are. They nurture the new generations of seedlings. And they do lots of other things in ecosystems, too, that many other people have studied. So they also store huge amounts of carbon, and they're also, you know, scaffolding for a lot of biodiversity. So birds love to live in these trees, squirrels. And, of course, the mycorrhizal network is hugely diverse, so they're a big source of biodiversity as well.

DAVIES: So one of the policy recommendations that flows from this, I suppose, is if you're going to harvest most of the trees in the forest, leave these old hub trees, mother trees.

SIMARD: Well, for the resilience of the forest, yes, that's right. To leave the old trees is like leaving the legacy - the genetic legacy that - of these trees that have actually lived for a long, long time through previous climatic regimes and extremes. And so their DNA is actually, you know, evolved to deal with, you know, changes in the future. But they also are legacies in that biodiversity that they store and that they're able to help other - the recovery of forests after they've been disturbed.

DAVIES: Let me reintroduce you. We're going to take another break here. We are speaking with Suzanne Simard. She's a professor of forest ecology at the University of British Columbia. Her new book is "Finding The Mother Tree: Discovering The Wisdom Of The Forest." She'll be back to talk more in just a moment. This is FRESH AIR.

(SOUNDBITE OF JAKE SHIMABUKURO'S "143 (KELLY'S SONG)")

DAVIES: This is FRESH AIR, and we're speaking with Suzanne Simard. She's a professor of forest ecology at the University of British Columbia. Her new book about trees and plants cooperating with one another in the forest is "Finding The Mother Tree: Discovering The Wisdom Of The Forest."

When you got into the forestry service, I mean, you know, the idea was, plant stuff. Clear everything out of the way because everything else is competition. I mean, all of this research suggests that, no, there's a lot of value in having diversity in the forest. To what extent have these ideas affected policy in handling all these magnificent forests in Canada?

SIMARD: Well, I would say not yet, but I think that we're coming to a change. But I'll explain why I say not yet. We're still clear-cutting our forests. That's still - in Canada. That's still the dominant what we call sylviculture system. It's how we've cleared, you know, the log, and it's the most profitable. So that means taking all the mother trees, all the young trees, everything all at once and then turning them into two-by-fours. And we've actually, you know, set the rate of cuts so high that, you know, we're actually - in British Columbia, we've only got, of the really productive forests, only 8% of those old forests left. So they're disappearing really, really rapidly.

And what I'm trying to get the government to do is save these old forests because, you know, they're so essential to carbon storage and biodiversity. But also, when they do log, you know, logging is going to continue to focus more on, you know, secondary forests that have been logged before and, when you do that, to save the big old trees so that they can provide seed. And the seed, which we've - you know, in our experiments show will produce a beautiful, diverse plantation from these old mother trees.

And, you know, it's not just a matter of saving one old tree. Those old trees, when you - if you clear-cut around and leave one tree, for example, which is the tendency, or to just leave a few seed trees - they're called seed trees - those trees are really vulnerable, left all alone because trees are social creatures. And they depend on each other for protection and all these things I've been talking about.

And so I've been trying to get them to leave old trees in patches so that the neighbors - they can continue to communicate with their neighbors and also that the neighbors can help protect them. And then so the trees will provide seed for natural regeneration and for conserving biodiversity and carbon as well.

DAVIES: You also looked at what a very old mature tree does when it's approaching the end of its life, that it - does it behave differently in some way in conjunction with its neighbors and the others that it is connected to?

SIMARD: Yes. So, you know, trees - they have a lifespan. They get old. They do eventually decline. And dying is a process, and it takes a long, long time. It can take decades for a tree to die. In the process of dying, there's a lot of things that go on. And one of the things that I studied was where does their energy - where does the carbon that is stored in their tissues - where does it go? And so we labeled some trees with carbon dioxide with C13, which is a stable isotope. And we watched as we actually caused these trees to die. We stressed them out by pulling their needles off and attacking them with blood worms and so on. And then we watched where - what happened to their carbon. And we found that about 40% of the carbon was transmitted through networks into their neighboring trees.

And so the carbon - the rest of the carbon would have just dispersed through natural decomposition processes, which happens in the soil. And when litter hits the ground, there will be all this food web of soil organisms that chew on that organic matter. And the CO2 just evolves back into the atmosphere. But some of it is directed right into the neighbors. And in this way, these old trees are actually, you know, having a very direct effect on the regenerative capacity of the new forest going forward.

So, yeah, this is a completely different way of understanding how old trees contribute to the next generations - you know, that they actually are - they have agency in the next generations and that, you know, our practices of salvage logging to get rid of dying trees or trees that have just died or been burned in wildfires - you know, if we go in and cut them right away, we're actually short-circuiting that natural process.

And it could have knock-on effects, or, you know, our studies suggest it would have knock-on effects to the regeneration coming up. They're not going to be as well-prepared for their lives coming forward. And so I've been trying to tell people, let - hold back on this salvage logging until trees have had the chance to pass on this energy and information to the new seedlings coming up.

DAVIES: Wow. So a dying tree senses it and begins to, in effect, give up some of its nutrients to other plants in the network.

SIMARD: Yeah. Well, we specifically looked at carbon or its energy, but, yes, that's correct.

DAVIES: You know, when you were doing the research on how trees that are approaching the end of their lives actually distribute some of their own carbon to others that they are connected with in this sort of act of - I don't know - mutual preservation and selflessness and - you write in the book that you were doing this when you had been diagnosed with breast cancer. And you'd had a mastectomy, and then they discovered that the cancer had spread to some lymph nodes. And it was pretty scary, and you got a heavy chemo treatment. And I'm just wondering - I don't know - if you felt a connection with these trees and that kind of research when you were also sort of dealing with maybe your own mortality.

SIMARD: Yeah, it definitely had a big influence on me. And my life has changed as a result, but it changed my research, too. So that was when I started working with kin recognition, seeing whether or not these old trees, especially when they were dying, could recognize and help their kin. And I had graduates come on to actually ask those questions. You know, if a tree is dying, do they send more to their kin? And we found that they do.

And then I also started some research because, you know, one of the main chemicals or the chemotherapy medicines that was administered to me was paclitaxel. And paclitaxel is a defense agent, actually, or a defense chemical that is produced by the yew tree, Pacific yew or all yews around the world, actually. You know, it was essential to my recovery - was this compound that, actually, trees produce to defend themselves against diseases.

And so I thought, you know what? I want to find out more about this. And so I started a study with a new graduate student, Eva. And she's looking at how the neighborhood of yews, whether they're associated with old cedars and maples - and how their neighbors might influence their ability to produce high-quality Taxol to increase their defense. And yeah, and we found out - you know, we just found out that these trees are all connected together by this arbuscular mycorrhizal network, which provides the avenues for them to communicate this information.

And so, yeah, we're embarking on that work. And I'm hopeful that it will help us to - you know, for one thing, to conserve these trees for their medicinal qualities because they are - you know, they're ingenious in what they've done. They've evolved these - what we call medicines. But they're for themselves to defend themselves against illness as well. But, yeah, that cancer treatment is what drove me to do this study. And I'm so excited to find out what we learn.

DAVIES: Well, Suzanne Simard, thank you so much for speaking with us.

SIMARD: Thank you so much. Those are really, really wonderful questions.

DAVIES: Suzanne Simard is a professor of forest ecology at the University of British Columbia. Her new book is "Finding The Mother Tree: Discovering The Wisdom Of The Forest." Coming up, Lloyd Schwartz reviews a collection of music from Bernard Herrmann, who wrote some of Hollywood's best-known film scores, including the music for Alfred Hitchcock's "Psycho." This is FRESH AIR.

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