An old tree is a marvel to gaze upon but it’s the unseen things an ancient tree does in a forest that are truly astounding. To talk about the vital importance of “mother trees” in regenerative forest ecosystems, my guest this week is ecologist Suzanne Simard, Ph.D.
Suzanne is a professor of forest ecology at the University of British Columbia, the author of more than 200 peer-reviewed articles and the leader of The Mother Tree Project. She pioneered the study of plant communication and intelligence, and her work has revealed how trees interact using fungal networks. She investigates how complex relationships among trees contribute to forest resiliency, adaptability and recovery. Her research informs foresters and policymakers on how to heal forests suffering from overlogging and climate change.
Suzanne Simard, Ph.D., is a forest ecologist whose work uncovered the importance of “mother trees.” (Photo Courtesy of Suzanne Simard)
Suzanne’s book “Finding the Mother Tree: Discovering the Wisdom of the Forest” was named one of The Wall Street Journal’s 10 Best Books of 2021. The New York Times review of her book said: “This book is a testament to Simard’s skill as a science communicator. Her research is clearly defined, the steps of her experiments articulated, her astonishing results explained and the implications laid bare: We ignore the complexity of forests at our peril.”
How Suzanne Got Her Start in a Logging Family
Suzanne’s grandfather and great-grandfather were both horse loggers. They would use horses to move timber from where the trees were cut to a log flume, which uses running water to move the logs down a mountain. They built the flumes by hand out of birch logs. When they logged, they only took what was needed, being sensitive to the forest and not greedy.
“These flumes snaked down the mountain and then deposited the logs that were sent down them into the lake below,” Suzanne says.
The horses would help move white pine logs that were as big as a meter in diameter. “So these were really big trees, and the horses were huge,” she adds.
Her family was camping on her logger uncle’s wooden, handmade houseboat on Mabel Lake in British Columbia when one night they were awoken by a howl. Everyone ran out of the house in their pajamas, and her uncle, who was in another houseboat, came running down the shoreline with his pickax. They realize that her family’s curious beagle, Jigs, had not only gotten into the outhouse, but he had also fallen in.
Jigs was woeful and apologetic, though Suzanne’s family found the humor in the situation. They spent all morning digging Jigs out.
“I was 6, so to me it was hilarious,” she recalls. “But it was also fascinating because as my uncles and my dad and grandfather dug Jigs out, they were carving through the soil straight down — 6 feet down.”
They were in an inland rainforest, where the great quantity of rainfall supports terrain that is productive and diverse.
“The mist and the rain pull through all the acids and the leachates from the canopy and then drip, drip, drip onto the forest floor,” Suzanne says.
When that water leaches down through the forest floor, it pulls organic matter and oxygen down into the soil. “As it does that, it creates all these colors in this soil. So you go from this deep, rich, organic matter, that’s like dark chocolate brown. And underneath, a white sandy layer that has been leached of iron and aluminum and organic matter. … And then underneath that is deposited all of these incredible minerals that are oxygenated, and they turn bright, bright red, like a beating heart.”
Beneath the red layers were yellows, like in a sunset, and white speckled boulders below that. “It was fascinating that it was so colorful to me, and I thought, ‘I need to know more about these soils’ — although I was only 6,” she says.
When she later stumbled upon the field of soil science, it was manna from heaven.
Untouched Forests Are Naturally Regenerative
Untouched or well-managed forests have many species and are beautifully structured, with big trees at the canopy, smaller trees underneath and seedlings below. The oldest trees fall over naturally sometimes or get logged, but the forest, on the whole, is regenerative, Suzanne says. “The seedlings find their place, and they grow up under the canopy of these great big old giants — these ancient trees.”
In order to regenerate itself, a forest needs those old trees. But when Suzanne entered the forestry industry, the professionals were not paying attention to that. “They weren’t looking at the ecology of those ecosystems. They had dollar signs in their eyes,” she says. “And so those dollar signs meant, ‘How do we make as much money as possible for as cheaply as possible?’ And that turned into clear-cut logging.”
The forest ecosystem requires old trees, seed sources, logs, forest floor — the basic parts of an ecosystem — to regrow when disturbed, according to Suzanne, and clear-cut logging doesn’t do that very well, if at all.
She questioned why the industry was not paying attention to the natural cycles of the forest when they were doing this kind of logging.
When clear-cut loggers replant, they only include one tree species in the new cohort. When Suzanne got her first job in forestry at 19 years old, she was tasked with replanting trees where the forest had been clear cut.
Replanting trees is one of the practices included in silviculture, “the art and science of controlling the establishment, growth, composition, health, and quality of forests and woodlands to meet the diverse needs and values of landowners and society such as wildlife habitat, timber, water resources, restoration, and recreation on a sustainable basis,” according to the U.S. Forest Service.
To prepare to plant a new cohort of young trees from a nursery, all of the existing native vegetation was removed. Where there was once biodiversity, a wall of conifers was planted. They grew fast and big so they could be cut down for two-by-fours in 100 years, Suzanne says, and on the west coast of British Columbia, the industry is now in its third rotation of doing that. “In other parts of the world, even more rotations have gone by,” she adds.
When Suzanne checked on her tree seedlings, she noticed they were very yellow and not doing well. When she compared the roots of the seedlings in the field she had planted and the seedlings in undisturbed forest, she saw a dramatic difference. Wild seedlings had vast roots connected to mycorrhizal hyphae, while the human-planted seedlings had nothing more than just a few roots.
Pulling out a seedling that germinated on its own in the wild is difficult, Suzanne points out. The roots go out in all directions, and it’s like pulling on a big spider web in the soil. But in a plantation, seedlings pop out of the ground like a carrot. Suzanne explains this is because seeds started in a nursery are grown in tubes that only allow their roots to grow in one direction, and not very far. Seedlings that germinated in the forest have been growing their roots out in all directions since they sprouted, while transplanted seedlings are just starting to explore — and that takes between two and five years.
Trees are most vulnerable during the early years, and many die because they can’t get the resources they need. “It’s absolutely crucial that they’re sending these roots out in all these directions and then getting inoculated with the fungal species — the mycorrhizal fungi that are in the soil, that are attached to all the other native plants and they just kind of link into these networks and that helps the roots even more to expand their world,” Suzanne says. “They tap into this massive network that’s already supported by the other plants.”
It makes me sick to my stomach to think that these trees that are hundreds and hundreds of years old — and really do play a significant role in the ecosystem — are just getting taken down without a thought. It’s just standard operating procedure for these companies doing this, and they’re not giving any credence to what they’re taking.
Why Trees Need Fungi
In the 1970s, the importance of mycorrhizal fungi in agriculture was just starting to be recognized. Mycorrhizae, as listeners to this podcast will know, grow through the soil and bring nutrients and water to plants’ roots in exchange for sugars. These sugars produced by plants are called photosynthate.
The agriculture industry began applying fungal inoculum to introduce beneficial fungi to soil and plants. Suzanne says forestry and nurseries followed suit in adopting this practice, but that’s about as far as it went for a while. Mycorrhizae weren’t being studied in natural ecosystems, but only thought of as a little boost given to seedlings in nurseries.
Advances in microscopy and molecular tools helped soil scientists see the things that live underground and aren’t visible to the naked eye.
“The more of those tools and the more precise they are, the more fascinating this world becomes because we’re realizing there are literally thousands and thousands of species of fungi,” Suzanne says. “And it’s not just fungi: It’s bacteria as well, and archaea and other members of the soil food web that are creating these vast networks through the soil that cycle nutrients and carbon and help plants grow. So now we just know so much more, and it is just so much more complex … than I think that anybody ever imagined back in the ’70s.”
The root system beneath the visible portion of a tree is colonized by fungi that allow the tree to communicate with other trees and share resources.
Complex to Simple
In clear-cut logging and replanting, a highly complex ecosystem would be replaced with something very simple, akin to a wheat field, according to Suzanne. It occurred to her that a replanted area was not a forest, but a simple plantation.
Suzanne says it took her quite a while before she spoke out because as a woman in an industry dominated by men, she was just trying to survive in that world. When she was in her mid-20s and had the opportunity to do some research on what the removal of native plants like birches, thimbleberries and huckleberries was doing to the forest. “It turns out it has a very negative effect because these plants have roles in the ecosystem,” she says.
In the 1980s and early 1990s, herbicides and pesticides were booming. They were expanding from use in agriculture to use in forestry. A lot of time had passed since Rachel Carson sounded the alarm about pesticides with her book “Silent Spring,” but chemicals were only being used more.
At the same time, the forestry industry was actively removing “competing plants, especially birches and aspens that were thought to be worth no money and only getting in the way. The other thought was that removing them would free up water and nutrients for the conifers that were planted for lumber.
Keeping Plant Communities Together
Herbicides were being sprayed throughout the forestry industry, which Suzanne recognized was wrong. She also realized that the conifers being babied along were not doing well — getting infested by pests and infected by pathogens. She wondered what the consequences were of ripping apart communities of firs, birch and cedar, which like to grow together.
Suzanne learned of studies in Europe that found mycorrhizae could link pine seedlings together in a greenhouse or laboratory. She wanted to know what happens in the wild when these connections are severed.
Working with colleagues, Suzanne constructed an experiment in which they grew birch, fir and cedar together and labeled them with different isotopes. The fir trees were shaded to different degrees to emulate how birch was supposedly shading Douglas fir in the wild. She found out that isotopes were going back and forth between the birches and fir, “like messages through the internet.” The more she shaded a fir, the more carbon the birches sent to the fir.
“Here we’re assuming that all that birch does is rob Douglas fir of light and therefore its ability to photosynthesize. And yet at the same time here, we’re seeing birch is also giving — providing sugars through its root system to Douglas fir. And that just completely changed how we saw the forest then,” Suzanne says. “Instead of just competing with each other, they’re actually in this much more intricate, nuanced relationship where they’re, yes, competing, but also giving back and forth between each other.”
Suzanne had to tell the herbicide industry that was created to kill these plants and the companies created to cut out plants and manicure forests that these plants are actually needed. The big shots in the industry did not want to hear it, but Suzanne persisted.
Though the industry is painfully slow to change and ignoring this information, the public is understanding it more, Suzanne says. “We know so much more now about biodiversity and its role in productivity of ecosystems and health of ecosystems.”
Saving Mother Trees
Mother trees, also called dominant trees or hub trees, are the biggest and most important trees in a forest, Suzanne’s research has found.
Foresters call them “wolf trees” because they are the top creatures of the forest. “They’ve got huge crowns, huge trunks,” Suzanne says. “They just dominate the landscape.”
Telling loggers and countries that they must preserve old-growth forests and leave mother trees alone falls on deaf ears more often than not. Suzanne says loggers are continuing to move into riskier areas, and this goes on every day all over the world.
“We’re moving into areas that are deeper in the Amazon or higher up the slope or deeper up the coast because the easily accessible stuff is now pretty much gone,” she points out. “… We all need to realize that this is, this is still happening and it still needs our attention in a big way.”
She says that in an extractive economy like Canada’s, it goes against the grain to say, “We’re going to manage these ecosystems for their wellbeing, or for their ecosystem services, or for their cleaning water and air.”
Now, climate change and other global changes are helping all of us — even the economists — come to grips with the fact that overlogging is causing problems and that we are undervaluing our forests.
The oldest trees in an old-growth forest are known as “mother trees.” They nurture younger trees by sharing sugar solutions and warnings from their roots into the fungi network that connects forests.
A Very Organized System
Imagine you’re in a forest. There are bigger trees, smaller trees, and massive trees. The massive trees aren’t necessarily the oldest, but they are the largest trees. The various sizes of trees give the forest its structure.
“The biggest trees generally also have the biggest crowns,” Suzanne points out. “So they have more needles or leaves, and when you have more needles or leaves, they also photosynthesize more. That means that they’re converting more light into sugars, and those sugars go into the production of the tree themselves. So more leaves, more branches, more stem, more roots —but a good portion of that sugar also feeds the mycorrhizas. So those are the fungi that are living on the tips of the roots of these trees.”
Mycorrhizal hyphae extend from the tips of the roots through the soil, building networks to explore for more nutrients to bring back to the roots.
Unlike roots that can live for a long, long time, mycorrhizal hyphae are short-lived. They grow, they die back, they grow, they die back, and repeat. That process of growing and dying back exudes carbon into the soil that feeds other beneficial fungi as well as mites, nematodes, springtails (Collembola), spiders and worms in the food chain.
All the microbes and creatures in the food chain spit out nutrients that the hyphae bring back to the trees, Suzanne says. “It’s a very organized system that is driven by the energy from photosynthesis,” she explains. “That’s what drives all these soil nutrient cycles.”
Some fungal species can link trees of the same species together, while generalist fungal species can link trees of different species together.
“It starts to paint a picture then of this very dynamic place with this huge web of connection that is constantly changing because of this ephemeral nature of the hyphae and all these other creatures feeding off of it,” Suzanne says.
The carbon cycle, the nitrogen cycle and the water cycle all work because of these microbes and creatures in the system that are in constant communication.
An old tree will have hundreds of different species of fungi associated with it, each filling a different niche. When an old-growth forest is cut down and the variety of tree species is exclusively replaced with pine trees that are only allowed to grow for 50 years before they are cut down, the forest won’t host the same variety of fungi. The species of fungi that only associate with old trees that provide them a lot of carbon just disappear from the forest.
“You’re basically down to a handful of fungi that only grow on young trees and only associate with pine,” Suzanne says.
Instead of an ecosystem that’s like a big city like Rome that’s full of art and culture and music and conversation, it’s more like a little prison where the trees can’t communicate.
“I know that’s a very stark picture, but that’s kind of how what we’re doing is we’re just simplifying these great societies below ground, into these little simple systems, and they’re not resilient. So they can’t capture all the resources in the soil. They can’t capture all the light. So the productivity goes down. And then, because they’re so simple, if some insect or pathogen comes along, that really likes that species, or is amplified by climate change, it can wipe them all out.”
An Idea Breaks Through
In 1997, Suzanne and her colleagues published an article in the scientific journal Nature, displacing a paper on sequencing the genome of fruit flies from the cover. Her article caught a lot of scrutiny and criticism.
Suzanne said ecologists and evolutionary biologists were focused on competition as the main driver of speciation and how ecological communities are constructed.
“There was a great deal of skepticism of any other ideas that were coming in,” she says.
People had glommed onto the idea that Darwin had set the stage for, and they excluded other ideas. She was saying that cooperation can be just as important in evolution, and people didn’t want to hear it. It was an idea promoted earlier by Lynn Margulis, who emphasized the importance of symbiosis in evolution.
“She came up with the endosymbiotic theory that said actually, species evolve when they interact and engulf each other,” Suzanne says.
She acknowledges that science is not perfect and that her own study could have had more data. Still, she moved the needle.
“Now the science is really developed, and it’s much more commonly recognized and understood, and the science is galloping along that these networks are common in ecosystems, and I’m just really glad that we’re at that space now, not where we were 30 years ago,” she says.
Old trees are also being recognized for the many other reasons they are important in ecosystems: They support the biodiversity of birds, lichens, mosses — even bears that live inside of them, Suzanne says. They also store huge amounts of carbon in their own mass and in soil.
Of all the trees in a forest, the big old trees are indeed the most highly connected to other plants, Suzanne’s research has shown.
“It really makes sense because they have these big root systems and many fine roots and many points of contact with their neighbors,” she says. “And so they really are the hubs of the forest and they also support this vast mycorrhizal network.”
When a seed falls to the ground from the crown of the forest and sprouts, the seedling’s roots are quickly colonized by the mycorrhizal fungi that are the network of these old trees.
“They benefit from that enormous resource uptake capacity of that existing network they’re feeding off — the nutrients and water that is being supported by this big old network that is supported by the old trees,” Suzanne explains.
Suzanne taught foresters and others that cooperation, not just competition, influences the evolution of trees.
Tree Siblings
Since seedlings benefit from tapping into the mycorrhizal network, Suzanne wondered if parent trees can recognize which of the seedlings are from their own seed. She and her graduate students started growing “full sibling” trees in a greenhouse together, some linked by mycorrhizal networks and some unlinked.
They found that siblings alter their behaviors when grown together, versus strangers that are grown together. Siblings have better nutrient exchange, bigger mycorrhizal networks and better survival.
“This recognition of each other as related, that’s the reason that they’re having this enhanced vigor and better outcomes,” Suzanne says. “So they’re really facilitating each other.”
They also found that the mother tree, if damaged, would send out signals to its offspring to put up their defenses against a threat.
“It passes down the line,” Suzanne says. “Whatever happened to the mother gives warning to the offspring and that offspring has better survival, and then that offspring could then transmit that information onto their own offspring in the future.”
And when mother trees are dying, they shuttle about 40% of their carbon into their offspring and neighboring plants.
Hope for Forests
Suzanne says there is a lot of hope to be had when it comes to the future of forests.
“Even though we’ve been exploiting them and damaging these ecosystems, they can recover,” she says. “They’re designed to recover. These hyphae and mycorrhizae can grow back really quickly if we let them. And if we leave old trees — old creatures — behind, they just speed that process up, and they provide more diversity for the future.”
She continues: “Once we get going and start changing that value system, we’re going to stop cutting our old-growth forests down really quickly, and we’re going to restore them and we’re going to start protecting our plantations better so that they are productive and healthy and able to sequester more carbon.”
To her, it’s just a matter of education and people understanding and pushing their politicians.
Suzanne Simard’s lauded book “Finding the Mother Tree” is a fascinating tale of scientific discovery.
I hope you enjoyed my conversation with Suzanne Simard. If you haven’t listened yet, you can do so now by clicking the Play button on the green bar near the top of this post.
And as a reminder, I want to take a moment to announce that I have a new book coming out in September, and it’s available for pre-order now. The title is “The Vegetable Gardening Book: Your complete guide to growing an edible organic garden from seed to harvest,” and I’m very excited for you to read it. It’s chock full of insider tips and new-to-you information that will help you step up your gardening game and tackle challenges.
And on tap for 2023 is my new Online Gardening Academy™ premium course, Organic Vegetable Gardening. Sign up for the waitlist here.
Links & Resources
Some product links in this guide are affiliate links. See full disclosure below.
Episode 118: The Power of Mushrooms: Its Potential to Cure Environmental Threats & More
Episode 185: Restoring The American Chestnut: An All-Out Effort to Save This Iconic Tree
Episode 206: Our Most Essential Trees: The Nature of Oaks, with Doug Tallamy
Episode 245: The Hidden Life of Trees, with Peter Wohlleben
joegardener Online Gardening Academy™: Popular courses on gardening fundamentals; managing pests, diseases & weeds; seed starting and more.
joegardener Online Gardening Academy Organic Vegetable Gardening: My new premium online course membership opens in 2023. Sign up for the wait list here.
joegardener Online Gardening Academy Beginning Gardener Fundamentals: Essential principles to know to create a thriving garden.
joegardener Online Gardening Academy Master Seed Starting: Everything you need to know to start your own plants from seed — indoors and out.
joegardener Online Gardening Academy Growing Epic Tomatoes: Learn how to grow epic tomatoes with Joe Lamp’l and Craig LeHoullier.
joegardener Online Gardening Academy Master Pests, Diseases & Weeds: Learn the proactive steps to take to manage pests, diseases and weeds for a more successful garden with a lot less frustration. Just $47 for lifetime access!
joegardener Online Gardening Academy Perfect Soil Recipe Master Class: Learn how to create the perfect soil environment for thriving plants.
“Finding the Mother Tree: Discovering the Wisdom of the Forest” by Suzanne Simard
“Finding the Mother Tree: Discovering the Wisdom of the Forest” on Bookshop.org
Suzanne Simard on Instagram: @drsuzannesimard
Suzanne Simard on Twitter: @drsuzannesimard
Suzanne Simard TED Talk: How Trees Talk to Each Other
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Disclosure: Some product links in this guide are affiliate links, which means we get a commission if you purchase. However, none of the prices of these resources have been increased to compensate us, and compensation is not an influencing factor on their inclusion here. The selection of all items featured in this post and podcast were based solely on merit and in no way influenced by any affiliate or financial incentive, or contractual relationship. At the time of this writing, Joe Lamp’l has professional relationships with the following companies who may have products included in this post and podcast: Rain Bird, Corona Tools, Milorganite, Soil3, Greenhouse Megastore, PittMoss, Territorial Seed Company, and TerraThrive. These companies are either Brand Partners of joegardener.com and/or advertise on our website. However, we receive no additional compensation from the sales or promotion of their product through this guide. The inclusion of any products mentioned within this post is entirely independent and exclusive of any relationship.
