Science

‘Mother Trees’ Are Intelligent: They Learn and Remember

Few researchers have had the pop culture impact of Suzanne Simard. The University of British Columbia ecologist was the model for Patricia Westerford, a controversial tree scientist in Richard Powers’s 2019 Pulitzer Prize–winning novel The Overstory. Simard’s work also inspired James Cameron’s vision of the godlike “Tree of Souls” in his 2009 box office hit Avatar. And her research was prominently featured in German forester Peter Wohlleben’s 2016 nonfiction bestseller The Hidden Life of Trees.

What captured the public’s imagination was Simard’s findings that trees are social beings that exchange nutrients, help one another and communicate about insect pests and other environmental threats.

Previous ecologists had focused on what happens aboveground, but Simard used radioactive isotopes of carbon to trace how trees share resources and information with one another through an intricately interconnected network of mycorrhizal fungi that colonize trees’ roots. In more recent work, she has found evidence that trees recognize their own kin and favor them with the lion’s share of their bounty, especially when the saplings are most vulnerable.

Simard’s first book, Finding the Mother Tree: Discovering the Wisdom of the Forest, was released by Knopf this week. In it, she argues that forests are not collections of isolated organisms but webs of constantly evolving relationships. Humans have been unraveling these webs for years, she says, through destructive practices such as clear-cutting and fire suppression. Now they are causing climate change to advance faster than trees can adapt, leading to species die-offs and a sharp increase in infestations by pests such as the bark beetles that have devastated forests throughout western North America.

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Simard says people can take many actions to help forests—the world’s largest terrestrial carbon sink—recover and, in doing so, slow global warming. Among her most unconventional ideas is the pivotal role that the ancient giants she calls “mother trees” play in the ecosystem and our need to zealously protect them.

[An edited transcript of the interview follows.]

People may be surprised that you grew up in a logging family—not exactly a bunch of tree huggers. How did your childhood in rural British Columbia prepare you for life as a scientist?

Spending time in the forest, as I did as a child, you know that everything is entwined and overlapping, things growing right next to each other. To me, it was always this incredibly connected place, even though I wouldn’t have been able to articulate that as a child.

In British Columbia today, loggers sacrifice birches and broadleaf trees, which they see as competing for sun and nutrients with the firs they harvest. As a young government tree scientist, you discovered that the birches were actually feeding the fir seedlings, keeping them alive.

That’s right. I was sent in to find out why some of the firs in the tree plantations were not doing as well as the healthy young fir trees in the natural forest. One thing we found is that in the natural forest, the more the birch trees shaded the Douglas fir seedlings, the more carbon in the form of photosynthetic sugars the birches provided to them through the mycorrhizal network belowground.

Birches are also full of nitrogen, which in turn supports bacteria that do all the work of cycling nutrients and creating antibiotics and other chemicals in the soil that counter pathogens and help to produce a balanced ecosystem.

But aren’t the soil bacteria creating the antibiotics for themselves, not for the trees? How do we know that they help the trees?

Birch supplies carbon and nitrogen to the soil, exuded by the roots and mycorrhizae, and this provides energy for bacteria in the soil to grow. One species of bacteria that grows in the rhizosphere of birch roots is a fluorescent pseudomonad. I conducted lab studies to show that this bacteria, plated with Armillaria ostoyae, a pathogenic fungus that attacks firs and to a lesser extent birch, inhibits the growth of the fungus.

You also found that birches give sugars to fir trees in the summer through the mycorrhizal networks and that firs return the favor by sending food to birches in the spring and fall, when the birches lack leaves.

Isn’t that cool? Some scientist were having trouble with this: Why would a tree send photosynthetic sugars to another species? And to me, that was so obvious. They are all helping one another to create a healthy community that is of benefit to everyone.

Are you saying that forest communities are in some respects more egalitarian, more efficient than our own society? Any lessons here?

Right, they foster diversity. Studies show that biodiversity leads to stability—it leads to resilience, and it’s easy to see why. Species collaborate. It’s a synergistic system. One plant has a high photosynthetic capacity, and it fuels all these soil bacteria that fix nitrogen. Then there’s this other deep-rooted plant, and it goes down and brings up water, which it shares with the nitrogen-fixing plant because that nitrogen plant needs a lot of water to carry out its activities. So suddenly the whole productivity of the ecosystem goes way up.

Because the species are helping one another?

Yes, this is such an important concept that we all need to learn about and embrace. It’s one that has evaded us.

So cooperation is equally important to, if not more important than, competition. Do we need to revise our views about how nature operates?

I think we do. [Charles] Darwin also understood the importance of cooperation. He knew that plants lived together in communities, and he wrote about it. It’s just that it never got the same traction as his natural-selection-based-on-competition theory.

Nowadays we look at things like the human genome and realize that a lot of our DNA is of viral or bacterial origin. We now know that we ourselves are consortiums of species that evolved together. It’s becoming more mainstream to think that way. Likewise, forests are multispecies organizations. Aboriginal cultures knew about these linkages and interactions and how sophisticated they were. Humans haven’t always had this reductionist approach. It’s a development of Western science that led us to this.

Do you mean that Western science has focused too much on the individual organism and not enough on the functioning of the larger community?

Yes, but I also think there’s been a progression of the science. We started very simply: we looked at single organisms, then we looked at single species, then we started to look at communities of species and then at ecosystems and then at even higher levels of organization. So Western science has gone from the simple to the complex. It’s changed naturally as we’ve become more sophisticated ourselves. It’s become more holistic.

Your use of the word “intelligent” to describe trees is controversial. But it seems like you are making an even more radical assertion—that there is an “intelligence” in the ecosystem as a whole.

You used the word “controversial.” That comes from me using a human term to describe a highly evolved system that works, that actually has structures that are very similar to our brain. They are not brains, yet they have all the characteristics of intelligence: the behaviors, the responses, the perceptions, the learning, the archiving of memory. And what is being sent through those networks are [chemicals] like glutamate, which is an amino acid that also serves as a neurotransmitter in our brain. I call the system “intelligent” because it is the most analogous word that I can find in the English language to describe what I am seeing.

Some people challenge your use of words like “memory.” What evidence do we have that trees are actually “remembering” what happened to them?

The memory of past events is stored in the tree rings and in DNA of the seeds. The width and density of the tree rings, as well as the natural abundance of certain isotopes, holds the memories of growing conditions of previous years, such as whether it was a wet or dry year, or whether there were nearby trees, or if they had blown over, creating more space for the trees to grow faster. In the seeds, the DNA evolves through mutations, as well as epigenetics, reflecting genetic adaptations to changing environmental conditions.

You write in the book, “I had learned so much more by listening instead of imposing my will and demanding answers.” Can you talk about that?

Being a scientist, we get really strongly trained. It can be quite rigid. There are very rigid experimental designs. I couldn’t just go and observe things—they wouldn’t publish my work. I had to use these experimental designs—and I did. But my observations were always so important to me in asking the questions that I asked. They always came from how I grew up, how I saw the forest, what I observed.

Your latest research effort is called the Mother Tree Project. What are “mother trees”?

Mother trees are the biggest, oldest trees in the forest. They are the glue that holds the forest together. They have the genes from previous climates; they are homes to so many creatures, so much biodiversity. Through their huge photosynthetic capacity, they provide food for the whole soil web of life. They keep carbon in the soil and aboveground, and they keep the water flowing. These ancient trees help forest recover from disturbances. We can’t afford to lose them.

The Mother Tree Project is trying to apply these concepts in real forests so that we can begin to manage forests for resilience, biodiversity and health, recognizing that we’ve actually pushed them to the brink of collapse with climate change and overharvesting. We are currently working in nine forests that span a 900-kilometer range from the U.S.-Canada border to Fort St. James, which is about halfway up British Columbia.

Patricia Westerford, the character in The Overstory who was inspired by you, becomes despairing at times. Do you also sometimes get discouraged?

Of course I do. But I don’t have time to be discouraged. As I started studying these forest systems, I realized that the way they are organized, they can recover really quickly. You can push them to the point of collapse, but they have a huge buffering capacity. I mean, nature is brilliant, right?

But the difference right now is that with climate change, we’re going to need to help nature along a bit. We’re going to have to make sure the mother trees are there to help the next generation come forward. We’re going to have to move some genotypes that are preadapted to a warmer climate into more northerly or higher-elevation forests that are rapidly warming. The velocity of climate change is far faster than the velocity at which trees can migrate on their own or adapt.

Isn’t there a risk in moving seeds from one integrated ecosystem to another?

Although regeneration of locally adapted seed is the best, we have changed climate so rapidly that forests will need help to survive and reproduce. We have to assist in the migration of seeds already preadapted from warmer climates. We need to become active agents of change—productive agents instead of exploiters.

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