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Full EpisodeWhat Plants Talk About

When we think about plants, we don’t often associate a term like “behavior” with them, but experimental plant ecologist JC Cahill wants to change that. The University of Alberta professor maintains that plants do behave and lead anything but solitary and sedentary lives. What Plants Talk About teaches us all that plants are smarter and much more interactive than we thought!

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NARRATOR: They don't have eyes or ears, but they can find their own food.

They lack a brain, but some scientists think they can communicate, cooperate, and even wage war.

Perhaps plants lead anything but solitary, sedentary lives... MAN: They actively respond to the nutrients, and the predators, and the herbivores that are around them.

NARRATOR: But not everyone believes there's a 'social side' to plants.

If you talk to a lay person about plant behaviors, they'll just think you're crazy.

If you talk to a scientist, they'll think you're crazy and wrong.

NARRATOR: From nurturing their young to eavesdropping on their neighbors, it seems plants are doing... Whoa!


And saying quite a bit.

MAN: It's the plants' way of calling for help.

NARRATOR: We just need to listen... NARRATOR: For the past few months, plant ecologist J.C. Cahill has been criss-crossing the continent, researching a new book that focuses on one central question -- do plants behave like animals?

An idea that seems a little far out to a lot of people.

If you talk to just a lay person about plant behaviors, they'll just think you're crazy.

If you talk to a scientist about plant behavior, they'll think you're crazy and wrong.

NARRATOR: And you can understand the skepticism.

Out in the field, observing plant behavior is a little bit like watching paint dry -- unless, of course, you speed things up.

They may not swing from branches or gallop across the savannah, but plants do move, and they do behave.

And one of the ways they behave is through growth.

But does all of this growth really constitute behavior?

Are the movements of plants in any way comparable to this?

Right now, this fox is hunting for mice, using every weapon in his evolutionary arsenal to find a meal.

And this plant is doing pretty much the same thing.

When an unsuspecting insect roams into a Venus Flytrap, all it takes is a brush with two of the plant's trip hairs, and the trap is sprung.

The bug is then slowly digested, providing the plant with much needed nutrients -- unless, of course, a lucky victim manages to escape.

For years, we just assumed that the flytrap was the exception that proved the rule -- 'plants don't behave.'

Turns out, we were wrong.

What people don't know is that all plants are doing this.

All plants are not necessarily eating living organisms, but they're having elaborate behaviors above ground and below ground, but they're slower than the snapping of the flytrap, or they are happening in the soil so we can't see them.

But all plants are complex, and all plants have complex feeding behaviors.

NARRATOR: In fact, every plant on Earth is on a constant hunt for food, including the light they need to photosynthesize.

And with the help of time-lapse cameras, we can now enter their world and see how they do it -- climbing upward and tracking the sun as it wheels across the sky.

But plants don't just need light to thrive -- they also need nutrients, food that lies in a hidden world that's just below our feet.

As much as 80% of a plant's total mass lives below the ground, in a secret world scientists once called the 'black box.'

But with the aid of new technology, we're now exploring that world and discovering that, when it comes to finding food, plant roots are a lot more animal-like than we ever imagined.

Not unlike this grizzly family, who are busy foraging for berries and other edible plants.

CAHILL: So, when an animal moves through the forest and it's foraging for berries, like a grizzly would, it will find a berry patch and it'll slow down and it'll spend more time there, maybe walking, without really going in a forward distance.

The plants do something roughly similar to that.

NARRATOR: Back in his lab at the University of Alberta, Cahill has been using this high-tech camera to explore the underground world of foraging plant roots.

Some have even nutrients and some have patches of nutrients.

Yeah, so we have... NARRATOR: These grow boxes have been seeded with nutrient patches, and Cahill and student, Pamela Belter, have taken thousands of pictures, documenting how long it takes the roots to reach the nutrients, and how they behave once they find them.

...the plant itself, about two and... NARRATOR: It takes long hours to review those images, but the surprises are worth the wait.

Let's go ahead a couple of days.

BOTH: Whoa!

So, this is huge growth over three days.

This goes, what... NARRATOR: The sudden root growth confirms their suspicions.

CAHILL: Almost three centimeters.

NARRATOR: Over three days, the growth rate of one root suddenly accelerates, as it homes in on a nutrient patch.

Then, just as suddenly, growth slows down, while the root, like the grizzlies, eats its fill.

Roaming legs or multiplying growth cells -- the mechanism may differ, but the foraging behavior is still the same.

The question is, how do they do it?

How do plants find the food they're looking for, both above the ground and below it, when they have no eyes, no ears, let alone no brain?

Well, the feeding habits of this strange, snake-like vine may hold the answer.

It's called the 'dodder vine,' the Count Dracula of the plant world.

The vine has no roots and can't produce its own food, so it lives entirely off a host plant.

And it has just 72 hours to find that host, or it dies.

Its tiny teeth-like probes pierce the stem and grow into its victim, draining it of its life-giving sap.

And this botanical 'vampire' seems to prefer some plants over others.

Tomatoes are among its favorite victims.

So, how does it find its host, and how does it choose between one plant or another?

J.C. Cahill has come to Pennsylvania to meet Consuelo De Moraes and Mark Mescher, the scientists who solved that mystery.

MESCHER: So, there's a patch here of our species that grows locally.

MORAES: We brought this plant to the lab, this parasitic plant, dodder.

We're looking at how these plants interact, but how do they find a host.

And we thought for sure somebody had already done that, and we went to back to the literature, and there was nothing on that.

So, what would happen to dodder if it just was really poor in its ability to detect its host?

Well, these guys are obligate parasites, so they're completely dependent on the host plants.

So, a seedling of dodder has to find a host plant within, you know, a few days, or they'll exhaust their energy resources and die.

So, really, we expect really intense pressure on these guys to be good at foraging and identifying their host.

NARRATOR: But while the dodder vine may be good at finding a victim, could it actually choose between two different host plants?

De Moraes and Mescher made it their mission to find out.

In a series of experiments, they placed wheat and tomato seedlings in the same pot, and planted a newly sprouted dodder vine between them.

Then, they set up a time-lapse camera to see if the seedling was actually making a choice.

For hours, it circles the air like a snake, as if sniffing out its victims.

And nine times out of ten, its preferred victim is the juicy tomato, a tender plant that's easier to attach to.

MORAES: You really get the sense of a behavioral response.

So, really, there is some fairly strong selection here for this plant to make the right decision, otherwise it will die.

NARRATOR: But how was the little stem making its choice?

The team decided to play a hunch.

They knew that all plants produce green leaf volatiles, chemical scents released by their leaves as they breathe.

So, maybe this predatory plant actually was sniffing out its victim.

To test that theory, the team devised another experiment.

First, they captured the scent of a tomato, essentially condensing the chemical odor released by the plant.

[ Gas hissing ] Once it's distilled, they present the tomato perfume to the vine, along with a real tomato it can't possibly smell.

Time after time, the dodder homes in on the chemical language that says, 'Yes, I'm a tomato.'

There's no doubt with the dodder there's choice.

There's choice involving the, uh, a suitable host or non-suitable host.

This is a very familiar thing in animal foraging behavior that we're seeing in this plant foraging behavior.

NARRATOR: But the dodder isn't the only plant that's exhibiting animal-like behavior.

Once it's under full-scale dodder attack, the tomato releases the chemical equivalent of a scream.

In fact, many plants emit a chemical SOS when they're under attack, and we've all caught a whiff of it.

It's the smell of freshly cut grass.

CAHILL: We all love the smell of freshly cut grass, we all love the smell of flowers that we put into a vase, we all love the smells of plants.

But those smells mean one thing to us than they mean to the other individuals in that environment, and we are causing stress, we are causing trauma to these plants.

It's the plant's way of calling out for help.

NARRATOR: So, if it's a cry for help, who or what are plants calling out to?

Well, if this unassuming desert plant is any indication, they may be calling in some pretty effective reinforcements... insects that eat the insects that eat them.

The desert isn't the most welcoming place for people, but it can be an ecological nightmare for some plants.

Unlike us, plants can't escape the heat or walk for miles to find water.

Nor can they run and hide when they're attacked by insects.

But it's precisely because they can't move that plants have evolved some pretty nifty methods of self-defense.

We used to think or used to view plants sort of as just sitting there, whatever happens, happens, they make their seeds, and they go on.

But we're realizing it's much more complex.

They're actively engaging with the environment in which they live.

They actively communicate.

They actively respond to the nutrients, and the predators, and the herbivores that are around them.

It's a really dynamic system.

So, when you take a look at a plant, and if you were to rip off a leaf, and then think about this from the plant's point of view, what just happened was something came around and ate some of its body.

And so this plant that was just damaged by me ripping it off, is likely to start changing its defensive chemistry, it can start communicating with its neighbors or insects, and all those processes begin.

NARRATOR: And here in the Utah desert, there's a wild species that's showing us just how dynamic plants are when it comes to self-defense.

It's called the wild tobacco plant.

For more than a decade, Ian Baldwin has been studying the wild tobacco and the amazing ways it responds to threats in its environment.

BALDWIN: This plant's genome has probably an order of magnitude more genes involved in environmental perception than most animals do.

Most plants have to, because they sit still and they have to really tune their physiology and biochemistry to what's going on, and they need a very sophisticated system of perception and response.

NARRATOR: And being able to respond quickly is essential for wild tobacco, because its seeds need wildfire to kick-start their growth, and they can wait for hundreds of years for that to happen.

So, when they finally do emerge, they may face enemies they've never seen before.

BALDWIN: It has no idea what it's gonna face when it germinates out of that seed bank and has to cope with whatever's there.

There are all these other organisms that rain in on this habitat that's just been cleared out by a fire.

Just about every part of the plant is attacked in a different way, by a specialist that feeds on that particular part.

It's a very complex problem they've got to solve.

NARRATOR: And it's not just one problem.

This plant's enemies are as plentiful as desert sunlight.

But it turns out that the wild tobacco has a secret chemical weapon to deploy.

As soon as an herbivore attacks, it ramps up a toxin -- one that some of us are all too familiar with.

It's evolved a toxin that will poison any organism that has a muscle, and that is this molecule we call nicotine, the one that human beings have such a relationship with.

So, anything that moves and wants to eat this plant is going to be poisoned by this thing.

NARRATOR: But while its nicotine cocktail poisons some bugs, it has absolutely no impact on this one.

In fact, the hornworm caterpillar can mow down a tobacco plant in a matter of days.

But this cunning little plant has a few more defensive tricks up its leaves.

Once the caterpillar starts feeding, the plant's leaves release an SOS -- chemical messages that drift up into the air where they're picked up by the enemies of their enemies -- predators that just love feasting on caterpillars.

And if you find it hard to believe that plants can call in insect mercenaries, Baldwin has proof.

In one experiment, he captured the chemical signals released by the leaves of plants that were under attack.

Then, he glued caterpillar eggs onto a leaf, smeared them with the chemical scent, and waited to see if anyone would show up.

Within a matter of hours, this insect has responded to the plant's call for help.

It's called the big-eyed bug, a pint-sized predator that devours eggs and larvae alike.

In fact, it's even been known to take a bite out of a full sized caterpillar.

But wait a minute -- how does the plant even know who's attacking it, let alone which predator to call in?

Well, the answer lies in yet another chemical message, this one delivered by the caterpillar itself.

BALDWIN: When the caterpillar chews on a plant, it has to have saliva in its mouth, and in that saliva there are these various compounds that provide information to the plant, and the plant uses those compounds to say, 'Ah, it is the hawk moth and not a negro bug that's feeding on me,' and so it adjusts and tunes its responses to that particular herbivore.

NARRATOR: And Baldwin has discovered that this plant has another secret weapon, specifically designed to rid itself of caterpillars.

This is a trichome, a sweet little treat deposited by the plant and irresistible to caterpillars.

Beautiful, yes, but it's as lethal as a land mine.

When this little guy chows down on a trichome, it gets a very bad case of body odor.

BALDWIN: Twenty minutes after eating a trichome, they're smelling.

So, what we've learned from these particular smells is that they inform predators, particularly ground foraging predators.

The plant is offering this nice, little sugary first meal for the caterpillars, but it's an evil lollipop, because the caterpillar gets tagged for predation.

It's the razor blade in the apple at Halloween time.

Plants, after all, can't run away, so they have to do this.

They have to be able to solve their environmental problems by changing the organism that they are.

NARRATOR: And being able to change who they are is critical to the tobacco plants' survival, because it turns out that the mother of these voracious caterpillars is also the plant's best friend -- its main pollinator, the hawk moth.

Tonight, Baldwin is in the field trying to lure the moth in through the irresistible draw of light.

Wild tobacco flowers bloom at dusk, the perfect draw for a nocturnal pollinator like the hawk moth.

As the moth sips nectar, it gathers pollen, spreading it from one plant to the next.

But while the moth happily does the plant's sexual bidding, it has its own reproductive agenda.

A single moth can lay as many as 200 eggs -- eggs that grow into plant-munching caterpillars.

So, sometimes, despite its best defenses, the wild tobacco can still get infested with caterpillars.

Even then, the plant has another card to play.

It simply switches pollinators.

Baldwin's colleague, Danny Kessler, was the first to observe this astonishing behavior.

He was out photographing tobacco plants in the early hours before dawn.

Most were infested with caterpillars.

As he worked, he began to notice something unusual.

Instead of blooming at night, some of the flowers were opening at dawn -- and the daytime flowers didn't look or smell the way they should.

KESSLER: They're different completely from the night opening flowers in terms of nectar volume, sugar concentration.

And what we found out later, even they didn't emit floral volatile as well.

When we walked around and we saw that almost any plant had caterpillars on them, it was really a huge outbreak, and we felt, 'Hmm, what's going on here?'

It's kind of... It was weird, right.

NARRATOR: And the weirdness continued.

Not only had the bloom's nectar and perfume changed, the shape of the flower itself had completely transformed.

Essentially, by changing its flowers and bloom time, the plant had stopped talking to its nocturnal pollinator, the hawk moth, and struck up a conversation with a daytime pollinator, the hummingbird.

BALDWIN: The eggs that hummingbirds lay don't hatch into caterpillars, they hatch into little baby hummingbirds, which don't eat plants.

So, by switching its pollinator, it avoids a whole group of herbivores that it would normally get.

NARRATOR: No one knows for sure why the plant doesn't permanently switch pollinators, but the switch can happen in less than eight days.

The ability to change the shape and smell and the quality of nectar in flowers, almost immediately, is incredible.

It's incredibly complex, and we have no idea how common this is across species.

It's a very novel and new finding.

NARRATOR: And the surprises don't end there.

When one plant is attacked and starts to signal, other plants can eavesdrop on its chemical messages, and may respond by ramping up their own defenses.

And Baldwin has also discovered that when you block the plant's ability to hear itself talk, it seems to go a little crazy.

If you basically plug the plant's ears so it can't hear that volatile that it's producing, it begins to scream louder.

For one thing, they don't know when they're pollinated.

They will produce floral scents continuously.

They'll yell and yell and yell for pollinators, even though they were pollinated a long time ago.

One could interpret that as evidence of self-awareness.

If they're not able to perceive themselves, everything goes wrong.

NARRATOR: So, if plants are basing their behavior on signals they receive from their environment, is it possible that they're also interacting with each other?

In other words, could plants have a secret social life?

For a pride of lions, social life isn't all about childcare and cooperative hunting.

Adult lions can be fierce competitors, fighting over everything from mates and meals to territory.

[ Lions snarling ] So, what about plants?

Do they fight over things like food and terrain?

Well, if you speed things up, you can actually see how plants compete -- pushing and shoving as they struggle to capture sunlight.

But here in Montana, there's a new plant in town, a beautiful, nasty weed that doesn't just compete with its neighbors -- underground, it's waging territorial war.

It's a foreign invader from Eastern Europe called 'spotted knapweed,' and it's killing off the native grasses the local cattle love to eat.

Bad news for rancher, Dave Mannix.

Dave's family has been raising cattle on this land for more than a hundred years.

And like all ranchers, he's fought the weather and taken on predators to keep his herd alive.

Never in his wildest dreams did he imagine that a single plant could take him down.

Succession with our children is a big thing, and the economic viability of our industry is a big thing, and then right there with that is knapweed.

That scares us worse than predators scare us, that's for sure.

And if we lose that battle to knapweed, you know, then we've lost the base for our whole enterprise.

You know, sometimes there's a dominant species out there, that's a plant that has an unfair advantage for a time, and right now is its time.

We focus a lot of our attention and monetary resources trying to take care of our range, battling knapweed.

NARRATOR: That battle has involved everything from chemical sprays to biological weapons, like these sheep.

A hired hand moves this herd across hundreds of acres of ranch land, because sheep are happy to mow down knapweed, unlike cattle and other herd animals.

MAN: Wildlife, like elk, deer, consume it, but it doesn't seem to be preferred.

The reality is, it's taken over a lot of territory, and it's both a destructive plant and it's also an absolutely fascinating organism.

NARRATOR: Fascinating because even the insects that eat knapweed don't seem to slow it down.

Imported from the plant's home range in Europe, this weevil's offspring burrow into knapweed's tap root, where they feed until fully grown.

But they seem to have very little impact on the health of the plant.

CALLAWAY: There's two of these insects in here, blasting away at the root, and look at the plant, it's healthy as hell.

NARRATOR: But a lack of natural enemies can't fully explain the phenomenal success of this land hungry weed.

As J.C. Cahill is about to discover, spotted knapweed has invaded some 4.5 million acres of Montana rangeland.

And no one knows more about its aggressive behavior than plant ecologist, Ray Callaway.

CALLAWAY: So, J.C., this is a pretty good example of a really dense knapweed monoculture.

It's not a very big patch.

But you notice it follows the road.

Knapweed loves disturbance.

Usually when you get something this dense, disturbance is a part of it, but there's nothing native in here.

So, this is maybe a little more typical of a spotted knapweed invasion.

You know there's a lot of knapweed, maybe it's more of it than anything else, but there's still a few natives left in here.

I mean, this kind of scene covers a lot of Montana.

CAHILL: It's been neat, and sort of sad, to see this, at the same time.

This isn't just a plant that academics use to answer neat academic questions.

This is a plant that is causing people hardship on the land.

But I would say again, this is a lot more typical.

So, there's a huge drop in biodiversity that follows this invasion, and so these plants are just knocking off the natives.


NARRATOR: So, if knapweed is the plant equivalent of the Terminator, how does it wage war?

Well, there may be a lot of factors giving this invader a leg up, but the root of the problem may, quite literally, be in the roots.

To uptake essential soil nutrients, knapweed roots deploy a variety of chemicals.

And when they're released, the chemicals have a nasty added benefit.

They seem to kill off a lot of native grasses, allowing knapweed to capture and hold huge swaths of territory -- a behavior that's very animal-like.

CALLAWAY: Interestingly enough, this could be one neat behavioral sort of analogy with plants, holding a territory through the release of chemicals that harm your neighbors, and then making the resources that are there available to you.

So, J.C., this experiment... NARRATOR: And experiments like this one show just how lethal those chemicals can be.

...competitive ability of this spotted knapweed.

Callaway planted native grass alone and others in pots with knapweed.

In the latter group, half the samples were treated with activated carbon, a substance that would neutralize at least some of the toxins released by the knapweed roots.

The results are astonishing.

So, these grasses that I'm showing you here were all planted at the same time.

This one's growing by itself, no big surprise, but it's huge, it doesn't have any competitors.

It's a happy plant.

It's a nice, happy plant, nothing affecting it.

This one is growing with the knapweed, obviously, and there's no activated carbon in the soil, so this plant is, you know, what, 50 times smaller.

CALLAWAY: Knapweed is absolutely waging war with its neighbors, but it's doing it not just in the standard light and nutrient sort of way, it's also doing it with these chemical interactions in the soil, and so it's adding this whole new dimension.

This chemical warfare is novel, and it has these abilities to kill the native plants, which is very rare among plant species.

NARRATOR: But there's one native plant that can fight back.

These colorful little flowers belong to the wild lupin.

And lupins can launch a chemical counterattack of their own.

Like their knapweed neighbors, lupin roots release a chemical, called oxalic acid, to get food from the soil.

And amazingly, that chemical also acts as a defensive shield against the knapweed toxins.

But these feisty little plants don't just defend themselves -- they also seem to protect the plants around them.

CALLAWAY: My lab has shown in the field that not only does lupin seem to do well against knapweed, if we experimentally plant grasses near lupin in a knapweed patch, they do much better next to lupin.

It's a really exciting thing to consider in terms of potential ways that plants may interact, that really has not been explored.

It's a totally behavioral sort of analogy.

NARRATOR: So, what does this complex behavior teach us about the social lives of plants?

Well, when you look around at the incredible diversity of the plant world, you start to realize that knapweed's style of 'killer competition' isn't the dominant form of social interaction among plants.

If it were, we'd be looking at far fewer plant species.

CAHILL: So, what we see in the natural world isn't just struggle for resources.

It's a balance of positive and negative interactions that sometime is beneficial, sometimes is detrimental, but it all depends on the environment.

There is no 'plants compete,' and that's the end of story.

That idea is dead.

NARRATOR: So, what other forms of social interaction are at play in the plant world?

If plants aren't the lone wolf competitors we once imagined them to be, are they as sociable as the animals that eat them?

For herd animals, like these elephants, recognizing family members is an important part of social life.

In fact, for many animals, kin recognition is an essential skill.

WOMAN: Animals use kin recognition for two things -- to recognize their relatives and avoid mating with them, and to benefit their relatives in social interaction.

So, animals may give off warning calls if there's a predator near, only if their relatives are near, but not if strangers are around, and animals would generally avoid mating with relatives.

It's well known that plants have a mechanism to at least avoid mating with themselves, so it's a kind of kin recognition.

But people hadn't known if plants could benefit their relatives in social interaction, so, and that's what we've been working on.

NARRATOR: When she began her research, Dudley knew that plants could sense the presence of other plants above the ground.

Using photo or light receptors in their leaves, they can sense when they're being shaded by a neighbor and respond by growing taller or in a different direction, to compete for light.

And plant roots have a similar ability.

DUDLEY: People have shown that plant roots would respond differently to their own roots than to a neighbor's root.

So, the question is, can do anything?

Do they know who they're interacting with?

So, I said, well, this is a good place to look for kin recognition, 'cause wouldn't it be cool?

NARRATOR: So, Dudley and student, Amanda File, headed for the shores of Lake Ontario on a hunt for a very special kind of plant, to see if its roots behaved differently when growing next to its kin.

It's called sea rocket, and its reproductive behavior made it ideal for a kinship study.

Here's a good-sized plant.

NARRATOR: Sea rocket produces two different kinds of seed pods, one of which clings to its mother, resulting in seedlings that often grow up side by side in a family group.

It's in flower, and it's got a few big fruits.

This is probably the biggest one.

And 'cause they're annual plants, the mother plant dies, and can get buried by, you know, by the wind and the waves, and so you have a whole bunch of seedlings potentially coming up from that buried, dead mother plant, so they would all be siblings, sharing the same mother.

NARRATOR: And family members were exactly what Dudley was after.

In a series of experiments, she and File planted some sea rocket siblings together, and others with unrelated plants.

After a few weeks, they cleaned the roots, weighed them, then checked to see if there was a difference between those growing with siblings versus strangers.

DUDLEY: I was extremely shocked because we got exactly what we predicted, and this doesn't happen often in science.

We're predicting a response, we found exactly that -- with siblings there's lower root allocation than with strangers.

NARRATOR: In other words, strangers grew more roots to compete for food, while siblings politely restrained their root growth.

So, was that evidence of a family sharing resources?

Was it proof of altruistic behavior?

DUDLEY: Altruism is defined as doing a benefit to others at some cost to yourselves.

And it's one of the things get involved by kin selection, but it's not the only one.

Two plants that do not compete avoid paying that cost of competition, and because they're benefiting a relative, their genes are passed on to a greater extent.

NARRATOR: But whether it's a case of enlightened self-interest or a selfless sharing of resources, no one knew how these plants were identifying their kin.

Dudley knew that all plants exude chemicals from their roots, so maybe sea rocket roots were identifying one another via chemical signals.

To test that theory, Dudley and her collaborators turned to Arabidopsis, a fast growing plant that exhibited the same kin response as sea rocket.

When they placed two siblings in water and blocked all chemical signals, the seedlings suddenly started ramping up their root growth.

Without the ability to chemically communicate, siblings had become strangers.

Is it communication?

Are they deliberately sending a signal?

That we don't know.

That's something that we can't answer yet.

NARRATOR: Since then, Dudley's lab has gone on to document sibling responses in at least three other species of plants.

Now, she and File are moving on to study some of the longest lived plants on Earth, to answer an even more surprising question -- do mother trees nurture their young?

It's a question that's been asked before.

On the other side of the continent, there's a species of tree that's defying all of our expectations about the social lives of plants.

In the feature film 'Avatar,' an ancient mother tree creates a magical network that sustains every living organism in the forest.

Here, in the rain forests of British Columbia, scientist Suzanne Simard has been studying the real mother trees and the vast underground networks they create to nurture their own kind.

SIMARD: When I saw the movie 'Avatar,' I was sitting in the audience with my 3D glasses on, and there is the mother tree, and the network growing out of the mother tree, and I just went -- [ Gasps ] -- 'They read my papers!'

NARRATOR: Those papers focused on the magnificent Douglas-fir, a tree that can live up to a thousand years and grow to a staggering height of more than 300 feet.

But like all trees, only part of its total mass can be seen above the ground.

SIMARD: I feel when I'm walking through the forest that I'm gliding past an iceberg, 'cause what I am seeing is just the tip of what's going on.

What we see above ground is just such a small fraction of the body of the ecosystem.

In a lot of ecosystems, over two-thirds of what you see is actually below ground.

We're seeing one-third of the body of the forest.

NARRATOR: That mysterious underground world has preoccupied Simard for more than a decade.

But while evolutionary theory told a story about competition, the trees themselves were telling her a different story.

SIMARD: We think of them as these individuals that are just competing against each other, you know, 'I'm gonna get this, and I'm gonna get it, and you're not gonna get it,' and that's really led a lot of the thinking.

And we've long ignored, you know, a lot of the other interactions other than competition.

But at the same time, there's a community effect that we haven't understood, and we're just starting to really look at this and understand that this is a system, it's not just a bunch of individuals competing against each other.

They're working together to make this system work.

NARRATOR: So, what does that community look like, and how do trees work together?

To answer these questions, Simard focused not only on the below ground world of the Douglas-fir, but on their relationship with some of the strangest looking organisms in the forest.

SIMARD: And these trees are all connected below ground by their roots and also by fungi.

And you can often see the fruiting bodies of the fungi right next to old trees like this.

NARRATOR: Neither plant nor animal, the fruiting bodies of fungi like this bloom in the spring and fall.

Each of their gills is filled with tiny spores, the source of the next generation.

And just like the fruiting bodies of trees, mushrooms represent only a tiny fraction of a vast organism that lives below the ground, in networks, made up of roots and organic material, like this bright yellow fungal tissue.

CAHILL: If we could see in that soil, we'd realize there's layer after layer of just networks, that -- and these are big organisms -- just like these trees are really, really big, so too are the mushrooms, they're just flat, and they happen to be predominantly in the soil.

SIMARD: When you're walking on the forest floor, you're walking on this massive amount of fungus material.

And, so, anything that grows or germinates in the forest floor can't help but be colonized.

NARRATOR: In fact, the massive underground roots of fir trees are colonized by below-ground fungi, because the fungi can't produce their own food.

Instead, their vast underground networks tap into the roots of trees and other plants.

The plants provide the fungi with carbon-based sugar, and the fungal network returns the favor, providing the trees with nutrients.

Many plant species are dependent on fungi for their survival, and the Douglas-fir is no exception.

In turn, the underground fungi are equally dependent on the trees' carbon.

SIMARD: It absolutely needs that carbon.

It cannot live without the carbon from the tree.

There's a signaling that goes on between the root and the fungus, and they say, 'Hey, we can help each other out.'

And the reason the trees associate and do this for the fungi is that the fungi are so small, right, they can crawl into little soil spaces that roots can't grow.

It costs a lot less in carbon to support the fungi than to build its own root system.

And so, it's a great trading system.

It shuttles a lot of carbon to the fungi, the fungi then shuttle it between themselves, from fungus to fungus, but also fungi can also connect the trees together.

NARRATOR: Simard has demonstrated that vast underground fungal networks link the Douglas-firs together into a kind of resource sharing community.

SIMARD: We found that the big, old Douglas-fir are hubs for this massive network that is connecting pretty much all the trees in the forest.

And the bigger the tree and the older the tree, the more roots that its growing, and the more mycorrhizal connections that it has, and what we found is that those connections are providing really a rearing ground for the new trees that are coming up.

NARRATOR: And experiments like this one demonstrate how that network operates.

First, Simard and graduate student, Marcus Bingham, bag the branches of an older 'mother tree,' then expose it to radioactive carbon-14, a gas the tree naturally absorbs to produce its food.

SIMARD: Okay, so, let's just inject this C-14 or radioactive gas, CO-2 gas, and within a few hours it's gonna be coming down the trunk, in its sugars and into the root system, and then in a day or two we should see the radioactivity.

NARRATOR: Days later, the team returns to track where the radioactive carbon has gone, using a Geiger counter.

The results are nothing short of amazing.

Let's pull out the Geiger counter, see what we've got.

[ Geiger counter beeping ] Oh, yeah.

NARRATOR: Not only has the underground network shuttled the mother tree's carbon-based food to surrounding trees, experiments like this one reveal that the biggest beneficiaries are the youngest, most vulnerable trees.

It's right in the foliage.

So, that parent tree is really giving lots of carbon to its offspring here.

That seedling over here, let's see if that's picked up any.

Sure, okay. Oh, yeah, this is all hot down here.

It's great, the whole network's lit up, yeah.

That provides this great source of nourishment for those little seedlings.

In an old forest, you can imagine that there's not a lot of light, it's quite shaded, they don't have the ability to fix their own carbon very well, they're really being nurtured and grown up as a community, as a family, almost.

And it's those relationships that really build the forest.

It's really a beautiful, self-organizing, complex system.

What we're finding is amazing.

And I know that there are nay-sayers out there, but those of us who are seeing this are just going, 'Wow, it's awesome.'

NARRATOR: So, if mother trees can nurture their own kind, if plants can recognize family members and communicate with their friends and foes, how are they doing it if they have no brain -- no way to organize or integrate the information they receive?

It's an interesting question, both philosophically but also biologically -- how do you integrate information if you don't have a nervous system?

Plants may have a parallel system.

There may -- there to be something that's doing this integration, we just don't know what to look for.

NARRATOR: Maybe we're not quite as smart as we thought we were, and perhaps plants are a lot more intelligent than we ever imagined.