(eerie music) - [Narrator] Hidden inside the Panama Canal, one of the busiest waterways on Earth, lies a special island closed to the outside world.

It is a living laboratory where scientists have been continuously studying one tropical forest ecosystem for more than a hundred years.

This island is full of giants, giant trees, like this one.

For decades, it slowly pulled carbon out of the atmosphere and turned it into wood.

But now it's fallen, and that carbon is slowly returning back to the atmosphere.

And it's not just this tree.

Giants like this one are dying all over the tropics faster than we expected.

And the question is why?

(insects chittering) (eerie music) - [Narrator] Hidden among them is a tree that seems unusually hard to kill.

Scientists on this island are starting to figure out what makes it different.

The mystery involves an invisible killer, technology that sees forests the way that no human can, and a tree that has somehow turned one of the most destructive forces in the atmosphere into an advantage.

- Today, we've come here to solve that mystery.

(footsteps crunching) (energetic music) - [Narrator] This is Barro Colorado Island.

A little over a century ago, this wasn't an island at all.

It was part of the rainforest along the Chagres River.

(gentle music) In 1914, when engineers dammed that river to build the Panama Canal, it created Gatun Lake.

The rising waters drowned a mountain, leaving only its tip exposed.

It's an island of ancient jungle, cut off from the mainland, now surrounded by one of the busiest shipping lanes on Earth.

(gentle music) Since 1923, this island has operated as one of the leading tropical research stations in the world.

It's likely that Barro Colorado is the most intensively studied piece of tropical forest on the entire planet.

The mammals, the birds, the bugs, the fungi, plants, everything else that lives here is cut off from the mainland by this wall of water.

It's isolated here, and this gives scientists the unique opportunity to study what nature does when it's left to work on its own.

But there's something strange happening here.

(pensive music) - In these tropical forests, the trees are starting to die more than they did in the past, and this is really concerning.

They're dying at faster rates than they were in the past, and we don't know why.

(birds chirping) - [Narrator] But among these giant trees that are dying, - But among these giant trees that are dying, one species on Barro Colorado seems It's thriving.

This is Dipteryx oleifera.

(pensive music) A Dipteryx like this can live for up to 300 years.

It's the largest tree in this forest and the one species that seems to outlast everything around it.

They rule the upper forest in a way that doesn't make sense, given how fiercely trees in this forest compete.

Scientists needed to understand not only what's killing giant trees, but also why Dipteryx is surviving.

And on Barro Colorado Island, one way scientists have begun to understand just how unusual Dipteryx is starts with the animals moving through the forest.

(monkeys chittering) (energetic music) - So one of the really exciting things that BCI has allowed us to do is pioneer new methods of studying animals, using increasingly methods that allow us to monitor every aspect of their life 24 hours a day.

So basically, using the kinds of sensors that we have on our mobile phones and put them in collars that we place on the monkeys' necks.

And that lets us both find them so we can track them actively, come out into the forest, we can listen for the signal, and walk right up to them.

But it also allows us to continuously collect data on where they are.

In some of our collars, we even have little microphones that allow us to know what they're saying.

(monkeys barking and squeaking) What I'm doing out here is trying to understand how other species that live in complex social groups achieve things together that they couldn't achieve alone.

(monkeys squeaks) (gentle music) - [Narrator] Meg combines that GPS data with a full three-dimensional LiDAR scan of the entire forest to build a portrait of animal life in the treetops, one that has never existed before.

- From the movement paths that we see in the GPS data, we see these animals moving consistently along the same routes.

And so we think that these are highways through the forest.

And one way that we have to actually see these highways, to see the traffic, to see how these paths bring animals together to interact, is using camera traps.

- [Narrator] And something interesting keeps showing up.

In a forest of hundreds of tree species where the animals could theoretically go anywhere, they keep coming back to Dipteryx.

- Each of these dots is a spider monkey moving through the forest on BCI with the colored blobs being crowns of Dipteryx trees, colored depending on how much sugar they have.

The more yellow, the more sugar.

And one of the things that we notice is that during the time when Dipteryx is fruiting, it seems to be the most important driver of all of the frugivores' movement decisions.

(birds chirping) (insects chittering) - [Narrator] The monkeys, the kinkajous, they've all organized their lives around this tree.

If Dipteryx fails to fruit, the failure ripples through the entire food web.

Everything that depends on it suffers.

Dipteryx oleifera aren't just producing the most important fruit, they dominate the canopy.

Their crowns are 30% wider and 50% taller than average.

And every June they erupt in purple-pink blossoms like a beacon above the forest.

Dipteryx is thriving even as giant trees across the tropics are dying at alarming rates.

(leaves rustling) - Why do these forest giants die?

It sounds like a simple question, but it's one that we still don't have a good answer for.

Remember, these giant trees, they hold something like half of the carbon and living biomass in this rainforest.

And when they fall and die, they slowly return that carbon back to the atmosphere.

So it's essential that we understand why it's happening and how fast it's happening if we want to understand the health of our planet.

(footsteps crunching) (eerie music) - Then with trees, it's actually really challenging because they have these tremendous lifespans.

They have these complex life cycles.

So we end up with this problem of scale where we suddenly understand what kills trees actually need to work at scales vastly larger than how humans operate.

(zipper zooms) Suhigante solves this problem by essentially imaging these forests at two to four centimeter resolution.

And from that, we can actually reconstruct the three-dimensional surface of our forests, and do this month over month and start to track when and where we're seeing these trees be damaged, when and where are they dying, something we've never been able to do before because we haven't been able to get to these trees in time to do this type of basic work.

We have people on our team who have built these really incredible systems that take those images, assemble them into products that we can then align and analyze over time to figure out, all right, from last month to this month, which trees across our landscape are starting to show major changes in their health?

Which trees are breaking?

Which trees are dying?

(grass rustling) (insects chittering) (Maikol speaks in foreign language) - [Narrator] This is essentially a crime scene investigation.

You're doing a postmortem on a tree.

(suspense music) - They go to each tree, and they start to record information about its health, how much of its crown has been lost?

Is it still having leaves on its branches that are green or has it lost all of its leaves?

And then they start recording other things that might contribute or lead to death.

So are they infested by beetles?

Do they have these vines colonizing their crowns?

Does it have heart rot at the place, you know, or rotten wood where it snapped?

And at the end, they take all that information and they make a subjective decision.

(equipment zooming) - When we took the wood from the drill, we see that the wood is dry, so that indicate us the tree is dying and the bark is separated out of the wood.

That is something that really indicate us.

Sometimes it's hard to say because the wood can get completely wet through the tree and we can like feel the wood wet.

(gentle music) - [Narrator] And there's one suspected killer that scientists never expected to find, one they didn't even think to look for for decades.

(thunder crashes) Lightning.

More lightning strikes in the tropics than almost anywhere on Earth.

During the rainy season, lightning storms can occur almost daily.

- Just in tropical forests alone, they experience somewhere between 30 to 60 million lightning strikes every single year.

Almost none of those lightning strikes are documented.

We find that over the last hundred years or so, maybe 10 lightning strikes were described out of about a billion.

- In this sort of a wet, shadowy environment, scientists originally assumed that lightning strikes didn't have much effect on the forest.

That turned out to be very wrong.

- So about 10 years ago, some colleagues and I started doing work studying how lightning affects forests, and we really thought we'd be figuring out how lightning wasn't killing trees.

And then, in the process of starting to track lightning strikes, where they're hitting forests, where they're entering forests, about 10 years ago, we figured out that actually lightning is a really important killer of large trees.

The only place we've actually been able to start precisely tracking lightning strikes, go in the forest and study them is in this forest, what we ended up finding, tracking about 100 lightning strikes and performing really detailed studies over many months and years is that lightning isn't producing scars or explosions or fires.

Instead, what we're seeing is this process called flashover.

It's a physics term.

It means the jumping of electric current from one object through an air gap to a second object.

And when we go out into the forest, what we see is that lightning comes down, attaches to a large canopy tree, and then from this canopy tree, jumps outwards to its neighbors.

It's little electric static shocks all over the forest.

And that's what lightning actually looks like.

So it basically forms this ring of death around that tree.

So it has these huge outcomes in this forest, but we can't identify them if we go looking for a scar or an explosion or a fire.

- [Narrator] It's now thought a single lightning strike in this forest damages an average of 23 trees and kills five of them outright.

And the largest trees are disproportionately among the victims.

(footsteps crunching) (leaves rustling) This is the aftermath of a lightning strike here in the rainforest.

Normally, this area behind me that's sunny and clear would be dark, covered by the canopy, but it's been cleared out by something.

You can see that there's damage all around us.

Trees are cracked and falling down.

I have to be careful where I walk.

These vines have died and pulled away from this main tree, and what's right at the center?

Dipteryx, the fruit-bearing keystone species of this rainforest.

(ominous music) - [Narrator] Evan and his colleagues ran lab tests on multiple tree species from this island, measuring electrical conductivity.

And what they found was something that no one expected.

Dipteryx came out on top.

It's the most electrically conductive tree they've tested.

So when lightning strikes it, the current flows through with less resistance, less heat than almost any other species here.

The tree isn't fighting the lightning, it's conducting it.

- They don't show almost any damage at all.

It's like no damage at all after receiving, on average, 30,000 amps of current into their tissues.

And not only do they survive, but we've really shown pretty conclusively they benefit from being struck by lightning.

So they get struck, and the parasitic liana is colonizing their crown, they get fried off of these trees.

And we also see that it damages and kills their neighbors just like it does for any other normal lightning strike, but instead of the Dipteryx dying as well, it survives.

So it essentially, if we anthropomorphize a little bit, Dipteryx uses lightning to kill its competitors and also lianas infesting its crown.

- Electricity, that is this tree's secret weapon.

It's turned the most powerful force in the atmosphere into a competitive advantage.

- Their neighbors are shorter.

They have fewer lianas.

They're experiencing less competition because they have these repeated benefits over and over.

- [Narrator] From above, the signature is unmistakable, a halo of open forest carved out around every ancient Dipteryx crown.

But by blasting away its competitors, this tree doesn't just secure light for itself, it completely reshapes the world on the ground below.

- This tree became a keystone species in this ecosystem, not simply because of the feast that it provides for birds and mammals and everything else, it's because it figured out how to turn destruction into an opportunity.

(insects chittering) (ominous music) - [Narrator] Understanding trees like Dipteryx isn't just about solving a local mystery, it's about understanding the fate of the planet's largest carbon vaults.

Tropical forests cover around 12% of Earth's land surface, but they store somewhere between 25 and 30% of all terrestrial carbon, more than any other ecosystem on land.

And the giants, the top 1% by size, store a disproportionate share of that, lose the giants, and the forest exhales centuries of stored carbon back into the atmosphere.

- For the first time, we're really understanding what causes the death of giant trees, but more importantly, we're starting to learn which trees, which species, which traits might actually help giant trees survive.

And this is going to be really important for figuring out how to conserve different places.

If we want that to be successful, we don't want trees to do well for 10 years.

We want them to do well for 200.

(ominous music) - [Narrator] What these researchers are building here together on this accidental island is a new kind of knowledge, not just about how Dipteryx survives, about what resilience for both plants and animals looks like in a tropical forest at a moment when resilience is the one thing that matters most.

(gentle music)