- Picture the largest things that have ever lived on our planet.

Giant sauropod dinosaurs, towering sequoia like General Sherman, or even the mighty blue whale.

But none of these can claim the record for the biggest heaviest thing ever.

The largest species on earth, you're in it.

(upbeat music) These are quaking aspens.

They're famous for the beauty of their fall leaves and the sound that those leaves make in the wind when millions of them shake.

(leaves rustling) Such a peaceful sound.

Their bark is famous too, it's white.

And that's because it's a living tissue.

It's actually capable of photosynthesis during winter even when the tree has lost all of its leaves.

Aspens hold a ton of records among trees.

They're the most widely distributed tree in North America.

The second most widely distributed tree in the world.

They grow everywhere from Canada to Mexico, from the Atlantic to the Pacific.

(upbeat music) You're probably aspen yourself.

Why should I care about a tree?

Well, it's pretty unbelievable, but members of this species are the largest organism on earth.

How could that possibly be true?

I mean, aspens are pretty average looking trees.

They're tens of meters high.

They don't have particularly thick trunks like a Sequoia or a Redwood.

How could any aspen be the largest organism on earth?

Well, because of what you can't see.

Groups of aspens like these are connected underground by a giant shared root system.

And that root system can even exchange water and nutrients among these individuals.

Groups of aspens like these connected underground, well, that makes them one giant organism.

They're like different limbs of the same body.

Each trunk that you can see is a genetically identical clone of the trunks around it.

Individuals up here on the surface, but one giant connected organism underneath.

A giant aspen clone in Utah is the current title holder for world's largest organism.

Its name is Pando.

The name Pando means I spread out in Latin, and the name fits.

It covers an area about the size of 100 football fields.

It's estimated to have more than 40,000 stems and is older than the last ice age.

It's a hundred times heavier than biggest dinosaurs, heavier than 30 blue whale, the largest animals to ever live and more than triple that of the largest giant Sequoia, General Sherman.

Well, Pando is just the biggest aspen clone that we know of.

This is not Pando.

Where I'm standing right now in Western Colorado, this is the largest continuous distribution of aspen in North America.

And researchers think that there might be a clone even bigger than Pando somewhere out here.

The hard part is finding them.

So how do these aspens grow to be so big?

Well, because of the special way that they grow.

Most plants have male and female parts on the same plant, but aspen clones are either all male or all female.

And they still exchange pollen, make seeds, would float off every spring in these cute little cottony fluffs.

A long, long long time ago before humans even set foot in North America, one of those seeds fell right here and it sprouted, became a seedling, and it chased the sun's light up to become a giant aspen.

But then that treated something unexpected.

Instead of making a new baby tree using seeds, it shot its roots outward, sending new stems up through the soil that grew into new trunks chasing the suns light upward.

And that process repeated over and over, over centuries in millennia to become this.

And this way of growing, well it gives aspens a big advantage in this kind of environment.

Why would a tree grow like this?

Well, like everything that evolves, aspens have adapted to their environment.

This is a landscape shaped by fire.

So aspens have evolved to succeed in these destructive conditions.

Imagine a wildfire comes through behind me.

Well, aspens have very thin bark.

They don't have that woody armor that other trees have.

So their sap basically boils from the inside and they die, but the roots structure underneath the ground, that survives.

And when all the other trees have been cleared out, they shoot up through the soil by the hundreds, even thousands.

And they completely take over.

Same with avalanches, even disease.

Anything that disrupts this environment, and aspens are uniquely adapted to spring back and take over.

Huge aspens like these are incredibly old.

How old is a tougher question though.

We can't read the rings to know how old these aspens are.

Any individual trunk might be 60 to 120 years old but these trunks are always dying and being replaced by younger new ones.

Just like the leaves falling every autumn, the trunks themselves return to the soil to feed the next generation.

History can give some clues to aspens' age.

Much of the area that aspen's loved to grow in today was covered by ice sheets during the last ice age.

So in those areas, aspens can only be as old as when the last ice sheets disappeared.

But aspens in areas like these were never covered by that ice and could be much older.

So aspens can grow to be the biggest organism on earth because of how old they are, because of where they grow, and because self cloning gives them a huge advantage over other trees.

While researchers estimate no individual trunk is more than a couple centuries old, these giant clones could be tens of thousands or hundreds of thousands of years old.

Their stems dying and reborn in a life cycle that renders them essentially immortal.

It's possible that aspens only ever die thanks to disease or extreme changes to their environment.

And that's happening more and more.

Aspens like these have survived countless challenges in the centuries, even millennia that they've been here but we are presenting them with new ones.

For one thing, the fires that aspens depend on to thrive, well, we're not letting them burn.

And we've thrown ecosystems completely out of whack.

So that grazing animals like cows and deer, well, they come in and eat all of these soft new shoots, and these young aspens never turn into old ones.

As a result, giants like Pando are shrinking.

But there is one threat that tops all of that, climate change.

Maybe you've heard of it.

And as areas like this get warmer and drier, it's really bad news for these aspens.

Now, the way that aspens grow by spreading their roots out through the ground, that means they can actually move.

They can move a up and down these mountains and valleys to chase their ideal climate conditions.

But if the climate changes too quickly, they might not be able to outrun it.

And as it gets warmer and drier, these trees have to turn down their photosynthesis while turning up their metabolism.

It's like being on a tread mill and holding your breath.

That's not a fun time.

As a result, researchers have seen ancient aspen stands die just like that.

They call it sudden aspen death syndrome or SAD.

And it is very sad.

These are ancient species, ancient stands, ancient giants and we're throwing them a challenge that they've never seen before in their entire existence.

Will they be able to survive?

We just don't know.

The story of earth's largest species is a story of survival, a story of resilience in the face of destruction and challenges.

And in this new changing world that's built by our hands, I hope that story of survival continues.

I think that by understanding these stories and these species, how they got to be the way that they are, it gives us another reason to work hard to save them so that the next person that walks through this giant 10 years, 100 years, 1000 years from now can learn that story too.

Stay curious.