Original subtitles

♪ ♪

[narrator] The toughest forests on our planet,

are the ones that grow where little else does.

These hostile environments create trees

with the ability to both save lives...

and destroy them.

Ghosts create life after death,

monsters turn trash into treasure,

and buildings get swallowed whole.

These are Earth's extreme forests.

And they will stop at nothing, to survive.

♪ ♪

Take a journey

through our planet's kaleidoscope of forests realms.

From the frozen boreal in the north,

to the tropical jungles that hug our equator...

from mountain peaks,

to the ocean depths...

Marvel at the magic,

and drama, that unfolds...

as we reveal... the Secrets of the Enchanted Forests.

♪ ♪

The Namib...

is one of our planet's driest deserts.

Dominated almost entirely by sand,

sources of water are scarce here.

Yet, it is home to one of the world's thirstiest animals.

The elephant.

[elephant grunting]

This herd, is led by its oldest female -

the matriarch.

She has one of the toughest jobs of any mother.

She must find water for her family,

in this parched wilderness.

The herd are all counting on her,

and her knowledge of this extreme environment

to lead the way.

[elephant grunting]

So, she looks to the most reliable guide she knows:

the desert's sparse and scattered forest.

Only specialist desert-adapted tree species,

such as Ana and Mopane, have any hope of surviving here.

The Ana tree's secret weapon,

is a long taproot that can seek out secret pockets of water

buried 200 feet below ground.

Keeping their thirst quenched and leaves green

for most of the year.

So they shine like a beacon, against the dry sand.

The elephants use these trees as signposts.

This dispersed forest,

lines the route of a river hidden deep underground.

And, the healthier the tree, the closer it must be to water.

To stave off dehydration, the elephants feed on moist leaves.

But, they eat sparingly.

Every tree is precious

and they can't risk destroying future supplies.

After days of walking,

the heat and physical effort of this make-or-break journey

...brings the herd to its knees.

[elephants grumbling]

The matriarch knows to trust the trees.

They tell her she's getting closer.

And soon, she spots the final sign.

A forest of lush, new-growth mopane trees

is a rare sight in this desert.

These young trees, have shallow roots.

If water is within their reach,

that means it's within the reach of elephants too.

♪ ♪

There's even enough water, for a cooling bath.

The Namib desert never stands still.

The coastal winds shift the sand dunes daily

so the location of these sporadic sources of water

are never guaranteed.

The green markers in this desert forest

may change their course over time,

but they will always lead to water.

Even in the gentlest of climes,

a forest must endure a daily onslaught...

...from hungry beasts.

In a dense, lush woodland with plenty to go around,

trees can share the burden of having their leaves munched

and branches stripped.

But in extreme forests, trees are few and far apart.

In a land with limited food and water,

a lone tree can be a rare source of sustenance

for wandering herbivores.

And there is a real danger of it being completely devoured.

So these trees, need to fight back.

One such tree, is the camelthorn acacia.

This desert stalwart is locked in an escalating arms race

with its greatest foe...

the giraffe.

A giraffe needs to eat

more than more than 60 pounds of vegetation per day.

And very little is out of its reach.

To counter these attacks, the tree has armed itself.

Covering its branches

in two and a half inch long punishing spikes.

But the giraffe evolved dexterous lips

and a one and a half foot long prehensile tongue

to deftly navigate around the spikes

and still pluck away at the leaves.

So the acacia, enlisted some back up.

[narrator] Its second line of defense

comes from within the thorns themselves.

Tiny bodyguards.

Each tree in this woodland,

has several ant species living within it.

Housed inside the hollow thorns, these ants will protect the tree

in return for a regular supply of nectar.

As the giraffe's lips move in,

the ants swarm,

delivering a barrage of painful stings.

But, this only works on young giraffes.

Adults have toughened, leathery lips

and appear undeterred by the pain.

For these hard-mouthed adults, the tree has one final tactic:

chemical warfare.

Once the tree senses it's under attack,

it floods its leaves with a bitter tasting toxin.

In high doses these chemicals could kill the giraffe,

but they take a huge amount of energy for the tree to produce,

so it releases just enough to render the leaves unpalatable.

Finally, this works.

But the giraffe, simply goes in search of another acacia to eat.

This may be a sparse forest,

but the trees within it still look out for each other.

So the beleaguered acacia, sends out a warning.

Through pores on its leaves,

it releases a waft of ethylene gas into the air.

The surrounding trees pick up this airborne message,

and follow suit, filling their leaves with toxins.

Within 15 minutes, all the leaves in this woodland

have turned distasteful.

The herd is forced to move on,

and the forest can stand down and reset.

The tree's defenses give the giraffe just enough time to eat,

but never enough to destroy any one tree.

Both sides have called a truce... for now at least.

Even a tree so hell bent on defense,

needs to soften up for the sake of its offspring.

Once a year,

the camelthorn acacias need the help of a certain herbivore,

if their woodland is to survive.

These, are acacia seed pods.

The tough outer casing is designed to protect the smaller,

more delicate seeds inside.

And the acid inside an animal's gut

is the most effective way to soften them up

and kick start their germination.

As the tree drops its ripened pods,

the fate of its offspring lies in the jaws of others.

And the crucial element of success,

comes down to who is doing the eating.

Something the tree, has no control over...

Many seed pods are doomed before they even hit the ground.

These round holes are the work of the bruchid beetle.

It lays its eggs directly onto the pod's outer casing.

The newly hatched larva,

burrows its way inside and starts eating...

By the time it re-emerges, all fattened up,

there is little left but a potholed empty husk.

The remaining healthy seeds

have caught the eye of a ground squirrel.

♪ ♪

But the squirrel's teeth and jaws are just too strong

and they grind down the pods;

husk, seeds, and all.

Turning any chance of a new seedling into dust.

But there is one set of animals in the desert

that can help the acacia.

This grey duiker, is a welcome diner.

A portion of the seeds

will safely pass through its four stomachs unharmed,

and reappear ready to be planted.

Duikers, along with gemsbok and springbok

are the Acacia woodland's salvation.

And the seeds are an important food source for them

during the long periods of drought.

As they continue their search for food,

they scatter the unscathed seeds far and wide across the desert.

Planting the next generation of tough little trees

to preserve this rare forest in the unforgiving Kalahari.

If a tree can't tempt an animal to eat its seed,

it must come up with an alternative solution.

One tree in particular,

has taken their unusual dispersal strategy

to a lethal level.

[narrator] 50 miles off the coast of Southern Australia...

lies Lady Elliot Island.

In the heart of the Great Barrier Reef,

this coral cay island is one of the few

that supports its own forest.

One species of tree has managed to get a foothold here.

The Pisonia Grandis.

An entire forest sprung from a few deposited seeds.

But on such a tiny secluded island,

once the forest has reached capacity,

how does it expand into new lands?

The nearest island is over 20 miles away,

too far for the seeds to be transported on the wind,

and they won't survive in saltwater.

The trees, need a seed carrier.

During the summer months, this forest plays host

to some 50,000 migratory seabirds,

as they flock to this island to find a mate.

[birds squawking]

The trees offer an attractive site to build a nest.

At over 60 feet tall, they provide shade

and a protective buffer

against the winds that batter the coastline.

But it's a small forest and there are many seabirds.

Over 40 pairs of noddies may nest in a single tree.

In return for acting as hosts,

the seabirds gift the trees tons of nutrient-rich droppings.

[birds tittering]

On a sandy, coral island

it is this guano that sustains the forest and fuels its growth.

It's a match made in paradise.

And these long distance seabirds,

make the perfect seed carrier.

They can fly from island to island

and spread the Pisonia's influence

throughout the archipelago.

But this is where the plan, hits a snag.

Seabirds don't eat fruit or seeds,

so the tree has to find another way

for the birds to carry its offspring...

Glue.

The Pisonia coats its seeds in a sticky substance.

Strong enough to hold firm on the bird's feathers,

whilst flying over the sea to another island,

but weak enough so the seed eventually drops off

in its new home.

However, in their ambitious quest to colonize new islands,

the tree has gotten overzealous,

and its dispersal strategy has spiraled.

The trees produce so many sticky seeds,

that they often smother the birds.

With their feathers glued together,

and loaded down with seeds,

they can no longer fly.

The forest is unwittingly sabotaging

its own expansion plan.

This remains the only forest for miles.

So they keep returning to this island,

despite the toll...

...Forest and bird,

stuck on an evolutionary path

that's hit a dark, dead end.

When forest and animal do work in harmony,

the result can bring a secret sweet relief

to life in a bitter landscape.

[narrator] There are very few species of tree,

that can survive in Australia's brutal interior.

Even the tough eucalypts have failed to conquer these lands.

Again, it is an acacia forest that dominates this desert.

This Australian species, is known as the Mulga tree.

Hardy and efficient,

it is one of this arid zones' most successful inhabitants.

Partly due to its own partnership

with not one, but two insect species.

The tree's branches, are sprinkled with lerps.

These crystallised honeydew bubbles,

protect juvenile psyllid bugs that live off the tree's sap.

Patrolling between the lerps, is the honeypot ant.

They won't harm the psyllids

but instead, they drink the honeydew

encased within the protective shells.

Diligently, the ants bring supplies of this sweet nectar

back to the nest located at the base of the mulga tree.

Down here, in their underground network of tunnels,

the ants stockpile their precious elixir

for when times are lean.

The honeydew is kept in living larders,

that hang from the burrow ceiling.

Specialist ants, known as repletes,

are overfed with this syrup,

until their dangling, extended bellies stretch to bursting.

These, are the eponymous, honeypots.

When the desert drought reaches extreme measures,

the worker ants stroke a replete's antennae

and receive a ration of the sugary liquid.

The mulga tree sustains the ant colony through tough times,

and in return, the ants help the tree to thrive.

Honeypot ants act like earthworms

in the Australian desert.

They till the soil,

recycling valuable nutrients and breaking down organic matter.

[thunder claps]

Their burrows act as funnels,

transporting precious drops of rain

straight to the tree's roots.

In the unpredictable and extreme nature

of the Australian outback,

this two-way relationship between insect and tree,

enables both to survive, where little else can.

As we walk through any forest,

we can be unaware of the extreme nature

of what lies just beneath our feet.

In broadleaf forests,

down amongst the layers of fallen leaves,

and broken tree trunks,

lies a world that is rotten to the core.

♪ ♪

This, is where hidden terrors live.

And no one, is safe.

This giant centipede in New Zealand

can only survive in a moist environment,

so the damp forest floor is his chosen realm.

At 8 inches long, it is the stuff of nightmares.

Its pincer-like legs, that move in a hypnotic rhythm.

The fact it has eyes, but can't see.

Its killer instinct.

The skink is safe...

...as long as it stays still.

The centipede can't see it,

but it can detect movement through its antennae.

Do.

Not.

Move.

The centipede subdues its victim

before using its specially adapted front limbs

to inject it with venom.

Having feasted on its innards,

the centipede returns to its darkened lair -

until it's time to hunt again.

In this underworld, nothing is what it seems.

Perfectly camouflaged against the forest floor,

this is a Brazilian wandering spider.

This spider doesn't build a web, to wait for her food to arrive.

She goes out and hunts.

And she uses the leaf litter, to help her.

Her fangs are packed with the most potent venom

of any spider in the world.

And her legs, can detect the slightest vibration

from several feet away.

So if any hiding insect rustles a single leaf...

It's game over.

But, as these mini monsters consume their next victims,

they are helping the forest deal with its litter problem.

A buildup of dead leaves could choke a forest.

All this leftover organic matter, needs to be reduced

and recycled.

Whilst the fungi and bacteria living in the soil

take on the bulk of the decomposing work,

spiders prop up the process

by hunting the insects that eat the fungi.

And also supporting this vital service

is a very specialist band of invertebrates,

known as The Detritivores.

Each member has its own role to play on the forest floor.

Millipedes feast on the tougher leaves,

breaking them down for the microorganisms to recycle.

Voracious grubs chew through the dead wood,

accelerating its decay

and returning its nutrients to the soil...

...where it continues to fuel the forest.

These hard working critters,

are paramount to the success of our forests worldwide.

By keeping the soil healthy and reworking every nutrient,

woodland ecosystems simply could not work without them.

A healthy, fully functioning forest can be a powerful force.

It can influence our climate, support life and even...

cover up our remains.

As humans, we have undoubtedly left our mark on this world.

But those marks, are not always indelible.

So what happens, when we're not there to maintain our dominance?

Over time,

nature returns to take back what is rightfully hers.

This former prison on Gorgona Island

off the coast of Colombia,

detained some of the country's most notorious offenders.

But 40 years after the last inmates left,

the surrounding forest has reclaimed the area.

Trees need to expand in order to grow.

Its living tissues simply flow around

and engulf anything in its way.

Binding itself to man-made structures,

until the two become one.

The forest, gradually swallowed the buildings

and the wildlife returned.

♪ ♪

This process of ecological succession,

is when nature is allowed to follow its own path

and regenerate an entire environment.

Across the globe, wherever humans move out,

fast growing pioneer species of trees move in.

In just a matter of years, empty urban spaces

revert back into forests.

Drawing in a different set of residents.

In the face of human encroachment,

life always finds a way.

Even a forest full of man-made ghosts,

can create a life worth fighting for.

[narrator] This forest in Yala National Park in Sri Lanka

was deliberately flooded.

The trees slowly and silently drowned

in the newly created lake.

Preserved in the saltwater, they remain upright,

stripped bare and knee deep in the water.

But a lifetime's worth of nutrients

once stored in their trunks, now leaches into the water.

Even as a ghost forest,

the trees have created a surprisingly fertile habitat.

[birds chirping]

Meaning it is still hot property for many animals.

[birds squawking]

And for a bird of prey, it's the perfect forest.

This juvenile white bellied sea eagle,

recently left his nest

and is now looking to establish his own territory.

And these ashen skeletons, will suit him very nicely.

The rigid branches support his weight,

and the lack of leaves and twigs

is far more accommodating of his six and a half foot wingspan.

There is no requirement

for a leafy canopy to hide from predators -

he is the predator.

And he has a clear line of sight

to pick out a target from the lake below.

But if he wants to claim this highly desirable area

as his own, he is going to have to fight for it.

A rival young male

has also set his sights on this forest graveyard,

so he attempts to run this new arrival out of town.

♪ ♪

In an effort to throw off his pursuer,

the trespasser lets go of the fish...

but this isn't about food, this is a battle for real estate.

When his competitor drops out,

the new arrival chalks up a win.

But his victory lap, does not last long.

The young eagles have failed to realize

they are squabbling over a territory that is already taken

by these resident adults.

With far superior flying - and fighting skills -

the fully grown sea eagles

have secured this coveted spot as their own,

and they are not prepared to share.

♪ ♪

The youngster will fare better

in a less competitive neighborhood.

Even a drowned forest is a useful one.

These grey ghosts, still have a huge ecological value

to the surrounding wildlife,

and competition to call them home

is just as rife as in a living forest.

Across the world,

forests have found a variety of ways to survive

in the toughest of landscapes,

but one of the greatest challenges they currently face

is an increasingly warming climate.

And in an effort to combat it, right now,

around the world, trees are on the move.

Individual trees might be rooted to the spot.

But, entire forests can travel.

By using their seeds.

As the planet warms,

it changes the conditions in and around forests.

Seeds might succeed in places, where they did not before.

Or start to fail, where they once thrived

as habitats become less hospitable.

The older trees start to die off

and the forest begins a slow journey, in a new direction -

sapling by sapling.

Essentially, the forest is migrating

towards a more comfortable life -

moving around 10 miles per decade.

But the rate of climate change is accelerating

and the trees are failing to keep up.

They need to increase their pace and move 10 times faster,

or risk extinction.

There is a danger, as forests are pushed beyond their limit,

some of our most remarkable tree species will suffer.

The slow growing giants, such as the iconic redwood,

some of which have been standing since Roman times,

could disappear forever.

As we lose our forests,

we lose a huge ally

in the battle to slow the warming of our planet.

Over millennia, they have fought to thrive in every environment -

however extreme.

But the limits of those extremes now lie in our hands.

If allowed,

our precious woodlands can continue to work their magic,

shape our world,

and nurture all life within them.