Original subtitles

[narrator] In a realm of constant change...

Earth's seasonal forests are the most dynamic of them all.

From armies of vigilante insects,

to a pilgrimage from ocean to tree canopy.

The march of the seasons inspire some to burn bright and brief,

and others to make an epic journey into the unknown.

This is a world of boom and bust.

All must dance to the beat of nature's seasonal drum.

Take a journey

through our planet's kaleidoscope of forest 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.

[Enchanting music]

These are the monsoon forests of Borneo.

Straddling the equator,

spring, summer, autumn and winter hold no sway here.

But these are seasonal forests all the same.

One dramatic annual shift

transforms this forest each year.

It's so powerful

that it has driven a once-social animal into a life of solitude.

This female orangutan gave birth just four months ago.

It's her first baby.

But like the rest of her species,

she's a single mum with no family

or troop to show her the ropes.

She relies on instinct

and an intricate knowledge of this forest,

passed down through generations.

Their bond is strong.

She is all things to him:

Provider...

Bridge...

and most importantly, she is his Teacher.

She has seven years

to teach him everything he needs to know about surviving

in this monsoon forest -

as he will spend the rest of his adult life alone.

It's the dry season...

so today's first lesson

is how to find a secret source of water...

in a tree nook, 20 feet above the ground.

But he'd rather go in search of food.

Mom has over 400 different types of fruit in this forest

to introduce him to.

but at this time of year, food of any kind is hard to find.

This giant male

has spotted one of the last remaining durian fruits.

From a safe distance, they hungrily watch the male tuck in.

And the baby learns one of the most important lessons

of them all.

Orangutans do not share food.

Unlike nearly every other primate species,

they are solitary creatures.

It wasn't always this way.

Like their ape cousins,

orangutans used to be social.

But after the last ice age, the forest changed.

Now, with food appearing seasonally,

it could no longer support large troops.

In a boom or bust world, each had to go it alone.

Without a troop for support,

this mother had to become an expert

in the ways of the forest.

And she knows change is coming.

Knowledge learned from her own mother,

tells her that once a year,

the Monsoon brings a lifesaving deluge of rain.

But the force is unpredictable...

and this year, the rains are late.

Orangutans have learned to be patient.

Eventually, the sky will deliver.

This change to wet season finally reawakens the forest.

And after a quick demonstration in umbrella construction...

it's finally time to teach her son

where to find the now ripened fruit.

With bellies full, they can practice making their beds.

Tomorrow will be yet another day of home-schooling.

By falling in sync with the powerful monsoon,

they have found a way to survive the all or nothing nature

of this seasonal forest.

Moving away from the equator,

temperate forests have four separate seasons to adapt to.

For a forest, every changing season presents an opportunity.

And spring in the Northern Hemisphere...

... means babies.

But too much 'new life' each spring

can pose a supply and demand problem.

Every now and then,

trees conspire together to act as nature's most synchronized

form of birth control.

In these woods,

reproduction requires teamwork between tree and animal.

And it all starts with a burst of spring blossom.

Pollinated flowers transform into fruit, nuts and seeds,

providing a feast for the woodland animals,

who spread them far and wide.

When the balance is tipped,

too many seeds are eaten, leaving too few to germinate.

The trees must find a way to take back control.

And they must do it together.

Every few years, the trees collude

to produce a bumper crop of fruits or nuts,

known as a 'mast year'.

A seasonal glut that yields more food

than the animals could possibly eat...

leaving more seedlings the chance to take root.

This strategy is not confined to temperate forests.

Trees across the planet use this method to gain the upper hand.

Now, the trees have to make sure this new bounty of food

does not encourage a corresponding baby boom

in the forest animals the following year,

that will still go on to eat it all!

So they cunningly follow their generous 'mast' years

with several 'meagre' ones.

The shift from abundance to scarcity

curbs any plans for legions of offspring

and more mouths to feed.

And balance is once again restored.

It's a master plan.

It leaves the animals guessing, keeps populations in check,

and most importantly, it shows who really holds the power.

Across hundreds of kilometres,

trees can precisely synchronize their 'mast years',

so the overabundance happens all at the same time.

How they do this is still one of the forest's greatest mysteries.

[narrator] As each season ripples through the forest,

orchestrating events like a musical conductor...

one of the forest's tiniest residents

is looking for a stage

to put on the performance of a lifetime.

For this firefly, late spring brings about

the most important night of her life.

She's just emerged from her underground lair

and is still getting used to her new adult beetle body.

She's looking for love...

...and tonight is her chance to shine.

But this isn't a solo show.

If she has timed it right,

she'll emerge with thousands of other fireflies.

She is looking positively radiant.

Her abdomen contains bioluminescent chemicals,

that, once mixed, she ignites by pumping in oxygen.

She then regulates this reaction

to turn her glow... into a pulse.

Winged males reveal themselves with their own dazzling display.

She's looking for the brightest light,

indicating a strong, virile male.

Bingo.

She adjusts her rhythm...

mimicking his flashes.

[Enchanting music]

Eventually, the pair are completely in sync.

She's won him over.

When her twilight tryst is over,

in the cold light of day,

her 'choice' is a little underwhelming.

Much smaller than she,

the male fireflies are far less glamorous than the females.

Dating in the dark definitely works in his favor!

In any case, a fireflies' 'relationship' is short lived.

Their entire lives are geared towards this seasonal light show

and he will die soon.

She'll carry on to lay her now fertilized eggs

and then she too will turn off her light forever.

Destiny fulfilled.

[Mysterious music]

For many, these glittering courtship displays

are a prelude to a new, radiant summer.

This change in season

can trigger an influx of picky visitors.

When seasonal extremes only allow

small patches of forest to thrive,

some animals could find themselves living tooth by jowl

with a monstrous neighbor.

In Colombia, a vast savannah

stretches for over 200,000 square miles.

Despite the extreme shifts

between the wet and dry seasons here,

trees have found a way to survive.

[Thunder clapping]

When the rains come, the rivers overflow...

bringing long awaited water and nutrients

to the green ribbons of forests that line their banks.

These unique 'gallery forests'

are the only strips of trees for hundreds of miles.

They are crucial for thousands of migratory birds,

visiting each year to this one spot of prime real estate.

But the birds are fussy.

Everyone wants waterside views,

stable foundations,

and a source of sturdy branches for nest building.

The higher the spot, the better.

This male great egret didn't bag the penthouse.

Not to worry.

He's looking particularly fabulous this year

and hoping his plumage impresses the incoming females.

[Fanciful music]

This potential mate shows an interest,

but she wants to check out his nest before committing.

It's first rate -

close enough to water to take advantage

of the early summer blooms of plant and fish life.

But also hidden away in the branches, for a little privacy.

She approves.

A month later,

the entire woodland is a frenzy of demanding chicks...

[Baby egrets squawking]

and frantic parents.

But as the egret chicks begin to fledge,

this perfect location might not be so perfect after all.

Beneath them... a monster is stirring.

[Ominous music]

[narrator] The arrival of the rains

has awoken this caiman from her state of torpor.

With the forest floor now a watery highway,

she has easy access to the nesting areas.

Weak from a season of not eating...

ideally her next meal would simply fall from the sky.

[Dramatic music]

For some, the dream riverside home...

has turned into a nightmare.

[Egrets chirping]

For the lucky ones,

life continues if they keep to the trees.

The forest wouldn't exist here,

if it weren't for nature's seasonal balancing act.

The narrow bands of forests bring with them

opportunity for everyone, however, ruthless it may seem.

Any tree weakened by the heat can be vulnerable to attack...

by caterpillars...

beetles...

and aphids.

But in this orchard, a winged army of protectors

are ready to keep any potential assailants at bay.

These hornets rule over all other insects in this forest.

Serving like pest control officers,

keeping every other bug population under control.

Their headquarters are located inside this hollowed out tree.

The hornets have spent the summer expanding their command.

Hundreds of new offspring requires a bigger nursery.

The workers recycle the dead wood from the forest

to construct the new paper-thin walls of their nest.

Their queen single handedly gave birth to this entire colony.

Her sole duty now is to deposit new eggs into each empty cell,

spawning the next generation of pest controllers.

Hornets are voracious hunters

and they target the food that is most abundant.

Summer swarms of insects tormenting the trees

will be exterminated.

[Hornets buzzing]

Powerful jaws dismember their prize.

Ironically for a hunter,

due to their extreme hourglass shape,

an adult hornet cannot digest its prey.

But the plump larvae can.

So the workers deliver the food to their immature grubs.

Their reward is a liquefied meat smoothie,

regurgitated back into their mouths.

[narrator] By summer's end, the colony has reached its peak.

The ageing queen has given the gift of 1500 eggs to her colony.

Now, she prepares to pass her crown to a new ruler.

Her final act of service

is to produce next year's batch of queens...

... before her body is returned to the forest.

As the days cool, the summer bounty of food is ending,

and the hornet empire starts to crumble.

Now redundant,

even the workers can sense their time is coming to an end.

The colony's only hope now rests with their future queens.

These princesses must build up their reserves

to see them through the winter.

So the last remaining larvae are sucked dry.

A sacrifice to the future queens.

The new queen leaves the ruins of the past empire behind,

never to return.

Next year, she'll start her own colony

and raise an army to keep her forest safe.

But her own fate is inevitably sealed, and one day,

she too will fall with the autumn leaves.

In one forest,

a seasonal pilgrimage leads millions of fish from the ocean,

up into the canopy.

And how they got there is a story like no other...

A true miracle of nature.

It's late summer in Alaska.

As the days grow shorter,

the trees here are preparing for winter.

They have slowed their growth and now plough all their energy

into building their winter stores.

So they need all the food and water

they can get their roots on.

In this forest, nutrients tend to wash downriver.

So nature had to find a solution to bring them back up stream.

And it originates

far, far away from North America's seasonal forests,

deep in the North Pacific...

These salmon have been spurred

to make the journey of a lifetime.

The changing season is pulling them

from their bountiful ocean home,

towards the land and the exact river where they were born,

years before.

Their instinct drives them thousands of miles upriver.

Using the earth's magnetic field like a compass

to help navigate their way.

But the sheer effort of fighting against the freshwater flow

takes its toll.

Humpbacked and disfigured,

these males barely resemble the sleek silver bullets

from their ocean life.

The fish carry valuable ocean nutrients with them,

essential for the growth of the trees.

But for the forest to access those nutrients, it needs help.

For Alaska's brown bears,

the annual arrival of the salmon

marks the start of their 'fishing' season.

[Exhilarating music]

During these high times,

the bears need only eat the most nutritious parts of the fish,

fat-rich brains,

skin,

and eggs.

The rest of the carcass is tossed aside.

Leaving plenty for others to take advantage of...

... including the forest.

[narrator] Here, the bears leave tons of leftover fish to

decompose, releasing rich nitrogen into the soil.

A key nutrient for the trees,

helping them grow three times faster than elsewhere,

making them stronger,

and more nutritious

for all that consume them.

And when the bears have digested the fish,

they further feed the forest with rich droppings and urine.

Bears fish all along these migration rivers,

feeding the forest as they go.

For the salmon that make it past the bear gauntlet,

they break off to spawn in the shallows.

[Enchanting music]

It's their final act.

They put every ounce of energy they had

into battling their way here,

for the chance to reproduce just once.

But their effort ensures a new generation of salmon

will continue the cycle,

providing a vital supply of nutrients to these forests.

Everything that lives here

benefits from the tireless efforts

of a very determined fish,

and an arduous 1800 mile journey,

bringing nourishment to an entire forest

from ocean to canopy.

Throughout the seasons,

much of the forests of Alaska remain evergreen.

But as summer draws to a close in temperate forests

around the world,

each tree must make a critical decision about its future.

The sun is losing its power and the drop in temperature

triggers a beautiful transformation.

The trees of a forest often work in synchronicity,

but this time, it's different.

As the daylight hours decrease,

the sun no longer provides enough energy

for the leaves to turn it into food.

Soon, a tipping point is reached

when holding onto their leaves

becomes more of a burden than a benefit.

Each makes its own judgement on when to shed.

It's now recognized that trees have 'memories'

and call on their previous experience

to make this crucial decision.

In fact, a more 'anxious' tree

may drop early based on past traumatic incidents.

No-one wants to get it wrong.

Too early and you lose the chance

to make the optimum amount of food for the winter.

Too late and the frost might damage the tree.

Any remnants of water in the leaves could cause them

to rupture like a burst frozen pipe.

When the decision is made, the green chlorophyll pigments

used to harvest the sun's energy are broken down...

revealing a blaze of gold...

and red.

[Enchanting music]

The tree, its crown now stripped bare,

is ready to sleep.

Protective buds appear

where the leaves once connected to the stems.

They contain a chemical that places the tree

into a state of silent slumber,

before the cold really sets in.

[narrator] As autumn creeps into winter,

the once buzzing forest takes on an eerie emptiness.

So, where do insects go at this time of year?

Our future hornet queen,

use the forest's natural insulation - leaf litter -

to wait out the winter in a secret hideaway.

Having already mated,

the new queens will emerge from their leafy lairs in spring.

But they won't return to their old home.

Hornets never reuse an abandoned nest.

The reign of this paper palace is now over.

Whilst winter sees one monarchy pass onto the next generation,

a different monarch is on the rise.

A new fleet of genetically elite, age defying,

supreme beings, have been born...

poised to invade the US.

In these secluded eucalyptus groves

on the coast of California,

Monarch butterflies are arriving...

in their thousands!

[Enchanting music]

Their monumental journey began

over 3000 miles away in Canada.

The drop in temperature, and changing tilt of the Earth,

triggered a mass exodus in search of some winter sun.

No ordinary monarch could fly that distance.

These butterflies are made from stronger stuff.

They are the 'super generation'.

Born bigger, more powerful and with the gift of longevity,

these formidable flyers live eight times longer

than the average monarch.

Built purely to make the migration

to this one small patch of forest.

These eucalyptus trees are not native to this area.

But the monarchs have chosen them

as their perfect home in the sun.

The trees create the ideal microclimate for them.

The canopy and branches provide wind protection.

And long thin leaves filter the California sunlight,

giving the butterflies dappled light and cooling shade

that they need.

And the slender leaves are easier

for their little legs to grip.

They didn't find this forest by chance.

Despite having never visited this site before,

Monarchs have been found to migrate

to the exact same tree as their ancestors,

years previously.

And they know how to find this very tree

through a remarkable sense of direction, and time.

These tiny creatures tell the time by using their antennae.

Coupled with super powered eyes

that decipher the sun's position, they rarely get lost.

This migration isn't all about the mild weather.

It also means more potential mates to choose from.

The air fills with the scent of female pheromones...

and long, elegant courtship dances commence.

[Magical music]

Eventually, this super generation's time

must come to an end.

And they will die shortly after laying their eggs.

Leaving them to hatch in the California sunshine.

It takes three more generations of butterflies

to relay-race each stage back to their Canadian homeland.

Just in time for summer.

By August, the next batch of superpowered monarchs

will be born.

And so the ticking clock of nature's changing seasons

starts its cycle all over again.

Every new season brings dramatic change.

From the boom of spring,

to the challenges of winter.

These unstoppable forces are what define the cycles of life

in our planet's seasonal forest.

[Enchanting music]