All language subtitles for 00020.track_4768

DAVID ATTENBOROUGH:
This is the Boreal Forest.

The largest forest on Earth.

750 billion trees.

This is
the northernmost boundary

of an extraordinary world.

Sandwiched
between the frozen poles

and the warm, wet tropics,

this s a very different world,

one dominated
by relentless change.

Here, there is an endless cycle
of four distinct seasons.

The short, freezing days
of winter

give way to the urgent
awakening of spring.

And the long, hot days
of summer

yield to the cooling of autumn.

Opportunities will be brief.

To survive in such places,

the plants that live
in these habitats

have not only
got to be hardy and resilient,

but many of them have developed
special strategies

in order to meet the demands
of this seasonal world.

To succeed, they must
get their timing just right.

For months now,
this world has been asleep.

But change is coming.

Spring is on its way.

(TICKING)

In Canada, the forests
are starting to stir.

Throughout the winter,
the sugar maples here

have kept stores of nutrients
ready for this moment.

(CREAKING)

Now, as temperatures
start to rise,

those nutrients must be
brought up from the roots.

(GURGUNG)

Sap is on the rise.

Fuel for new green growth.

But just as growth reaches
full speed...

...robbers arrive.

(HAMMERING)

(TAPPING)

A sapsucker.

A thief in search
of maple syrup.

The tree can tolerate
hundreds of wounds.

But if they completely encircle
the trunk,

the tree will eventually die.

The sapsucker's break-in
hasn't gone unnoticed.

(wmes VIBRATE)

Hummingbirds,

squirrels

and insects

all invade
the sapsucker's territory,

trying to steal a meal.

But unwittingly,
they help the tree.

(SAPSUCKER CALLS)

Time spent chasing
the competition...

...means less time
to drill new holes.

- (SQUAWKING)
- (SQUEAKING)

The tree gets a chance
to repair its wounds...

...and the sap
can rise freely again.

So this year at least,

the tree will once more
produce its leaves.

Hundreds of miniature
solar panels

that collect energy
from the sunlight.

Each leaf is covered

by millions
of microscopic pores -

stomata.

They take in carbon dioxide...

...and release oxygen
and water vapour

in a process that produces
high-energy fuel.

All the trees here can now
use that fuel to grow

over the coming months.

As spring proceeds,
the power of the sun increases.

On river banks across Europe,

many plants have been
lying dormant underground...

...waiting for the earth
to warm.

And now a race begins.

Among the first to start
are nettles.

The sprinters.

Then, onlyjust behind,
come the brambles.

They're much more aggressive,
and have backward-pointed hooks

with which to scramble
over their rivals.

Last to come, the climbers.

Hops and bryony.

They lasso their way
towards the light.

Soon, every inch of space
and every patch of light

has been claimed.

But hidden in the shadows,

another plant has been
waiting to make its move.

This...

...is dodder.

A hunter...

...with an exceptional sense
of smell.

Moving swiftly,
it searches for its prey.

It detects the scent
of a young nettle.

The dodder punctures
the nettle's stem

and sucks out
the nutritious sap.

And it doesn't stop
at just one victim.

With its energy supply assured,
it multiplies.

It's a parasite

with an insatiable appetite.

(BIRDSONG)

(CAWING)

But remarkably,

its victims can now
exploit the dodder...

...when other springtime
enemies appear.

Caterpillars.

When under attack,
a leaf sends out a signal

telling the rest of the plant
to start building defences...

...foul-tasting toxins
that deter the caterpillars.

In an extraordinary twist,

the stems of the dodder
interconnecting the plants

are now being used
as lines of communication

to relay these warning signals
from plant to plant.

They respond by producing
their own protective chemicals.

Eventually,
the whole river bank

becomes one huge
defensive network...

...and the one-time rivals
are now better off together.

(WATER RIPPLES)

The first warm days of spring
encourage some plants to flower

and get ahead
of the competition...

...creating beautiful displays

throughout
the seasonal world...

...from the spectacular
cherry blossoms of Japan...

...to the goldfield flowers
of California.

But spring flowering is risky.

If they flower too early,
it may not be warm enough

for pollinating insects
to be active.

But the common daisy
gives them every opportunity.

They're closed up
tightly at night

to protect
against the elements.

But once they feel the warmth
and light of the sun,

they spring into action.

They turn, absorbing
as much heat as they can.

(BUZZING)

It's a behaviour
called heliotropism.

A thermal camera can reveal

just how effective
this strategy can be.

The blues show that the air
is 12 degrees Celsius...

...while the yellow colour
at the centre of the flower

indicates a temperature
of 21 degrees.

Pollinators
such as bees and wasps

prefer the warmed-up flowers.

As a consequence,
they can collect more nectar

from more flowers,
pollinating as they go.

And the daisies,

with the help
of their pollinators,

are able to have

a particularly long
flowering season.

But when the window
of opportunity is brief,

even more ingenious tactics
are necessary.

(BIRDSONG)

Here in south-west Australia,

summer temperatures can soar
to up to 40 degrees Celsius.

Now, for a brief moment
in the cooler spring,

flowering plants must get busy.

And this hammer orchid,
with its strange flower,

is doing exactly that.

It needs to attract
a pollinator,

but has no nectar,

and doesn't even
look like a flower...

...unlike these neighbouring
huge grass trees.

(BUZZING)

These produce thousands
of tiny white flowers,

all dripping with nectar.

But the orchid
has a different strategy.

It synchronises its emergence

with the brief mating season

of the thynnid wasp.

The flightless female wasp
produces pheromones

to attract males.

This male detects a scent
that seems to be hers...

...yet flies right on by.

The orchid not only has
the same shape and colour

as a female wasp,

but it sits at the same height
as she does.

It even mimics her smell.

And the beguiled male
attempts to mate with it.

It hammers against
the orchid's pollen sacs.

And the wasp leaves,
doubtless a little shaken...

...only to be duped once again.

The pollen on his back

now sticks
to this different orchid...

...and the deception
has worked.

The orchid's imitation
is even more intoxicating,

it seems,

than the real thing.

Eventually, the male wasps tire
of the orchid's enticements.

Now the wasps,
both male and female,

find the flowers
of the grass trees to feed.

And there, at long last...

...they mate.

And as for the hammer orchid...

...pollination is complete.

As summer approaches,

the need to flower
becomes more urgent.

And nowhere more so
than here...

...in South Africa.

This is the fynbos, part of
the Cape Floral Kingdom,

a great expanse
of open heathlands

just north of the Cape.

Here, there are more
different species of plant

than anywhere else
in the world -

nearly 9,000 -
many needing pollinators.

They compete with one another

for such help
by flaunting extravagant shapes

and vivid colours...

...all promising nectar
as a sugary reward.

But there is a plant here

that avoids
this crowded competition.

And the rising temperatures
of summer

bring just what it needs.

(WHOOSHING)

This is the first fire here
for 15 years.

Nearly all the plants
are destroyed.

Incinerated.

It might seem from the smoke

and the still-smouldering
embers

that no plant
could survive such an inferno.

But just four days
after the flames...

...rising from the ashes...

...a fire lily.

It has been lying dormant
underground for 15 years,

but now, awakened by the smoke,

it flowers.

The blooms may be small
and unshowy,

but that's all they need to be.

In this charred landscape,

pollinators such as
these sunbirds

can spot
these little red beacons

from great distances.

They're the only source
of nectar around.

And the more visitors
the flowers attract,

the more likely they are
to be pollinated.

(BIRDS TWEET)

It's just in time.

Within a few months,

the whole landscape
is alive once more.

In fact, all these plants
need fire to survive.

And as competitors return...

...the fire lily fades.

It now returns underground,

and will rest there as a bulb
until another fire awakens it.

- (BIRDSONG)
- (INSECTS BUZZ)

By the time the long, hot days
of summer arrive,

wildflower meadows
are bursting with life.

But autumn is not far away.

So now pollinated plants
must use their remaining energy

to produce the next generation.

Seeds.

A dandelion clock contains
around 200 seeds...

...each with its own
tiny parachute.

Few seeds can fly
as far as these.

Some are known to have
travelled over 60 miles.

But to travel
any distance at all,

they need the wind
to be just right.

However,
before they can get away,

there is a risk.

Voracious hunters
live in this meadow.

(SNIFFS)

Tiny harvest mice.

- They love dandelion seeds.
- (CREAKING)

But they don't like to share.

Finally, the sun warms
the ground

and a gentle breeze blows in,

creating an ideal updraught.

(SNIFFS)

Air flowing
between the bristles

creates a vortex
that lifts the seeds up.

Heading off
on the gentle breeze,

this seed now starts
its travels.

Plants spread their seeds
as far as possible

to avoid competing
with their own offspring.

This unassuming plant
is called ecballium.

It's a relative
of the cucumber.

For several weeks,

water pressure
has been building up

inside this seed pod,

It's as taut as
a well-pumped-up bicycle tyre.

Primed and ready,
all it needs now

is a nudge.

The tiny hole
where the stern was attached

funnels a high-pressured
jet of water,

carrying with it
the precious seeds.

This Himalayan balsam

has seed pods
that act rather differently -

like catapults.

As they dry out,
the ripe pods start to strain

along precisely positioned
lines of weakness.

The slightest disturbance,
and the pod splits...

...with a force big enough

to cast its seeds
up to seven metres away.

(SEA BIRDS CALL)

The east coast of South Africa.

Summer here
brings scorching temperatures.

This ceratocaryum has to get
its seeds underground

before it gets too hot.

And it has acquired
somewhat unlikely allies.

Dung beetles.

They have a particular fondness
for antelope droppings.

At this time of the year,
these beetles bury the dung

and lay their eggs on it.

And the fresher...

...the better.

Ceratocaryum's spindly stems...

...send their seeds flying.

They're the same size, shape,

and even smell
remarkably like...

...antelope dung.

The beetles
simply can't resist them.

Dung beetles always bury
dung balls at the same depth,

and it's one which suits
the seeds very well.

So effective
is the seeds' deception

that the beetles come back
for more time and time again.

Most of the ceratocaryum's
seeds get safely buried...

...exactly where
they need to be.

As the sun begins to retreat,
so autumn arrives.

The cooler, shorter days
are the cue

for many seasonal forests

to prepare
for the winter shutdown.

The trees start
to divert their nutrients

back into their roots.

And there,
other organisms await them.

FungL

Mushrooms are
the familiar face of fungi.

But they are merely
the fruiting bodies.

Some of them only last
for a few days.

They are, however,
evidence of the giant organism

that lies in the soil beneath.

Just a single handful of soil

may contain
several thousand metres

of their microscopic filaments.

It's only recently
that scientists have discovered

the extraordinary role
these fungi play

in a forest like this.

Their filaments plug into
the tips of the tree's roots...

...and nutrients pass
between tree and fungus

throughout the year.

Some fungi have
the ability to link

with not just a single tree,

but a whole group of trees,

so that the entire forest

may be linked together
by these microscopic threads...

...to form a wood-wide web.

Hundreds of trees can be
interconnected by these webs.

Scientists call it
the mycorrhizal network.

And it might look
something like this.

They've discovered that trees

not only send nutrients
along it,

but chemical
and electrical signals...

...allowing them to communicate
with one another.

But some trees
can also be selfish

and steal from their rivals...

...or even wage war,

by sending out toxins
that will harm competitors.

It seems, however,

that most trees
do try to help each other.

They raise the alarm
when attacked by leaf-eaters,

leaving other trees time
to produce defensive chemicals.

Those that are dying

may send their food reserves
to their neighbours.

And some individuals,
known as mother trees,

recognise
their own offspring...

...and will channel resources
to them...

...so giving their young

the best possible start
in life.

(BIRDSONG)

As the harshness of winter
approaches...

...temperate woodlands
from Russia

to Canada...

...are now in a race
to shut down.

(TICKING)

The green pigment in the leaves
starts to break down,

and nutrients are withdrawn
into the branches.

The chemical substances
that are left behind

then create one of the most
spectacular displays of colour

in the whole of nature.

(TICKING)

The first freezing nights
of winter

bring with them a killer.

Frost.

(CRACKLING)

Stems, when frozen, rupture...

...and flowers of ice form
in the chilly air.

For some,
winter brings dormancy.

For others, it brings death.

Another cycle of the seasons
comes to an end.

(BIRDSONG)

All across the seasonal world

plants have developed
an array of strategies

for success,

but they all depend on
the seasons changing reliably,

year after year.

Whether you live for years
or just a few months,

everything depends on you
getting the timing just right.

But in our changing world,

this is becoming
a greater challenge

every year...

...and even the most hardy
and resilient of plants

are starting to struggle.

These sequoia trees
are the giants

of the natural world.

The largest living things
on the planet.

They can grow
to almost 100 metres tall

and 11 metres across.

Their bark alone can be
over a half a metre thick.

They're also
among the oldest...

...some living
to over 3,000 years.

And to achieve such age
and size,

they need very particular
living conditions...

...such as those found here,

in the Sierra Nevada mountains
of California.

Not only do they need energy
from the sun,

but, critically,

they also require

up to 4,000 litres of water
a day,

so they are
almost entirely reliant

on the seasonal snowmelt.

But recent years have brought
longer, hotter summers,

and their source of water

is becoming
increasingly unreliable.

Scientists are discovering that

even these seemingly
indestructible giants

are now starting to show
signs of vulnerability.

Some are shedding their needles
and branches

as a way of conserving
precious water.

But for others, climate change
has already been fatal.

10% of them have been lost
in just the last few years.

A single giant sequoia
in its lifetime

can produce 100 million seeds.

Just one cone

contains more than enough

to start
a completely new forest.

But they can only do
such a thing

in a world where the seasons
change with some reliability.

Today, our climate is changing,

bringing an unprecedented level
of unpredictability

all across the seasonal world.

The question is, can we curb
climate change sufficiently

to ensure that the seasons
will continue?

Only if we can do that

will the future
of seasonal plants,

including these magnificent
trees, be assured.