All language subtitles for BBC.The.Green.Planet.4of5.Desert.Worlds.1080p.HDTV.x265.AAC.MVGroup.org

The Taklamakan Desert
in northern China,

a constantly shifting landscape
of sand.

WIND WHISTLES

Temperatures that swing from -20

to more than
40 degrees centigrade...

..and, most critically,
almost entirely without rain.

Yet, here on the dunes...

..a Euphrates poplar tree.

And it's not alone.

Some of these trees have lived
here for 1,000 years.

They have exceptionally long roots
with which to collect water,

and what is more, those roots are
connected to neighbouring trees,

so that if one strikes water,

others can share it.

In every desert across the planet,

plants have found ways
to not only survive,

but flourish.

This is the Gran Desierto
of Mexico and the United States.

These dunes may appear
to be totally barren.

In fact, they are full of life.

In the sand beneath my feet

there are seeds
of many different kinds.

In fact, you could say that the dune
itself is one great seed bank,

and when it rains,
it bursts into life.

WIND WHISTLES

THUNDER CRACKLES

But rain may come only once
a decade, and even then,

the long-awaited storm
may be very brief.

So seeds must respond immediately.

This is sand verbena.

It can grow from a seed to a
sweetly-scented flowering plant

in just a few weeks.

Primroses and many other plants
soon join the race

to flower before the sand dries.

Desert blooms like this,
however, are rare.

This is the first year for 20 years.

The combination of vibrant colour

and powerful scent
attracts migrating pollinators,

such as these
Painted Lady butterflies,

which fly into the middle of what
were, only recently, barren dunes.

Everything is rushing
to complete their lives

before the moisture has gone.

Such spectacular blooms transform
deserts all around the world.

From the Atacama in South America...

..to the dusty plains
of southern Africa.

Rain in deserts, however,
never lasts long,

and all too soon,
the flowers wither and die.

But not before they've produced
the next generation...

..the seeds that will now wait
in the sand for the next rains.

In the Sonoran Desert
of North America,

the huge saguaro cacti
have a different strategy.

They store water in quantity

and can live to a great age.

But in their early years,

they are extremely vulnerable.

This little saguaro cactus
is about ten-years-old.

When they're really small and
growing out in the open,

there's a real chance that they may
shrivel up and die.

But this one has been lucky.

It's been growing in the shade
of this mesquite tree,

and it's got a very good chance
of surviving to maturity.

The young saguaro is protected
by the mesquite's branches.

They halve the amount of scorching
sunlight reaching the cactus...

..and so, keep it cool.

And the mesquite's extremely
long roots draw up water,

bringing it within reach
of the young saguaro.

So the mesquite
is known as a 'nurse plant',

and a very effective one it is, too.

In fact, this particular mesquite
has already nurtured

seven young saguaros
over the past 30 years.

As a young cactus grows,

it needs the protection of its nurse
plant, not only from the heat,

but from the other hazards
of desert life.

Temperatures can drop
to -10 degrees overnight...

..and very occasionally...

..it even snows.

If the water stored inside a young
cactus should freeze,

the cactus will die.

But the nurse plant traps a blanket
of slightly warmer air around it,

just enough to keep it alive.

Eventually, saguaros outgrow
their nurses,

but by that time,

they are robust enough to face
the elements by themselves.

No matter how old a desert plant
is,

water is always precious,

whether gathered
from melting snow...

THUNDER BOOMS

..or a shower of rain.

So, cacti have developed
extraordinary adaptations

that enable them
to not only collect water...

..but to retain it.

Instead of leaves,

which would lose precious
moisture through evaporation,

they have spines.

Each spine
has a tiny pad at its base

where the water is absorbed...

..and then stored
in the great swollen trunk.

A large saguaro can
hold 5,000 litres of water

and is able to do so because it
has another special adaptation.

The ridges on its surface
are like the pleats on an accordion.

They allow the saguaro
to change its shape.

After rain has fallen,
the pleats expand

and the saguaro fills
up its water tank.

In the dry times,
it uses its water to grow,

produce flowers and,
eventually, seeds.

Fully-loaded with thousands
of litres of water,

this saguaro won't need to drink
a single drop for another year.

But such valuable stores
of water attract thieves.

Now the spine's function
changes from collecting...

..to guarding.

The spines of some species
are a quarter of a metre long.

Others are needle-like barbs

that grow in clusters and easily
break off in the skin of any animal

that touches them.

But perhaps the most vicious cacti

belong to a group
called the chollas.

This is called a teddy-bear cholla

because of the thick coating
of spines on it.

But don't be deceived by the name,

there is nothing cuddly
about THIS particular teddy bear.

In fact, it's the most dangerous
plant in the desert,

and I wouldn't dream of
putting my hand anywhere near it

without proper protection.

Brush against it...

..this can happen.

Ow!

This can happen,
even with this glove on.

One of them has just gone through,
I can feel it.

It's quite painful!

Look closely at the spine
and you can see very clearly

why they're so dangerous.

Each is like a splinter of glass,

sharp enough to pierce flesh...

..and they're covered
with backward-pointing barbs.

So getting them out, even with a
pair of pliers, is quite hard.

This is not pleasant at all.

It won't come off, or without.

Oh, look at that!

It's hard to imagine a more
aggressive defence than this,

and it makes both the plant and its
buds virtually invulnerable.

Most animals know to keep clear.

Cholla buds grow like tiny barrels
from the top of the adult plant

and then drop off.

BARKS

If the young cholla
put down roots here,

it would compete with
its parent for water.

Night falls...

..and this one is on the move.

A pack rat. She knows how to deal
with a cholla.

She avoids the spines by gripping
it at the place

where it broke off from its parent.

And she works fast.

There are pack rat hunters here.

She uses the cholla to build a spiny
wall around her nest.

The flesh of the cholla
supplies her with water...

..and the severed spines
further reinforce the defences.

This cholla bud...

..might be next.

But one accidental nudge...

..and it escapes.

DRAMATIC MUSIC PLAYS

The bud starts to put down roots.

So, the cholla,
thanks to the pack rats,

finds new territory

and sets about claiming it.

Few plants deal with the problems
of desert living better than cacti.

There are almost 2,000 different
species of them.

They're spread across the deserts
of the American West,

from Arizona,
all the way to Mexico and beyond.

In South America, the ice-covered
peaks of the Andes

act as a rain barrier...

..beyond which
lies the world's driest desert -

the Atacama.

In the desert world, water thieves
can come in many forms

to exploit even the smallest chink
in a plant's defence.

One of the strangest
travels within the gut

of a fruit-eating mockingbird.

These are the seeds
of Tristerix,

a kind of mistletoe.

Their goal is the water
inside this hedgehog cactus.

Using the spines as anchors,

the seeds start to germinate.

Each produces a long probe

with which to try and locate
the cactus' skin.

For most,
that's a stretch too far,

and they perish.

But for this one, the cactus'
surface is within reach.

It clamps onto it
with a special sucker...

..and then waits for darkness.

At night, the cactus opens its pores
in order to respire.

Oxygen goes out,

carbon dioxide goes in,

and so does Tristerix.

Once within, its tissues spread
throughout the body of the cactus,

sustained by the precious store
of water that they find there.

Then, a year later,

it breaks
through the cactus' skin...

..and bursts into flower.

Hummingbirds come to drink
their nectar

and pollinate them as they do so.

And, then, to complete the cycle,

Tristerix produces hundreds
of white, eye-catching seeds,

ready to be carried away by a bird

to invade another cactus.

The Karoo Desert in southern
Africa.

And although it may look bare,

its rocky ground contains an
unrivalled variety of plants that,

one way or another,
store water in their tissues.

They belong to many different
families, but as a group,

they're known as succulents.

Some are small and low
and barely distinguishable

from their surroundings.

These look like little pebbles.

They resemble them so closely

that animals which might be only too
glad to steal their water

just pass them by.

When rain does fall,
they absorb it and quickly expand.

But even this
doesn't spoil their disguise.

They just look like larger pebbles.

Nor are they green.

The cells on their top surface
are transparent

and allow sunlight to pass through.

Deep within and out of sight

are the green cells
where photosynthesis occurs -

the process which uses this light
to make food for the plant.

When the time comes to reproduce,
however,

the stone plant
abandons its disguise.

And now, it blooms.

The flowers open and close
every 24 hours.

So for a few dangerous days,

the plant advertises for pollinators

before returning to life
as a pebble.

Some desert plants
have developed a very different way

of attracting pollinators.

This is a Stapelia.

It produces what is perhaps
the desert's strangest disguise.

It uses water stored in its stems

to grow buds
the size of tennis balls.

The flower, once opened,
is called a desert starfish.

Instead of releasing millions
of loose pollen grains

as most flowers do,

the desert starfish produces them
packed in five tiny sacks.

But if its strategy is successful,

just one of them
will produce hundreds of seeds...

..and this depends on deception.

The flower appears to have hair...

..wrinkly skin...

..and it produces a stench
like the carcass of a dead animal.

And when a carrion fly
investigates...

..the flower tempts a tiny sack
of pollen to its proboscis.

It isn't easy to feed
with such encumbered mouthparts.

But try as it might,
the fly can't get rid of it.

And it's still there when the fly
leaves to try and feed

from another bogus carcass.

This time, however,
when its clamped-up proboscis

slots into the flower,
the pollen sack is released.

With pollination complete,

the fly is no longer needed...

..and just as well.

Some deserts can be so dry
that plants must find techniques

of surviving for long periods
without any water whatsoever.

One of them
is to grow extremely slowly,

and few plants grow more slowly
than this one -

the creosote bush.

It is inactive for most of its life

and only wakes up and grows
for a brief period,

if and when
there is a fall of rain.

I've seen evidence of this
"grow slow" strategy for myself.

40 years ago, I came here
to California's Mojave Desert

to visit one particular plant.

ARCHIVE: An individual
creosote bush

tends to spread
not by setting seeds

and producing a new generation,

but by sending out new stems
around its base.

This plant started growing between
10,000 and 12,000 years ago.

NOW: That was in 1982.

Since then, careful measurement
has shown that it has increased

its size by less than one inch.

Its ability to endure
is truly extraordinary.

So efficient is creosote
at collecting what little rain

falls here, that few other plants
can compete with it.

As a result,
over the last 12,000 years,

it's come to completely
dominate this landscape.

The Chihuahuan Desert
in north Mexico.

Here, one particular plant
plays the waiting game so well

that it spends much of its life
looking dead

and certainly not worth eating.

And it can survive like this
for a decade.

This is the resurrection plant.

It's a kind of moss.

It barely has roots, and it
certainly can't store much water,

but it can travel.

After a particularly long drought...

..it breaks away from its roots...

..and becomes...

..a tumbleweed.

Blowing across the desert,

it can travel a mile in a week.

With luck, it may find water.

THUNDER RUMBLES

Just a shower of rain
can bring it back to life.

As its fronds soak up the water,
they unfurl.

In its protected centre,

it still has green cells

which absorb both the water
and sunlight

and rapidly produce the food
it needs to resume its growth.

It will grow for just as long
as there is moisture...

..but when that disappears...

..it closes up once more

and resumes its travels.

Here in the Canyonlands of Utah

lives a plant that has developed

a finely-balanced relationship

with the animals with which
it shares this dramatic desert.

Rain does occasionally fall here

and turns dust into mud.

But that doesn't last long.

A brief window of opportunity opens.

Seeds that have been buried
for years

may now be exposed to light

and come to life.

This is coyote tobacco.

In just a few weeks,
it grows a metre tall

and produces dozens of flowers.

The night air becomes heavy
with their fragrance.

Soon, they attract hawk moths

which sip their nectar,

and in doing so, pollinate them.

But the moths also
lay their eggs on them.

Soon, the caterpillars have hatched

and are munching the leaves.

Their nibbles expose the plant's
sap to the drying air.

But the tobacco plant has a defence.

The leaves under attack
produce nicotine.

This chemical
sedates the caterpillars

and slows them down.

And what is more, it makes them give
off a particular scent...

..one that summons others

to come to the plant's aid.

Big-eyed bugs,

miniature assassins,
only two millimetres long...

..and whiptail lizards.

Big or small, they make
a meal of the caterpillars.

It's certainly effective...

..but there's more to this strategy
than meets the eye.

When the leaf of a tobacco plant
is attacked by a caterpillar,

all the rest of the leaves
prepare to defend themselves.

But how does this leaf know

that that leaf there
is under attack?

Well, scientists here
in the United States

have specially genetically modified
these tobacco plants

so that under special
lighting conditions,

this microscope can show us
exactly what is going on.

I'm going to attack one
of the leaves of this plant

with these tweezers which,
to the plant,

will seem as if it's being nibbled
by a caterpillar.

Signals are being transmitted
along the veins

that link the leaf
to the rest of the plant.

It's rather like
a very simple nervous system.

From that initial injury,

the whole of this little plant is
aware that something has happened.

This signal warns each leaf
of the danger,

so that it is ready
to produce nicotine

the moment it's attacked.

With this defence at the ready,

the tobacco plant
can continue to grow

until eventually, it produces seeds.

It's particularly important
in deserts

for seeds to be distributed as
widely as possible

so that some will have a chance
of reaching moisture.

And deserts have an excellent
agent to help them do that -

the wind.

Many seeds have adaptations
to help them exploit it.

They have shell to protect
the seeds within from abrasion

or wings to help them catch the air.

As the temperature rises
throughout the day,

desert winds increase in strength.

Here in Arizona,

the land is regularly swept by what
is known as a haboob.

It's a giant sandstorm,

but also, in effect, a seed storm.

Countless millions of them
are swept up into the air.

Some seeds can travel
thousands of miles on the wind...

..so that plants may eventually
reach even the most isolated desert.

Some have landed on an island

in the middle of the world's
largest salt flat in Bolivia.

In the Galapagos, they sprout
on fields of recently-erupted lava.

They've even reached one of the most
inhospitable of all sites -

the tiny island of San Pedro Martir,

a scorched, lonely rock

off the coast of Mexico.

This is the home of
the giant cardon,

a species of huge cactus

that can weigh up to 12 tonnes.

They're able to thrive here because
of an extraordinary partnership...

..with brown and blue-footed
boobies.

The cardones here
can become so broad

that they provide cooling
shade for nesting birds.

As the booby chicks get older,

they repay the cardones...

..with their droppings...

..guano -

the digestive remains
of vast shoals of fish.

This guano is of such
strength and quantity

that most plants
would be poisoned by it.

These cardones, however,
have evolved the ability

to tolerate the toxins in the
guano and digest the nutrients.

As a result, the cacti
now grow in a dense forest

over a million strong.

But such relationships
are very finely balanced

and can only too easily
tip into catastrophe...

..as is now happening
in northern Zimbabwe.

For six months of the year,

the savannah here is kept lush
and green by daily rains.

ELEPHANTS TRUMPET

But when the rainy season is over,

it becomes as dry as any desert.

So to survive here, trees must be
able to tolerate both conditions.

And these giants
are adapted to do just that.

They are baobabs.

This one might be over
1,000 years old.

It survives here in part
thanks to its ability

to store thousands
of litres of water

within the spongy wood of its trunk.

But its battered surface is evidence

of a very finely-balanced
relationship.

These huge trees

are a focus for animals
of all kinds.

And they're particularly
important for elephants.

In the wet season,
they eat the baobab's fruit

and disperse the
seeds in their dung.

Now as the dry season begins...

ELEPHANT RUMBLES

..they migrate
to distant watering holes.

The baobabs have damp inner wood...

..and the elephants use it to
quench their thirst on the journey.

ELEPHANT ROARS

This relationship can only
work because baobabs

have a remarkable
ability to heal themselves.

Between each damaging attack,

they expand their spongy
wood and grow new skin.

And they've done this
time and time again over centuries.

Today, however, it's harder
for the baobabs to recover

as dry seasons become longer
and drier due to climate change.

Not only that, but the elephants
are forced to take ever more wood

from the trees in order to survive.

In some parts of Africa,

many of the largest
and oldest baobabs

have fallen in the last decade.

The loss of a vital species
like the baobab

strikes a blow at all life
in the desert.

In these hostile lands,

few living organisms can survive
without help from others.

You can find an extraordinary
illustration of this

in Arizona's saguaro country.

If you wander off
the beaten track here,

you may be lucky enough
to find one of these.

It might look like an old boot,

but in fact,
it's part of a saguaro cactus.

Almost every saguaro has one.

There's one up there.

It has been produced
indirectly by woodpeckers,

which regularly dig homes for
themselves in the bloated trunks

of the saguaros.

In the next six months,

the cactus heals the wound

and so creates a safe,
cool, and watertight nest hole.

Its tough lining
will persist for years,

even after the cactus itself
has died and rotted away.

A single saguaro may hold several
of these extraordinary homes.

So over its lifetime, it may provide
accommodation for some 3,000 chicks

of several different species.

BIRDS SCREECH

But in the long term,
the saguaro benefits,

as their lodgers repay the cactus

by pollinating its flowers...

..and dispersing its seeds.

It's relationships like these

that enable life to flourish

in some of the world's
harshest landscapes.

Over millions of years,

plants have become superbly adapted

to hostile desert conditions.

But it's a very
finely-balanced existence

and one that makes them
uniquely vulnerable.

However, our growing understanding
of the complex ways

by which desert animals and plants
rely on one another

is now helping us to understand
how best we can protect them.

90 years ago, a photograph
was taken from this very spot

that shows a population
of saguaro cactus

that was very different
from what it is today.

In the last 50 years,

the population of saguaros
here has greatly diminished,

not because of a direct
assault on the cactus,

but because many of the
shade-giving nurse trees

were harvested for firewood, leaving
young saguaro to die in the sun.

Now that this relationship
is understood

and the nurse trees are protected,

there are already signs
that the saguaro are recovering.

Wherever there's a desert, plants
have evolved to meet its challenge.

But everywhere, they need our help.

As we understand more about them

and their intimate and
complex relationships...

..we will be better able to
protect them and all life

in these beautiful but
increasingly fragile worlds.

The most remote shoot
for the deserts team

is to the island of
San Pedro Martir

in the Sea of Cortez, Mexico.

They are to film the giant cacti

that have found a unique way
to thrive here.

It takes 48 hours
to travel the 260 miles...

..over unpleasantly choppy seas.

It's not made some of
us feel very well.

I think one of us is quite seasick.

It's nothing but big blue out there,

and the island is
somewhere in that direction.

Can't see it yet.

As they draw close to the island,

there's a spectacular reminder of
how rich life in the sea is here.

Oh, look in front of us!
Look at that for a pod!

Oh! They're everywhere!

Show us the way.

HE CHUCKLES

Meeting them here
is desert scientist Ben Wilder.

It's through his research
that the team first heard

about the island's
peculiar residents.

I'll never forget
the first time I came

to, actually, exactly where
we're standing right now.

It was April of 2006,

and saw this view.

And it both settled in my
heart and captivated my mind.

And so that started a process
of trying to understand,

you know, what can produce this?

The secret is in the
relationship between the cactus

and a type of
sea bird called a booby.

All the boobies that we need
to film are right up there

at the top of the island.

So, it's going to be
a bit of a scramble.

The crew soon discover how
harsh the conditions are here.

I'm pretty tired. It's pretty hot.

It's a great view.

There's very few places on Earth

where you're going to
see this many cactus.

I mean, it's absolutely amazing.

I would say this is the only
place you're going to see this.

Well, there you go, then.

Ben's research is uncovering
the ingenious ways the cacti

have adapted to the conditions here.

The waters just offshore here
are some of the most productive

marine waters in the world.

And so it's kind of an ideal
habitat for sea birds to roost.

But when they do so,
they deposit tonnes of guano.

And so those nutrients, really high
in nitrogen and phosphorus,

actually are toxic
to most plant species.

The cardones thrive because
they're uniquely able

to process the guano and extract
what they need to fuel their growth.

One of the first shots
the crew need to get

is of the boobies
nesting under the cacti.

The lack of predators means
the birds aren't afraid of people.

It should make filming
them up close a bit easier.

That's the theory.

So we've positioned this camera here
to try and get a good perspective

on the chick on the nest.

It's taken quite a
liking to our camera.

Let's hope he doesn't break it.

Bull's-eye.

It's pooed right on
the front of the lens!

It's kind of a shot we need,
but unfortunately,

my camera wasn't rolling
at the time it did it.

So now, I've just got a dirty lens.

After a thorough lens clean,
Ollie eventually gets the shot

to reveal the extraordinary
relationship between bird and plant.

But it's not just the
bird guano that influence

how the cardones grow.

They're way shorter,
they're dwarfed here,

throughout the rest of their range,

they usually get in
upwards of 50-60 feet.

But here, on average,
they're 20-24 feet in height.

They stop growing up,
and they grow out.

Ben's research suggests
that they go wide here

as an adaptation
to the violent winds.

Too tall, and they'd blow over.

A gusting wind isn't
helping the drone crew, either.

We really need it
to calm down a bit,

otherwise it's
going to be impossible.

WIND GUSTS

BIRD CHIRPS

On the Sonoran mainland, where the
cardon is also found,

you have on average between
50 to 150 plants per hectare.

Here on this island, you have
over 2,500 plants per hectare.

A lull in the wind, and the
team get the chance to reveal

the remarkable density of the cacti,

almost 20 times greater
than anywhere else.

It appears that here
they're doing very well.

But even this island isn't isolated

from the effects
of a changing planet.

Every cactus you see there,

its body is filled with
nutrients from the sea.

Given that we know that the
cardones are linked to the ocean,

what happens in the oceans
affects what happens to the cardon.

So a concern we have right now is
that there's a lot of overfishing,

and we know there are less sea birds
here than there were 20 years ago.

And we have reason to believe
then that will ripple and affect

the nutrients that fuel
the cardones as well.

This finely-balanced relationship
is at risk,

and knowing that
makes leaving the island

particularly thought provoking
for the crew.

The cardones look beautiful.

I've never seen so
many cactus in my life.

It's absolutely amazing.

I'm just hoping that when
we leave this place,

it stays as it is, and things
don't affect it that are negative.

It's an absolutely wonderful thing,
and, yeah, I don't want to go.

I want to stay.

This remote cactus forest is
a reminder of the adaptability

of plants that enables them
to establish green worlds

almost anywhere on Earth.

Next time on The Green Planet...

..our extraordinary relationship
with plants.

From those who eat...

..to those we help...

..plants are our greatest allies.

The Open University
has produced a poster

that explores the vital role
that plants have for our planet.

To order your free copy call...

Or go to...

..and follow the links
to the Open University.