we finally ventured to the moon.

For the very first time, we looked back at our own planet.

Since then, the human population has more than doubled.

This series will celebrate the natural wonders that remain

and reveal what we must preserve

to ensure people and nature thrive.

Forests.

Over half of all the world's trees, evergreen and deciduous,

stand in these great assemblies.

[birds chirping]

[narrator] For many of us, they are places of mystery and darkness.

They are key to our climate, and home to countless unique species.

In the past, we have destroyed them without hesitation.

Yet, forests do have an astonishing ability to recover.

This is the southern edge of the boreal forest,

the forest that dominates the far north.

It stretches eastwards across Russia for thousands of kilometers,

and now, in the middle of winter, it's largely silent,

seemingly empty of animals of any kind.

But this is the home of an animal so rare it's almost mythical.

A Siberian tiger.

There are less than 600 of them.

[caws]

[tiger growls]

[narrator] These are the most intimate  pictures of them in the wild yet taken.

A male Siberian tiger patrols a territory of almost 2,000 square kilometers,

and it has to do so if it is to find enough prey

to keep itself alive during the long winter.

This is an impoverished land where food of any kind is scarce.

A pine cone.

Pine nuts provide vital energy during the winter months.

Wild boar depend on these nuts.

For them, it has to be grab and go,

for they themselves are food for a tiger.

[squeals]

[narrator] This game of hide-and-seek is played out over vast areas of forest,

as all the animals search for the life-giving stands of pine.

Poaching has taken the Siberian tiger to the brink of extinction,

but, since the 1980s, their numbers have slowly increased.

These rare glimpses reveal

that their future still depends on having vast areas of forest in which to hunt.

The boreal forest extends from Russia in the east,

across Europe, to North America.

It contains 750 billion trees,

and it stores over 40 percent of the world's carbon,

making it a vital element in the fight against climate change.

During the winter, the days are so short and the temperature so low,

that growth is at a standstill.

This far north, the forest can support very few animals

compared to the rain forests  of the tropics.

Many are dependent on the supplement to be found in its rivers.

[birds caw]

[narrator] In western Alaska, both in spring and, as now, in autumn,

salmon, having left the ocean,

swim up rivers to reach their ancestral spawning grounds.

Their journey from the sea into the forest represents

the greatest transfer of nutrients from one habitat to another

anywhere on Earth.

The fish are crucial seasonal food

for all the predators that live in this forest.

Bald eagles live here the year round and nest close to the river.

[piercing chirp]

[narrator] Young eagles,  still without white heads,

spend their first years searching for food in the forest.

But, like this four-year-old female,

they're now big enough to try and claim a place on the river.

At first, all she can get are the scraps left by others.

And she can't get even these without a fight.

[eagles chittering]

[narrator] She will have to  look elsewhere.

The boreal forest may be vast,

but the places where the salmon spawn are known to all the predators.

As the rivers begin to freeze over, competition becomes even more intense.

This youngster is lucky, but her success is noticed immediately.

[chittering]

[narrator] She does have one advantage.

Females, even when young, are larger than the males.

And here, size matters.

[cawing]

[narrator] As winter advances,

young and old have to meet the challenge of the intense cold.

If she can survive, she will have gained a place

in one of the greatest and harshest forests on Earth.

A little further south

stand the last  of the great redwood forests.

Not so long ago, these great trees grew throughout the Pacific Northwest.

Now, only five percent of them remain.

Here, conditions are less harsh.

Winds bring in warm, humid air from the Pacific Ocean,

so the trees are able to grow the year round.

Hidden in the vegetation of the moist forest floor

is a richly populated underworld.

A male rough-skinned newt.

Every spring, he is driven by a mysterious urge

to return to the pool where he hatched as a tadpole.

And he needs to get there quickly.

Because there are lots of others like him...

with exactly the same idea.

They're all searching for a female,

but trekking across hundreds of meters of old-growth forest is not easy.

There is strong competition.

And the female is anxious to get going.

Once paired, he hangs on,

for she may take several hours to discharge all her eggs.

Other males are only too eager to displace him if they can.

Competition is so vigorous

that the females in the center of these writhing balls

have been known to drown for lack of air.

But her first partner maintains his grip.

Within weeks, their tadpoles will hatch.

And none too soon, for the forest is about to change dramatically.

By the end of the summer, the sun has baked the forest so intensely

that they are as dry as tinder.

Strong winds drive the flames,

and temperatures rise to 700 degrees Celsius.

Its aftermath is seemingly total destruction.

The wind, scouring the newly exposed forest floor,

kicks up dust devils of ash.

The ground appears to be devoid of life.

But the forest is far from dead.

Within only a few months,

flowers and tree seedlings will rise from the soil.

Many, in fact, would not have germinated had they not received a baptism of fire.

[birds chirping]

[narrator] Light streaming down through the newly opened canopy

provides the energy for a surge of new growth.

The older, well-established redwoods have survived,

protected by their thick, fire-resistant bark.

This natural resilience is essential to the continued health of these forests.

Most forests, however, cannot recover on their own.

Many are helped to recover by animals.

The trees and their inhabitants are interdependent.

And nowhere is this dependence more apparent

than in India's Western Ghats.

This globally important habitat

contains a third of all animal species found in India.

[animals whoop and chirp]

[narrator] Lion-tailed macaques.

They feed on the fruits of many kinds of tree,

and, in doing so, they distribute the seeds,

which will emerge unscathed with their droppings.

But few monkeys travel very far.

If plants are to distribute their seeds across great distances,

they need another kind of transport.

Wings.

Great hornbills have wings that are almost two meters across,

and they enable the birds to travel long distances in search of food.

Hornbills feed on the fruit of 40 or so different kinds of trees,

and transport the indigestible seeds they contain

all over the Western Ghats.

So, it's to the trees' advantage to attract the hornbill's attention.

The figs themselves, although they taste nice,

are not, in fact, very nutritious,

so the hornbills have to eat great quantities of them.

It can be tiring work.

But it's not a free-for-all.

A single fig tree in fruit attracts great numbers of hornbills.

Competition between them is intense.

This aerial jousting between males has never been filmed before

and its purpose is not really understood.

It may be that the birds are simply squabbling over food,

but it seems rather more than that.

[squawking]

[narrator] The winner  is able to  demonstrate his strength and skill

to visiting females.

The males certainly seem keen to ingratiate themselves.

It's particularly important for hornbills to get the best pairing

because, once established, the bond between male and female

will last a lifetime.

The forest also benefits from these squabbles,

because even a defeated hornbill

will spread seeds over wide areas as he searches for his next meal.

The relationship between trees and animals in a forest

is not always so harmonious.

This is Africa's largest forest, the Miombo,

named after one of its common trees.

It stretches for over a thousand miles,

from Angola in the west to Mozambique in the east.

At the height of the dry season,

the Miombo attracts animals from all across southern Africa.

[trumpeting]

[narrator] Elephants prefer grass  if they can get it. It's very nutritious.

In its absence,

they browse on the abundant leaves and branches of the Miombo.

[elephants huffing]

[narrator] But they're not the only  hungry ones here.

These mopane worms

are not worms, of course, but caterpillars.

They hatch simultaneously in huge numbers.

And just as the forest is putting out new leaves,

they begin their attack.

They feed so voraciously

that in just six weeks they increase their size 40 times.

By the time they're fully developed, this million-strong army

will have stripped the entire forest of its foliage.

But the Miombo bounces back.

With the caterpillars gone,

the trees produce a second growth of leaves.

This fresh feast then attracts elephants.

They are less fussy than the caterpillars.

They will eat every part of the tree.

And a hungry elephant can munch through 200 kilos of vegetation in a day.

Yet, even this destruction has its benefits.

It shapes the forest in a way that helps one of Africa's most endangered animals.

Hunting dogs.

They're seen most frequently, and most easily, on grasslands.

But, in fact, these open forests are their preferred habitat.

[grunts]

[narrator] Browsing elephants  open up a forest,

and that attracts the animals on which the dogs prey.

[barks]

[dogs yipping]

[narrator] For the dogs, the Miombo forest is perfect hunting country.

And it's also an excellent place in which to bring up pups.

The pups greet the adults returning from a hunt

with great excitement.

[squealing]

[narrator] Food.

The first three months of the pups' lives are spent sheltering underground.

[soft growling]

[narrator] Now, they are confident enough

to stay out in the open for much of the time.

And then, like all puppies, they have fun.

Playing is important for the youngsters,

for as they do so, they establish the social bonds

that they will need when they start to hunt together as a team.

-[bird squawks] -[squealing]

[narrator] The Miombo has always been  important for hunting dogs,

both as a place to find prey and as a refuge.

Never has it been more crucial

for the survival of this endangered species than now.

But they will only survive if other creatures are here

to create the kind of habitat they need.

[elephant lows]

[narrator] Madagascar has a forest

dominated uniquely by one of the oddest of trees.

Baobabs.

The island has been isolated for over 80 million years.

During that immensity of time, its animals and plants

have evolved into forms quite different from any elsewhere.