Original subtitles

Trees, surely among the most magnificent of all living things.

Some of the largest organisms on Earth, dwarfing all others

and these are the tallest of them all.

The deciduous and coniferous woodlands that grow in the seasonal parts of our planet

are the most extensive forests on Earth.

Their sheer extent stuns the imagination.

The barren snows of the Arctic.

A thousand miles from the North Pole and heading south.

This is the very first place that trees can grow.

To begin with, the conifers are sparse but soon they dominate the land.

This is the taiga forest.

There are as many trees here as in all the world's rainforests combined.

The taiga circles the globe and contains a third of all the trees on Earth.

It produces so much oxygen that it refreshes the atmosphere of the entire planet.

At the taiga's northern extent,

the growing season can last for just one month a year.

It can take 50 years for a tree to get bigger than a seedling.

It's a silent world where little stirs.

But there are occasional signs of life, stories written in the snow.

The prints of an Arctic fox and the hare it might have been stalking.

A female polar bear and her two cubs.

Some animals are so difficult to glimpse that they're like spirits.

One could live a lifetime in these woods and never see a lynx.

The cat must roam hundreds of miles in search of prey

and may never visit the same patch of forest twice.

It's the very essence of wilderness.

With so few prey animals here, life for a hunter is particularly hard.

Creatures are scarce because few can eat conifer needles.

The moose is an exception.

Growth is so difficult

that conifers protect their precious leaves by filling them with resin.

That reduces water loss, but it also make them very distasteful.

At least the conifer's seeds are edible.

But they're protected within armour-plated cones and it takes a specialist to reach them.

The crossbill's extraordinary beak can prise apart the scales

so that its tongue can extract the seeds.

Birds are fortunate.

When the seasonal crop is gathered, they can fly south.

But one animal is so expert at survival in this frozen forest

that it stays here and is active all year long.

In local folklore, the wolverine is a link to the spirit world

and a cross between a bear and a wolf.

In reality, it's a huge weasel.

Its bulk helps to conserve body heat and also broadens its menu.

It's so big and powerful, it can even bring down an adult caribou.

For its size, it's said that the animal can eat more in one sitting than any other.

Which is why it's also known as the glutton.

Being gluttonous here is a very effective strategy.

It's wise to eat all you can when you can.

And when even a glutton can't eat more,

it stores what's left for later in the surrounding deep freeze.

Spring in the ice forest.

The capercaillie can also digest conifer needles.

But feeding is not its priority at the moment.

Like gladiators, the males square up for a battle.

Each may have just a single chance to impress a female.

Neither can afford a lapse in concentration.

The injured loser may not survive.

The inhabitants of this great wilderness

may live and die without ever having contact with humanity.

Long may it be that way.

The northern forests may be the largest on Earth,

but to see coniferous trees that have reached their full potential

you must travel 1,000 miles south of here.

The Pacific coast of North America.

The land of hemlock, Douglas fir and giant redwood.

Here water is never locked up in ice.

And even if rains fail,

the needles can extract moisture from the fogs that roll in from the sea.

The sun's energy powers these forests, not for one month as it does in the taiga,

but for half the year.

These conifers grow at ten times the rate of those near the Arctic

and they live for thousands of years.

One grove of redwoods in California contains three of the tallest trees on Earth.

This one is over 100 metres high, the size of a 30-storey building.

These forests were growing here long before humans walked the Earth.

They were in their prime 20 million years ago

and existed before the Swiss Alps or the Rocky Mountains were even raised.

There is more living matter in a forest of giant conifers

than in any tropical rainforest.

But it's all contained within the trees.

These are as inedible as those in the taiga.

So animals are still scarce, but they are present.

A pine marten.

It's spring, the best time of year for a marten to find food.

Birds' eggs are a seasonal snack and for a short time there's plenty of them.

Sometimes perhaps too many.

But to live here permanently, the marten needs a more reliable food source.

Squirrels fit the bill.

They thrive here on the pine cones

and although these are also seasonal, they can be stored and eaten throughout the year.

The squirrels are busy mating.

Good news for the hunter. A distracted squirrel is a vulnerable squirrel.

But this time the amorous couple are safe.

There is a loner stocking his larder who will be the easier target.

Early summer and great grey owl chicks are fledging.

Adults can only raise young here

in years when the seasonal vole crop is big enough to support them.

The moment has arrived for their first flight.

Leaping from the world's tallest trees is not for the faint-hearted.

If you're going to fall here, it's quite a good idea to do it in stages.

The ground is no place for an owl.

If he's to climb to the top of his class, he'll need to persevere.

So now, let's have another go.

The American conifer forests may not be the richest in animal life

but their trees are extraordinary.

This giant sequoia, a relative of the redwood, is the largest living thing on Earth.

Known as General Sherman, it's the weight of ten blue whales.

Higher up in the nearby mountains, bristlecone pines.

The oldest organisms on the planet.

Some have been here for 5, 000 years.

They were alive before the pyramids were built

and were already 3, 000 years old when Christ was born.

Across the equator in the southern hemisphere, there are forests that mirror those of the north.

Here in South America, araucaria trees, or monkey puzzles, are like the conifers of the taiga.

They have waterproof scales instead of needles and their cones look a little different,

but the principles are the same.

Slender-billed parakeets rather than crossbills extract their seeds.

Where the growing season is longer, there are alerce trees, the redwoods of the south.

As in the frozen north,

the Valdivian forests of Chile support very few animals.

But that is the end of the similarity.

This is a bizarre world of miniature creatures.

The pudu, the world's smallest deer, feeds on the giant leaves of the gunnera plant.

The female is just 30 centimetres high at the shoulder

and her infants are hardly bigger than kittens.

The male must stay alert.

There are hunters here who would snatch his young.

Another miniature, the kodkod cat.

It's the smallest cat in all the Americas and a young pudu would be a feast for it.

But with the male on guard, the kodkod must lower his sights.

Moths are hatching. They're the last of the summer.

The tiny cat should be able to score with these.

No one knows why the creatures here are so small, but at least they can survive on meagre rations.

You might call this a game of cat and moth.

As winter approaches in Chile, spring is arriving in the northern hemisphere.

These are the deciduous forests of home.

Dormant throughout the winter,

they now undergo one of the most magical transformations in the natural world.

By late spring, the landscape is wrapped in a vibrant fresh green.

Here, instead of conifers, there are broad-leaved trees

and that makes the character of this forest entirely different.

Being broad, these leaves trap much more light than needles.

But they're also thin, soft and edible.

And others can eat the leaf-eaters.

It's spring in the great broad-leaved forests of Eastern Europe and Asiatic Russia.

The mandarin ducks are calling.

The female mandarin nests in a tree hole.

And when it's time for everyone to leave, she leads the way.

The ducklings are only 24 hours old.

It's a long drop and a few calls of encouragement are required.

Two down, seven to go.

There are still two missing.

All present and correct.

But they won't be safe until they reach water.

And the forest pool is almost a mile away.

By June, the days are at their longest

and all across the northern hemisphere the broadleaves are hard at work.

On the east coast of North America, it seems like any other summer's evening.

But tonight is special.

After 17 years underground, creatures are stirring.

The nymphs of the periodical cicada have been biding their time.

Now they march like zombies towards the nearest tree and start to climb.

At first there are merely thousands.

But soon more than a billion swarm all over the forest.

The biggest insect emergence on the planet is underway.

They invade the upper branches where they climb out of their external skeletons

and assume their adult winged form.

At first, they're white and soft.

But they have until dawn to complete their transformation.

After an absence of 17 years, the forest is now overrun by cicadas.

The adults are clumsy and very edible.

For turtles and other inhabitants of the forest,

this is a feast they're lucky to see once in their lifetime

and they gorge themselves while they can.

Times have never been so good.

The cicadas have no defences and virtually offer themselves to their attackers.

The stream of insects is so relentless

that soon all the predators are full to the point of bursting.

And still the cicadas come.

With the predators overwhelmed, the survivors can achieve their purpose.

After mating, the adults lay their eggs and then their job is done.

In just a few days they will all die and the forest will fall silent.

The cicadas here will not be heard again for another 17 years.

Having fed the predators, the cicadas leave one final gift for the forest itself.

The nutrients in a generation of cicadas are returned to the soil all at once.

And the trees enjoy a marked spurt in growth.

This may be the single largest dose of fertiliser in the natural world.

In the great broadleaf forests of Eastern Europe,

the days are beginning to shorten

and a primeval sound heralds the onset of autumn.

Male red deer are starting their rut.

The air is heavy with the scent of females.

The rules are simple, winner takes all.

Across the northern hemisphere, the deciduous forests are changing.

Leaves that have provided food and shelter since the spring, are now shed.

In the broadleaf forests of Russia, winter is particularly severe.

But there will always be some who benefit from hardship.

Black vultures scavenge from the carcass of a seka deer

that has died of cold or starvation.

These endangered birds are visitors,

they've come down from the north to escape the even colder conditions in Siberia.

An Amur leopard, the rarest cat on Earth.

Winter is a difficult time for this hunter.

There are no leaves for cover and no young prey animals.

This female has the added pressure of having to provide for her one-year-old cub.

It will be another 12 months before he'll be able to fend for himself.

The bickering vultures have abandoned the carcass.

It's a valuable discovery for the leopards.

But the cub doesn't share its mother's sense of urgency.

The vultures have left behind plenty of good meat, but it's stiff with frost.

The mother works to open the hide and make feeding a little easier for her cub.

There are only 40 Amur leopards left in the wild

and that number is still falling.

The harshness of the winter here, hinders their increase in numbers.

It takes one of these females longer to raise her young to independence

than it does a leopard in Africa.

If the mother can sustain her cub for a few more weeks,

spring will bring an increase in prey and her task will lighten.

For all the inhabitants of this seasonal forest,

the long, cold wait is nearly over.

Spring in a deciduous woodland is special.

With no leaves overhead, the rays of the sun strike the forest floor directly

and their warmth rouses plants from their winter sleep.

The ground-living plants are in a hurry.

Before long, the trees above will come into leaf and steal their light.

Their flowers decorate the forest floor

as they advertise their sweet nectar to the newly emerged insects.

The spring blooms of the deciduous woodlands have no equivalent

in either the great conifer forests or the tropicaljungles.

Within a matter of weeks, the canopy has closed

and only a few wheeling shafts of light penetrate the woodland.

In the tree tops, the broad leaves rapidly expand to their full size

to make the most of summer while it lasts.

Then, after a few months, the days begin to shorten again

and the trees must shut down and shed their leaves

in preparation for the cold, dark time ahead.

Great tracts of North America flush red as the season progresses.

The effect is so spectacular and so extensive that it can be seen from space.

The threat of winter frost is not the only reason for trees to shed leaves.

These forests stand in the tropics.

Here, day length never changes.

But the dry season is so severe

that the trees can't afford to lose the amount of water

that would evaporate from their broad leaves,

so the leaves must be shed.

The forest resembles a European woodland in midwinter

but the heat is overpowering and its inhabitants unfamiliar.

For the creatures of India's teak forests these are desperate times.

But salvation is at hand.

The mahua tree is about to bloom.

Its flowers are full of liquid, making them irresistible.

The mahua is an oasis in a hot, dry desert.

Those that fly or climb are not the only ones to get a share.

Chital deer follow the Langur monkeys, collecting the flowers that fall.

The monkeys welcome the deer, for deer are unrivalled at spotting predators.

If they are relaxed, it must be safe to come down to the ground

and gather the food that lies there.

But it's not wise to travel far from the sentinels.

Tropical Madagascar, the wet season.

It's now that the baobab trees re-grow their leaves

and collect water to store in their huge trunks ready for the dry season ahead.

The prehistoric shape of these trees is rightly famous

but few have ever witnessed the baobab's real magic.

For that happens at night and high in the tree tops.

Flush with water, the baobab prepares itself for an unforgettable display.

Once started, the foot-long flowers can open fully in less than a minute.

As the flowers open, the creatures of the forest wake.

The mouse lemurs have been hibernating throughout the dry season.

With the return of rains, it's time to get busy.

A dozen share this tree-hole, but there's plenty of room.

The world's smallest primate is no bigger than your hand.

High in the branches above, the baobab's nectar is starting to flow.

A drink of this sugary, energy-packed liquid is an ideal way for the lemurs to start their day.

Liquid oozes from the flower's centre and trickles down the petals.

But the nectar is not intended for lemurs.

These giant hawk moths are the drinkers the tree needs to attract.

As they sip, moving from tree to tree,

so they transfer pollen and fertilise the flowers.

Nectar was an excellent first course for lemurs,

but moths are the main dish.

The moths are very important to the lemurs for they will replenish the fat reserves

that the lemurs need to survive the barren dry season.

The lemurs might seem to be a pest for the baobab,

they kill its pollinators and rob it of its nectar but they do give something in return.

For as they wrestle with the moths, their fur inevitably becomes dusted with pollen.

So they too become pollinators.

As the alternation of wet and dry seasons brings change to some tropical forests,

so the progression of summer to winter dictates life in more temperate regions.

Whether trees have needles or broad leaves,

it is their ability to survive annual change

that has enabled them to cover such vast areas of the Earth

and made the seasonal forests the greatest forests of all.