BBC How Nature Works: Seasonal Forest

Planet Earth is home to 10 million different species.

But none can survive on its own.

This lynx needs a caterpillar.

This fungus needs a flying squirrel.

And this whole forest needs a fish.

Unexpected relationships like these enable animals and plants to survive in a world of extremes.

They power all life in the seasonal forests.

Seasonal forests wrap around the northern hemisphere,

and cover a quarter of all land on the planet.

Crucial to all life on earth

they release 9 percent of all the oxygen that we breathe.

But for these vital ecosystems to survive

the animals and plants that live here must overcome a deadly challenge.

As the earth moves round the sun, over the course of a year,

the amount of the sun's energy reaching higher latitudes varies,

powering dramatic seasonal change.

But some creatures have formed relationships that allow them to thrive against the odds.

Two of the most successful can be found in the great forests of North America.

On the east coast are the great North Woods, the land of the lynx.

On the west is the rain coast forest, the land of the grizzly bear.

These predators play a pivotal role in the success of all life in their forests.

But surprisingly they depend on an organism that's not an animal or a plant.

The best time to see it is in the autumn.

It's fungus.

Feeding on rotting leaves, fungi of all kinds are bursting into life.

Fungi are the ultimate decomposers,

without them forests would suffocate under layer upon layer of dead leaves, animals and plants.

But more importantly fungi unite all the plants and animals in every forest.

Across North America they have a helping hand from a spectacular performer.

One that only reveals itself at night.

The flying squirrel.

By gliding from tree to tree he's covering a greater distance and avoiding predators.

Like all other mammals he's in a race against time to find food before the winter.

From the safety of the tree, he's searching for fungus on the forest floor.

But his favourite part is actually underground.

It's a truffle, the fruiting body of the fungus, packed with spores.

He continues his search carrying the spores in his gut.

Gliding up to 30 metres at a time from tree to tree.

As he defecates the spores disperse throughout the forest.

Back amongst the leaf litter the new spores are taking root.

Extending underground a long network of fungus threads tap in to the roots of trees

where they extract sugars.

In return the tree is using these threads like root extensions,

allowing it to suck up nutrients from a much wider area.

A single fungus can spread for almost 10 square kilometres connecting thousands of trees.

The largest organism on the planet.

Created by billions of physical connections beneath the soil.

This vast network has been called the wood-wide-web, and every forest has one.

The alliance between fungi and plants is over half a billion years old

and is what enabled plants to colonise the land.

As the winter approaches the animals in these forests are on the move.

Many are hiding away.

Wood frogs are burrowing down into damp leaf litter.

Garter snakes leave the forest to take cover in underground dens.

In the great north woods this newly hatched budworm caterpillar must find a place to retreat.

And this lynx must grow a thicker coat.

Those that can escape abandon the cooling forests.

This monarch butterfly is detecting the weakening sun,

triggering earth's most spectacular insect migration.

Millions of butterflies are travelling up to 4,800 kilometres

to the warmer forests of Mexico and California.

But while many animals flee the seasonal forests,

on the west coast of Canada millions of animals are travelling in the opposite direction.

Their arrival in the coastal forest of Canada will provide a life-line.

Spreading for over 70,000 square kilometres here lies the greatest seasonal rainforest on Earth.

Trees tower 90 metres high and are up to 6 metres in diameter.

But the autumn brings a huge problem,

up to 6 metres of drenching rain pours every year,

and this is the wettest time.

Vital nutrients flush away.

And the wood-wide-web is starving.

For the ecosystem to survive and the fungus to grow,

it needs a big dose of nutrients, and fast.

Help is on its way.

Salmon.

Out at sea they've been waiting for a signal.

Now, with the rivers full,

millions of salmon are swimming as far as 2,000 kilometres

heading back to the forest streams where they were born.

Under the shade of the towering trees and secure in the gravel they spawn.

Here in the cool running water is a constant supply of oxygen.

And the nutrients flushing out of the forest are helping to nourish the young salmon.

But this nourishing nursery comes at a price.

Hungry bald eagles.

At this time of year even wolves prefer salmon to anything else.

But there is an even more formidable fishing expert, one that has been waiting all year.

The grizzly bear.

In just a few weeks she'll eat almost one and a half thousand kilograms

and will almost double her body weight.

Feeding on a plentiful supply of salmon

she could be up to 80 percent bigger than bears that don't fish.

On the bank she has three young cubs and this is their first salmon run.

They're busily digging under rocks looking for salmon eggs.

But if they're to survive the winter

they must learn how to catch big fish by watching their mother.

It's more difficult than it looks,

and they don't have long to learn.

This salmon run will be over soon,

and every salmon caught will increase the cubs' chances of surviving the winter.

But catching fish isn't their only worry.

Big males roam these shores, unrelated to the cubs they would kill them on sight.

To eat in safety mum leads her cubs into the cover of trees,

this is the key to how the salmon can feed the forest.

A bear needs up to 60,000 calories per day, the equivalent of 550 chocolate bars.

So the bears prefer to eat the brains and ovaries packed with eggs,

the fattiest parts of the fish,

giving them maximum calories per bite.

They dump the rest on the forest floor.

So many bears hunt here that before long

3 tonnes of salmon can litter an area the size of a football field.

The smell of fish attracts this banana slug.

It looks like a banana, and it can grow just as long as a banana.

One of the largest slugs in the world.

They're crucial decomposers in this forest and they thrive in the damp conditions,

up to 5 per square metre.

But this rotting salmon is not just food, it also makes a cosy nursery for flies.

They're desperate to mate.

Soon the frosts will come and these flies will die.

But there's a rival.

The female is worth fighting for.

Having lost one battle he goes off to 'bug' another couple.

The victors' offspring will fatten up on the salmons' flesh.

Full of decaying fish this banana slug leaves a trail of salmon-rich fertiliser in its wake.

As the salmon rots its nutrients enter the soil.

This boosts the wood-wide-web and serves as a life-support system

nourishing the trees through the harshest time of the year,

the winter.

The earth's Northern hemisphere faces away from the sun.

It's winter in the north.

Triggered by snowfall in the rain coast forest,

this mother bear is leading her cubs to the safety of a den.

Plump on salmon, they will survive here for the next 5 months.

Meanwhile in the great North Woods as the temperature plummets wood frogs freeze solid.

Antifreeze in their blood keeps them alive.

These painted turtles have just hatched

but they too freeze in their nests until food comes along with the spring.

The seasonal forest is in suspended animation.

But not all sleeps.

Small mammals and birds venture into the cold to search for food.

With oily fur resistant to frost, this Wolverine is looking for a meal.

He may need to travel up to 30 kilometres a day to find it.

This moose is roaming its territory.

Weighing half a ton and standing up to two and a half metres tall

this is the largest land mammal in North America.

Its long legs are perfectly adapted to walk through deep snow.

And for extra insulation its thick fur is made of hollow hairs.

Moose can survive the winter by browsing on barren twigs and bark.

White tailed deer have a much more difficult time.

They are adapted to life in warm climates further south,

these forests are as far north as they can survive.

One quarter the size of a moose their small bodies feel the cold.

And their short legs make walking in snow treacherous.

Beneath conifer trees the snow is shallower

and the canopy helps to keep the air a few degrees warmer.

Despite this many will succumb to the winter.

And those animals that perish provide a crucial source of food for scavengers.

The wolverine has found fresh carrion.

His voracious appetite earns him his nickname, Glutton.

This capacity to feast will help him to survive the winter until he finds another meal.

But these forests aren't just home to scavengers.

In the great North Woods a deadly predator has made a big comeback.

The lynx.

It owes its success to an extraordinary connection.

Scientists from Maine Department of Inland Fisheries and Wildlife

are tracking lynx to understand how it's possible for so many to live here.

And they hope to catch a female to retrieve valuable data from her radio collar.

These elusive cats can have home ranges as big as 700 square kilometres.

From the air the radio signal is detected.

24-12 I have the coordinates for you.

It's the female lynx that wildlife biologist Jen Vashon is hoping to catch.

Now it's time to go in on the ground.

These fresh tracks are a sure sign that this is a good place to set a trap.

A chunk of meat to draw it in.

All that's needed now is patience, and a lot of luck.