The Magical Forest Secrets of our Living Planet

Ten million species live on planet earth.

Each one is remarkable.

But none can survive on its own.

All life depends upon connections.

Unexpected, invariably complex, beautiful relationships between millions of plants and animals.

This time, in our seasonal forests,

why does this lynx need a caterpillar?

Why does the tree need the fish?

And why does this truffle fungus need one of these?!

Flying squirrel!

Connections like these form the planet's great ecosystems.

They're vital for all life.

I want to show you our world as you've never seen it before.

JAUNTY MUSIC

New England in autumn.

There really can't be a more magical place anywhere on earth,

to appreciate that dramatic transition between summer and winter.

But we mustn't get blinded by this natural fiesta,

because such an extreme transformation is a huge challenge for life.

And autumn is just one of many transformations the forest must face.

From summer to winter, this land of plenty will appear to collapse,

before attempting to rebuild itself all over again in the spring.

To see how, I'm going to, what is for me, the greatest seasonal forest on the planet.

The wooded wilderness that stretches right across North America.

From the land of the Canadian lynx,

to the land of the grizzly bear.

Our story begins in autumn.

As the days are drawing shorter, less light is feeding the forests.

Deciduous trees are shedding their leaves.

Many creatures are burrowing away to escape the cold.

Others are simply leaving.

But there's one animal with a crucial job to do.

Now, before the winter sets in.

It's a job the entire forest depends upon.

The best time to see them is in the first couple of hours after dark.

And what I'm hoping is, if I stand here and stay really quiet, I'll be in for a real treat.

It's a creature I've waited all my life to see.

But they move so fast!

Oh! Did you see that?!

That was amazing, it went right past my face!

Flying squirrel!

They really are expert gliders.

They can glide for up to 200 metres!

When I was a kid, I was obsessed with things that were, you know, not meant to fly.

Flying fish, flying frogs, flying lizards, flying squirrels.

This is the first time I've ever seen them.

It was worth a 45-year wait. Honestly!

Did you... Did you see that?!

I felt it. It went right through my hair. Seriously!

Centre parting!

It was like a sheet of A4 coming right over my face

and as soon as they hit the tree they're running and up they go.

They're just criss-crossing all the trees.

They immediately scamper up to the top, then take off and glide again,

and sometimes, I've noticed, they can even change direction in flight.

Arghh!

One hit me in the chest!

It doesn't come better than that, does it?

It doesn't get more exciting.

But what on earth have they got to do with our story?

Well, at the moment, these flying squirrels are in the woods

trying to find as much food as possible before the weather turns nasty and the winter kicks in.

But what are they after? Well, they're after these.

Truffles.

They're the fruiting bodies of fungi and they appear in the damp cool of autumn.

In preparation for winter,

the hungry squirrel needs to hoard food such as truffles.

But the truffles also need the squirrel to eat them.

As the squirrel moves through the forest, the spores are dispersed.

And that's crucial, not just for the truffle, but for the trees.

What's so special about these truffles?

They certainly don't look much, and the smell can be said to be an acquired taste.

And they're not just here as another organism to be eaten by hordes of hungry squirrels either,

because without these truffles, and all the other fungi here in the wood,

this woodland simply couldn't function. It couldn't exist.

Why?

Well, take a look beneath the soil down here.

Each truffle has thread-like roots extending from it.

The threads extract nutrients in the soil, from rotting material like leaves.

And, cunningly, they also tap into the roots of the trees to siphon off sugars.

But this is not a one-way relationship,

because the tree can now tap into the nutrients extracted by the fungal threads.

This symbiotic relationship between the trees and the fungus,

where each is dependent on the other,

clearly helps the tree grow, but it's not only that.

It greatly extends the reach of its roots

because, in effect, they become as extensive as the fungal network that they're connected to.

In autumn, throughout the northern hemisphere, trees use fungi to extend their roots

and absorb sufficient nutrients for the big freeze ahead.

I love this web of relationships - the squirrels, the fungi, the trees.

It ensures that they're all ready to face the winter.

But for me, one of the most magical relationships of all is seen on the far west coast of Canada,

as one of the world's most ancient forests

prepares for the oncoming challenge.

Here, I can stand at the foot of 1,000-year-old cedars

and 90-metre-tall Sitka spruce trees.

The combination of large mountains

and ocean winds generates unusually heavy rainfall -

earning this place the title The Raincoast.

There's so much rain in autumn that the rivers are swollen.

And that is vital to the forest's survival.

There's a significant event happening here,

which allows the whole forest not only to survive the winter,

but also to flourish throughout the course of the year.

But you know, the really incredible thing is this key to life

is not here in the forest at all at the moment, but it will be soon.

After years at sea, salmon are returning to spawn

in the same forest streams in which they were hatched.

The swollen rivers make it easy for them to swim deep into the forest.

But the scent of home also draws them irresistibly towards danger.

Predators make the most of this banquet of seafood.

But none of them compare to the most formidable fish eater of all.

Grizzly bears. Just look at this - there's a female here,

about 40 metres in front of me, in the shallows, fishing for salmon.

Behind her, on the bar over there, she's got three cubs.

They're not struggling to catch the fish here.

There's such a tremendous surfeit of tired salmon out there.

All she really has to do is wander into those shallows

until one comes close.

And then she can grab it, much to the delight of her cubs.

For these cubs, it's the first salmon run.

They've got to learn how to catch fish by watching their mother.

Look at this! Look!

This is the adult grizzly,

that's just leapt off the island there,

and caught a salmon. Look at that! Right in its mouth.

Over just six weeks in autumn,

tens of millions of salmon are going to return to these rivers.

And during the course of a day

one adult bear like this can eat 40kg of salmon,

and during the course of a salmon run 1400kg -

that's just one bear's intake.

But every salmon caught by these bears

increases their chances of survival.

They are incredibly important to these bears,

particularly at this time of year,

when it's essential that they bulk up as quickly as possible

before they slip into hibernation with the winter coming.

This cub hasn't quite got the hang of it yet,

but he hasn't got long -

the salmon run has only got a couple of weeks to go.

Until they learn, Mum has to work even harder.

Every salmon caught makes a real difference.

These bears, those that are close to a huge amount of salmon,

grow 80% larger than those in other areas.

They have 25% more cubs,

and occur at densities 50 times greater.

So salmon, frankly, are great news for bears.

Exciting as it is to watch,

there's a lot more going on here

than simply bears catching fish.

And the reason is thanks to what happens next.

And the only way to see it is with remote cameras,

positioned deep in the forest.

These younger bears have carried fish 30m from the river.

Because here they are less likely to be challenged by hungry adults.

So they can eat in peace.

There's so much fish available, they just eat the richest bits

to lay down enough fat for hibernation.

The rest appears to be wasted, abandoned on the forest floor!

Along with our camera.

The aftermath of this feast is unbelievable.

Up to four tonnes of carcasses are left in an area the size

of a football pitch.

But what have dead fish

got to do with the forest preparing for winter?

Well, this is where it starts to get really intriguing,

because the catching of the salmon is just the start of it.

Bears aren't the only creatures

attracted by such a feast.

A banana slug.

At 25cm long, it's one of the largest slugs in the world.

And masses of insects.

These flies won't survive the winter,

but if they plant their eggs in the salmon's flesh,

their offspring might.

This flurry of activity

eventually breaks the flesh down into simple nutrients

that are absorbed into the soil.

The significance of all of this decaying fish

goes far beyond it being just a feast for scavengers.

Without all of these rotting salmon accumulating here every autumn,

this forest would be a very different place.

The salmon nutrients in the soil are taken up by the fungi.

So this ancient forest is better equipped to face

the almighty change that's fast approaching.

For forests in the Northern Hemisphere, time has run out.

Every day the sun sinks lower in the sky.

Winter.

And on the face of it,

all of the life here seems to have just gone away.

Those truffles and the seeds,

they're locked away underneath all of this snow.

The salmon run is over.

The vegetation... look at it. It appears to have shut down.

Even the water is in short supply - it's all frozen.

All of those connections appear to be broken.

The fungi have reduced their recycling to a bare minimum.

And the trees they're connected to are producing little in return.

The deciduous trees pre-empted the winter by shedding their leaves.

The conifers are slowing down, too.

The waxy coating on the needles protects their leaves from the cold.

But not everything here can

exist in a state of suspended animation.

Some of the animals have to remain active,

and surviving in conditions like this isn't easy.

The icy cold is the cue for the bears

to leave the forest altogether.

With the salmon run over,

they are retreating to their winter dens, up in the mountains.

They must spend the entire winter living off their fat reserves

gained by feeding on all of those salmon.

The squirrels,

and other small mammals, must keep activity to a minimum,

only occasionally venturing out to retrieve their autumn caches.

The lower the temperature falls,

the more vulnerable creatures become.

Winter has been too brutal for this young white-tailed deer,

but at least it's an opportunity for some nocturnal scavengers.

A racoon.

Out of the forest, a fisher - a relative of martens and weasels.

And it's smart enough to keep this meal to itself!

But there is more to this lifeless-looking forest

than just the scavengers.

For most creatures, winter is a brutal and unforgiving time.

But others actually thrive in these conditions.

You see, for animals that are adapted to live in winter,

this stripped-down forest ecosystem,

well, it's a wonderland.

In winter, here, there are beautiful connections,

between some of the forest's most enchanting characters.

There is one predator here,

an incredibly important animal

that has no intention of avoiding the snow,

because, unlike me, it's perfectly adapted to it.

But it's an enigma, a really, really shy animal,

one that's difficult to study.

Having said that,

scientists have been tracking them through the forest here

for more than a decade.

Scientists from the Maine Department of Fisheries and Wildlife

have set a trap to catch one alive.

With those distinctly pointed ears, it can only be a Canadian lynx.

Lynx are the world's most northerly-dwelling cats.

And this particular lynx is well-known

to chief scientist Jen Vashon.

The ear tags are blue with white.

That indicates it's L1-11.

He's called L1-11 and was born in May 2004.

They've discovered that he is just one of hundreds of lynx living here.

It's too intimidated with everybody right there.

In fact, there are more lynx living in these Eastern forests

than anywhere else in North America.

Now, the fact that L1-11 has lived all of his life

in this frozen forest

has to mean that this is a perfect place for a lynx to live.

But how can a top predator like this

survive in such a stripped-down environment,

when there appears to be so little else here?

The tracks of their prey are everywhere,

but actually finding one is a real challenge

because its winter camouflage is perfect.

It's taken some finding, but it's there - the snowshoe hare!

You can just make out its beady little black eye,

and the black tips to its ears.

And these things form 80% of a lynx's diet.

But, as you can see, they don't make it easy for that lynx.

Their camouflage is astonishing.

In the summertime they're brown,

but in the winter they moult through to a white coat.

But they also use this thick brush.

It provides them with excellent cover

to hide from the lynx, also hide from the elements,

but it's also crucial in keeping them alive,

because they climb on top of the snow and nibble at

all of the shoots and the bark growing from all of this brush.

Look at that! Beautiful, and with those big snowshoe feet,

it just sort of floats across the surface of the snow.

But what the snowshoe hare needs most to survive the winter

is a specific type of vegetation.

It must be the right height to eat,

and provide enough cover to hide from all those lynx.

So, the vegetation, here, must be perfect.

And the reason that it does grow into this perfect environment,

well, you could never guess.

The most beautiful thing about this story

is that the lynx, the hare,

patches of cover like this amongst the forest, didn't happen by chance.

They're all controlled by the most unlikely of creatures,

a tiny thing, less than the size of one of my fingernails.

And at the moment it's hiding,

having burrowed into the bark of one of these trees,

or perhaps in a crack in a log lying on the forest floor,

covered with frozen snow.

But it's there, and it's waiting.

It's waiting for springtime.

As the hours of daylight increase and the ground thaws,

as if by magic, the northern forests change again.

As new leaves appear, trees start producing sugars,

and that's good for the fungus in the soil.

Entwined with the trees' roots, they can siphon off some of these sugars.

But not everything appears so harmonious.

In the land of the lynx,

something extraordinary is happening to the forest.

These trees may have endured the winter,

but now it's spring, they're under attack.

Some are even dying.

But what's happening here now is vital

for how this ecosystem functions over the year.

This defoliation is entirely natural.

And I might be able to find one of the culprits down here,

although they're quite tricky to spot.

They live in these fresh, green shoots.

Yes, here we are.

Now, wrapped delicately in these leaves

is a species that is single-handedly

influencing the ecology of this entire forest.

Inside this nest is the caterpillar of the spruce moth. The budworm.

And it hasn't only wrapped itself up in those leaves

to hide from predators,

because it's eating them as well.

And it doesn't just eat the leaves,

it also eats the buds, the flowers, and the cones on the tree here.

Up in the canopy, a tiny budworm caterpillar

has just emerged from hibernation.

It's racing to fatten itself up.

When it's finished on one branch, it releases a strand of silk,

and abseils down to the next.

It's a risky business being a juicy, fat caterpillar -

you're in danger of being spied by all of those birds,

just back from migration.

But the caterpillar has a plan -

it uses its silk

to weave the needles together and hide in a dense web.

Now, the springtime assault by these caterpillars

is bad news for the trees.

But for other inhabitants of these forests,

these caterpillars are heroes.

It's thanks to the behaviour of this species

that one of North America's most elusive and charismatic predators -

the Canadian lynx - is enjoying a bit of a renaissance

in forests like this one.

But the caterpillar lives all the way up there, in the canopy.

Whilst the cat with the pointed ears is prowling around down here.

So, how can a humble insect like this have any impact

on a formidable thing like that?

I bet the lynx never even sees the caterpillar

throughout the course of its life.

Why does the lynx need the caterpillar?

The clue is how they affect the lynx's prey on the forest floor.

For decades, scientists have studied budworm caterpillars,

and a remarkable pattern has emerged.

They've discovered

that the population of caterpillars fluctuates dramatically.

And at the peak of a cycle

there can be tens of thousands of budworms in a single tree.

And this has devastating consequences.

Whilst these dramatic natural events might be a catastrophe

for the established trees, for anything trying to grow

on the forest floor they are an absolute bonus.

In here, where it's dark, there's very little,

very poor diversity - just some mosses and a few ferns.

But as soon as there's a break in the canopy

and the sunlight can flood in, well, look at the difference.

Lots of wild flowers, there's a young maple coming through here,

mountain ash, and, most importantly of all, regenerating spruce and fir.

The hares essentially need these regenerating conifers as shelter.

And of course what's good for the hares is also good for the lynx.

It's such an elegant connection.

Without the spring emergence of the hungry caterpillars

to chew holes in a dense canopy,

there wouldn't be enough light flooding the forest floor.

And, with less light down here, there would be less growing

for our snowshoe hare to forage and to hide in,

and then there would be nothing for L1-11

and all of those hundreds of other lynx to eat.

And that's why the lynx needs the caterpillar.

And now it's spring,

there's no better time to see what the future holds

for the lynx population.

Wildlife biologist Jen is doing a count.

She has detected a signal from a radio-collared female.

There she is.

But there might be something else here.

Safe inside her den, a lynx cub.

He's just a few weeks old.

His eyes aren't even open.

Jen must work fast before Mum returns.

Thanks to the timing of the budworm opening the canopy this spring,

there's going to be enough prey for these lynx to hunt next winter.

Far away to the west, The Raincoast forest is coming back to life.

Thanks to the richness of the autumn salmon run,

the bears have survived the winter.

They've now returned to the forest, looking for something to eat.

They'll survive on vegetation until the next salmon run.

The emergence of the bears is a cue for scientists

to conduct a rather risky experiment.

They need a large, hungry, bear.

It's the only way to measure the impact of all those salmon

on this ecosystem

and to understand why this vast,

ancient forest has thrived for so long.

For such a big question,

the methods employed by senior researcher Chris Darimont

seem a bit curious.

He's equipped with a can full of old salmon guts,

effusing, probably, the most disgusting smell known to man.

This is wonderful stuff.

But he hopes the bears are going to love it.

He's made an aerial lure.

So, the wind will carry this distinctive perfume

deep into the forest.

Wind, extra boost.

Now they surround the area with barbed wire.

And it's this

that they hope will collect what they're so interested in -

a single hair from a visiting bear.

Now the site is prepared,

it's time to set some remote cameras,

and beat a hasty retreat.

Personally, I'm very happy to watch from a safe distance -

it's not the smell - some of those bears are huge!

Look at the size of this bear!

As a trap, this is the perfect bait, it's working brilliantly.

The bears have come in,

and they're snagging themselves on the wire there.

You can see it vibrating about. And that's just what we want.

Fresh out of hibernation,

it seems they can't resist this pile of stinking salmon.

This one's even rolling around in the stuff now.

No doubt it values the scent - I'm not sure we would.

The bear's coat has been growing for nearly a year.

But soon it will be moulted and lost.

The more bears we can attract, the better.

Now the coast is clear,

and it's time to retrieve any fur from the barbed wire.

So, what on earth can a hair tell us about this forest ecosystem?

Well, hair is made of protein,

sourced from whatever the bear has been eating over the last year.

And by analyzing this hair,

science can reveal an astonishing level of detail about a bear's life.

We can learn so much from a single bear's hair.

So I know, for instance, that this one has come from a female grizzly.

I know exactly what it's been eating even on a week-by-week basis,

where that food has come from

and even the impact on the quality of its life.

So this bear has been getting most of its protein,

not from the forest around here,

but actually from the deep ocean, via the salmon.

And we know that throughout the course of the year

80% of that bear's protein has come from these salmon.

And that's surprising because, remember,

they're only available to the bear for a few weeks during the autumn.

And yet the impact is clearly lasting all year.

So, how come the bears appear to be so full of salmon?

Well, back in autumn, we saw the bears scattering fish carcasses

all around the forest floor.

As the protein in those rotting salmon broke down,

nitrogen from it accumulated in the soil.

And this salmon nitrogen

is like fertiliser!

So, in spring,

nutrients all the way from the ocean gradually appear

in all the vegetation growing here.

Just in time for the hungry bears

to eat as they emerge from hibernation.

And hungry bears have huge appetites -

they'll eat a third of their body weight every day!

So that's why their bodies appear to contain so much salmon.

But the impact of this ocean-born nitrogen

extends far beyond bears and their food.

This particular form of nitrogen

can be found in almost all of the animals and plants

that appear here in the spring.

These Rufous Hummingbirds have migrated to the forest to breed.

They're drinking nectar from plants

that have been fertilised by rotted fish.

So, they'll carry the same salmon nutrients with them

as they fly through the forest.

Many of the insects pollinating the plants now

were incubated in that decaying flesh back in the autumn.

As they themselves are eaten,

the salmon nutrients are spread even further.

Thanks to the bears, the insects, and the birds,

this salmon fertiliser is spread deeper and deeper into the forest -

sometimes as much as 800m from the river,

and this pulse of nutrients then allows the organisms

which define the forest itself to prosper - its trees.

Doctor Tom Reimchen can measure exactly how much

these vast old trees need the fish.

The secret is to look inside the tree, by taking a core sample.

Written on it is the entire story of this 300-year-old tree.

The rings I see are two, three, even four millimetres,

which continues back

to even the early parts of the 1800s, late 1700s.

Tom has taken similar samples from thousands of trees.

An entire forest is lined up in his lab,

each tree waiting to tell its own story.

In this seasonal climate, annual rings are created as the tree grows.

From these rings, he can determine not just the age of the tree,

but also the amount of growth in each year.

Some of the rings are thicker than others,

showing that the tree has grown more.

Like the bear fur, each annual ring can be analyzed.

Tom can search for the same type of nitrogen

that's found in the bear's hairs.

It comes from the ocean

and it's called nitrogen-15.

And the data will tell us just how much nitrogen in those trees

comes from all those salmon.

I think this is really exciting.

You see, the annual growth rings here

show the presence of the stable isotope Nitrogen-15,

which significantly comes from the oceans.

It could only have been carried here by the salmon.

Now look, here is the present, the bark on the outside of the tree,

so these rings represent perhaps the last 15 years

and they're very closely packed together.

But here, back in the 1980s, the rings are twice as thick.

The trees have been growing twice as much during the course of a year.

So, perhaps the salmon runs then were even more productive

than they have been recently.

But that's not the best thing. Come and have a look at this.

By measuring the abundance of that nitrogen isotope in this material,

I can tell you that majestic old giants like this beauty here

are actually composed of up to 85% material

that's derived from salmon.

Now, when I was a teenager,

I remember learning that I was made of carbon,

and carbon could only be formed when stars died.

Effectively I was made of dead stars

and that struck me as terribly romantic.

But look at this.

This is a forest made of the ocean!

That's why the tree needs the fish.

Without this unlikely-sounding relationship,

this magnificent ancient forest just wouldn't be the place it is today.

But there is one more relationship,

crucial to seasonal forests all around the world,

when it comes to surviving constant change.

And it's one that reaches its greatest intensity now,

at the height of summer.

It's the driest time of year, and the trees need water.

Fortunately, united with their fungal partners,

the trees have massively extended their roots.

Fungal threads in the soil are absorbing water

and passing it to the tree.

But what has only recently been discovered

is the sheer scale of these fungal root networks.

A single cubic centimetre of the soil here

can have a mile of these white fungal threads running through it.

They're called mycorrhizae.

And, for me,

it's these organisms that are the real secret of the forest here.

In the lab, the genetic fingerprints

of individual mycorrhizae have been identified.

By mapping an area 30m across,

it's been discovered

that individual fungi connect to more than a single tree.

Just one fungus can be joined to 80% of all of the plants growing here.

And, amazingly, these physical links enable different species of plants

to exchange nutrients.

Older established plants are even nurturing

younger weaker ones.

It acts like an underground welfare system!

These giant webs

connect all of the trees in this forest,

and keep them, and all of the things that are dependent upon them, alive.

That's why scientists are calling this the Wood Wide Web.

It's thanks to this natural phenomenon, the Wood Wide Web,

that, together, the trees in the forest ecosystems are resilient -

resilient enough to cope with the dramatic changes

they encounter every year.

And what's really amazing is how the web is built.

It's thanks to hungry mammals like our flying squirrels

that this essential life support system is effectively maintained.

It's actually fair to say that these trees wouldn't be standing here,

wouldn't be thriving, unless a squirrel had eaten a truffle.

And that is fantastic!

It is fantastic.

It's fantastic to think that what animals do in one season

influences the forest ecosystem throughout the year.

It's almost as if all of these stories are choreographed.

The arrival of the salmon at exactly the right time

to fatten the bears for winter.

Then, the emergence of the lush green vegetation

fertilised by those salmon

to sustain the bears when they emerge from hibernation.

The squirrels - foraging for truffles in the autumn time,

and sowing their spores throughout the forest

to grow a fungal network

that joins all of the trees and all of the plants,

and provides them with nutrients.

The budworm - chewing a hole in springtime in the canopy,

so that in summer, sunlight floods down to the forest floor

and produces the perfect hunting habitat for lynx.

It's all in the timings.

And it's this that makes these temperate forests

such magical places.

Join me next time,

when I'll be travelling to some of the world's greatest water habitats.

Subtitles by Red Bee Media Ltd