0:00:04 > 0:00:08Ten million species live on planet earth.
0:00:09 > 0:00:11Each one is remarkable.
0:00:13 > 0:00:15But none can survive on its own.
0:00:19 > 0:00:22All life depends upon connections.
0:00:25 > 0:00:31Unexpected, invariably complex, beautiful relationships between millions of plants and animals.
0:00:36 > 0:00:39This time, in our seasonal forests,
0:00:39 > 0:00:43why does this lynx need a caterpillar?
0:00:43 > 0:00:47Why does the tree need the fish?
0:00:47 > 0:00:51And why does this truffle fungus need one of these?!
0:00:52 > 0:00:54Flying squirrel!
0:00:58 > 0:01:02Connections like these form the planet's great ecosystems.
0:01:02 > 0:01:06They're vital for all life.
0:01:06 > 0:01:11I want to show you our world as you've never seen it before.
0:01:41 > 0:01:42JAUNTY MUSIC
0:01:51 > 0:01:53New England in autumn.
0:01:54 > 0:02:00There really can't be a more magical place anywhere on earth,
0:02:00 > 0:02:05to appreciate that dramatic transition between summer and winter.
0:02:08 > 0:02:13But we mustn't get blinded by this natural fiesta,
0:02:13 > 0:02:18because such an extreme transformation is a huge challenge for life.
0:02:18 > 0:02:23And autumn is just one of many transformations the forest must face.
0:02:27 > 0:02:33From summer to winter, this land of plenty will appear to collapse,
0:02:33 > 0:02:40before attempting to rebuild itself all over again in the spring.
0:02:51 > 0:02:57To see how, I'm going to, what is for me, the greatest seasonal forest on the planet.
0:03:05 > 0:03:11The wooded wilderness that stretches right across North America.
0:03:11 > 0:03:14From the land of the Canadian lynx,
0:03:14 > 0:03:17to the land of the grizzly bear.
0:03:23 > 0:03:26Our story begins in autumn.
0:03:37 > 0:03:42As the days are drawing shorter, less light is feeding the forests.
0:03:46 > 0:03:49Deciduous trees are shedding their leaves.
0:03:54 > 0:03:59Many creatures are burrowing away to escape the cold.
0:03:59 > 0:04:01Others are simply leaving.
0:04:03 > 0:04:08But there's one animal with a crucial job to do.
0:04:08 > 0:04:11Now, before the winter sets in.
0:04:13 > 0:04:18It's a job the entire forest depends upon.
0:04:19 > 0:04:23The best time to see them is in the first couple of hours after dark.
0:04:23 > 0:04:30And what I'm hoping is, if I stand here and stay really quiet, I'll be in for a real treat.
0:04:33 > 0:04:38It's a creature I've waited all my life to see.
0:04:38 > 0:04:40But they move so fast!
0:04:48 > 0:04:51Oh! Did you see that?!
0:04:51 > 0:04:53That was amazing, it went right past my face!
0:04:54 > 0:04:56Flying squirrel!
0:05:01 > 0:05:04They really are expert gliders.
0:05:04 > 0:05:08They can glide for up to 200 metres!
0:05:12 > 0:05:17When I was a kid, I was obsessed with things that were, you know, not meant to fly.
0:05:17 > 0:05:21Flying fish, flying frogs, flying lizards, flying squirrels.
0:05:21 > 0:05:24This is the first time I've ever seen them.
0:05:24 > 0:05:27It was worth a 45-year wait. Honestly!
0:05:32 > 0:05:35Did you... Did you see that?!
0:05:35 > 0:05:38I felt it. It went right through my hair. Seriously!
0:05:38 > 0:05:40Centre parting!
0:05:40 > 0:05:42It was like a sheet of A4 coming right over my face
0:05:42 > 0:05:46and as soon as they hit the tree they're running and up they go.
0:05:50 > 0:05:53They're just criss-crossing all the trees.
0:05:53 > 0:05:57They immediately scamper up to the top, then take off and glide again,
0:05:57 > 0:06:01and sometimes, I've noticed, they can even change direction in flight.
0:06:10 > 0:06:12Arghh!
0:06:12 > 0:06:14One hit me in the chest!
0:06:14 > 0:06:16It doesn't come better than that, does it?
0:06:16 > 0:06:19It doesn't get more exciting.
0:06:19 > 0:06:23But what on earth have they got to do with our story?
0:06:23 > 0:06:27Well, at the moment, these flying squirrels are in the woods
0:06:27 > 0:06:32trying to find as much food as possible before the weather turns nasty and the winter kicks in.
0:06:32 > 0:06:36But what are they after? Well, they're after these.
0:06:36 > 0:06:38Truffles.
0:06:38 > 0:06:44They're the fruiting bodies of fungi and they appear in the damp cool of autumn.
0:06:46 > 0:06:48In preparation for winter,
0:06:48 > 0:06:53the hungry squirrel needs to hoard food such as truffles.
0:06:53 > 0:06:56But the truffles also need the squirrel to eat them.
0:07:04 > 0:07:09As the squirrel moves through the forest, the spores are dispersed.
0:07:13 > 0:07:18And that's crucial, not just for the truffle, but for the trees.
0:07:24 > 0:07:28What's so special about these truffles?
0:07:28 > 0:07:33They certainly don't look much, and the smell can be said to be an acquired taste.
0:07:33 > 0:07:39And they're not just here as another organism to be eaten by hordes of hungry squirrels either,
0:07:39 > 0:07:44because without these truffles, and all the other fungi here in the wood,
0:07:44 > 0:07:49this woodland simply couldn't function. It couldn't exist.
0:07:49 > 0:07:50Why?
0:07:50 > 0:07:54Well, take a look beneath the soil down here.
0:07:59 > 0:08:03Each truffle has thread-like roots extending from it.
0:08:03 > 0:08:09The threads extract nutrients in the soil, from rotting material like leaves.
0:08:19 > 0:08:26And, cunningly, they also tap into the roots of the trees to siphon off sugars.
0:08:27 > 0:08:30But this is not a one-way relationship,
0:08:30 > 0:08:35because the tree can now tap into the nutrients extracted by the fungal threads.
0:08:42 > 0:08:47This symbiotic relationship between the trees and the fungus,
0:08:47 > 0:08:49where each is dependent on the other,
0:08:49 > 0:08:53clearly helps the tree grow, but it's not only that.
0:08:53 > 0:08:56It greatly extends the reach of its roots
0:08:56 > 0:09:02because, in effect, they become as extensive as the fungal network that they're connected to.
0:09:04 > 0:09:11In autumn, throughout the northern hemisphere, trees use fungi to extend their roots
0:09:11 > 0:09:15and absorb sufficient nutrients for the big freeze ahead.
0:09:23 > 0:09:28I love this web of relationships - the squirrels, the fungi, the trees.
0:09:28 > 0:09:33It ensures that they're all ready to face the winter.
0:09:41 > 0:09:49But for me, one of the most magical relationships of all is seen on the far west coast of Canada,
0:09:49 > 0:09:52as one of the world's most ancient forests
0:09:52 > 0:09:55prepares for the oncoming challenge.
0:10:07 > 0:10:12Here, I can stand at the foot of 1,000-year-old cedars
0:10:12 > 0:10:16and 90-metre-tall Sitka spruce trees.
0:10:23 > 0:10:25The combination of large mountains
0:10:25 > 0:10:30and ocean winds generates unusually heavy rainfall -
0:10:30 > 0:10:34earning this place the title The Raincoast.
0:11:07 > 0:11:11There's so much rain in autumn that the rivers are swollen.
0:11:17 > 0:11:21And that is vital to the forest's survival.
0:11:29 > 0:11:32There's a significant event happening here,
0:11:32 > 0:11:37which allows the whole forest not only to survive the winter,
0:11:37 > 0:11:41but also to flourish throughout the course of the year.
0:11:41 > 0:11:45But you know, the really incredible thing is this key to life
0:11:45 > 0:11:49is not here in the forest at all at the moment, but it will be soon.
0:12:00 > 0:12:03After years at sea, salmon are returning to spawn
0:12:03 > 0:12:06in the same forest streams in which they were hatched.
0:12:11 > 0:12:17The swollen rivers make it easy for them to swim deep into the forest.
0:12:21 > 0:12:24But the scent of home also draws them irresistibly towards danger.
0:12:28 > 0:12:32Predators make the most of this banquet of seafood.
0:12:46 > 0:12:50But none of them compare to the most formidable fish eater of all.
0:12:56 > 0:13:02Grizzly bears. Just look at this - there's a female here,
0:13:02 > 0:13:09about 40 metres in front of me, in the shallows, fishing for salmon.
0:13:09 > 0:13:15Behind her, on the bar over there, she's got three cubs.
0:13:15 > 0:13:17They're not struggling to catch the fish here.
0:13:17 > 0:13:21There's such a tremendous surfeit of tired salmon out there.
0:13:21 > 0:13:23All she really has to do is wander into those shallows
0:13:23 > 0:13:26until one comes close.
0:13:26 > 0:13:29And then she can grab it, much to the delight of her cubs.
0:13:31 > 0:13:35For these cubs, it's the first salmon run.
0:13:35 > 0:13:38They've got to learn how to catch fish by watching their mother.
0:13:43 > 0:13:48Look at this! Look!
0:13:48 > 0:13:49This is the adult grizzly,
0:13:49 > 0:13:52that's just leapt off the island there,
0:13:52 > 0:13:55and caught a salmon. Look at that! Right in its mouth.
0:13:55 > 0:13:58Over just six weeks in autumn,
0:13:58 > 0:14:02tens of millions of salmon are going to return to these rivers.
0:14:02 > 0:14:04And during the course of a day
0:14:04 > 0:14:07one adult bear like this can eat 40kg of salmon,
0:14:07 > 0:14:11and during the course of a salmon run 1400kg -
0:14:11 > 0:14:15that's just one bear's intake.
0:14:17 > 0:14:20But every salmon caught by these bears
0:14:20 > 0:14:22increases their chances of survival.
0:14:46 > 0:14:48They are incredibly important to these bears,
0:14:48 > 0:14:50particularly at this time of year,
0:14:50 > 0:14:53when it's essential that they bulk up as quickly as possible
0:14:53 > 0:14:57before they slip into hibernation with the winter coming.
0:14:59 > 0:15:02This cub hasn't quite got the hang of it yet,
0:15:02 > 0:15:04but he hasn't got long -
0:15:04 > 0:15:08the salmon run has only got a couple of weeks to go.
0:15:11 > 0:15:16Until they learn, Mum has to work even harder.
0:15:21 > 0:15:25Every salmon caught makes a real difference.
0:15:35 > 0:15:38These bears, those that are close to a huge amount of salmon,
0:15:38 > 0:15:42grow 80% larger than those in other areas.
0:15:42 > 0:15:44They have 25% more cubs,
0:15:44 > 0:15:47and occur at densities 50 times greater.
0:15:47 > 0:15:52So salmon, frankly, are great news for bears.
0:16:11 > 0:16:13Exciting as it is to watch,
0:16:13 > 0:16:15there's a lot more going on here
0:16:15 > 0:16:18than simply bears catching fish.
0:16:18 > 0:16:22And the reason is thanks to what happens next.
0:16:31 > 0:16:35And the only way to see it is with remote cameras,
0:16:35 > 0:16:38positioned deep in the forest.
0:16:51 > 0:16:55These younger bears have carried fish 30m from the river.
0:16:58 > 0:17:02Because here they are less likely to be challenged by hungry adults.
0:17:02 > 0:17:07So they can eat in peace.
0:17:09 > 0:17:13There's so much fish available, they just eat the richest bits
0:17:13 > 0:17:16to lay down enough fat for hibernation.
0:17:17 > 0:17:22The rest appears to be wasted, abandoned on the forest floor!
0:17:24 > 0:17:26Along with our camera.
0:17:31 > 0:17:36The aftermath of this feast is unbelievable.
0:17:37 > 0:17:42Up to four tonnes of carcasses are left in an area the size
0:17:42 > 0:17:43of a football pitch.
0:17:45 > 0:17:47But what have dead fish
0:17:47 > 0:17:50got to do with the forest preparing for winter?
0:17:53 > 0:17:57Well, this is where it starts to get really intriguing,
0:17:57 > 0:18:00because the catching of the salmon is just the start of it.
0:18:00 > 0:18:03Bears aren't the only creatures
0:18:03 > 0:18:06attracted by such a feast.
0:18:06 > 0:18:07A banana slug.
0:18:07 > 0:18:13At 25cm long, it's one of the largest slugs in the world.
0:18:15 > 0:18:18And masses of insects.
0:18:26 > 0:18:28These flies won't survive the winter,
0:18:28 > 0:18:31but if they plant their eggs in the salmon's flesh,
0:18:31 > 0:18:32their offspring might.
0:18:38 > 0:18:40This flurry of activity
0:18:40 > 0:18:45eventually breaks the flesh down into simple nutrients
0:18:45 > 0:18:47that are absorbed into the soil.
0:18:55 > 0:18:58The significance of all of this decaying fish
0:18:58 > 0:19:02goes far beyond it being just a feast for scavengers.
0:19:02 > 0:19:07Without all of these rotting salmon accumulating here every autumn,
0:19:07 > 0:19:10this forest would be a very different place.
0:19:12 > 0:19:19The salmon nutrients in the soil are taken up by the fungi.
0:19:22 > 0:19:26So this ancient forest is better equipped to face
0:19:26 > 0:19:29the almighty change that's fast approaching.
0:19:43 > 0:19:46For forests in the Northern Hemisphere, time has run out.
0:19:51 > 0:19:56Every day the sun sinks lower in the sky.
0:20:27 > 0:20:29Winter.
0:20:29 > 0:20:31And on the face of it,
0:20:31 > 0:20:35all of the life here seems to have just gone away.
0:20:35 > 0:20:37Those truffles and the seeds,
0:20:37 > 0:20:40they're locked away underneath all of this snow.
0:20:40 > 0:20:43The salmon run is over.
0:20:43 > 0:20:46The vegetation... look at it. It appears to have shut down.
0:20:46 > 0:20:52Even the water is in short supply - it's all frozen.
0:20:52 > 0:20:57All of those connections appear to be broken.
0:20:59 > 0:21:03The fungi have reduced their recycling to a bare minimum.
0:21:04 > 0:21:10And the trees they're connected to are producing little in return.
0:21:13 > 0:21:18The deciduous trees pre-empted the winter by shedding their leaves.
0:21:18 > 0:21:22The conifers are slowing down, too.
0:21:22 > 0:21:27The waxy coating on the needles protects their leaves from the cold.
0:21:31 > 0:21:33But not everything here can
0:21:33 > 0:21:36exist in a state of suspended animation.
0:21:36 > 0:21:40Some of the animals have to remain active,
0:21:40 > 0:21:43and surviving in conditions like this isn't easy.
0:21:54 > 0:21:57The icy cold is the cue for the bears
0:21:57 > 0:22:00to leave the forest altogether.
0:22:02 > 0:22:04With the salmon run over,
0:22:04 > 0:22:08they are retreating to their winter dens, up in the mountains.
0:22:08 > 0:22:13They must spend the entire winter living off their fat reserves
0:22:13 > 0:22:16gained by feeding on all of those salmon.
0:22:18 > 0:22:20The squirrels,
0:22:20 > 0:22:26and other small mammals, must keep activity to a minimum,
0:22:26 > 0:22:30only occasionally venturing out to retrieve their autumn caches.
0:22:37 > 0:22:39The lower the temperature falls,
0:22:39 > 0:22:42the more vulnerable creatures become.
0:22:44 > 0:22:48Winter has been too brutal for this young white-tailed deer,
0:22:48 > 0:22:53but at least it's an opportunity for some nocturnal scavengers.
0:22:56 > 0:22:58A racoon.
0:23:00 > 0:23:05Out of the forest, a fisher - a relative of martens and weasels.
0:23:05 > 0:23:08And it's smart enough to keep this meal to itself!
0:23:23 > 0:23:26But there is more to this lifeless-looking forest
0:23:26 > 0:23:29than just the scavengers.
0:23:36 > 0:23:42For most creatures, winter is a brutal and unforgiving time.
0:23:44 > 0:23:49But others actually thrive in these conditions.
0:23:49 > 0:23:54You see, for animals that are adapted to live in winter,
0:23:54 > 0:23:59this stripped-down forest ecosystem,
0:23:59 > 0:24:02well, it's a wonderland.
0:24:20 > 0:24:23In winter, here, there are beautiful connections,
0:24:23 > 0:24:30between some of the forest's most enchanting characters.
0:24:36 > 0:24:38There is one predator here,
0:24:38 > 0:24:41an incredibly important animal
0:24:41 > 0:24:44that has no intention of avoiding the snow,
0:24:44 > 0:24:48because, unlike me, it's perfectly adapted to it.
0:24:48 > 0:24:52But it's an enigma, a really, really shy animal,
0:24:52 > 0:24:54one that's difficult to study.
0:24:54 > 0:24:55Having said that,
0:24:55 > 0:24:59scientists have been tracking them through the forest here
0:24:59 > 0:25:00for more than a decade.
0:25:11 > 0:25:14Scientists from the Maine Department of Fisheries and Wildlife
0:25:14 > 0:25:19have set a trap to catch one alive.
0:25:30 > 0:25:35With those distinctly pointed ears, it can only be a Canadian lynx.
0:25:38 > 0:25:43Lynx are the world's most northerly-dwelling cats.
0:25:47 > 0:25:50And this particular lynx is well-known
0:25:50 > 0:25:52to chief scientist Jen Vashon.
0:25:54 > 0:25:58The ear tags are blue with white.
0:25:58 > 0:26:01That indicates it's L1-11.
0:26:01 > 0:26:06He's called L1-11 and was born in May 2004.
0:26:16 > 0:26:19They've discovered that he is just one of hundreds of lynx living here.
0:26:24 > 0:26:27It's too intimidated with everybody right there.
0:26:42 > 0:26:46In fact, there are more lynx living in these Eastern forests
0:26:46 > 0:26:49than anywhere else in North America.
0:27:08 > 0:27:13Now, the fact that L1-11 has lived all of his life
0:27:13 > 0:27:16in this frozen forest
0:27:16 > 0:27:21has to mean that this is a perfect place for a lynx to live.
0:27:21 > 0:27:24But how can a top predator like this
0:27:24 > 0:27:28survive in such a stripped-down environment,
0:27:28 > 0:27:32when there appears to be so little else here?
0:27:44 > 0:27:47The tracks of their prey are everywhere,
0:27:47 > 0:27:51but actually finding one is a real challenge
0:27:51 > 0:27:54because its winter camouflage is perfect.
0:28:15 > 0:28:21It's taken some finding, but it's there - the snowshoe hare!
0:28:21 > 0:28:25You can just make out its beady little black eye,
0:28:25 > 0:28:28and the black tips to its ears.
0:28:28 > 0:28:33And these things form 80% of a lynx's diet.
0:28:33 > 0:28:37But, as you can see, they don't make it easy for that lynx.
0:28:37 > 0:28:40Their camouflage is astonishing.
0:28:41 > 0:28:43In the summertime they're brown,
0:28:43 > 0:28:46but in the winter they moult through to a white coat.
0:28:46 > 0:28:48But they also use this thick brush.
0:28:48 > 0:28:51It provides them with excellent cover
0:28:51 > 0:28:54to hide from the lynx, also hide from the elements,
0:28:54 > 0:28:57but it's also crucial in keeping them alive,
0:28:57 > 0:29:01because they climb on top of the snow and nibble at
0:29:01 > 0:29:06all of the shoots and the bark growing from all of this brush.
0:29:11 > 0:29:16Look at that! Beautiful, and with those big snowshoe feet,
0:29:16 > 0:29:19it just sort of floats across the surface of the snow.
0:29:30 > 0:29:35But what the snowshoe hare needs most to survive the winter
0:29:35 > 0:29:38is a specific type of vegetation.
0:29:38 > 0:29:43It must be the right height to eat,
0:29:43 > 0:29:46and provide enough cover to hide from all those lynx.
0:29:48 > 0:29:52So, the vegetation, here, must be perfect.
0:29:58 > 0:30:03And the reason that it does grow into this perfect environment,
0:30:03 > 0:30:06well, you could never guess.
0:30:10 > 0:30:14The most beautiful thing about this story
0:30:14 > 0:30:17is that the lynx, the hare,
0:30:17 > 0:30:22patches of cover like this amongst the forest, didn't happen by chance.
0:30:22 > 0:30:28They're all controlled by the most unlikely of creatures,
0:30:28 > 0:30:31a tiny thing, less than the size of one of my fingernails.
0:30:31 > 0:30:34And at the moment it's hiding,
0:30:34 > 0:30:38having burrowed into the bark of one of these trees,
0:30:38 > 0:30:42or perhaps in a crack in a log lying on the forest floor,
0:30:42 > 0:30:44covered with frozen snow.
0:30:44 > 0:30:48But it's there, and it's waiting.
0:30:48 > 0:30:50It's waiting for springtime.
0:31:10 > 0:31:16As the hours of daylight increase and the ground thaws,
0:31:16 > 0:31:20as if by magic, the northern forests change again.
0:31:44 > 0:31:48As new leaves appear, trees start producing sugars,
0:31:48 > 0:31:51and that's good for the fungus in the soil.
0:31:51 > 0:31:56Entwined with the trees' roots, they can siphon off some of these sugars.
0:31:58 > 0:32:02But not everything appears so harmonious.
0:32:02 > 0:32:03In the land of the lynx,
0:32:03 > 0:32:06something extraordinary is happening to the forest.
0:32:09 > 0:32:11These trees may have endured the winter,
0:32:11 > 0:32:14but now it's spring, they're under attack.
0:32:17 > 0:32:19Some are even dying.
0:32:22 > 0:32:25But what's happening here now is vital
0:32:25 > 0:32:30for how this ecosystem functions over the year.
0:32:32 > 0:32:36This defoliation is entirely natural.
0:32:36 > 0:32:39And I might be able to find one of the culprits down here,
0:32:39 > 0:32:43although they're quite tricky to spot.
0:32:43 > 0:32:46They live in these fresh, green shoots.
0:32:46 > 0:32:48Yes, here we are.
0:32:48 > 0:32:53Now, wrapped delicately in these leaves
0:32:53 > 0:32:56is a species that is single-handedly
0:32:56 > 0:33:01influencing the ecology of this entire forest.
0:33:01 > 0:33:06Inside this nest is the caterpillar of the spruce moth. The budworm.
0:33:06 > 0:33:09And it hasn't only wrapped itself up in those leaves
0:33:09 > 0:33:11to hide from predators,
0:33:11 > 0:33:13because it's eating them as well.
0:33:13 > 0:33:16And it doesn't just eat the leaves,
0:33:16 > 0:33:21it also eats the buds, the flowers, and the cones on the tree here.
0:33:21 > 0:33:26Up in the canopy, a tiny budworm caterpillar
0:33:26 > 0:33:30has just emerged from hibernation.
0:33:30 > 0:33:33It's racing to fatten itself up.
0:33:42 > 0:33:46When it's finished on one branch, it releases a strand of silk,
0:33:46 > 0:33:49and abseils down to the next.
0:34:10 > 0:34:13It's a risky business being a juicy, fat caterpillar -
0:34:13 > 0:34:17you're in danger of being spied by all of those birds,
0:34:17 > 0:34:19just back from migration.
0:34:27 > 0:34:29But the caterpillar has a plan -
0:34:29 > 0:34:32it uses its silk
0:34:32 > 0:34:35to weave the needles together and hide in a dense web.
0:34:43 > 0:34:48Now, the springtime assault by these caterpillars
0:34:48 > 0:34:49is bad news for the trees.
0:34:49 > 0:34:53But for other inhabitants of these forests,
0:34:53 > 0:34:55these caterpillars are heroes.
0:34:59 > 0:35:01It's thanks to the behaviour of this species
0:35:01 > 0:35:05that one of North America's most elusive and charismatic predators -
0:35:05 > 0:35:10the Canadian lynx - is enjoying a bit of a renaissance
0:35:10 > 0:35:12in forests like this one.
0:35:12 > 0:35:18But the caterpillar lives all the way up there, in the canopy.
0:35:18 > 0:35:22Whilst the cat with the pointed ears is prowling around down here.
0:35:22 > 0:35:27So, how can a humble insect like this have any impact
0:35:27 > 0:35:30on a formidable thing like that?
0:35:30 > 0:35:33I bet the lynx never even sees the caterpillar
0:35:33 > 0:35:35throughout the course of its life.
0:35:35 > 0:35:38Why does the lynx need the caterpillar?
0:35:41 > 0:35:46The clue is how they affect the lynx's prey on the forest floor.
0:35:48 > 0:35:53For decades, scientists have studied budworm caterpillars,
0:35:53 > 0:35:55and a remarkable pattern has emerged.
0:35:55 > 0:35:57They've discovered
0:35:57 > 0:36:01that the population of caterpillars fluctuates dramatically.
0:36:03 > 0:36:05And at the peak of a cycle
0:36:05 > 0:36:09there can be tens of thousands of budworms in a single tree.
0:36:17 > 0:36:20And this has devastating consequences.
0:36:24 > 0:36:28Whilst these dramatic natural events might be a catastrophe
0:36:28 > 0:36:32for the established trees, for anything trying to grow
0:36:32 > 0:36:35on the forest floor they are an absolute bonus.
0:36:35 > 0:36:38In here, where it's dark, there's very little,
0:36:38 > 0:36:42very poor diversity - just some mosses and a few ferns.
0:36:42 > 0:36:44But as soon as there's a break in the canopy
0:36:44 > 0:36:49and the sunlight can flood in, well, look at the difference.
0:36:49 > 0:36:53Lots of wild flowers, there's a young maple coming through here,
0:36:53 > 0:36:59mountain ash, and, most importantly of all, regenerating spruce and fir.
0:37:01 > 0:37:07The hares essentially need these regenerating conifers as shelter.
0:37:08 > 0:37:13And of course what's good for the hares is also good for the lynx.
0:37:16 > 0:37:20It's such an elegant connection.
0:37:20 > 0:37:23Without the spring emergence of the hungry caterpillars
0:37:23 > 0:37:25to chew holes in a dense canopy,
0:37:25 > 0:37:30there wouldn't be enough light flooding the forest floor.
0:37:30 > 0:37:34And, with less light down here, there would be less growing
0:37:34 > 0:37:38for our snowshoe hare to forage and to hide in,
0:37:38 > 0:37:41and then there would be nothing for L1-11
0:37:41 > 0:37:45and all of those hundreds of other lynx to eat.
0:37:45 > 0:37:50And that's why the lynx needs the caterpillar.
0:37:50 > 0:37:51And now it's spring,
0:37:51 > 0:37:54there's no better time to see what the future holds
0:37:54 > 0:37:56for the lynx population.
0:38:13 > 0:38:17Wildlife biologist Jen is doing a count.
0:38:19 > 0:38:24She has detected a signal from a radio-collared female.
0:38:40 > 0:38:44There she is.
0:38:44 > 0:38:47But there might be something else here.
0:38:50 > 0:38:56Safe inside her den, a lynx cub.
0:38:56 > 0:38:58He's just a few weeks old.
0:39:11 > 0:39:13His eyes aren't even open.
0:39:13 > 0:39:17Jen must work fast before Mum returns.
0:39:34 > 0:39:39Thanks to the timing of the budworm opening the canopy this spring,
0:39:39 > 0:39:43there's going to be enough prey for these lynx to hunt next winter.
0:40:00 > 0:40:07Far away to the west, The Raincoast forest is coming back to life.
0:40:20 > 0:40:23Thanks to the richness of the autumn salmon run,
0:40:23 > 0:40:25the bears have survived the winter.
0:40:32 > 0:40:36They've now returned to the forest, looking for something to eat.
0:40:36 > 0:40:39They'll survive on vegetation until the next salmon run.
0:40:44 > 0:40:48The emergence of the bears is a cue for scientists
0:40:48 > 0:40:51to conduct a rather risky experiment.
0:40:51 > 0:40:54They need a large, hungry, bear.
0:40:54 > 0:41:00It's the only way to measure the impact of all those salmon
0:41:00 > 0:41:02on this ecosystem
0:41:02 > 0:41:05and to understand why this vast,
0:41:05 > 0:41:09ancient forest has thrived for so long.
0:41:15 > 0:41:17For such a big question,
0:41:17 > 0:41:22the methods employed by senior researcher Chris Darimont
0:41:22 > 0:41:23seem a bit curious.
0:41:26 > 0:41:30He's equipped with a can full of old salmon guts,
0:41:30 > 0:41:36effusing, probably, the most disgusting smell known to man.
0:41:36 > 0:41:39This is wonderful stuff.
0:41:42 > 0:41:46But he hopes the bears are going to love it.
0:41:46 > 0:41:51He's made an aerial lure.
0:41:51 > 0:41:53So, the wind will carry this distinctive perfume
0:41:53 > 0:41:55deep into the forest.
0:41:55 > 0:41:56Wind, extra boost.
0:42:04 > 0:42:07Now they surround the area with barbed wire.
0:42:08 > 0:42:10And it's this
0:42:10 > 0:42:13that they hope will collect what they're so interested in -
0:42:13 > 0:42:18a single hair from a visiting bear.
0:42:19 > 0:42:21Now the site is prepared,
0:42:21 > 0:42:24it's time to set some remote cameras,
0:42:24 > 0:42:26and beat a hasty retreat.
0:42:32 > 0:42:36Personally, I'm very happy to watch from a safe distance -
0:42:36 > 0:42:40it's not the smell - some of those bears are huge!
0:42:43 > 0:42:46Look at the size of this bear!
0:42:51 > 0:42:56As a trap, this is the perfect bait, it's working brilliantly.
0:42:56 > 0:42:57The bears have come in,
0:42:57 > 0:43:00and they're snagging themselves on the wire there.
0:43:00 > 0:43:04You can see it vibrating about. And that's just what we want.
0:43:04 > 0:43:06Fresh out of hibernation,
0:43:06 > 0:43:11it seems they can't resist this pile of stinking salmon.
0:43:11 > 0:43:14This one's even rolling around in the stuff now.
0:43:14 > 0:43:17No doubt it values the scent - I'm not sure we would.
0:43:21 > 0:43:24The bear's coat has been growing for nearly a year.
0:43:24 > 0:43:27But soon it will be moulted and lost.
0:43:31 > 0:43:34The more bears we can attract, the better.
0:43:44 > 0:43:47Now the coast is clear,
0:43:47 > 0:43:51and it's time to retrieve any fur from the barbed wire.
0:43:56 > 0:44:04So, what on earth can a hair tell us about this forest ecosystem?
0:44:09 > 0:44:11Well, hair is made of protein,
0:44:11 > 0:44:16sourced from whatever the bear has been eating over the last year.
0:44:27 > 0:44:30And by analyzing this hair,
0:44:30 > 0:44:35science can reveal an astonishing level of detail about a bear's life.
0:44:42 > 0:44:47We can learn so much from a single bear's hair.
0:44:47 > 0:44:51So I know, for instance, that this one has come from a female grizzly.
0:44:51 > 0:44:54I know exactly what it's been eating even on a week-by-week basis,
0:44:54 > 0:44:56where that food has come from
0:44:56 > 0:44:59and even the impact on the quality of its life.
0:44:59 > 0:45:03So this bear has been getting most of its protein,
0:45:03 > 0:45:05not from the forest around here,
0:45:05 > 0:45:08but actually from the deep ocean, via the salmon.
0:45:08 > 0:45:12And we know that throughout the course of the year
0:45:12 > 0:45:1680% of that bear's protein has come from these salmon.
0:45:16 > 0:45:18And that's surprising because, remember,
0:45:18 > 0:45:21they're only available to the bear for a few weeks during the autumn.
0:45:21 > 0:45:25And yet the impact is clearly lasting all year.
0:45:26 > 0:45:31So, how come the bears appear to be so full of salmon?
0:45:42 > 0:45:46Well, back in autumn, we saw the bears scattering fish carcasses
0:45:46 > 0:45:48all around the forest floor.
0:45:56 > 0:46:00As the protein in those rotting salmon broke down,
0:46:00 > 0:46:03nitrogen from it accumulated in the soil.
0:46:03 > 0:46:05And this salmon nitrogen
0:46:05 > 0:46:09is like fertiliser!
0:46:11 > 0:46:13So, in spring,
0:46:13 > 0:46:17nutrients all the way from the ocean gradually appear
0:46:17 > 0:46:20in all the vegetation growing here.
0:46:21 > 0:46:23Just in time for the hungry bears
0:46:23 > 0:46:28to eat as they emerge from hibernation.
0:46:28 > 0:46:33And hungry bears have huge appetites -
0:46:33 > 0:46:35they'll eat a third of their body weight every day!
0:46:38 > 0:46:44So that's why their bodies appear to contain so much salmon.
0:46:52 > 0:46:56But the impact of this ocean-born nitrogen
0:46:56 > 0:46:59extends far beyond bears and their food.
0:47:02 > 0:47:04This particular form of nitrogen
0:47:04 > 0:47:07can be found in almost all of the animals and plants
0:47:07 > 0:47:10that appear here in the spring.
0:47:20 > 0:47:25These Rufous Hummingbirds have migrated to the forest to breed.
0:47:25 > 0:47:27They're drinking nectar from plants
0:47:27 > 0:47:29that have been fertilised by rotted fish.
0:47:29 > 0:47:33So, they'll carry the same salmon nutrients with them
0:47:33 > 0:47:35as they fly through the forest.
0:47:43 > 0:47:47Many of the insects pollinating the plants now
0:47:47 > 0:47:50were incubated in that decaying flesh back in the autumn.
0:47:53 > 0:47:56As they themselves are eaten,
0:47:56 > 0:47:59the salmon nutrients are spread even further.
0:48:02 > 0:48:06Thanks to the bears, the insects, and the birds,
0:48:06 > 0:48:11this salmon fertiliser is spread deeper and deeper into the forest -
0:48:11 > 0:48:14sometimes as much as 800m from the river,
0:48:14 > 0:48:19and this pulse of nutrients then allows the organisms
0:48:19 > 0:48:27which define the forest itself to prosper - its trees.
0:48:41 > 0:48:45Doctor Tom Reimchen can measure exactly how much
0:48:45 > 0:48:49these vast old trees need the fish.
0:48:51 > 0:48:58The secret is to look inside the tree, by taking a core sample.
0:49:05 > 0:49:09Written on it is the entire story of this 300-year-old tree.
0:49:16 > 0:49:20The rings I see are two, three, even four millimetres,
0:49:20 > 0:49:22which continues back
0:49:22 > 0:49:28to even the early parts of the 1800s, late 1700s.
0:49:28 > 0:49:34Tom has taken similar samples from thousands of trees.
0:49:34 > 0:49:38An entire forest is lined up in his lab,
0:49:38 > 0:49:41each tree waiting to tell its own story.
0:50:01 > 0:50:06In this seasonal climate, annual rings are created as the tree grows.
0:50:12 > 0:50:16From these rings, he can determine not just the age of the tree,
0:50:16 > 0:50:19but also the amount of growth in each year.
0:50:19 > 0:50:22Some of the rings are thicker than others,
0:50:22 > 0:50:26showing that the tree has grown more.
0:50:32 > 0:50:38Like the bear fur, each annual ring can be analyzed.
0:50:38 > 0:50:41Tom can search for the same type of nitrogen
0:50:41 > 0:50:44that's found in the bear's hairs.
0:50:44 > 0:50:46It comes from the ocean
0:50:46 > 0:50:50and it's called nitrogen-15.
0:50:54 > 0:51:00And the data will tell us just how much nitrogen in those trees
0:51:00 > 0:51:03comes from all those salmon.
0:51:08 > 0:51:12I think this is really exciting.
0:51:12 > 0:51:15You see, the annual growth rings here
0:51:15 > 0:51:18show the presence of the stable isotope Nitrogen-15,
0:51:18 > 0:51:21which significantly comes from the oceans.
0:51:21 > 0:51:25It could only have been carried here by the salmon.
0:51:25 > 0:51:28Now look, here is the present, the bark on the outside of the tree,
0:51:28 > 0:51:31so these rings represent perhaps the last 15 years
0:51:31 > 0:51:34and they're very closely packed together.
0:51:34 > 0:51:39But here, back in the 1980s, the rings are twice as thick.
0:51:39 > 0:51:43The trees have been growing twice as much during the course of a year.
0:51:43 > 0:51:47So, perhaps the salmon runs then were even more productive
0:51:47 > 0:51:50than they have been recently.
0:51:50 > 0:51:53But that's not the best thing. Come and have a look at this.
0:51:53 > 0:51:58By measuring the abundance of that nitrogen isotope in this material,
0:51:58 > 0:52:04I can tell you that majestic old giants like this beauty here
0:52:04 > 0:52:09are actually composed of up to 85% material
0:52:09 > 0:52:13that's derived from salmon.
0:52:13 > 0:52:15Now, when I was a teenager,
0:52:15 > 0:52:19I remember learning that I was made of carbon,
0:52:19 > 0:52:22and carbon could only be formed when stars died.
0:52:22 > 0:52:24Effectively I was made of dead stars
0:52:24 > 0:52:28and that struck me as terribly romantic.
0:52:28 > 0:52:30But look at this.
0:52:30 > 0:52:32This is a forest made of the ocean!
0:52:35 > 0:52:39That's why the tree needs the fish.
0:52:39 > 0:52:44Without this unlikely-sounding relationship,
0:52:44 > 0:52:48this magnificent ancient forest just wouldn't be the place it is today.
0:52:51 > 0:52:56But there is one more relationship,
0:52:56 > 0:53:00crucial to seasonal forests all around the world,
0:53:00 > 0:53:03when it comes to surviving constant change.
0:53:03 > 0:53:08And it's one that reaches its greatest intensity now,
0:53:08 > 0:53:10at the height of summer.
0:53:12 > 0:53:16It's the driest time of year, and the trees need water.
0:53:20 > 0:53:23Fortunately, united with their fungal partners,
0:53:23 > 0:53:26the trees have massively extended their roots.
0:53:26 > 0:53:29Fungal threads in the soil are absorbing water
0:53:29 > 0:53:31and passing it to the tree.
0:53:33 > 0:53:37But what has only recently been discovered
0:53:37 > 0:53:42is the sheer scale of these fungal root networks.
0:53:42 > 0:53:46A single cubic centimetre of the soil here
0:53:46 > 0:53:52can have a mile of these white fungal threads running through it.
0:53:52 > 0:53:55They're called mycorrhizae.
0:53:55 > 0:53:57And, for me,
0:53:57 > 0:54:01it's these organisms that are the real secret of the forest here.
0:54:03 > 0:54:06In the lab, the genetic fingerprints
0:54:06 > 0:54:10of individual mycorrhizae have been identified.
0:54:16 > 0:54:19By mapping an area 30m across,
0:54:19 > 0:54:21it's been discovered
0:54:21 > 0:54:27that individual fungi connect to more than a single tree.
0:54:27 > 0:54:33Just one fungus can be joined to 80% of all of the plants growing here.
0:54:34 > 0:54:41And, amazingly, these physical links enable different species of plants
0:54:41 > 0:54:44to exchange nutrients.
0:54:44 > 0:54:47Older established plants are even nurturing
0:54:47 > 0:54:49younger weaker ones.
0:54:49 > 0:54:53It acts like an underground welfare system!
0:54:54 > 0:54:56These giant webs
0:54:56 > 0:55:01connect all of the trees in this forest,
0:55:01 > 0:55:05and keep them, and all of the things that are dependent upon them, alive.
0:55:05 > 0:55:11That's why scientists are calling this the Wood Wide Web.
0:55:22 > 0:55:27It's thanks to this natural phenomenon, the Wood Wide Web,
0:55:27 > 0:55:32that, together, the trees in the forest ecosystems are resilient -
0:55:32 > 0:55:36resilient enough to cope with the dramatic changes
0:55:36 > 0:55:40they encounter every year.
0:55:48 > 0:55:52And what's really amazing is how the web is built.
0:55:55 > 0:55:58It's thanks to hungry mammals like our flying squirrels
0:55:58 > 0:56:04that this essential life support system is effectively maintained.
0:56:05 > 0:56:11It's actually fair to say that these trees wouldn't be standing here,
0:56:11 > 0:56:16wouldn't be thriving, unless a squirrel had eaten a truffle.
0:56:16 > 0:56:19And that is fantastic!
0:56:22 > 0:56:23It is fantastic.
0:56:23 > 0:56:27It's fantastic to think that what animals do in one season
0:56:27 > 0:56:33influences the forest ecosystem throughout the year.
0:56:35 > 0:56:42It's almost as if all of these stories are choreographed.
0:56:45 > 0:56:49The arrival of the salmon at exactly the right time
0:56:49 > 0:56:52to fatten the bears for winter.
0:56:52 > 0:56:56Then, the emergence of the lush green vegetation
0:56:56 > 0:56:58fertilised by those salmon
0:56:58 > 0:57:02to sustain the bears when they emerge from hibernation.
0:57:02 > 0:57:06The squirrels - foraging for truffles in the autumn time,
0:57:06 > 0:57:09and sowing their spores throughout the forest
0:57:09 > 0:57:11to grow a fungal network
0:57:11 > 0:57:15that joins all of the trees and all of the plants,
0:57:15 > 0:57:17and provides them with nutrients.
0:57:17 > 0:57:23The budworm - chewing a hole in springtime in the canopy,
0:57:23 > 0:57:28so that in summer, sunlight floods down to the forest floor
0:57:28 > 0:57:33and produces the perfect hunting habitat for lynx.
0:57:33 > 0:57:36It's all in the timings.
0:57:36 > 0:57:40And it's this that makes these temperate forests
0:57:40 > 0:57:42such magical places.
0:57:59 > 0:58:01Join me next time,
0:58:01 > 0:58:05when I'll be travelling to some of the world's greatest water habitats.
0:58:21 > 0:58:23Subtitles by Red Bee Media Ltd