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  • [narrator] Just 50 years ago,

  • we finally ventured to the moon.

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

  • Since then, the human population has more than doubled.

  • This series will celebrate the natural wonders that remain

  • and reveal what we must preserve

  • to ensure people and nature thrive.

  • Forests.

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

  • stand in these great assemblies.

  • [birds chirping]

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

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

  • In the past, we have destroyed them without hesitation.

  • Yet, forests do have an astonishing ability to recover.

  • This is the southern edge of the boreal forest,

  • the forest that dominates the far north.

  • It stretches eastwards across Russia for thousands of kilometers,

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

  • seemingly empty of animals of any kind.

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

  • A Siberian tiger.

  • There are less than 600 of them.

  • [caws]

  • [tiger growls]

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

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

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

  • to keep itself alive during the long winter.

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

  • A pine cone.

  • Pine nuts provide vital energy during the winter months.

  • Wild boar depend on these nuts.

  • For them, it has to be grab and go,

  • for they themselves are food for a tiger.

  • [squeals]

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

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

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

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

  • These rare glimpses reveal

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

  • The boreal forest extends from Russia in the east,

  • across Europe, to North America.

  • It contains 750 billion trees,

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

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

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

  • that growth is at a standstill.

  • This far north, the forest can support very few animals

  • compared to the rain forests  of the tropics.

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

  • [birds caw]

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

  • salmon, having left the ocean,

  • swim up rivers to reach their ancestral spawning grounds.

  • Their journey from the sea into the forest represents

  • the greatest transfer of nutrients from one habitat to another

  • anywhere on Earth.

  • The fish are crucial seasonal food

  • for all the predators that live in this forest.

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

  • [piercing chirp]

  • [narrator] Young eaglesstill without white heads,

  • spend their first years searching for food in the forest.

  • But, like this four-year-old female,

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

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

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

  • [eagles chittering]

  • [narrator] She will have to  look elsewhere.

  • The boreal forest may be vast,

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

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

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

  • [chittering]

  • [narrator] She does have one advantage.

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

  • And here, size matters.

  • [cawing]

  • [narrator] As winter advances,

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

  • If she can survive, she will have gained a place

  • in one of the greatest and harshest forests on Earth.

  • A little further south

  • stand the last  of the great redwood forests.

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

  • Now, only five percent of them remain.

  • Here, conditions are less harsh.

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

  • so the trees are able to grow the year round.

  • Hidden in the vegetation of the moist forest floor

  • is a richly populated underworld.

  • A male rough-skinned newt.

  • Every spring, he is driven by a mysterious urge

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

  • And he needs to get there quickly.

  • Because there are lots of others like him...

  • with exactly the same idea.

  • They're all searching for a female,

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

  • There is strong competition.

  • And the female is anxious to get going.

  • Once paired, he hangs on,

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

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

  • Competition is so vigorous

  • that the females in the center of these writhing balls

  • have been known to drown for lack of air.

  • But her first partner maintains his grip.

  • Within weeks, their tadpoles will hatch.

  • And none too soonfor the forest is about to change dramatically.

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

  • that they are as dry as tinder.

  • Strong winds drive the flames,

  • and temperatures rise to 700 degrees Celsius.

  • Its aftermath is seemingly total destruction.

  • The wind, scouring the newly exposed forest floor,

  • kicks up dust devils of ash.

  • The ground appears to be devoid of life.

  • But the forest is far from dead.

  • Within only a few months,

  • flowers and tree seedlings will rise from the soil.

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

  • [birds chirping]

  • [narrator] Light streaming down through the newly opened canopy

  • provides the energy for a surge of new growth.

  • The older, well-established redwoods have survived,

  • protected by their thick, fire-resistant bark.

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

  • Most forests, however, cannot recover on their own.

  • Many are helped to recover by animals.

  • The trees and their inhabitants are interdependent.

  • And nowhere is this dependence more apparent

  • than in India's Western Ghats.

  • This globally important habitat

  • contains a third of all animal species found in India.

  • [animals whoop and chirp]

  • [narrator] Lion-tailed macaques.

  • They feed on the fruits of many kinds of tree,

  • and, in doing so, they distribute the seeds,

  • which will emerge unscathed with their droppings.

  • But few monkeys travel very far.

  • If plants are to distribute their seeds across great distances,

  • they need another kind of transport.

  • Wings.

  • Great hornbills have wings that are almost two meters across,

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

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

  • and transport the indigestible seeds they contain

  • all over the Western Ghats.

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

  • The figs themselves, although they taste nice,

  • are not, in fact, very nutritious,

  • so the hornbills have to eat great quantities of them.

  • It can be tiring work.

  • But it's not a free-for-all.

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

  • Competition between them is intense.

  • This aerial jousting between males has never been filmed before

  • and its purpose is not really understood.

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

  • but it seems rather more than that.

  • [squawking]

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

  • to visiting females.

  • The males certainly seem keen to ingratiate themselves.

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

  • because, once established, the bond between male and female

  • will last a lifetime.

  • The forest also benefits from these squabbles,

  • because even a defeated hornbill

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

  • The relationship between trees and animals in a forest

  • is not always so harmonious.

  • This is Africa's largest forest, the Miombo,

  • named after one of its common trees.

  • It stretches for over a thousand miles,

  • from Angola in the west to Mozambique in the east.

  • At the height of the dry season,

  • the Miombo attracts animals from all across southern Africa.

  • [trumpeting]

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

  • In its absence,

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

  • [elephants huffing]

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

  • These mopane worms

  • are not worms, of course, but caterpillars.

  • They hatch simultaneously in huge numbers.

  • And just as the forest is putting out new leaves,

  • they begin their attack.

  • They feed so voraciously

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

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

  • will have stripped the entire forest of its foliage.

  • But the Miombo bounces back.

  • With the caterpillars gone,

  • the trees produce a second growth of leaves.

  • This fresh feast then attracts elephants.

  • They are less fussy than the caterpillars.

  • They will eat every part of the tree.

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

  • Yet, even this destruction has its benefits.

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

  • Hunting dogs.

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

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

  • [grunts]

  • [narrator] Browsing elephants  open up a forest,

  • and that attracts the animals on which the dogs prey.

  • [barks]

  • [dogs yipping]

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

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

  • The pups greet the adults returning from a hunt

  • with great excitement.

  • [squealing]

  • [narrator] Food.

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

  • [soft growling]

  • [narrator] Now, they are confident enough

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

  • And then, like all puppies, they have fun.

  • Playing is important for the youngsters,

  • for as they do so, they establish the social bonds

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

  • -[bird squawks] -[squealing]

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

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

  • Never has it been more crucial

  • for the survival of this endangered species than now.

  • But they will only survive if other creatures are here

  • to create the kind of habitat they need.

  • [elephant lows]

  • [narrator] Madagascar has a forest

  • dominated uniquely by one of the oddest of trees.

  • Baobabs.

  • The island has been isolated for over 80 million years.

  • During that immensity of time, its animals and plants

  • have evolved into forms quite different from any elsewhere.

  • This makes them one of the most precious forests on our planet.

  • This is not a monkey, but a distant relative