In the lands between the Arctic Circle and the tropics,
each year brings a great change between winter and summer,
imposing a rhythm in the lives of animals and plants.
Up north in the great evergreen forests,
conditions in mid-winter are cripplingly severe.
Life, if it is to flourish, has three needs:
Light, warmth and moisture.
And the reason trees like these don't grow much farther north
is not only the extreme cold,
but with the long months of winter darkness,
there is not enough light in the year for them to grow.
Here in northern Norway, 300 miles, 500km north of the Arctic Circle,
there is just enough light, but it does get extremely cold.
70 degrees of frost have been measured here,
and in winter there are very heavy snowfalls.
The cold threatens to freeze the liquid within the trees,
and denies them one of their essential supplies: Water.
Although snow and ice lie all around,
the trees can't tap that water while it's frozen.
So this land is effectively as parched as a desert
and the pine trees have as great a need to conserve water as a cactus.
All plants lose some water through their leaves,
but pine needles are protected by a near-impermeable rind.
The pores through which they breathe,
and from which water can evaporate,
are kept out of the wind by being placed
along the groove that runs the length of the needle,
each in a tiny pit ringed with a ridge.
These dry, waxy leaves are almost inedible,
but the seeds in the cones are not,
and are one of the few kinds of food available in the forest in winter.
The crossbill's special beak enables it to separate the cone's segments
and prise out the nutritious seeds.
This winter feast is never certain.
Some years every branch of the trees will be laden with cones,
in others there will only be a handful.
Then the seed-eaters must move on or die.
The few remaining cones can then shed their seeds into the snow
when there are few animals around.
Even so, there are some.
Voles make their runways through the snow and collect what they can.
Moose get little nourishment from pine trees,
apart from the shaggy moss that hangs from the branches.
They chew the sappy twigs and bark of birch,
but there's not enough to keep them going.
If it wasn't for the fat reserves they built up in summer, few would survive.
The winter forests can support very few plant-eaters,
but there are just enough to feed one or two hardy hunters.
The great grey owl's legs are ideal for grabbing prey in snow:
Long and covered with warm feathers.
It regularly patrols the snow,
for it can't afford to miss a single opportunity of a meal.
And this is an incautious move.
Lynx seek bigger prey.
The female has young, which, though large,
are not yet skilled enough to hunt for themselves, so they rely on her.
The cost-efficiency of hunting is precisely calculated.
If the lynx doesn't catch a hare within 200 yards,
the meat it might provide is not enough to warrant the effort,
and the lynx gives up.
Bigger prey are worth much longer chases,
and the lynx pursue roe deer with great persistence.
A single deer will provide food for the whole lynx family.
In this bleak land, even the most ferocious and capable hunters
do not scorn to scavenge.
An eagle owl will take cold deer flesh
just as eagerly as the warm bodies of voles.
A wolverine, the biggest of the weasel family,
and more than a match for an eagle owl.
The coniferous forest grows right round the globe
in a belt that, in places, is 1,200 miles across.
From Scandinavia, it extends across northern Europe and Siberia
to the shores of the Pacific.
During the last ice age, when the seas were lower,
the Bering Strait did not exist,
so the trees continued into North America,
across northern Canada to the Atlantic.
Consequently, all the trees in this vast forest
and its permanent inhabitants in America, Asia and Europe, are much the same.
But when spring comes, visitors journey up from warmer parts
and each forest takes on its own individual character.
In Scandinavia, a hawk owl,
a nomad that has spent the winter farther south,
comes cruising up north again looking for food and a nest site.
Unlike other owls, it's primarily a daytime hunter,
and relies not so much on its acute hearing as its sharp eyesight
as it waits for the melting snow to reveal rodents.
In pine trees, from Norway to Siberia,
the cock capercaillie claims his territory.
This giant grouse is one of the few creatures that eats pine needles.
His hen takes them too.
Now is the time for nesting.
The hawk owl is in search of a hole in a tree,
for it's already found its partner.
But many tree holes are occupied,
for great numbers of owls have travelled up to feed on the voles.
No owl can dig a hole for itself.
They rely mainly on woodpeckers, and none is a more expert carpenter
than the black woodpecker of northern Europe.
Their sharp beak serves as an excellent chisel,
but most prefer to work in dead trees where the wood is softer.
There are ants near this tree too,
which the woodpeckers rely on for food in winter.
Not all owls use nest holes.
The eagle owl nests on the ground, among rocks.
It already has a clutch of three eggs,
for, being a permanent resident of these forests, it paired early.
Plants now have their chance to breed.
The wood anemones are already in flower, as are the pines.
Each tree produces male and female flowers,
which mature at different times,
so the female flowers are likely to be fertilised by pollen from other trees.
Now it is as warm as it ever will be in the northern forests.
Summer visitors are arriving, and the trees echo with their song.
This willow warbler, singing so vigorously in Scandinavia,
has come all the way from the savannah country south of the Sahara.
So has the winchat.
And the lure that has brought them so far
is the sudden emergence of myriads of insects.
This bedraggled creature is hardly recognisable, for its wings have not yet expanded.
It's a pine beauty moth,
and its first priority is to leave the forest floo which is full of danger.
But not all the moths have such a clear run.
Shrews are among the first to feed on them.
Up among the pine needles, the pine beauty pumps fluid out of its body
and into the veins of its wings.
Here the moths will lay their eggs so their caterpillars can feed on the young shoots
The wood ants have missed their chance to catch the adult moth,
but now they're looking for the caterpillars among the branches.
The colour and the pattern of the caterpillar conceals it from birds which hunt by sight,
but is no protection against ants which search by smell and touch.
Finally the body is hauled down to the nest for all to consume.
The caterpillars of the sawfly are also swarming on the pine shoots.
They do have a defence against ants: A chemical one.
As they chew, they store some of the resin from the pine needles in a pouch inside their mouth.
When a foraging ant discovers them, they dab a spot of this resin on its head, like th
The resin damages the ant's eyes and antennae, so disorientating it that it can hardly walk.
Even if it finds its way back to the nest,
it smells so strongly and strangely that the other ants treat it as an intruder and kill it
The ants themselves are food for others.
The wryneck is a member of the woodpecker family that has specialised in eating ants,
and particularly relishes their cocoons.
Like its cousins, the wryneck nests in holes in trees, but it doesn't excavate them for itself.
It is yet another tenant of vacated woodpecker holes.
With a long tongue you can even collect insects from the bark without leaving your nest.
Here in the far north, close to the Arctic Circle,
the sun during the summer hardly sinks below the horizon and the nights are brief.
The eagle owl hunts just as effectively in the twilight as in the dark.
It has a rabbit. The season is a good one and game is abundant.
Down in the nest on the forest floor, there is only one chick left.
The other two may have been taken by foxes.
Eagle owls often kill rival species, and this chick's last meal was a short-eared owl,
which it's not yet finished.
The single survivor has a superabundance of food.
It has grown fast and its adult feathers are already appearing through its down.
The tail of a red squirrel is left over from a previous meal,
and it even takes that too.
The voles are swarming on the forest floor.
Last winter, the pines produced great quantities of seed,
so many adult voles survived till spring
and now they're all breeding at an extraordinary rate.
This female produced her four young only three weeks ago,
but she is already pregnant again and will soon abandon this family and start a new one.
All the owls, some visitors, some residents,
scour the forest for voles.
Tengmalm's owl, up in a tree hole, has three chicks, all flourishing and all demanding voles.
The number of voles varies considerably.
It gradually builds up over a period of five to six years
until finally there are so many that they eat out their food supply and the population crashes.
These changes have their effect on the owl population.
More voles mean better-fed owls,
which produce bigger clutches of eggs and rear more chicks.
And as the number of owls increases, so they spread out into new territory.
I'm in Finland, very close to the Russian border.
In fact, those pine forests behind me are actually in Russia.
But the frontier is no barrier to the bird they call the phantom of the north,
the great grey owl,
and in years when the vole population is high,
the owl comes across these frontiers and into the Finnish pine forests.
And I know they are here already because I have just picked up this.
This is an owl pellet.
All owls, as part of their natural digestion, throw up the fur and bones of their prey.
And this, I can see, has actually got vole skulls in it.
But to discover exactly what the state of the vole population is at the moment,
I'll have to look inside the nest of a great grey and to do that I'll need this.
All owls are fairly ferocious and the great grey owl certainly can be,
so as part of the standard equipment of looking for owl nests you need this.
Up there is one of their nests, and the female has just flown off.
She's perching in that tree over there, keeping a very close eye on me.
If I go up and have a look in the nest,
I may be able to get some idea as to how the vole cycle is going.
And... come on...
there is just one chick.
If the voles had been at the height of their population,
there would probably be four chicks in such a nest as this,
but the fact there is only one makes it pretty clear
that the vole population is already beginning to crash.
So it is very likely the female and her mate will soon be on their way back to Russia.
There's now just a month left of the short northern summer.
Many of the birds that came up here to harvest the insects and to breed
will soon be moving back again to avoid the severities of the coming winter.
Some, like the redwing, will go to open pastureland down south.
The brambling prefers beech woodland,
and will leave almost as soon as it has finished its summer moult.
The hawk owl is driven south by hunger,
for as the forest gets colder, there is less food to be found.
As it flies south, so the trees beneath change character.
The ranks of dark conifers are replaced by the brighter green of the broadleaved trees:
Oak and ash, birch and beech.
Down here, the weather's warmer, the summers are longer, and the woodlands are free of frost,
not forjust two or three months in the year, but for eight or nine,
and the shape of the trees is very different.
Instead of their branches drooping down, and so shedding the snow,
these branches spread out widely,
carrying tier upon tier of leaves with which to catch the abundant energy of the sun.
And the leaves are very different.
They are not covered with a thick, protective rind but are thin, delicate structures.
During the summer water is more accessible,
so there is less need to take rigorous measures to conserve it.
Indeed, during hot days the trees evaporate large quantities to keep cool.
So the pores through which they breathe are numerous,
and not in pits as they are in the pines.
These succulent, soft leaves, unlike pine needles, are relished as food by all kinds of creatures.
Large animals, like deer, take many of them,
but the greatest quantity by far is gathered by insects.
The forest canopy in late summer has more birds in it than at any other time of the year.
There are returning migrants newly arrived from the north,
resident breeders gathering food to feed their second families of the season,
and young fledglings starting to forage for themselves
and still not sure what is edible and what isn't.
Nearly all of them are hunting for insects, and the crop they take is huge.
Not surprisingly, the insects have evolved many ways of protecting themselves.
They snip off half-eaten leaves so as to give the minimum sign of their presence.
They disguise themselves as a blob of cuckoo spit or a bird dropping,
but if they move, as eventually they must, their concealment is lost.
Some hang in places which are difficult to reach.
This might baffle a fledgling, but an adult great tit is both experienced and agile.
The tree creeper specialises in insects that live on bark.
A poplar hawk moth tries to defend itself by pretending to be fierce.
The nuthatch habitually works its way down the trunk,
and that way may see insects that have been overlooked
by tree creepers that habitually come up it.
One of the most expert of all bark-feeders,
and in some ways the most specialised of all the birds living in the tall trees of these forest
are the woodpeckers.
The greater spotted woodpecker is typical of them.
Its hearing is excellent and it locates the grubs it seeks
by the tiny sounds they make as they move inside the bark.
Its tall feathers have strong quills and serve as props for its body.
Its bill has a resilient pad at the base which cushions its brain from the shock of its drilling.
Its feet give it a grip in all directions, with two toes pointing forwards and two backwards.
Each continent has its own range of woodpeckers.
Europe has ten species,
but here in North America there are over twice as many.
This one, a sapsucker, drills holes in trees not for insects, but for sap.
It digs lines of these wells in many kinds of trees.
Each little hole points slightly downwards so that the sap does not trickle out
but collects in a small pool inside,
and the sapsucker collects it with its tongue.
And so do other birds.
A hummingbird.
Most of its family live in the tropics and feed on nectar,
but this one comes north in the summer and finds tree sap just as acceptable.
Flies, too, come to the sweet sap.
In late summer, the parent sapsuckers lead their fledglings to the wells
and leave them to feast not only on the sap but on the insects it attracts
This American woodpecker uses its drilling skills to bore neat sockets in dead tree trunks.
Acorns are its main food, but during the season,
there are far more acorns than the woodpeckers can eat immediately.
But they don't leave them for others.
Several birds share a communal acorn treasury, like this one.
They hammer the acorns into the holes so firmly that few other creatures can get them out,
and the store will keep the acorn woodpeckers supplied throughout the year.
The ripening acorns herald the end of summer and the beginning of autumn.
Trees and bushes proffer their seeds to the forest animals.
Some are wrapped in soft and tasty flesh
to tempt the animals to eat them and so transport them to new sites.
Others are packed with nourishment,
not for animals, but to provide food for the germinating seedling,
but the animals eat them just the same.
Even the hard and unpromising-Iooking acorns of the American pin oak are collected by racoons.
The squirrel's habit of burying acorns for a winte store has been the start of many an oak.
The black bear, on occasion, will eat fish and voles and even carrion,
but much of its diet is vegetable.
It will dig for roots and even eat pine cones,
but it has a very sweet tooth and just now it relishes the fruit.
All sorts of mammals are now clambering around in the trees in search of fruit.
The possum, a strange primitive animal of the Americas
related more closely to kangaroos than to rats, eats almost anything.
Few of them can get to the very tops of the trees or the thinnest twig,
but a chipmunk can.
The chills of autumn presage the coming of winter.
The delicate leaves worked efficiently throughout the warm moist summer,
but they are not suited to cold weather.
Frost will damage them.
Their abundant pores would lose too much water.
So the green chlorophyll in them is broken down and withdrawn into the tree,
revealing the red and brown waste products,
and the leaves fall.
And they, too, provide food for another woodland community,
the inhabitants of the leaf litter.
There may be 100,000 box mites in every cubic yard.
And there are many other creatures too,
chewing their way through the dead leaves,
extracting what nutriment they can
and leaving the remainder to be dealt with by fungi and bacteria.
They themselves are hunted by monsters in miniature,
pseudoscorpions,
horrific in close-up, but, perhaps fortunately, the size of a pinhead.
Snails are giants in comparison and, since they carry their shells around with them,
they might seem to be fairly well protected against any creatures smaller than a bird.
But one particular beetle has specialised equipment for dealing with them.
Its head and jaws are long and thin.
Almost hidden in the leaves of these American woods
are some spectacularly coloured little creatures hardly bigger than worms.
They are amphibians: Salamanders.
Almost every mountain range in the US has its own species with its own colours,
but, being nocturnal, they're rarely seen.
Shrews eat most small living things they come across, and they are formidable hunters,
for they are one of the few mammals that has a poisonous bite.
The salamander's only defence is to produce an acrid liquid from glands on its tail
The first time a shrew encounters this, it usually takes no notice and eats the salamander,
but apparently the taste is not very nice,
for on later encounters, like this one,
one sniff reminds the shrew the meal won't be a good one
and it leaves the salamander alone.
The summer visitors have departed.
The woods have fallen silent.
The days are shortening and the temperature falling.
Eventually the land is gripped tight by frost.
It's late winter.
The once-resplendent trees are now mere skeletons
and life in these woodlands has come almost to a standstill.
The trees, without their leaves, can't grow.
The birds that came visiting up here during the summer have now retreated south,
and some of these small mammals have crawled into holes and gone to sleep.
Their heartbeat has almost stopped, their bodies have become as cold as stone.
They're hibernating.
But that sleep doesn't last throughout the winter.
They wake up every four or five days and go and look for food.
Like, for example, those small chipmunks over there.
Not only warmth but intense cold will bring them out,
for although their body temperature falls while they hibernate,
if it drops to freezing point, they will die.
So in really cold spells, they must get up and warm themselves with a little exercise,
even though it dangerously depletes their fat reserves.
But in these American woodlands there is one spectacular sleeper
who dozes for months on end.
Just look at this.
A black bear.
She retired to this den in early autumn, and after a month or so of drowsiness,
produced her cubs.
In the colder northern parts of these woods, she may spend six or seven months here,
during which time she suckles her cubs
but neither feeds herself nor urinates nor defecates.
So she spends the majority of her life half-asleep.
When spring at last comes, the brown carpet of rotting leaves is suddenly flooded with colour.
The plants that live close to the ground now make haste to sprout and flower
and soak up the spring sunshine
before the trees above produce their own leaves and cut out the light.
The bear's den is empty,
but the owners haven't gone far.
There's still not much to eat, only a few leaves,
nor will there be until the first of the berries come into fruit in summer,
but meanwhile at least the sun is warm.
Another mother spends the spring up in a tree:
A wood duck, only she is about to leave.
The hole has provided a secure nest,
but all ducklings follow their mothers as soon as they hatch.
And now new forms appear from among the dead leaves.
The spring showers soak the woodlands
and create just the moist, warm conditions needed by the fungi
to produce their fruiting bodies.
These must be mature and ready to discharge their microscopic spores
by the time the dry winds of summer begin to blow,
so that their spores, like dust, will be carried all through the forest.
Once, the woods of North America stretched over the eastern half of the continent
in an almost continuous band hundreds of miles deep.
Today, the majority has been felled to make space for farmland and cities,
but enough remains to make plain their splendour.
And now we've come farther south still.
I'm on the borders of Florida and Georgia in the southern United States,
and here it's very hot in the summer and the winters are very mild,
with only a few frosts, and none of them severe.
So some of the broadleaved trees here, like this oak, don't shed all their leaves in the autum
but keep them throughout the year and continue growing.
And these aren't the only evergreens that are here, either.
There are pines.
In some parts where the soil is very rocky or sandy and poor in nutrients,
the pines will grow because nothing else can survive there.
But this pine forest owes its existence to another factor altogether.
Oak saplings are killed within minutes by fire.
But the terminal buds of young pines are surrounded by a shock of needles.
They burn at a relatively low temperature, and by the time the flames have consumed them,
the main fire has swept by
and the bud at the top of the stem, from which new growth will come, is still unharmed.
Fires like these are notjust the work of careless people, they occur naturally.
The spark that regularly sets fire to these forest is lightning.
In this part of the southern States, violent thunderstorms are common
and lightning often strikes the taller trees,
scoring a deep groove down the length of the trunk as it flashes down to earth.
And this at my feet is the tinder which set it aflame.
These are pine needles, and they're so full of resin and they're so dry
that they flame up very easily.
But the fire they produce is not very hot, and it's also very short-lived,
so that if any creature can survive fire forjust one or two minutes,
then it can survive a fire like this.
The rattlesnake, like many other ground-living animals,
regularly takes refuge from the midday sun in holes,
so now it knows exactly where to go to escape the fire.
But this hole is already occupied by its digger and owner,
a gopher tortoise.
Rattlesnake and tortoise do not normally interfere with one another...
...and that seems to be the way things are going to stay.
But in the back of the burrow lies another refugee, an indigo snake,
and it, on occasion, eats rattlesnakes.
But the fire is passing and the rattlesnake can return to the forest.
Some insects don't avoid fire, they actively seek it.
Beetles find it difficult to lay their eggs in the pines
because the trees swamp them with resin.
But a tree killed by fire can't resist,
and these beetles take advantage of the situation.
They have pits behind their legs which are sensitive to infra-red rays,
and therefore they can detect the slightest rise in temperature,
and with these to guide them, they travel from all over the forest to the wake of the fire
and arrive in hundreds.
Quickly they mate.
The females crawl all over the scorched trunks,
seeking crevices in the bark into which they can lay their eggs,
so ensuring that their grubs will have some nice nutritious bark to chew.
As insects assemble in the burnt forest,
the insect-eaters follow.
The oak toad almost exactly matches the colour of the charred forest floor.
Other more conspicuous hunters wait on newly emerged shoots.
Within a couple of months of a summer fire, the forest has more than recovered,
it is rejuvenated.
The fire has cleared away the old growth on the ground,
and by reducing the pine needles to ash has released their nutrients into the soil,
and now the ground sprouts more flowers than at any other time.
Because of regular fires, big bushes can't establish themselves here,
so swampy areas are not colonised and sucked dry by them as happens elsewhere,
and open marshes remain where pitcher plants can grow
and where frogs can swim and breed.
Indeed, one species of frog lives nowhere else but in these pools in the American pine barrens.
The woodpeckers here can't excavate their nest in dead trees as do woodpeckers elsewhere,
for in this fire-ravaged forest they would risk incineration,
so the red-cockaded woodpecker drills its holes in living pines.
But the wood is so hard, it takes several woodpeckers about two years to dig the hole.
Resinous sap seeps out around the hole where the outer layers of the tree have been breached.
So the birds make their hole low down on the trunk
where the inner sap-free heartwood is thick enough to accommodate the entire nest.
The flow of resin is diverted to the outside
by drilling pits like sap wells above and below the hole.
It's in these laboriously excavated holes that the red-cockaded woodpecker raises its young.
The holes are very conspicuous, for each is surrounded by a sheet of yellow congealed resin.
The rat snake is a great robber of nests and stealer of chicks.
It's an extremely skilful tree climber.
Since the woodpecker's hole in the living tree has to be fairly low down on the trunk,
it is within easy reach of the snake and therefore might seem to be in considerable danger.
But now the other function of all that resin,
deliberately produced around the nest by the woodpecker,
is about to become clear.
The chemicals in the resin seem to irritate the snake beyond endurance,
and it arches its body away.
Eventually it's too much.
So fire, one way or another, influences the whole community of animals and plants
in the pine forests of the south.
This injury was also caused by fire, and this is also a coniferous tree,
but a very different one.
To start with, it's over 40 feet across along its base and it's 267-1/2 feet high.
This is a giant sequoia.
It's thought to be about 2,500 years old,
but the largest individual tree of all is this one known as the General Sherman.
It's just taller and it's estimated to weigh 1,385 tons,
which makes it the most massive living organism in the world.
Although these trees are growing almost as far south as the southern pines,
the climate here, 2,000 metres up in the Sierra Nevada mountains,
is much colder and snow lies on the ground for almost half the year.
It's as though, by climbing to this height,
we have returned climatically to the great forests of the north.
During the Ice Age, these sequoias grew over much of North America.
But when, some 8,000 years ago, the earth began to warm,
they died out except for these isolated groups high up in the mountains.
We've travelled some 2,000 miles southwards
since we started at the tree line near the Arctic Circle,
and in all that vast territory the majority of the forest trees have been conifers,
so it seems only right and proper that we should end with these, the noblest of them all.
As a group, the conifers owe much of their success
to their ability to cope with the changeable northern climate.
They can survive both the short, dark days of winter with their bitter cold,
as well as the long sunny days of summer with their raging fires.
But if we continue a further 1,000 miles southwards, we come to the tropics,
and there the climate is radically different.
It's no longer very variable but remarkably constant,
with much the same amounts of light and rain and heat throughout the year.
There the other great group of forest trees, the broadleaved trees, come into their own.
That is the jungle, and that's where we'll be in the next programme.