0:00:55 > 0:00:58In the lands between the Arctic Circle and the Tropics,
0:00:58 > 0:01:02each year brings a great change between winter and summer.
0:01:02 > 0:01:07This in turn imposes an annual rhythm on the lives of animals and plants.
0:01:07 > 0:01:09Up in the north in the great evergreen forests,
0:01:09 > 0:01:12conditions in mid-winter are cripplingly severe.
0:01:21 > 0:01:24Life, if it is to flourish, has three needs -
0:01:24 > 0:01:28light, warmth and moisture.
0:01:28 > 0:01:32And the reason trees like these don't grow much farther north
0:01:32 > 0:01:35is not only because of the extreme cold,
0:01:35 > 0:01:38but because with the long months of winter darkness,
0:01:38 > 0:01:42there is simply not enough light in the year for them to grow.
0:01:42 > 0:01:45Here in northern Norway,
0:01:45 > 0:01:48300 miles, 500 kilometre, north of the Arctic Circle,
0:01:48 > 0:01:53there is just enough light, but it does get extremely cold.
0:01:53 > 0:01:5770 degrees of frost have been measured in these northern forests
0:01:57 > 0:02:01and during the winter, there are very heavy falls of snow.
0:02:02 > 0:02:06The cold not only threatens to freeze the liquid within the trees,
0:02:06 > 0:02:10it also denies them one of their essential supplies - water.
0:02:10 > 0:02:12Although snow and ice lie all around,
0:02:12 > 0:02:15the trees can't tap that water while it's frozen.
0:02:15 > 0:02:19So in winter this land is effectively as parched as a desert
0:02:19 > 0:02:23and the pine trees have as great a need to conserve their water
0:02:23 > 0:02:24as a cactus.
0:02:28 > 0:02:31All plants lose some water from the surface of their leaves,
0:02:31 > 0:02:37but the long thin pine needles are protected by a near-impermeable rind.
0:02:37 > 0:02:39And the pores through which they breathe,
0:02:39 > 0:02:42and from which they could lose water by evaporation,
0:02:42 > 0:02:45are kept out of the wind by being placed in lines
0:02:45 > 0:02:47along the groove that runs the length of the needle,
0:02:47 > 0:02:50each in a tiny pit ringed with a ridge.
0:02:54 > 0:02:57These dry, waxy leaves are almost inedible,
0:02:57 > 0:03:01but the seeds in the cones are a different matter.
0:03:01 > 0:03:05And what's more they are one of the few kinds of food
0:03:05 > 0:03:08available in the forest during the winter.
0:03:08 > 0:03:11The crossbill has a special beak
0:03:11 > 0:03:14which enables it to separate the brown segments of the closed cone
0:03:14 > 0:03:16and prise out the nutritious seeds.
0:03:22 > 0:03:25This winter feast is never certain.
0:03:25 > 0:03:28Some years every branch of the trees will be laden with cones,
0:03:28 > 0:03:31in others there will only be a handful.
0:03:31 > 0:03:34Then the seed-eaters must move on or die.
0:03:37 > 0:03:40The few remaining cones will then have a chance to shed their seeds
0:03:40 > 0:03:43into the snow at a time when there are few animals around.
0:03:49 > 0:03:50Even so, there are some.
0:03:54 > 0:03:58Voles make their runways through the snow and collect what they can.
0:04:02 > 0:04:05Moose get little nourishment from pine trees,
0:04:05 > 0:04:10apart from the shaggy moss that here and there hangs from the branches.
0:04:22 > 0:04:25They chew the sappy twigs and bark of birch,
0:04:25 > 0:04:30but there's not enough to keep them going.
0:04:30 > 0:04:33If it wasn't for the fat reserves they built up during the summer,
0:04:33 > 0:04:35few would survive.
0:04:39 > 0:04:42The winter forests can support very few plant-eaters,
0:04:42 > 0:04:46but there are just enough to provide meals for one or two hardy hunters.
0:05:06 > 0:05:08The great grey owl has legs well suited
0:05:08 > 0:05:10for grappling with its prey in snow,
0:05:10 > 0:05:14particularly long and covered with warm feathers.
0:05:14 > 0:05:16It regularly patrols the snow,
0:05:16 > 0:05:20for it can't afford to miss a single opportunity of a meal.
0:05:45 > 0:05:47And this is an incautious move.
0:06:23 > 0:06:26Lynx seek bigger prey.
0:06:38 > 0:06:41The female has young, which, though large,
0:06:41 > 0:06:44are not yet skilled enough to hunt for themselves,
0:06:44 > 0:06:46so they are relying on her.
0:06:56 > 0:06:59The cost-efficiency of hunting is precisely calculated.
0:07:01 > 0:07:04If the lynx doesn't catch a hare within 200 yards,
0:07:04 > 0:07:09the meat it might provide is not enough to warrant the effort,
0:07:09 > 0:07:11and the lynx gives up.
0:07:16 > 0:07:19Bigger prey are worth much longer chases,
0:07:19 > 0:07:22and the lynx pursue roe deer with great persistence.
0:07:34 > 0:07:42A single deer will provide food for the whole lynx family.
0:07:47 > 0:07:51In this bleak land, even the most ferocious and capable hunters
0:07:51 > 0:07:54do not scorn to scavenge.
0:07:59 > 0:08:02An eagle owl will take cold deer flesh
0:08:02 > 0:08:04just as eagerly as the warm bodies of voles.
0:08:08 > 0:08:11A wolverine, the biggest of the weasel family,
0:08:11 > 0:08:14and more than a match for an eagle owl.
0:08:50 > 0:08:53The coniferous forest grows right round the globe
0:08:53 > 0:08:58in a belt that, in places, is 1,200 miles across.
0:09:01 > 0:09:05From Scandinavia, it extends across northern Europe and Siberia
0:09:05 > 0:09:07to the shores of the Pacific.
0:09:10 > 0:09:13During the last ice age, when the seas were lower,
0:09:13 > 0:09:16the Bering Strait did not exist,
0:09:16 > 0:09:19so the trees continued without interruption into North America,
0:09:19 > 0:09:23across northern Canada to the Atlantic.
0:09:23 > 0:09:25And because of this one-time continuity,
0:09:25 > 0:09:27all the trees in this vast forest
0:09:27 > 0:09:32and all its permanent inhabitants in America, Asia and Europe,
0:09:32 > 0:09:34are very much the same.
0:09:36 > 0:09:40But when spring comes, visitors journey up from warmer parts
0:09:40 > 0:09:43and the forest on each continent
0:09:43 > 0:09:46takes on its own individual character.
0:09:48 > 0:09:50In Scandinavia, a hawk owl,
0:09:50 > 0:09:54a nomad that has spent the winter in the gentle conditions farther south,
0:09:54 > 0:09:58comes cruising up north again looking for food and a nest site.
0:10:05 > 0:10:08Unlike other owls, it's primarily a daytime hunter,
0:10:08 > 0:10:12and relies not so much on its acute hearing as its sharp eyesight
0:10:12 > 0:10:15as it waits for the melting snow to reveal rodents.
0:10:44 > 0:10:47In pine trees from Norway to Siberia,
0:10:47 > 0:10:51the cock capercaillie is starting to claim his territory.
0:10:51 > 0:10:55This giant grouse is one of the few creatures that eats pine needles.
0:10:57 > 0:10:59His hen takes them too.
0:11:15 > 0:11:17Now is the time for nesting.
0:11:17 > 0:11:20The hawk owl is in search of a hole in a tree,
0:11:20 > 0:11:22for it's already found its partner.
0:11:25 > 0:11:28But many tree holes are already occupied,
0:11:28 > 0:11:33for great numbers of owls have travelled up to feed on the voles.
0:11:37 > 0:11:39No owl can dig a hole for itself.
0:11:39 > 0:11:43They rely mainly on woodpeckers, and none of that family are more
0:11:43 > 0:11:47expert carpenters than the black woodpecker of northern Europe.
0:11:47 > 0:11:49The sharp beak with which they pick out insects
0:11:49 > 0:11:52also serves as an excellent chisel,
0:11:52 > 0:11:57but even so, most prefer to work in dead trees where the wood is softer.
0:11:59 > 0:12:00There are ants near this tree too.
0:12:00 > 0:12:05The woodpeckers rely on them for food during the winter.
0:12:17 > 0:12:19Not all owls use nest holes.
0:12:19 > 0:12:24The eagle owl nests on the ground, often among rocks.
0:12:24 > 0:12:27It already has a clutch of three eggs,
0:12:27 > 0:12:30for, being a permanent resident of these forests, it paired early.
0:12:41 > 0:12:43Plants now have their chance to breed.
0:12:43 > 0:12:46The wood anemones are already in flower, and so are the pines.
0:12:46 > 0:12:50Each tree produces both male and female flowers,
0:12:50 > 0:12:53which mature at slightly different times,
0:12:53 > 0:12:55so the female flowers are likely to be fertilised
0:12:55 > 0:12:57by pollen from other trees.
0:13:00 > 0:13:03Now it is as warm as it ever will be in the northern forests.
0:13:03 > 0:13:08Summer visitors are arriving, and the trees echo with their song.
0:13:14 > 0:13:17This willow warbler, singing so vigorously in Scandinavia,
0:13:17 > 0:13:20has come all the way from the savannah country south of the Sahara.
0:13:20 > 0:13:22So has the whinchat.
0:13:25 > 0:13:27And the lure that has brought them so far
0:13:27 > 0:13:30is the sudden emergence of myriads of insects.
0:13:41 > 0:13:45This bedraggled creature is hardly recognisable, for its wings have not
0:13:45 > 0:13:47yet expanded. It's a pine beauty moth,
0:13:47 > 0:13:51and its first priority is to get away from the forest floor
0:13:51 > 0:13:52which is full of danger.
0:13:52 > 0:13:55But not all the moths have such a clear run.
0:14:01 > 0:14:03Shrews are among the first to feed on them.
0:14:06 > 0:14:10Up among the pine needles, the pine beauty pumps fluid out of its body
0:14:10 > 0:14:12and into the veins of its wings.
0:14:22 > 0:14:23Here the moths will lay their eggs
0:14:23 > 0:14:26so their caterpillars can feed on the young shoots
0:14:27 > 0:14:31The wood ants have missed their chance to catch the adult moth,
0:14:31 > 0:14:35but now they're looking for the caterpillars among the branches.
0:14:35 > 0:14:38The colour and the pattern of the caterpillar conceals it from birds
0:14:38 > 0:14:40which hunt by sight,
0:14:40 > 0:14:45but is no protection against ants which search by smell and touch.
0:15:11 > 0:15:15Finally the body is hauled down to the nest for all to consume.
0:15:23 > 0:15:27The caterpillars of the sawfly are also swarming on the pine shoots.
0:15:27 > 0:15:31They do have a defence against ants - a chemical one. As they chew,
0:15:31 > 0:15:34they store some of the resin from the pine needles
0:15:34 > 0:15:37in a pouch inside their mouth.
0:15:37 > 0:15:39When a foraging ant discovers them,
0:15:39 > 0:15:42they dab a spot of this resin on its head, like this.
0:15:46 > 0:15:49The resin damages the ant's eyes and antennae,
0:15:49 > 0:15:53so disorientating it that it can hardly walk straight.
0:15:53 > 0:15:56Even if it finds its way back to the nest,
0:15:56 > 0:15:58it smells so strongly and so strangely
0:15:58 > 0:16:01that the other ants treat it as an intruder and kill it.
0:16:07 > 0:16:11The ants themselves are food for others.
0:16:11 > 0:16:15The wryneck is a member of the woodpecker family
0:16:15 > 0:16:17that has specialised in eating ants,
0:16:17 > 0:16:20and particularly relishes their cocoons.
0:16:24 > 0:16:28Like its cousins, the wryneck nests in holes in trees,
0:16:28 > 0:16:31but it doesn't excavate them for itself.
0:16:31 > 0:16:34It is yet another tenant of vacated woodpecker holes.
0:16:43 > 0:16:47With a long tongue, you can even collect insects from the bark
0:16:47 > 0:16:49without leaving your nest.
0:16:51 > 0:16:55Here in the far north, close to the Arctic Circle,
0:16:55 > 0:16:58the sun during the summer hardly sinks below the horizon
0:16:58 > 0:16:59and the nights are brief.
0:17:05 > 0:17:08The eagle owl hunts just as effectively in the twilight
0:17:08 > 0:17:10as in the dark.
0:17:10 > 0:17:13It has a rabbit. The season is a good one and game is abundant.
0:17:16 > 0:17:20Down in the nest on the forest floor, there is only one chick left.
0:17:20 > 0:17:23The other two may have been taken by foxes.
0:17:27 > 0:17:30Eagle owls often kill rival species,
0:17:30 > 0:17:33and this chick's last meal was a short-eared owl,
0:17:33 > 0:17:35which it's not yet finished.
0:17:35 > 0:17:38The single survivor has a superabundance of food.
0:17:38 > 0:17:40It has grown fast
0:17:40 > 0:17:44and its adult feathers are already appearing through its down.
0:17:45 > 0:17:48The tail of a red squirrel is left over from a previous meal,
0:17:48 > 0:17:50and it even takes that too.
0:18:04 > 0:18:06The voles are swarming on the forest floor.
0:18:06 > 0:18:10Last winter, the pines produced great quantities of seed,
0:18:10 > 0:18:13so many adult voles survived till spring
0:18:13 > 0:18:16and now they're all breeding at an extraordinary rate.
0:18:16 > 0:18:19This female produced her four young only three weeks ago,
0:18:19 > 0:18:21but she is already pregnant again
0:18:21 > 0:18:24and will soon abandon this family and start a new one.
0:18:51 > 0:18:54All the owls, some visitors, some residents,
0:18:54 > 0:18:56scour the forest for voles.
0:19:06 > 0:19:09Tengmalm's owl, up in a tree hole, has three chicks,
0:19:09 > 0:19:12all flourishing and all demanding voles.
0:19:42 > 0:19:44The number of voles varies considerably.
0:19:44 > 0:19:48It gradually builds up over a period of five to six years
0:19:48 > 0:19:52until finally there are so many that they eat out their food supply
0:19:52 > 0:19:54and the population crashes.
0:19:54 > 0:19:57These changes have their effect on the owl population.
0:19:57 > 0:20:00More voles mean better-fed owls,
0:20:00 > 0:20:03which produce bigger clutches of eggs and rear more chicks.
0:20:03 > 0:20:06And as the number of owls increases,
0:20:06 > 0:20:09so they spread out into new territory.
0:20:09 > 0:20:12I'm in Finland, very close to the Russian border.
0:20:12 > 0:20:17In fact, those pine forests behind me are actually in Russia.
0:20:17 > 0:20:20But the frontier is no barrier to the bird
0:20:20 > 0:20:22they call the phantom of the north,
0:20:22 > 0:20:27the great grey owl, and in years when the vole population is high,
0:20:27 > 0:20:33the owl comes across these frontiers and into the Finnish pine forests.
0:20:33 > 0:20:36And I know they are here already because I have just picked up this.
0:20:36 > 0:20:39This is an owl pellet.
0:20:39 > 0:20:45All owls, as part of their natural digestion, throw up the fur and bones
0:20:45 > 0:20:51of their prey. And this, I can see, has actually got vole skulls in it.
0:20:51 > 0:20:56But to discover what the state of the vole population is at the moment,
0:20:56 > 0:21:01I'll have to look inside the nest of a great grey
0:21:01 > 0:21:04and to do that I'll need this.
0:21:06 > 0:21:12All owls are fairly ferocious and the great grey owl certainly can be,
0:21:12 > 0:21:15so as part of the standard equipment of looking for owl nests,
0:21:15 > 0:21:17you need this.
0:21:22 > 0:21:26Up there is one of their nests, and the female has just flown off.
0:21:26 > 0:21:29She's perching in that tree over there,
0:21:29 > 0:21:31keeping a very close eye on me.
0:21:31 > 0:21:33If I go up and have a look in the nest,
0:21:33 > 0:21:38I may be able to get some idea as to how the vole cycle is going.
0:21:51 > 0:21:54And...come on.
0:21:55 > 0:21:57There is just one chick.
0:21:57 > 0:22:01If the voles had been at the height of their population,
0:22:01 > 0:22:06there would probably be about four chicks in such a nest as this,
0:22:06 > 0:22:09but the fact there is only one makes it pretty clear
0:22:09 > 0:22:13that the vole population is already beginning to crash.
0:22:13 > 0:22:17So it is very likely that the female and her mate
0:22:17 > 0:22:22will soon be on their way back to Russia.
0:22:26 > 0:22:30There's now just a month left of the short northern summer.
0:22:30 > 0:22:34Many of the birds that came up here to harvest the insects and to breed
0:22:34 > 0:22:36will soon be moving back again
0:22:36 > 0:22:38to avoid the severities of the coming winter.
0:22:39 > 0:22:43Some, like the redwing, will go to open pastureland down south.
0:22:43 > 0:22:47The brambling prefers beech woodland,
0:22:47 > 0:22:50and will leave almost as soon as it has finished its summer moult.
0:22:55 > 0:22:59The hawk owl is driven south by hunger,
0:22:59 > 0:23:01for as the forest gets colder,
0:23:01 > 0:23:04there is less and less food to be found.
0:23:04 > 0:23:08As it flies south, so the trees beneath change character.
0:23:08 > 0:23:13The ranks of dark conifers are replaced by the brighter green
0:23:13 > 0:23:18of the broadleaved trees - oak and ash, birch and beech.
0:23:43 > 0:23:47Down here, the weather's warmer, the summers are longer,
0:23:47 > 0:23:49and the woodlands are free of frost,
0:23:49 > 0:23:53not for just two or three months in the year, but for eight or nine,
0:23:53 > 0:23:55and the shape of the trees is very different.
0:23:55 > 0:24:00Instead of their branches drooping down, and so shedding the snow,
0:24:00 > 0:24:03these branches spread out widely,
0:24:03 > 0:24:05carrying tier upon tier of leaves
0:24:05 > 0:24:08with which to catch the abundant energy of the sun.
0:24:08 > 0:24:10And the leaves are very different.
0:24:10 > 0:24:14They are not covered with a thick, protective rind but are thin,
0:24:14 > 0:24:16delicate structures.
0:24:16 > 0:24:19During the summer water is more accessible,
0:24:19 > 0:24:22so there is less need to take rigorous measures to conserve it.
0:24:22 > 0:24:24Indeed, during hot days,
0:24:24 > 0:24:27the trees evaporate large quantities to keep cool.
0:24:27 > 0:24:30So the pores through which they breathe are numerous,
0:24:30 > 0:24:32and not in pits as they are in the pines.
0:24:36 > 0:24:40These succulent, soft leaves, unlike pine needles, are relished
0:24:40 > 0:24:42as food by all kinds of creatures.
0:24:48 > 0:24:51Large animals, like deer, take many of them,
0:24:51 > 0:24:55but the greatest quantity by far is gathered by insects.
0:25:21 > 0:25:23The forest canopy in late summer
0:25:23 > 0:25:26has more birds in it than at any other time of the year.
0:25:26 > 0:25:30There are returning migrants newly arrived from the north,
0:25:30 > 0:25:32resident breeders gathering food
0:25:32 > 0:25:34to feed their second families of the season,
0:25:34 > 0:25:37and young fledglings starting to forage for themselves
0:25:37 > 0:25:40and still not sure what is edible and what isn't.
0:25:40 > 0:25:43Nearly all of them are hunting for insects,
0:25:43 > 0:25:45and the crop they take is huge.
0:25:57 > 0:25:58Not surprisingly,
0:25:58 > 0:26:02the insects have evolved many ways of protecting themselves.
0:26:02 > 0:26:05They snip off half-eaten leaves
0:26:05 > 0:26:07so as to give the minimum sign of their presence.
0:26:07 > 0:26:10They disguise themselves as a blob of cuckoo spit or a bird dropping,
0:26:10 > 0:26:14but if they move, as eventually they must, their concealment is lost.
0:26:26 > 0:26:29Some hang in places which are difficult to reach.
0:26:29 > 0:26:33This might baffle a fledgling, but an adult great tit is both
0:26:33 > 0:26:35experienced and agile.
0:26:45 > 0:26:49The tree creeper specialises in insects that live on bark.
0:26:52 > 0:26:56A poplar hawk moth tries to defend itself by pretending to be fierce.
0:27:10 > 0:27:14The nuthatch habitually works its way down the trunk,
0:27:14 > 0:27:16and that way may see insects that have been overlooked
0:27:16 > 0:27:20by tree creepers that habitually come up it.
0:27:37 > 0:27:40One of the most expert of all bark-feeders
0:27:40 > 0:27:43and, in some ways, the most specialised of all the birds
0:27:43 > 0:27:47living in the tall trees of these forests are the woodpeckers.
0:27:47 > 0:27:50The greater spotted woodpecker is typical of them.
0:27:50 > 0:27:53Its hearing is excellent and it locates the grubs it seeks
0:27:53 > 0:27:56by the tiny sounds they make as they move inside the bark.
0:28:00 > 0:28:04Its tail feathers have strong quills and serve as props for its body.
0:28:04 > 0:28:08Its bill has a resilient pad at the base which cushions its brain
0:28:08 > 0:28:10from the shock of its drilling.
0:28:13 > 0:28:16Its feet give it a grip in all directions,
0:28:16 > 0:28:19with two toes pointing forwards and two backwards.
0:28:19 > 0:28:22Each continent has its own range of woodpeckers.
0:28:22 > 0:28:24Europe has ten species,
0:28:24 > 0:28:28but here in North America, there are over twice as many.
0:28:28 > 0:28:30This one, a sapsucker,
0:28:30 > 0:28:34drills holes in trees not for insects, but for sap.
0:28:34 > 0:28:37It digs lines of these wells in many different kinds of trees.
0:28:37 > 0:28:39Each little hole points slightly downwards
0:28:39 > 0:28:44so that the sap doesn't trickle out, but collects in a small pool inside,
0:28:44 > 0:28:48and the sapsucker collects it with its tongue.
0:28:48 > 0:28:49And so do other birds.
0:28:53 > 0:28:55A hummingbird.
0:28:55 > 0:28:58Most of its family live in the tropics and feed on nectar,
0:28:58 > 0:29:00but this one comes north in the summer
0:29:00 > 0:29:03and finds tree sap just as acceptable.
0:29:03 > 0:29:07Flies, too, come to the sweet sap.
0:29:23 > 0:29:27In late summer, the parent sapsuckers lead their fledglings to the wells
0:29:27 > 0:29:29and leave them to feast
0:29:29 > 0:29:32not only on the sap but on the insects it attracts.
0:29:36 > 0:29:39This American woodpecker uses its drilling skills
0:29:39 > 0:29:41to bore neat sockets in dead tree trunks.
0:29:41 > 0:29:45Acorns are its main food, but during the season,
0:29:45 > 0:29:49there are far more acorns than the woodpeckers can eat immediately.
0:29:49 > 0:29:51But they don't leave them for others.
0:29:51 > 0:29:55Several birds share a communal acorn treasury, like this one.
0:29:55 > 0:29:58They hammer the acorns into the holes so firmly
0:29:58 > 0:30:01that few other creatures can get them out,
0:30:01 > 0:30:04and the store will keep the acorn woodpeckers supplied
0:30:04 > 0:30:06throughout the rest of the year.
0:30:10 > 0:30:15The ripening acorns herald the end of summer and the beginning of autumn.
0:30:15 > 0:30:19Trees and bushes proffer their seeds to the forest animals.
0:30:19 > 0:30:21Some are wrapped in soft and tasty flesh
0:30:21 > 0:30:25to tempt the animals to eat them and so transport them to new sites.
0:30:27 > 0:30:29Others are packed with nourishment,
0:30:29 > 0:30:32not for animals, but to provide food for the germinating seedling,
0:30:32 > 0:30:35but the animals eat them just the same.
0:30:36 > 0:30:40Even the hard and unpromising-looking acorns of the American pin oak
0:30:40 > 0:30:41are collected by racoons.
0:30:51 > 0:30:54The squirrel's habit of burying acorns for a winter store
0:30:54 > 0:30:57has been the beginning of many an oak.
0:30:59 > 0:31:01The black bear, on occasion,
0:31:01 > 0:31:04will eat fish and voles and even carrion,
0:31:04 > 0:31:07but much of its diet is vegetable.
0:31:07 > 0:31:10It will dig for roots and even eat pine cones,
0:31:10 > 0:31:13but it has a very sweet tooth and just now it relishes the fruit.
0:31:28 > 0:31:31All sorts of mammals are now clambering around in the trees
0:31:31 > 0:31:33in search of fruit.
0:31:33 > 0:31:35The possum, a strange primitive animal of the Americas
0:31:35 > 0:31:39related more closely to kangaroos than to rats, eats almost anything.
0:31:44 > 0:31:46Few of them can get to the very tops of the trees
0:31:46 > 0:31:50or the very thinnest twig, but a chipmunk can.
0:31:54 > 0:31:56The chills of autumn presage the coming of winter.
0:31:56 > 0:32:00The delicate leaves worked efficiently
0:32:00 > 0:32:01throughout the warm moist summer,
0:32:01 > 0:32:04but they are not suited to cold weather.
0:32:04 > 0:32:05Frost will damage them.
0:32:05 > 0:32:07Their abundant pores would lose too much water.
0:32:07 > 0:32:11So the green chlorophyll in them is broken down
0:32:11 > 0:32:13and withdrawn into the tree,
0:32:13 > 0:32:18revealing the red and brown waste products, and the leaves fall.
0:32:18 > 0:32:22And they, too, provide food for another woodland community,
0:32:22 > 0:32:24the inhabitants of the leaf litter.
0:32:28 > 0:32:32There may be 100,000 box mites in every cubic yard.
0:32:32 > 0:32:35And there are many other creatures too,
0:32:35 > 0:32:38chewing their way through the dead leaves,
0:32:38 > 0:32:40extracting what nutriment they can
0:32:40 > 0:32:44and leaving the remainder to be dealt with by fungi and bacteria.
0:32:51 > 0:32:54They themselves are hunted by monsters in miniature,
0:32:54 > 0:32:56pseudoscorpions,
0:32:56 > 0:33:02horrific in close-up, but, perhaps fortunately, the size of a pinhead.
0:33:36 > 0:33:38Snails are giants in comparison
0:33:38 > 0:33:42and, since they carry their shells around with them,
0:33:42 > 0:33:44they might seem to be fairly well protected
0:33:44 > 0:33:47against any creatures smaller than a bird.
0:33:47 > 0:33:49But one particular beetle
0:33:49 > 0:33:53has specialised equipment for dealing with them.
0:33:53 > 0:33:56Its head and jaws are long and thin.
0:34:24 > 0:34:27Almost hidden in the leaves of these American woods
0:34:27 > 0:34:30are some spectacularly coloured little creatures
0:34:30 > 0:34:31hardly bigger than worms.
0:34:35 > 0:34:37They are amphibians - salamanders.
0:34:37 > 0:34:42Almost every range of mountain in the US has its own species
0:34:42 > 0:34:44with its own particular colours,
0:34:44 > 0:34:47but, being nocturnal, they're rarely seen.
0:35:07 > 0:35:10Shrews eat most small living things they come across,
0:35:10 > 0:35:12and they are formidable hunters,
0:35:12 > 0:35:15for they are one of the few mammals that has a poisonous bite.
0:35:21 > 0:35:23The salamander's only defence
0:35:23 > 0:35:26is to produce an acrid liquid from glands on its tail.
0:35:26 > 0:35:29The first time a shrew encounters this,
0:35:29 > 0:35:32it usually takes no notice and eats the salamander,
0:35:32 > 0:35:34but apparently the taste is not very nice,
0:35:34 > 0:35:36for on later encounters, like this one,
0:35:36 > 0:35:38one sniff is enough to remind the shrew
0:35:38 > 0:35:42that the meal won't be a good one and it leaves the salamander alone.
0:35:49 > 0:35:51The summer visitors have departed.
0:35:51 > 0:35:53The woods have fallen silent.
0:35:53 > 0:35:57The days are shortening and the temperature falling.
0:36:28 > 0:36:31Eventually the land is gripped tight by frost.
0:36:58 > 0:37:00It's late winter.
0:37:00 > 0:37:03The once-resplendent trees are now mere skeletons
0:37:03 > 0:37:07and life in these woodlands has come almost to a complete standstill.
0:37:07 > 0:37:10The trees, without their leaves, can't grow.
0:37:10 > 0:37:14The birds that came visiting up here during the summer
0:37:14 > 0:37:16have now retreated south,
0:37:16 > 0:37:21and some of these small mammals have crawled into holes and gone to sleep.
0:37:21 > 0:37:23Their heartbeat has almost stopped,
0:37:23 > 0:37:28their bodies have become as cold as stone. They're hibernating.
0:37:28 > 0:37:31But that sleep doesn't last throughout the winter.
0:37:31 > 0:37:36They wake up every four or five days and go and look for food.
0:37:36 > 0:37:39Like, for example, those small chipmunks over there.
0:37:39 > 0:37:43Not only warmth but intense cold will bring them out,
0:37:43 > 0:37:46for although their body temperature falls while they are hibernating,
0:37:46 > 0:37:49if it drops to freezing point, they will die.
0:37:49 > 0:37:53So in really cold spells, they must get up and warm themselves
0:37:53 > 0:37:54with a little exercise,
0:37:54 > 0:37:58even though it dangerously depletes their fat reserves.
0:37:58 > 0:38:01But in these American woodlands, there is one spectacular sleeper
0:38:01 > 0:38:04who dozes for months on end.
0:38:05 > 0:38:07Just look at this.
0:38:15 > 0:38:16A black bear.
0:38:18 > 0:38:21She retired to this den in early autumn,
0:38:21 > 0:38:24and after a month or so of drowsiness, produced her cubs.
0:38:24 > 0:38:27In the colder northern parts of these woods,
0:38:27 > 0:38:29she may spend six or seven months here,
0:38:29 > 0:38:31during which time she suckles her cubs
0:38:31 > 0:38:34but neither feeds herself nor urinates nor defecates.
0:38:34 > 0:38:37So she spends the majority of her life half-asleep.
0:38:41 > 0:38:43When spring at last comes, the brown carpet of rotting leaves
0:38:43 > 0:38:46is suddenly flooded with colour.
0:38:49 > 0:38:51The plants that live close to the ground
0:38:51 > 0:38:54now make haste to sprout and flower and soak up the spring sunshine
0:38:54 > 0:38:59before the trees above produce their own leaves and cut out the light.
0:39:03 > 0:39:07The bear's den is empty, but the owners haven't gone far.
0:39:17 > 0:39:21There's still not much to eat, only a few leaves, nor will there be
0:39:21 > 0:39:24until the first of the berries come into fruit in summer,
0:39:24 > 0:39:27but meanwhile at least the sun is warm.
0:39:33 > 0:39:36Another mother spends the spring up in a tree -
0:39:36 > 0:39:39a wood duck, only she is about to leave.
0:39:46 > 0:39:48The hole has provided a secure nest,
0:39:48 > 0:39:52but all ducklings follow their mothers as soon as they hatch.
0:40:23 > 0:40:27And now new forms appear from among the dead leaves.
0:41:03 > 0:41:06The spring showers soak the woodlands
0:41:06 > 0:41:10and create just the moist, warm conditions needed by the fungi
0:41:10 > 0:41:12to produce their fruiting bodies.
0:41:12 > 0:41:16These must be mature and ready to discharge their microscopic spores
0:41:16 > 0:41:19by the time the dry winds of summer begin to blow,
0:41:19 > 0:41:21so that their spores, like dust,
0:41:21 > 0:41:23will be carried all through the forest.
0:41:26 > 0:41:28Once, the woods of North America
0:41:28 > 0:41:31stretched over the eastern half of the continent
0:41:31 > 0:41:34in an almost continuous band hundreds of miles deep.
0:41:34 > 0:41:38Today, the majority has been felled
0:41:38 > 0:41:40to make space for farmland and cities,
0:41:40 > 0:41:43but enough remains to make plain their splendour.
0:41:46 > 0:41:49And now we've come farther south still.
0:41:49 > 0:41:51I'm on the borders of Florida and Georgia
0:41:51 > 0:41:52in the southern United States,
0:41:52 > 0:41:57and here it's very hot in the summer and the winters are very mild,
0:41:57 > 0:42:00with only a few frosts, and none of them severe.
0:42:00 > 0:42:04So some of the broadleaved trees here, like this oak,
0:42:04 > 0:42:07don't shed all their leaves in the autumn,
0:42:07 > 0:42:11but keep them throughout the year and continue growing.
0:42:11 > 0:42:15And these aren't the only evergreens that are here, either.
0:42:15 > 0:42:17There are pines.
0:42:17 > 0:42:21In some parts, where the soil is very rocky or sandy and poor in nutrients,
0:42:21 > 0:42:25the pines will grow because nothing else can survive there.
0:42:25 > 0:42:28But this pine forest owes its existence
0:42:28 > 0:42:30to another factor altogether.
0:42:40 > 0:42:44Oak saplings are killed within minutes by fire.
0:42:47 > 0:42:51But the terminal buds of young pines are surrounded by a shock of needles.
0:42:51 > 0:42:55They burn at a relatively low temperature,
0:42:55 > 0:42:57and by the time the flames have consumed them,
0:42:57 > 0:43:00the main fire has swept by, and the bud at the top of the stem,
0:43:00 > 0:43:04from which new growth will come, is still unharmed.
0:43:06 > 0:43:10Fires like these are not just the work of careless people,
0:43:10 > 0:43:11they occur naturally.
0:43:12 > 0:43:16The spark that regularly sets fire to these forests is lightning.
0:43:16 > 0:43:20In this part of the southern States, violent thunderstorms are common
0:43:20 > 0:43:23and lightning often strikes the taller trees,
0:43:23 > 0:43:25scoring a deep groove down the length of the trunk
0:43:25 > 0:43:28as it flashes down to earth.
0:43:30 > 0:43:35And this at my feet is the tinder which set it aflame.
0:43:35 > 0:43:41These are pine needles, and they're so full of resin and they're so dry
0:43:41 > 0:43:43that they flame up very easily.
0:43:43 > 0:43:47But the fire they produce is not very hot, and it's also very short-lived,
0:43:47 > 0:43:51so that if any creature can survive fire for just one or two minutes,
0:43:51 > 0:43:54then it can survive a fire like this.
0:43:58 > 0:44:01The rattlesnake, like many other ground-living animals,
0:44:01 > 0:44:04regularly takes refuge from the midday sun in holes,
0:44:04 > 0:44:08so now it knows exactly where to go to escape the fire.
0:44:25 > 0:44:29But this hole is already occupied by its digger and owner...
0:44:33 > 0:44:34..a gopher tortoise.
0:44:49 > 0:44:53Rattlesnake and tortoise do not normally interfere with one another.
0:45:06 > 0:45:09And that seems to be the way things are going to stay.
0:45:12 > 0:45:16But in the back of the burrow lies another refugee, an indigo snake,
0:45:16 > 0:45:19and it, on occasion, eats rattlesnakes.
0:45:40 > 0:45:45But the fire is passing and the rattlesnake can return to the forest.
0:45:52 > 0:45:55Some insects don't avoid fire, they actively seek it.
0:45:55 > 0:45:58Beetles find it difficult to lay their eggs in the pines
0:45:58 > 0:46:01because the trees swamp them with resin.
0:46:01 > 0:46:03But a tree killed by fire can't resist,
0:46:03 > 0:46:07and these beetles take advantage of the situation.
0:46:07 > 0:46:09They have pits behind their legs
0:46:09 > 0:46:11which are sensitive to infra-red rays,
0:46:11 > 0:46:14and therefore they can detect the slightest rise in temperature,
0:46:14 > 0:46:17and with these to guide them, they travel from all over the forest
0:46:17 > 0:46:20to the wake of the fire, and arrive in hundreds.
0:46:25 > 0:46:27Quickly they mate.
0:46:33 > 0:46:36The females crawl all over the scorched trunks,
0:46:36 > 0:46:39seeking crevices in the bark into which they can lay their eggs,
0:46:39 > 0:46:43so ensuring that their grubs will have nice nutritious bark to chew.
0:46:45 > 0:46:49As insects assemble in the burnt forest,
0:46:49 > 0:46:51the insect-eaters follow.
0:46:51 > 0:46:53The oak toad almost exactly matches
0:46:53 > 0:46:55the colour of the charred forest floor.
0:46:57 > 0:47:01Other more conspicuous hunters wait on newly emerged shoots.
0:47:06 > 0:47:09Within a couple of months of a summer fire,
0:47:09 > 0:47:12the forest has more than recovered, it is rejuvenated.
0:47:12 > 0:47:16The fire has cleared away the old growth on the ground,
0:47:16 > 0:47:18and, by reducing the pine needles to ash,
0:47:18 > 0:47:21has released their nutrients into the soil,
0:47:21 > 0:47:24and now the ground sprouts more flowers than at any other time.
0:47:37 > 0:47:42Because of regular fires, big bushes can't establish themselves here,
0:47:42 > 0:47:45so swampy areas are not colonised
0:47:45 > 0:47:47and sucked dry by them as happens elsewhere,
0:47:47 > 0:47:51and open marshes remain where pitcher plants can grow
0:47:51 > 0:47:53and where frogs can swim and breed.
0:47:53 > 0:47:57Indeed, one species of frog lives nowhere else
0:47:57 > 0:48:00but in these pools in the American pine barrens.
0:48:16 > 0:48:20The woodpeckers here can't excavate their nest in dead trees
0:48:20 > 0:48:21as do woodpeckers elsewhere,
0:48:21 > 0:48:25for in this fire-ravaged forest they would risk incineration,
0:48:25 > 0:48:30so the red-cockaded woodpecker drills its holes in living pines.
0:48:30 > 0:48:31But the wood is so hard,
0:48:31 > 0:48:36it takes several woodpeckers about two years to dig the hole.
0:48:38 > 0:48:41Resinous sap seeps out around the hole
0:48:41 > 0:48:43where the outer layers of the tree have been breached.
0:48:43 > 0:48:46So the birds make their hole low down on the trunk
0:48:46 > 0:48:50where the inner sap-free heartwood is thick enough to accommodate
0:48:50 > 0:48:51the entire nest.
0:48:53 > 0:48:55The flow of resin is diverted to the outside
0:48:55 > 0:48:58by drilling pits like sap wells above and below the hole.
0:49:05 > 0:49:08It's in these laboriously excavated holes
0:49:08 > 0:49:13that the red-cockaded woodpecker raises its young.
0:49:19 > 0:49:21The holes are very conspicuous,
0:49:21 > 0:49:24for each is surrounded by a sheet of yellow congealed resin.
0:49:26 > 0:49:31The rat snake is a great robber of nests and stealer of chicks.
0:49:46 > 0:49:48It's an extremely skilful tree climber.
0:49:48 > 0:49:51Since the woodpecker's hole in the living tree
0:49:51 > 0:49:54has to be fairly low down on the trunk,
0:49:54 > 0:49:56it is within easy reach of the snake
0:49:56 > 0:49:59and therefore might seem to be in considerable danger.
0:49:59 > 0:50:03But now the other function of all that resin,
0:50:03 > 0:50:06deliberately produced around the nest by the woodpecker,
0:50:06 > 0:50:07is about to become clear.
0:50:37 > 0:50:41The chemicals in the resin seem to irritate the snake beyond endurance,
0:50:41 > 0:50:43and it arches its body away.
0:50:46 > 0:50:48Eventually it's too much.
0:50:52 > 0:50:56So fire, one way or another, influences the whole community
0:50:56 > 0:50:59of animals and plants in the pine forests of the south.
0:51:01 > 0:51:06This injury was also caused by fire, and this is also a coniferous tree,
0:51:06 > 0:51:09but a very different one.
0:51:09 > 0:51:14To start with, it's over 40 feet across along its base
0:51:14 > 0:51:20and it's 267.5 feet high.
0:51:20 > 0:51:22This is a giant sequoia.
0:51:25 > 0:51:27It's thought to be about 2,500 years old,
0:51:27 > 0:51:32but the largest individual tree of all is this one
0:51:32 > 0:51:35known as the General Sherman.
0:51:35 > 0:51:42It's just taller and it's estimated to weigh 1,385 tons,
0:51:42 > 0:51:46and that makes it the most massive living organism in the world.
0:51:48 > 0:51:50Although these trees are growing
0:51:50 > 0:51:53almost as far south as the southern pines, the climate here,
0:51:53 > 0:51:572,000 metres up in the Sierra Nevada mountains, is much colder
0:51:57 > 0:52:01and snow lies on the ground for almost half the year.
0:52:01 > 0:52:03It's as though, by climbing to this height,
0:52:03 > 0:52:06we have returned climatically to the great forests of the north.
0:52:06 > 0:52:11During the Ice Age, these sequoias grew over much of North America.
0:52:11 > 0:52:16But when, some 8,000 years ago, the Earth began to warm, they died out,
0:52:16 > 0:52:20except for these isolated groups high up in the mountains.
0:52:28 > 0:52:32We've travelled some 2,000 miles southwards
0:52:32 > 0:52:36since we started at the tree line near the Arctic Circle,
0:52:36 > 0:52:38and in all that vast territory,
0:52:38 > 0:52:42the majority of the forest trees have been conifers,
0:52:42 > 0:52:44so it seems only right and proper
0:52:44 > 0:52:48that we should end with these, the noblest of them all.
0:52:48 > 0:52:52As a group, the conifers owe much of their success
0:52:52 > 0:52:56to their ability to cope with the changeable northern climate.
0:52:56 > 0:53:01They can survive the short, dark days of winter with their bitter cold,
0:53:01 > 0:53:06as well as the long sunny days of summer with their raging fires.
0:53:06 > 0:53:10But if we continue a further 1,000 miles southwards,
0:53:10 > 0:53:14we come to the tropics, and there the climate is radically different.
0:53:14 > 0:53:17It's no longer very variable but remarkably constant,
0:53:17 > 0:53:23with much the same amounts of light, rain and heat throughout the year.
0:53:23 > 0:53:27There the other great group of forest trees, the broadleaved trees,
0:53:27 > 0:53:29come into their own.
0:53:29 > 0:53:33That is the jungle, and that's where we'll be in the next programme.