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Howling storms and hurricanes, devastating droughts and raging forest fires
may come only once or twice in our lives, so we think of them as exceptional happenings.
But for those plants with much longer lives than ours,
such events may be crucial for survival.
And those with briefer lives may only be able to exist at all in the aftermath of such disasters.
The greatest British hurricane of recent times struck on the night of October 16th, 1987.
Ancient woodlands that had stood for centuries were devastated.
For individual trees like this 250-year-old beech it was, of course, a catastrophe.
But for other plants, it was an opportunity they'd been waiting for for decades.
The seeds from which these young plants are springing
may have come from adults growing in a clearing elsewhere in the wood over a century ago.
Since then, they have lain dormant in the soil,
but now the light has triggered them into life.
They won't have this clearing to themselves for long, so they grow swiftly,
and the soil, enriched by rotting woodland leaves, feeds them well.
In their second year, they flower. They're foxgloves.
The seeds from these flowers will not sprout here, there's no room for them.
They will have to find a new clearing, and in their turn may have to wait decades for it.
Willowherb competes with the foxgloves for this kind of territory,
so it, too, needs to reproduce urgently.
The willowherb seeds, supported by no more than fluff,
will be carried in millions, far and wide, by the wind.
After a few years, other claimants appear
and contest the ownership of the clearing - young birches.
The willowherb may not be able to stay here for much longer.
A few years later, the willowherb has gone. The birches, it seems, have won.
But the soil is now losing its richness.
Most of these birches started their lives about ten years ago,
but there were a lot of them and competition was intense.
So most of them didn't do well at all. This one is pretty well dead.
This is one of the winners that has overshadowed the rest.
But even this one will not be the final holder of this territory.
That prize will go to a little seedling like this, an oak.
Birches, on the timescale of a wood, have short lives.
After 40 or 50 years, they begin to succumb to fungus and disease.
Oaks grow more slowly but more strongly.
Eventually, they overtake the ailing birches and capture the sunshine.
The social struggle in the woodland is over, at least for the time being. The oaks rule.
But, as befits ruling monarchs, they have to support a multitude of lesser mortals.
The wide, spreading expanses of leaves, all full of starches and sugars,
are rich meadows in which vast numbers of creatures can graze throughout the summer.
The oaks protect themselves to some degree by generating toxins in their leaves,
but that doesn't deter weevils.
Or bush crickets.
Sawfly larvae munch away within the thickness of the leaves.
The caterpillar of the yellow-tailed moth.
The tortrix moth caterpillar converts a leaf into a tube, eats it from within,
and then uses it as a shelter while it changes into an adult.
The bagworm caterpillar tunnels inside a leaf,
but shelters inside a portable tube which it hangs beneath the leaf it's currently consuming.
The meals the oak provides for these insects then nourish others,
when the insects themselves become meals for bigger diners.
The great tit with chicks to feed will collect at least 300 caterpillars a day.
The banquet only lasts a few months.
As summer gives way to autumn, the oaks drop their leaves and halt all activities.
So do their lodgers within their cocoons, or hibernating in crevices.
As the year turns, sunshine warms the soil, and plants that spent winter as bulbs below ground
race to make use of the light before the oaks can regrow their leaves.
First are the snowdrops.
As spring proceeds and the sunlight strengthens, bluebells take over.
These glorious carpets of uninterrupted blue are a British speciality.
20,000 years ago, glaciers advanced over most of Britain,
driving many plant species southwards in front of them.
When at last the ice melted, the English Channel began to form
and became a barrier that prevented many plants from spreading back from Europe into Britain.
The bluebell was one of the few that made it.
In the woodlands of North America, things are rather different.
These lilies are growing in the Appalachian Mountains.
Spring here is even richer than it is in Britain.
Instead of just two or three species of flower in a square yard, as in an English woodland,
here there are a dozen or even more.
Glaciers once covered these mountains, too,
but no arm of the sea cut them off from the rest of the American continent.
So when the glaciers melted,
nothing prevented all the spring plants from returning from warmer areas down south.
And here, instead of the uniform carpet created by bluebells,
there is one with a rich pattern of many colours.
Trillium lilies.
Dutchman's breeches.
Wild geranium.
A member of the lily family, the bellwort.
Blue flox.
But the glories of spring do not last long.
In April, English oaks rebuild their canopy.
The bluebells' time is over.
In their few weeks of activity, they manufactured enough food not only to set seed,
but, below ground, to bud off new bulbs.
So next spring, if there's room, they will extend their splendid carpet.
A young beech reinforces the canopy by adding another, lower layer.
Shade returns to the woods.
In tropical forests, it's never winter, and the trees are in leaf throughout the year.
If a plant on the ground beneath a permanent canopy like this needs sunshine,
it will have to climb to get it.
These youngsters search for some kind of ladder by lashing around with their whip-like tendrils.
Once one of them gets a grip, it puts a coil in the tendril to shorten it,
pulling itself closer to the branch up which it might climb.
Other plants ascend by twining their main stem around their support.
As a climber gets nearer the canopy, and the light, it expands its leaves.
There are no more determined competitors in this upward scramble
than the rattans that live in the forests of South-East Asia and tropical Australia.
A mature rattan produces the longest stem of any plant.
One has been measured at 560 feet.
The mature plant doesn't develop leaves on its stem here on the forest floor,
it only does that up in the canopy.
This luxuriant growth,
basking in the full sunshine 200 feet above the ground, is the crown of the rattan.
And it makes the plant's character quite plain - it's a kind of palm.
The tendrils with which it climbs are so thin they are easily overlooked.
But snag one of these on your arm and it'll rip your clothes and your flesh.
The tendrils are rigid enough to reach up and hook onto the branches of established trees.
They then hold the stouter, heavier main stem in position
while it grows upwards from the fearsomely protected bud at its tip.
There is, of course, an easier way to get up here. You can float up, as a seed.
Other plants among these squatters arrived here as seeds stuck to the fur of a monkey,
or within the gut of a bird.
Orchids have seeds as fine as dust,
so small they can be lifted by even the faintest breath of air.
As a consequence, even the highest branches of the tallest trees
may carry spectacular displays of breathtaking blooms.
But living up here has its problems.
Water must be collected and stored.
And, away from the ground, it's difficult to get mineral nutriment.
This orchid deals with those difficulties
by wrapping its green roots around the branch it sits on.
They then intercept rainwater trickling down the bark
that carries with it a trace of nourishing dust.
In some forests at higher altitudes, there are almost permanent mists,
and the canopies of all rainforests
get intermittent supplies of water from regular storms.
Many plants store water as permanent pools in their centres,
and have little difficulty keeping them topped up.
Orchids may keep water in swollen, bulb-like stems.
The ponds in the centre of bromeliads are more than water reservoirs,
they're a source of nourishment.
Tiny animals such as mosquito larvae take up residence in them,
and their excrement and dead bodies accumulate at the bottom as a rich sludge.
So the bigger a plant grows, the more food it accumulates.
I'm 200 feet above the ground, suspended in the branches of a koompassia tree,
the tallest of all the trees in the South-East Asian forest.
And here beside me is a magnificent basket fern.
As it grew and put out leaves, so it collected moisture.
As the leaves got bigger, they in turn collected other leaves falling from above.
They decayed to form a rich leaf-mould.
And as they grew, so more leaves, and with them, more seeds.
So that now there's a fig tree growing here.
So here you have a complete garden, 200 feet up in the forest,
with no part of it touching the ground in any way.
But some of these squatters can become murderers.
A young fig tree like this may arrive here as a seed carried by a bird.
At first, it grows quite slowly.
As it gains in strength, its roots crawl downwards over its landlord's branches.
Some dangle free but keep on growing.
Eventually, they reach the ground.
Now, supplied with nutrients from the soil, the fig grows really fast.
The rootlets wrapped around the main trunk thicken and fuse into a lattice.
The host tree's fate is now sealed, for it is in the clutches of a strangler fig.
As years pass, the fig thickens its roots, embracing the trunk ever more completely.
Trees grow by increasing their girth. For the host tree, that is now impossible.
But growth is difficult anyway, because the fig has a huge crown in the canopy
that cuts off sunshine from its host,
and its roots in the ground are stealing most of the soil's nutrients.
Eventually, the host tree is killed, and its trunk rots away.
But the fig does not fall.
Its roots now form a hollow cylinder that is quite capable of standing upright by itself.
This strangler is about 300 years old.
In fact, it may be misleading to refer to it as a single tree.
It's probable that, 300 years ago, there were several young figs up in the canopy.
Now, centuries later, their roots have grown down to the ground,
they've got rid of the body of their victim,
and they're clinging to one another in this extraordinary interlace of pillars and buttresses
in order to maintain their dominance of this part of the forest.
Nor are these monsters always satisfied with just one victim.
This 500-year-old, having strangled its first victim and lost its support,
toppled sideways into a second, killed that, and then a third,
and now its roots are ready to embrace a fourth.
Dead tree trunks are not wasted, and neither are dead leaves when they fall.
Both are food for fungi.
Some leaves are captured even before they reach the ground.
The fungus has constructed a net, stretched between the twigs of the undergrowth.
Once they've caught their leaf, the threads put out white filaments.
These produce powerful acid which dissolves the cellulose in the leaves.
Why doesn't it dissolve the fungus, too?
Because fungi are not plants, their bodies don't contain cellulose.
They're constructed from a material much more akin to that of animal horn and hooves.
Fungi are neither plant nor animal, they belong to a category of life that is all their own.
Nourished by the liquefied tissues of leaves, this fungus puts out more threads.
But fungi do require moisture.
They can only live out in the open like this in the moist atmosphere of the rainforest.
In cooler, drier woodlands, they have great difficulty living out in the open.
Instead, they hide in the ground or within the tissues of the bodies they feast on.
A fungus has no stem, no root, no leaves.
For most of the time, it's nothing more than a tangled tissue of branching threads.
These produce digestive acids, absorb the resulting soup,
and use it to construct more threads and widen their search for more dead plant tissue.
But cellulose is very low in nitrogen.
To get that, some fungi trap living animals.
The microscopic threads develop tiny lassoes.
These give off a chemical that attracts microscopic worms, nematodes.
One of them nuzzles into the ring.
And the fungus suddenly draws its lasso tight.
The worms are killed and the fungus has its nitrogen.
All this takes place out of sight below ground or within the body of a dead plant.
Only when a fungus is ready to reproduce does it make itself more visible.
From such apparitions as these come spores, the fungal equivalent of seeds.
They're so small that they drift away like smoke.
But the appearance of these spectacular constructions is brief.
As soon as their spores have been shed, sometimes after only a few days, they collapse.
Now they are merely food for maggots.
So the corpses of plants do not retain their nutriment forever.
Some of it is consumed by fungi,
and the remainder, now in soluble form,
seeps back into the soil to sustain the next generation.
But it's not always easy for that new generation to get a start.
On the north-west coast of America and in British Columbia,
fir, spruce and hemlock grow so densely and tall
that very little light filters down to the world below, except when one of them dies.
This giant tree fell about ten years ago.
The gap it left in the canopy above is still open,
but there's still very little light on the forest floor itself -
these ferns and mosses are so very thick.
But up here on the fallen trunk, things are very different.
This is the next generation.
Bare bark doesn't hold the moisture.
Thick moss could bury a seedling.
But in moss like this, growing in such a position,
a young plant can get all the moisture and the light that it needs.
Its threadlike roots grow downwards over the surface of the trunk.
Even before they reach the ground, they find plenty of nourishment,
from the soil accumulating around the clumps of moss,
and from the bark that is already being broken down by fungi.
Eventually, the roots make contact with the earth, and, one after the other,
a whole group of vigorous saplings establish themselves along the trunk.
So, eventually, a row of giant trees stands in the forest
in a line of almost regimental straightness.
Each is propped up on arching roots that never were beneath ground,
but which still hold, between them, the rotting remains of the huge trunk
that 150 years ago nursed them into life.
Southern Australia, in the rolling mountains outside Melbourne.
It's drier than the American North-West, but there's still enough rain to support tall forests.
And with them, a rich, dense undergrowth.
These ferns are even taller than those in British Columbia,
these are tree ferns, but they're still only understorey.
Above them rise one of the tallest of all trees.
These are called locally mountain ash, but they're no relation of the European ash.
In fact, they are eucalypts, and they stand over 300 feet tall.
They carry their tiny seeds in small capsules,
and shed them, a few at a time, throughout the year.
But in the deep shade, the seeds stand no chance.
Even if they had landed on the fallen trunk of an adult,
they would not have been high enough above the ground to be clear of the tree ferns.
The fact is, in mature forests like this,
the mountain ash has a severe problem of regeneration.
The solution could not be more dramatic.
It's another of those events that for human beings are major catastrophes,
a forest fire.
Oil in the bark and leaves of eucalyptus make them extremely inflammable.
As the flames leap higher,
the seed capsules up in the crowns are singed and shed their contents.
The fire grows in strength and becomes a fire-storm.
In the wake of such a huge burn, little is left alive.
But, safe in the soil, below the worst of the heat, many of the eucalyptus seeds have survived.
In the sunshine, they sprout with extraordinary speed.
Few, if any, of their competitors can match them.
Nourished by the rich dressing of ash,
they grow around the charred logs as thick and as uniform as a crop of wheat.
Within a year or so, they're so tall that no other competitors are able to invade their land.
This stump was burning exactly ten years ago.
These saplings around me sprouted from seeds that germinated immediately after that fire.
The saplings are growing so close together that their roots interlock,
forming a dense mat into which few other tree seedlings can get a root-hold.
This land still belongs to the mountain ash.
As the trees begin to spread their canopies, the less vigorous saplings are thinned out.
Now there is space on the ground for the tree ferns to return.
Only the giant redwoods of California exceed the mountain ash in height,
and that may only be because of the activities of loggers.
Back in the 19th century, one ash was felled here that measured 435 feet.
That's the tallest tree ever known.
The greatest threat to the survival of this forest
is that it should grow for hundreds of years without fire.
Should that happen, then these huge trees will eventually die from sheer old age,
fall and lie buried among the tree ferns without ever having reseeded.
The paradox is that this magnificent forest can only survive if it is first almost destroyed.
Another fire, this time in the west of Australia.
Here, the soil is so poor, and the climate so dry, that tall trees can't grow.
Instead, there's a rich variety of bushy plants.
The grass tree has a shock of long leaves that burn fiercely but quickly.
It is neither a grass nor a tree, but a strange relation of the lilies.
What looks like a woody trunk is a fibrous stem that has a special protection.
The grass tree sheds its leaves each year, but their bases remain attached to the stem,
and produce a thick gum that glues the whole lot together into a very effective fire-guard.
Even if it doesn't save every one, most will survive.
Indeed, every species here has to have the ability to live through at least a brief fire.
Otherwise, it would quickly lose its place in the community.
It's 8 months since the fire, and the bush has recovered in a most dramatic way.
The fire stimulated the various plants in at least four different ways.
Its sheer heat baked these fruits of the banksias,
opening the windows on the sides, allowing the seeds to fall to the ground.
The fire also produced great volumes of ethylene gas,
and that triggered the grass tree, after it had regrown its burnt leaves,
to put up this huge green spike which will ultimately carry its flowers.
The smoke of the fire...
..was the stimulus which caused seeds that had lain dormant in this sand
for the last twenty years or so since the last fire,
it triggered them to produce the annual plants which are now springing up.
And it was the sudden superabundance of nutrients produced by the fire in its ash
which allowed the perennial plants like this lovely cat's-paw
to cover the whole ground with colour.
Within a few years, the whole community is fully restored.
The fire was an opportunity for the social struggle to begin again.
Each individual plant had to fight once more for living space,
profiting or suffering from its speed of recovery,
or its suitability to any slight change in the terrain that the fire created.
But, overall, the character of the bush remains unchanged.
The grasslands of East Africa are not so stable.
Here, fire, or its absence, can trigger great change.
This fire is advancing quite fast
because the grass is so dry that as soon as the flames reach the leaves,
they are consumed within seconds.
And as long as there's wind behind it, it'll travel.
But in fact, the line of the fire is very thin,
and if I want to, it's quite easy to cross it.
The land certainly looks ravaged and destroyed, but in fact, little damage has been done.
The leaves of these grasses, certainly, have disappeared.
But the roots are undamaged.
The heat is at its least intense close to the ground,
and this part of the root, close to the surface of the earth, is totally undamaged.
And it's from here that the new growth will come.
And there are many around to relish the succulent young leaves.
Grasses can survive such cropping because their leaves readily break at the base when pulled,
leaving the horizontal stems and their buds undamaged.
Plants with vertical stems grow from the top. If cropped, they're likely to be killed.
And if they're pulled, they come up roots and all.
So the great herds of Africa in effect weed out the grass's competitors,
and it has vast acres to itself, which suits the grazers as well as the grass.
But something can upset this happy arrangement.
Another of those environmental catastrophes.
This time it's drought.
The sun turns the soil to dust.
The animals, having eaten the last dry stalks, move away to look for food elsewhere.
The land is left bare.
Scavengers come to clean up the corpses of those that starved to death,
but they too will soon leave.
When the rains at last return, the grass springs again,
as do other plants whose seeds, one way or another, have landed here.
With no animals to graze them, acacia seedlings grow into bushes.
And they grow very fast.
Once they are a few feet high, they're tall enough to survive a fast-moving grass fire.
And their thorns will protect them from most grazers.
By the time they're about ten years old,
their spreading branches cut off most of the light from the ground,
their thrusting roots are sucking up most of the moisture,
and the grass has virtually disappeared.
A patch of acacia scrub has established itself.
All the trees, significantly, are of the same age and height.
They all took the same opportunity to sprout.
But this situation isn't permanent either.
A hungry elephant that fancies a mouthful of acacia leaves
has no problem, and no hesitation either, in pushing over the whole tree to get it.
Strangely, it's not just acacias that they knock over.
They also push down commiphera trees.
They don't even like their leaves, and seldom eat them.
So is that just wanton destruction,
or can they possibly know what the effect of removing the trees will be?
Whether they know it or not,
a group of elephant, once they have taken up residence,
can turn a promising acacia woodland back to grass in only four or five years.
The grazers return,
and the elephants, once more, have supplies of their favourite food, which is grass.
Elephant are, without any doubt, prime factors in removing thorn scrub from the plain
and allowing grass to spread.
You could argue that, in effect, they are farming the grass,
knocking over trees they don't like and browsing the thorn scrub into destruction.
But you could also argue things the other way round.
You could say it is the grass which is exploiting the elephants.
By providing them with food year after year,
the grass remain the dominant plants on the plains.
And if you look at things that way,
then that is a trick which other grasses have played on a world-wide scale.
Wheat once grew only around the Mediterranean.
10,000 years ago, its seeds were accepted by a small group of human beings as food.
It was so much to their liking that they sowed it wherever they settled
and carried it with them as they overran the earth.
So a few species of grass,
by recruiting the aid of animals, and in particular ourselves, the human animal,
have succeeded in interrupting the ecological cycles
that have operated for millions of years in so many parts of the earth.
They've managed to claim for their own exclusive use
not only wide open plains but fertile, well-watered lands
that once supported rich communities of animals and plants.
These grasses have solved the social struggle.
They have got rid of their competitors.
Eventually, they reach the ground.
Now, supplied with nutrients from the soil, the fig grows really fast.
The rootlets wrapped around the main trunk thicken and fuse into a lattice.
The host tree's fate is now sealed, for it is in the clutches of a strangler fig.
As years pass, the fig thickens its roots, embracing the trunk ever more completely.
Trees grow by increasing their girth. For the host tree, that is now impossible.
But growth is difficult anyway, because the fig has a huge crown in the canopy
that cuts off sunshine from its host,
and its roots in the ground are stealing most of the soil's nutrients.
Eventually, the host tree is killed, and its trunk rots away.
But the fig does not fall.
Its roots now form a hollow cylinder that is quite capable of standing upright by itself.
This strangler is about 300 years old.
In fact, it may be misleading to refer to it as a single tree.
It's probable that, 300 years ago, there were several young figs up in the canopy.
Now, centuries later, their roots have grown down to the ground,
they've got rid of the body of their victim,
and they're clinging to one another in this extraordinary interlace of pillars and buttresses
in order to maintain their dominance of this part of the forest.
Nor are these monsters always satisfied with just one victim.
This 500-year-old, having strangled its first victim and lost its support,
toppled sideways into a second, killed that, and then a third,
and now its roots are ready to embrace a fourth.
Dead tree trunks are not wasted, and neither are dead leaves when they fall.
Both are food for fungi.
Some leaves are captured even before they reach the ground.
The fungus has constructed a net, stretched between the twigs of the undergrowth.
Once they've caught their leaf, the threads put out white filaments.
These produce powerful acid which dissolves the cellulose in the leaves.
Why doesn't it dissolve the fungus, too?
Because fungi are not plants, their bodies don't contain cellulose.
They're constructed from a material much more akin to that of animal horn and hooves.
Fungi are neither plant nor animal, they belong to a category of life that is all their own.
Nourished by the liquefied tissues of leaves, this fungus puts out more threads.
But fungi do require moisture.
They can only live out in the open like this in the moist atmosphere of the rainforest.
In cooler, drier woodlands, they have great difficulty living out in the open.
Instead, they hide in the ground or within the tissues of the bodies they feast on.
A fungus has no stem, no root, no leaves.
For most of the time, it's nothing more than a tangled tissue of branching threads.
These produce digestive acids, absorb the resulting soup,
and use it to construct more threads and widen their search for more dead plant tissue.
But cellulose is very low in nitrogen.
To get that, some fungi trap living animals.
The microscopic threads develop tiny lassoes.
These give off a chemical that attracts microscopic worms, nematodes.
One of them nuzzles into the ring.
And the fungus suddenly draws its lasso tight.
The worms are killed and the fungus has its nitrogen.
All this takes place out of sight below ground or within the body of a dead plant.
Only when a fungus is ready to reproduce does it make itself more visible.
From such apparitions as these come spores, the fungal equivalent of seeds.
They're so small that they drift away like smoke.
But the appearance of these spectacular constructions is brief.
As soon as their spores have been shed, sometimes after only a few days, they collapse.
Now they are merely food for maggots.
So the corpses of plants do not retain their nutriment forever.
Some of it is consumed by fungi,
and the remainder, now in soluble form,
seeps back into the soil to sustain the next generation.
But it's not always easy for that new generation to get a start.
On the north-west coast of America and in British Columbia,
fir, spruce and hemlock grow so densely and tall
that very little light filters down to the world below, except when one of them dies.
This giant tree fell about ten years ago.
The gap it left in the canopy above is still open,
but there's still very little light on the forest floor itself -
these ferns and mosses are so very thick.
But up here on the fallen trunk, things are very different.
This is the next generation.
Bare bark doesn't hold the moisture.
Thick moss could bury a seedling.
But in moss like this, growing in such a position,
a young plant can get all the moisture and the light that it needs.
Its threadlike roots grow downwards over the surface of the trunk.
Even before they reach the ground, they find plenty of nourishment,
from the soil accumulating around the clumps of moss,
and from the bark that is already being broken down by fungi.
Eventually, the roots make contact with the earth, and, one after the other,
a whole group of vigorous saplings establish themselves along the trunk.
So, eventually, a row of giant trees stands in the forest
in a line of almost regimental straightness.
Each is propped up on arching roots that never were beneath ground,
but which still hold, between them, the rotting remains of the huge trunk
that 150 years ago nursed them into life.
Southern Australia, in the rolling mountains outside Melbourne.
It's drier than the American North-West, but there's still enough rain to support tall forests.
And with them, a rich, dense undergrowth.
These ferns are even taller than those in British Columbia,
these are tree ferns, but they're still only understorey.
Above them rise one of the tallest of all trees.
These are called locally mountain ash, but they're no relation of the European ash.
In fact, they are eucalypts, and they stand over 300 feet tall.
They carry their tiny seeds in small capsules,
and shed them, a few at a time, throughout the year.
But in the deep shade, the seeds stand no chance.
Even if they had landed on the fallen trunk of an adult,
they would not have been high enough above the ground to be clear of the tree ferns.
The fact is, in mature forests like this,
the mountain ash has a severe problem of regeneration.
The solution could not be more dramatic.
It's another of those events that for human beings are major catastrophes,
a forest fire.
Oil in the bark and leaves of eucalyptus make them extremely inflammable.
As the flames leap higher,
the seed capsules up in the crowns are singed and shed their contents.
The fire grows in strength and becomes a fire-storm.
In the wake of such a huge burn, little is left alive.
But, safe in the soil, below the worst of the heat, many of the eucalyptus seeds have survived.
In the sunshine, they sprout with extraordinary speed.
Few, if any, of their competitors can match them.
Nourished by the rich dressing of ash,
they grow around the charred logs as thick and as uniform as a crop of wheat.
Within a year or so, they're so tall that no other competitors are able to invade their land.
This stump was burning exactly ten years ago.
These saplings around me sprouted from seeds that germinated immediately after that fire.
The saplings are growing so close together that their roots interlock,
forming a dense mat into which few other tree seedlings can get a root-hold.
This land still belongs to the mountain ash.
As the trees begin to spread their canopies, the less vigorous saplings are thinned out.
Now there is space on the ground for the tree ferns to return.
Only the giant redwoods of California exceed the mountain ash in height,
and that may only be because of the activities of loggers.
Back in the 19th century, one ash was felled here that measured 435 feet.
That's the tallest tree ever known.
The greatest threat to the survival of this forest
is that it should grow for hundreds of years without fire.
Should that happen, then these huge trees will eventually die from sheer old age,
fall and lie buried among the tree ferns without ever having reseeded.
The paradox is that this magnificent forest can only survive if it is first almost destroyed.
Another fire, this time in the west of Australia.
Here, the soil is so poor, and the climate so dry, that tall trees can't grow.
Instead, there's a rich variety of bushy plants.
The grass tree has a shock of long leaves that burn fiercely but quickly.
It is neither a grass nor a tree, but a strange relation of the lilies.
What looks like a woody trunk is a fibrous stem that has a special protection.
The grass tree sheds its leaves each year, but their bases remain attached to the stem,
and produce a thick gum that glues the whole lot together into a very effective fire-guard.
Even if it doesn't save every one, most will survive.
Indeed, every species here has to have the ability to live through at least a brief fire.
Otherwise, it would quickly lose its place in the community.
It's 8 months since the fire, and the bush has recovered in a most dramatic way.
The fire stimulated the various plants in at least four different ways.
Its sheer heat baked these fruits of the banksias,
opening the windows on the sides, allowing the seeds to fall to the ground.
The fire also produced great volumes of ethylene gas,
and that triggered the grass tree, after it had regrown its burnt leaves,
to put up this huge green spike which will ultimately carry its flowers.
The smoke of the fire...
..was the stimulus which caused seeds that had lain dormant in this sand
for the last twenty years or so since the last fire,
it triggered them to produce the annual plants which are now springing up.
And it was the sudden superabundance of nutrients produced by the fire in its ash
which allowed the perennial plants like this lovely cat's-paw
to cover the whole ground with colour.
Within a few years, the whole community is fully restored.
The fire was an opportunity for the social struggle to begin again.
Each individual plant had to fight once more for living space,
profiting or suffering from its speed of recovery,
or its suitability to any slight change in the terrain that the fire created.
But, overall, the character of the bush remains unchanged.
The grasslands of East Africa are not so stable.
Here, fire, or its absence, can trigger great change.
This fire is advancing quite fast
because the grass is so dry that as soon as the flames reach the leaves,
they are consumed within seconds.
And as long as there's wind behind it, it'll travel.
But in fact, the line of the fire is very thin,
and if I want to, it's quite easy to cross it.
The land certainly looks ravaged and destroyed, but in fact, little damage has been done.
The leaves of these grasses, certainly, have disappeared.
But the roots are undamaged.
The heat is at its least intense close to the ground,
and this part of the root, close to the surface of the earth, is totally undamaged.
And it's from here that the new growth will come.
And there are many around to relish the succulent young leaves.
Grasses can survive such cropping because their leaves readily break at the base when pulled,
leaving the horizontal stems and their buds undamaged.
Plants with vertical stems grow from the top. If cropped, they're likely to be killed.
And if they're pulled, they come up roots and all.
So the great herds of Africa in effect weed out the grass's competitors,
and it has vast acres to itself, which suits the grazers as well as the grass.
But something can upset this happy arrangement.
Another of those environmental catastrophes.
This time it's drought.
The sun turns the soil to dust.
The animals, having eaten the last dry stalks, move away to look for food elsewhere.
The land is left bare.
Scavengers come to clean up the corpses of those that starved to death,
but they too will soon leave.
When the rains at last return, the grass springs again,
as do other plants whose seeds, one way or another, have landed here.
With no animals to graze them, acacia seedlings grow into bushes.
And they grow very fast.
Once they are a few feet high, they're tall enough to survive a fast-moving grass fire.
And their thorns will protect them from most grazers.
By the time they're about ten years old,
their spreading branches cut off most of the light from the ground,
their thrusting roots are sucking up most of the moisture,
and the grass has virtually disappeared.
A patch of acacia scrub has established itself.
All the trees, significantly, are of the same age and height.
They all took the same opportunity to sprout.
But this situation isn't permanent either.
A hungry elephant that fancies a mouthful of acacia leaves
has no problem, and no hesitation either, in pushing over the whole tree to get it.
Strangely, it's not just acacias that they knock over.
They also push down commiphera trees.
They don't even like their leaves, and seldom eat them.
So is that just wanton destruction,
or can they possibly know what the effect of removing the trees will be?
Whether they know it or not,
a group of elephant, once they have taken up residence,
can turn a promising acacia woodland back to grass in only four or five years.
The grazers return,
and the elephants, once more, have supplies of their favourite food, which is grass.
Elephant are, without any doubt, prime factors in removing thorn scrub from the plain
and allowing grass to spread.
You could argue that, in effect, they are farming the grass,
knocking over trees they don't like and browsing the thorn scrub into destruction.
But you could also argue things the other way round.
You could say it is the grass which is exploiting the elephants.
By providing them with food year after year,
the grass remain the dominant plants on the plains.
And if you look at things that way,
then that is a trick which other grasses have played on a world-wide scale.
Wheat once grew only around the Mediterranean.
10,000 years ago, its seeds were accepted by a small group of human beings as food.
It was so much to their liking that they sowed it wherever they settled
and carried it with them as they overran the earth.
So a few species of grass,
by recruiting the aid of animals, and in particular ourselves, the human animal,
have succeeded in interrupting the ecological cycles
that have operated for millions of years in so many parts of the earth.
They've managed to claim for their own exclusive use
not only wide open plains but fertile, well-watered lands
that once supported rich communities of animals and plants.
These grasses have solved the social struggle.
They have got rid of their competitors.
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