This is the boreal forest...
..the largest forest on Earth.
750 billion trees...
..smothered by snow throughout the winter.
This is the northernmost boundary of an extraordinary world.
I'm standing at the edge of the Arctic Circle.
To the north of me lies a land dominated for most of the year
by snow and ice.
3,000 miles to the south - the tropics,
bathed the year round in warm sunshine.
And in between, a very different world,
dominated by relentless change.
An endless cycle of four distinct seasons,
each with its own challenges.
The short, freezing days of winter
give way to the urgent awakening of spring.
And the long, hot days of summer
yield to the cooling of autumn.
Opportunities will be brief.
To survive these extremes,
plants have not only got to be hardy and resilient,
but many of them have developed special strategies
in order to meet the demands of this seasonal world.
To succeed, they must get their timing just right.
For months now, this world has been asleep.
But change is coming.
Spring is on its way.
In Canada, the forests are starting to stir.
Throughout the winter, the sugar maples here
have kept stores of nutrients ready for this moment.
Now, as temperatures start to rise,
those nutrients must be brought up from the roots.
Sap is on the rise.
Fuel for new green growth.
But just as growth reaches full speed...
..robbers arrive.
A sap sucker.
A thief in search of maple syrup.
The tree can tolerate hundreds of wounds.
But if they completely encircle the trunk,
the tree will eventually die.
The sap sucker's break-in hasn't gone unnoticed.
Hummingbirds, squirrels and insects
all invade the sap sucker's territory,
trying to steal a meal.
But unwittingly, they help the tree.
Time spent chasing the competition...
..means less time to drill new holes.
The tree gets a chance to repair its wounds.
And the sap can rise freely again.
So this year, at least, the tree will once more produce its leaves.
Hundreds of miniature solar panels
that collect energy from the sunlight.
Each leaf is covered by millions of microscopic pores.
Stomata.
They take in carbon dioxide...
..and release oxygen and water vapour
in a process that produces high-energy fuel.
All the trees here can now use that fuel
to grow over the coming months.
As spring proceeds, the power of the sun increases.
On river banks across Europe,
many plants have been lying dormant underground,
waiting for the earth to warm.
And now a race begins.
Among the first to start are nettles - the sprinters.
Then, only just behind, come the brambles.
They're much more aggressive and have backward-pointed hooks
with which to scramble over their rivals.
Last to come - the climbers.
Hops...
..and bryony.
They lasso their way towards the light.
Soon, every inch of space and every patch of light has been claimed.
But hidden in the shadows,
another plant has been waiting to make its move.
This...
..is dodder.
A hunter...
..with an exceptional sense of smell.
Moving swiftly, it searches for its prey.
It detects the scent of a young nettle.
The dodder punctures the nettle's stem
and sucks out the nutritious sap.
And it doesn't stop at just one victim.
With its energy supply assured,
it multiplies.
It's a parasite with an insatiable appetite.
But remarkably, its victims can now exploit the dodder...
..when other springtime enemies appear.
Caterpillars.
When under attack, a leaf sends out a signal
telling the rest of the plant to start building defences...
..foul-tasting toxins that deter the caterpillars.
In an extraordinary twist,
the stems of the dodder interconnecting the plants
are now being used as lines of communication
to relay these warning signals from plant to plant.
They respond by producing their own protective chemicals.
Eventually, the whole river bank becomes one huge defensive network.
And the one-time rivals are now better off together.
The first warm days of spring encourage some plants to flower
and get ahead of the competition...
..creating beautiful displays throughout the seasonal world.
From the spectacular cherry blossoms of Japan...
..to the goldfield flowers of California.
But spring flowering is risky.
If they flower too early, it may not be warm enough
for pollinating insects to be active.
But the common daisy gives them every opportunity.
The warmer the flowers are, the more attractive they are -
and they have a remarkable strategy in order to maximise this.
They were closed up tightly throughout the night
as a protection against the elements.
But once they feel the warmth and the light of the sun,
they spring into action.
It's a behaviour called heliotropism.
They turn to keep facing the sun,
absorbing as much heat as they can.
This is a thermal camera,
and it will tell me the difference between the surrounding temperature
and the temperature in the centre of a daisy flower.
The surroundings - 12 degrees.
In the centre of a flower...
..21.
Pollinators such as bees and wasps prefer the warmed-up flowers
because they can get a share of the heat for themselves.
As a consequence, they can collect more nectar from more flowers,
pollinating as they go.
And the daisies, with the help of their pollinators,
are able to have a particularly long flowering season.
But when the window of opportunity is brief,
even more ingenious tactics are necessary.
Here in south-western Australia,
summer temperatures can soar to up to 40 degrees Celsius.
Now, for a brief moment in the cooler spring,
flowering plants must get busy.
And this hammer orchid, with its strange flower,
is doing exactly that.
It needs to attract a pollinator,
but has no nectar and doesn't even look like a flower.
Unlike these neighbouring huge grass trees.
These produce thousands of tiny white flowers,
all dripping with nectar.
But the orchid has a different strategy.
It synchronises its emergence with the brief mating season
of the thynnid wasp.
The flightless female wasp produces pheromones to attract males.
This male detects a scent that seems to be hers...
..yet flies right on by.
The orchid not only has the same shape and colour as a female wasp,
but it sits at the same height as she does.
It even mimics her smell.
And the beguiled male attempts to mate with it.
He hammers against the orchid's pollen sacks.
And the wasp leaves, doubtless a little shaken...
..only to be duped once again.
The pollen on his back now sticks to this different orchid.
And the deception has worked.
The orchid's imitation is even more intoxicating, it seems,
than the real thing.
Eventually, the male wasps tire of the orchid's enticements.
And now, the wasps, both male and female,
find the flowers of the grass trees to feed.
And there, at long last...
..they mate.
And as for the hammer orchid?
Pollination is complete.
As summer approaches, the need to flower becomes more urgent.
And nowhere more so than here in South Africa.
This is the fynbos, part of the Cape floral kingdom,
a great expanse of open heathlands just north of the cape.
Here there are more different species of plant
than anywhere else in the world.
Nearly 9,000, many needing pollinators.
They compete with one another for such help
by flaunting extravagant shapes and vivid colours.
All promising nectar as a sugary reward.
But there is a plant here that avoids this crowded competition.
And the rising temperatures of summer bring just what it needs.
This is the first fire here for 15 years.
Nearly all the plants are destroyed.
Incinerated.
It might seem from the smoke and the still-smouldering embers
that no plant could survive such an inferno.
But just four days after the flames...
..rising from the ashes...
..a fire lily.
It has been lying dormant underground for 15 years,
but now, awakened by the smoke, it flowers.
The blooms may be small and unshowy, but that's all they need to be.
In this charred landscape, pollinators, such as these sunbirds,
can spot these little red beacons from great distances.
They're the only source of nectar around.
And the more visitors the flowers attract,
the more likely they are to be pollinated.
It's just in time.
Within a few months, the whole landscape is alive once more.
In fact, all these plants need fire to survive.
And as competitors return,
the fire lily fades.
It now returns underground
and will rest there as a bulb until another fire awakens it.
By the time the long, hot days of summer arrive,
wild flower meadows are bursting with life.
But autumn is not far away.
So now pollinated plants must use their remaining energy
to produce the next generation.
Seeds.
A dandelion clock contains around 200 seeds...
..each with its own tiny parachute.
Few seeds can fly as far as these.
Some are known to have travelled over 60 miles.
But to travel any distance at all, they need the wind to be just right.
However, before they can get away, there is a risk.
Voracious hunters live in this meadow.
Tiny harvest mice.
They love dandelion seeds.
But they don't like to share.
Finally, the sun warms the ground
and a gentle breeze blows in,
creating an ideal updraught.
Air flowing between the bristles creates a vortex
that lifts the seeds up.
Heading off on a gentle breeze, this seed now starts its travels.
Plants use a variety of different techniques
to spread their seeds as far as possible from themselves.
And one of the most remarkable is this one.
It's called ecballium, and it's a relative of the cucumber.
For several weeks now, pressure has been building up inside the pods.
Until now, they're as taut as a well-pumped-up bicycle tyre.
All they need is just a slight nudge.
HE LAUGHS
The further the plant can fire its seeds, the better.
If a seedling germinates close to its parent,
the two will have to compete for nutrients and light.
This Himalayan balsam has seed pods that react rather differently,
like catapults.
As they dry out, they begin to strain along precisely positioned
lines of weakness.
And then, just the slightest disturbance,
even a single raindrop, is enough to trigger the mechanism.
The east coast of South Africa.
Summer here brings scorching temperatures.
This Ceratocaryum has to get its seeds underground
before it gets too hot.
And it has acquired somewhat unlikely allies.
Dung beetles.
They have a particular fondness for antelope droppings.
At this time of the year, these beetles bury the dung
and lay their eggs on it.
And the fresher...
..the better.
Ceratocaryum's spindly stems send their seeds flying.
They're the same size, shape
and even smell remarkably like...
..antelope dung.
The beetles simply can't resist them.
Dung beetles always bury dung balls at the same depth.
And it's one which suits the seeds very well.
So effective is the seed's deception
that the beetles come back for more time and time again.
Most of the Ceratocaryum seeds get safely buried...
..exactly where they need to be.
As the sun begins to retreat, so autumn arrives.
The cooler, shorter days are the cue for many seasonal forests
to prepare for the winter shutdown.
The trees start to divert their nutrients back into their roots.
And there, other organisms await them.
Fungi.
Mushrooms are the familiar face of fungi,
but they are merely the fruiting bodies.
Some of them only last for a few days.
They are, however, evidence of the giant organism
that lies in the soil beneath.
Just a single handful of soil may contain several thousand metres
of their microscopic filaments.
It's only recently that we have discovered the extraordinary role
that these fungi play in a forest like this.
Their filaments plug into the tips of the tree's roots
and nutrients pass between tree and fungus throughout the year.
Some fungi have the ability to link with not just a single tree,
but with a whole group of trees,
so that the entire forest may be linked together
by these microscopic threads,
to form what you might call a wood-wide web.
Hundreds of trees can be interconnected by these webs.
Scientists call it the mycorrhizal network.
And it might look something like this.
They've discovered that trees not only send nutrients along it
but chemical and electrical signals,
allowing them to communicate with one another.
But some trees can also be selfish and steal from their rivals.
Or even wage war by sending out toxins that will harm competitors.
It seems, however, that most trees do try to help each other.
They raise the alarm when attacked by leaf-eaters,
leaving other trees time to produce defensive chemicals.
Those that are dying may send their food reserves to their neighbours.
And some individuals known as mother trees
recognise their own offspring,
and will channel resources to them...
..so giving their young the best possible start in life.
As the harshness of winter approaches...
..temperate woodlands from Russia to Canada
are now in a race to shut down.
The green pigment in the leaves starts to break down
and nutrients are withdrawn into the branches.
The chemical substances that are left behind
then create one of the most spectacular displays of colour
in the whole of nature.
The first freezing nights of winter bring with them a killer.
Frost.
Stems, when frozen, rupture.
And flowers of ice form in the chilly air.
For some, winter brings dormancy.
For others, it brings death.
Another cycle of the seasons
comes to an end.
Plants all across the seasonal world
have developed many different strategies for success.
But they all depend on the seasons changing reliably year after year.
It doesn't matter whether you live for 3,000 years
or just a few months,
everything depends on you getting the timing just right.
But in our changing world,
this is becoming a greater challenge every year.
And even the most hardy and resilient of plants
are starting to struggle.
These sequoia trees are the giants of the natural world,
the largest living thing on the planet.
They can grow to almost 100 metres tall
and 11 metres across.
Their bark alone can be over a half-a-metre thick.
They're also among the oldest, some living to over 3,000 years.
And to achieve such age and size,
they need very particular living conditions,
such as are found here in the Sierra Nevada mountains
of California.
Not only do they need energy from the sun but,
critically, they also require up to 4,000 litres of water a day.
So they are almost entirely reliant on the seasonal snowmelt.
But recent years have brought longer, hotter summers
and their source of water is becoming increasingly unreliable.
Scientists are discovering
that even these seemingly indestructible giants
are now starting to show signs of vulnerability.
Some are shedding their needles and branches
as a way of conserving precious water.
But for others, climate change has already been fatal.
10% of them have been lost in just the last few years.
A single giant sequoia in its lifetime
can produce 100 million seeds.
These, in my hand, from one cone,
are more than enough to start a completely new forest.
But they can only do such a thing in a world where the seasons change
with some reliability.
Today, our climate is changing, bringing an unprecedented level
of unpredictability all across the seasonal world.
The question is, can we curb climate change sufficiently
to ensure that the seasons will continue?
Only if we can do that will the future of seasonal plants,
including these magnificent trees, be assured.
During filming The Green Planet,
it wasn't only the plants that struggled to get their timing right.
The increasing unpredictability of the seasons
made it frustratingly difficult for the crew
to be in the right place at the right time.
Whether the height of summer...
We are in the middle of a biblical rainstorm.
..or a quiet autumnal afternoon...
MAN: This is crazy. Hurricanes are no joke.
..they were always kept on their toes.
That's really scary. Ooh!
Sometimes very numb ones.
It's been the most snow they've had for 38 years,
and I think this is now our fifth filming trip in a row
where the seasons haven't done what they're predictably meant to do
every year.
And these challenges continue in attempting to find and film
an elusive plant with a remarkable survival strategy.
The fire lily.
Somewhere out there is a tiny, tiny red plant...
..in all of that.
Travelling to South Africa at the height of summer...
..the crew witnessed many extraordinary flowers
bursting into bloom.
But if they are to see a fire lily, they need a fire.
None of the team have filmed wildfire before -
not something to be underestimated.
In most cases, with wildfires, is that, when they go wrong,
they go wrong radically, and it's very unforgiving.
If you get caught on the wrong side of a fire
because you want to get that special shot,
there's a possibility you can get killed.
That's really what it comes down to.
And after some understandably studious note-taking...
..it's time to put their training to the test.
The fire's just racing up a gully behind me.
The heat is intense. You just... You can't even stand to be, like,
five metres from it. It's insane.
Whoa!
This is what any fire lilies here should be waiting for.
Jeepers.
But fynbos wildfires are starting to become fiercer than in the past.
Something fire chief Reinard Geldenhuys
knows first-hand.
Nowadays, we're experiencing more extreme temperature conditions
and more extreme wind conditions,
which exacerbates a fynbos fire.
Fynbos fire in full tilt, with a strong wind behind it,
is an animal that's just running wild.
It's a very frightening sight if you're standing in front of it.
Guided by the experience of the fire crew,
the team are able to safely capture the full fury of the blaze.
It's given me a complete new appreciation for fire.
How intensely it burns and how quickly it roars
through this environment.
And the fact that any plants can survive afterwards
is just... Yeah, it's crazy.
As destructive as it seems,
the fire has been part of this ecosystem for millennia.
Fires should refresh and regenerate the landscape.
Worryingly, however, this fine balance between plant and fire
is beginning to shift.
More extreme wildfires may spell disaster.
If you disturb the soil in a fynbos system,
you're going to lose a lot of species.
Bigger, hotter fires can burn the soil there
and burn out all the seeds.
And for the species that have these really intricate strategies,
like the fire lily, it really is a difficult game.
Every year is getting hotter and potentially drier.
So it's kind of becoming Russian roulette
for a lot of the species here.
In the fire's aftermath,
the crew must now try to find a tiny flower...
..but have any survived the blaze?
It just looks completely dead, the whole valley.
There's nothing for it but to get their hands dirty.
Searching in the ash is a grubby business.
I feel...
..disgusting.
After scouring this mountainside just days after it burned...
..they finally find the first signs of regeneration.
Wow. That's what it's all about. That's the fire lily.
It's like a pop of bright red against all the black.
I mean, that's amazing. There's another one over there,
and there's another one behind us.
This place hasn't burned for 15 years,
so they've been sat underground waiting for this precise moment
for that entire time. And then, within two weeks,
they're up and they're flowering. It's incredible.
This time, the fire lilies here succeeded in flowering.
But in the 15 years since they last bloomed,
the Earth has experienced ten of the warmest years on record.
You know, there's huge debates about climate change -
how real is it, really, and so on.
Erm, we experience it in the fire.
It's more intense, burns easier, burns longer.
In the past, we used to have one big fire per season,
and we said, "Ja, that was a big one."
Now we have five to six big ones per season.
If we don't change our ways, we are going to reach a point where
fires are burning with such ferocity that it will destroy the landscape.
And in recent times, a rapidly changing climate
has led to some of the biggest fires in living memory -
not just in South Africa, but around the world.
The next five to ten years are going to be pretty scary
to actually see what this has done to the ecosystem.
Let's hope that the next time this fire lily blooms,
it will find a world in which it can still survive.
Next time on The Green Planet...
..desert worlds.
A landscape of extremes...
..where water equals life.
It only takes a drop.
The Open University has produced a poster
that explores the vital role that plants have for our planet.
To order your free copy, call...
..or go to...
..and follow the links to the Open University.