Original subtitles

One tree is an icon of the British countryside.

It is, of course, the oak.

Today, we begin an extraordinary experiment -

we want to understand this species as never before and to do that,

we will film this one remarkable specimen for an entire year.

Armed with the latest technology, we will investigate how our oak

battles to survive through four very different seasons.

In autumn, we go underground to see how its root

stocks up on precious resources.

What we're looking at is a highly dynamic system.

In winter, we discover the sophisticated strategies

our tree uses to take on everything the elements can throw at it.

In spring, we find out how it senses the world

and how it even has its own form of language. It talks to itself.

There's a chattering that goes on across the whole canopy.

And in summer, we'll see it fight predators

hellbent on eating it alive.

Over the next 12 months, I want to see the world as our tree does

and tell its amazing story.

Wow!

In the coming year, I can't predict exactly how well it will fare,

badly or well, but I can promise you one thing -

you will never look at an oak tree in the same way again.

The oak we've chosen to follow for our year-long experiment

stands in Wytham Woods, just outside Oxford.

It's a rather special tree.

For a start, it's almost 400 years old.

That means it was a sapling during the battles of the English Civil War.

It was laying down its roots as Isaac Newton described gravity.

And it matured as Britain underwent its Industrial Revolution.

What's more, our oak is in a rather special place.

In 1942, the University of Oxford

acquired Wytham Woods, our oak's home,

specifically so scientists could research British woodland.

And I know these woods extremely well.

I taught biology at Oxford for 20 years

and my students and I used to come and study

the countless insects that live here.

The climate, bird populations, the soil -

scientists know a lot about Wytham

and this will help us better understand our tree

and how it changes as we follow it through the year.

Our year-long experiment begins in late August

and the first task is to assess our tree's condition.

To do this, forestry scientists Dr Mat Disney and Dr Eric Casella

will create an incredibly accurate three-dimensional map of our tree.

This is done by firing almost two billion pulses

of laser light at our oak.

The end result is this beautiful image - a virtual oak.

A year from now, it will help us find out how our oak has fared,

how much it's grown and how much new wood it's made,

even how much oxygen it's released.

We get some information straight away.

Our tree is some 19 metres tall and 30 metres wide.

But for me, there's another quite astonishing

and quite unexpected detail.

One of the really interesting things that we can get from these data

is we can estimate the total number of leaves on the tree

and then from that... Without counting them?

Without counting them manually and I tell you,

I have manually counted leaves on an oak tree just recently

and it's not a fun job and being able to do it in an automatic way

without having to get your hands dirty is far preferable.

Come on, tell me, how many?

Well, we think there are around 700,000 leaves on this tree.

To me, the fact that you can tell how many leaves this tree has got

is just incredible. It's amazing, isn't it?

The total area of those leaves is about 700 metres squared so,

to put that in a bit of context,

that's about three tennis courts worth of leaf area.

Our oak needs all these leaves because they capture sunlight,

the source of all its energy.

But now, in late August, our tree is acutely aware that sunlight

will soon become a precious commodity.

As autumn approaches, the days shorten

and the temperature begins to drop.

To survive, our oak must transform itself.

Well, it may look as if not much is happening,

but all across our tree, a dramatic process is taking place.

Our oak is beginning a colossal redistribution of its resources.

Well, it's been through this process hundreds of times before,

but each time is no less challenging.

To see what our oak is really up to,

we need to see what is going on beneath its bark.

BUZZING

As autumn begins, throughout all of the branches and leaves,

a hubbub of chemical messages are now being sent and received.

These chemicals are known as hormones and our tree is producing them

to prepare itself for the autumn.

Well, it may seem odd, but just like us, trees have hormones.

These chemical messengers flow through the body of the tree,

controlling and managing all sorts of important processes.

In us, these hormones are responsible for some of the biggest changes

we'll go through in life, like pregnancy and puberty.

In our tree, they're responsible for an equally crucial change.

As autumn gets underway, driven by hormonal signals,

trees begin to break down pigments and nutrients in their leaves

to store over the winter.

They begin to eat themselves.

The result is a spectacular change in the colour of the leaves.

Once the nutrients have been extracted,

trees like our oak will start to shed their leaves to conserve water

and energy in the coming months.

But how exactly does our tree know

when it's time to begin this huge change?

For a very long time, people assumed changes in autumn

were triggered simply by a drop in temperature.

But what happens if there's an unusually cold spell in summer?

How does our tree know not to drop all its leaves?

It turns out that trees rely on a far more sophisticated method

than temperature alone to sense the changing seasons.

In their own way, they can SEE what's going on.

Well, this should give you an idea of how most plants see the world.

While you and I can perceive a wide range of colours,

trees like our oak are only able to sense the red light

in the spectrum and they can do this thanks to an incredible

chemical pigment in their leaves called phytochrome.

Phyocrome, a substance in our oak's leaf cells,

is incredibly sensitive to the red light

that makes up part of the sun's rays.

It's a kind of chemical stopwatch

that is also able to measure the hours of sunlight and darkness.

So, as the nights get longer, the phytochrome acts like a signal,

telling the tree that autumn has begun.

This means that all the hormones that prepare the tree for the cold months

kick in at exactly the right time.

It's now October and our tree is not only dropping leaves,

it's also time for our oak to release its most precious cargo.

Autumn is not just a time for preparing for the cold,

it's also when our oak releases its offspring out into the world.

We're all familiar with acorns,

but this really is a masterpiece of evolution.

Inside this little capsule is not only the genetic code

to make one of these, it also comes packed with food

and protection from the elements,

meaning this seed has all it needs to survive the winter.

To spread acorns, the tree relies on the help of animals like jays

and squirrels, who often store them underground

and then forget where they are.

But the oak has an ingenious trick to improve its acorn's chances.

It varies the number of acorns it produces from year to year.

Some years, there are thousands.

Others, like this year, there are very few.

While acorns are the perfect food for our friend here,

the fact that she can't rely on oaks all the time

means she has to find other sources of food.

But every five or ten years, oaks have what is called a mast year.

They produce such a deluge of acorns

that all the acorn eaters simply can't cope -

they're overwhelmed, no matter how hard they try.

And this means that the chances of one acorn germinating

and surviving becomes dramatically increased.

With help from the local wildlife, at least one of our oak's acorns

dropped this autumn is likely to germinate next year.

And when it does, it will be a spectacular event.

Fuelled by nutrients locked up within the acorn,

our tree's offspring is brought to life.

A shoot reaches upwards to find sunlight...

..while a root penetrates beneath to find water.

In just a few months,

this acorn has developed into an infant oak with its very own leaves.

This tiny organism is now able to fend for itself.

The success of the oak is largely dependent on the animals

that help disperse its acorns.

And there's one species that, in the last 300 years,

has been particularly helpful.

And that's us.

In the 18th and early 19th century,

there was a frenzy of oak planting in Britain.

In just six years, it was reported that one military officer

managed to plant 922,000 oaks.

The reason for this surge was simple -

Britain had the world's most powerful navy

and nearly all of our ships were made of oak.

This is the HMS Victory,

famous for defeating the French fleet at the Battle of Trafalgar.

The ship is a product of almost 6,000 oak trees,

reimagined by some of Britain's finest shipwrights.

This vessel and hundreds like it

were the reason for Britain's insatiable demand for oak.

Climb inside and you see oak everywhere.

This is the lower gun deck of the HMS Victory.

Many of these sturdy oak timbers have been here

since the ship first set sail in 1765.

At night, hundreds of men would sleep jammed together in hammocks

slung from oak beams and at meal times,

they would eat together at these oak tables.

For the crew of HMS Victory, oak surrounded them.

It encased them and it kept them alive against the elements.

The oak timbers of the Victory

withstood the terrifying power of the sea.

They managed to cross the Atlantic Ocean in hurricane season.

They survived furious battles and innumerable volleys of cannon fire.

They saw death and destruction on a colossal scale.

And it was an oak hull that cradled Lord Nelson as he bled to death.

Each of these spectacular oak planks

has borne witness to and survived the many violent

and dangerous battles on board HMS Victory, but this wood

actually predates the building of this ship by hundreds of years.

This wood is a product of medieval acorns that dropped all over Britain

and, if you look closely, you can still see the story of their lives

etched into the grain.

The way oaks live, the battles they face in the natural world

and their incredible adaptations are what makes this species

so uniquely useful for building ships.

The curved boughs of the oak,

evolved to support the vast canopies of leaves,

allow ships to be curved yet maintain the strength

to withstand the full force of the ocean.

By planting and cultivating oaks,

humans have been able to travel between continents...

..and spread our species to almost every corner of the planet.

Harnessing the strength of this unique organism,

we have been able to overcome even the most treacherous of oceans.

Back at our tree, it's now late October and autumn is well underway.

Our oak is now getting six hours less sunlight per day

than it was in peak summer

and, as the sun is the tree's only energy source,

it must stock up and store resources for the winter.

Crucial to how it does this is the tree's root system -

a hidden subterranean world every bit as complex as the world above.

I'm extremely keen to investigate how this works, but that's no easy task.

Digging up our tree to see its roots would kill it, so to investigate,

we're going to excavate the root system of an oak sapling...

in its entirety.

This is East Malling Research in Kent.

For over 100 years, they have been experimenting with roots

and plants to help develop better yields and they have given us

a unique opportunity to get an insight

into what's going on beneath the ground.

The process begins by digging a metre-and-a-half deep trench.

It's only then the REAL hard work can begin.

An oak's root system, even a very young one like this,

is incredibly complex and fragile

and that means it can only be excavated by hand.

We are trying to ensure that no root, no matter how small, is damaged.

And that means the team must be meticulous in their work.

It's a painstaking process

that will take ten people almost two weeks to complete.

But once it's done,

we can begin to understand the subterranean world of the oak.

Well, this is absolutely incredible.

Look at how much soil they've had to remove

to expose the root system of this tree.

It's only 15 years old

and several tonnes of earth have had to be shifted.

This is something you'll never see in a month of Sundays

and it's something I haven't seen ever before.

But just look at the size of this, look how far they go out

and as they go farther and farther out,

these rootlets get finer and finer and finer

until you're further out than the tree is tall, virtually.

With the roots exposed, we can get a glimpse into their hidden world.

Under extreme magnification, we can see these strange threads.

They are known as mycorrhizal fungi.

They grow all over the oak's roots

and help them extract phosphates,

a vital nutrient locked inside rocks in the soil.

Now, I've just pulled out this little piece of rock here.

I think I can see fungal threads

that were actually attached onto this rock.

Yes, so there's plant inaccessible phosphate in that rock

and what the mychorriza do is they go inside of the rock

and they pull out the phosphate

and they can transport that then into the plant

and into the root system,

whereas the plant wouldn't be able to do that on its own.

So the oak tree simply isn't able to access the phosphate

in this without the fungi.

No, it's much smaller and it can penetrate inside of the rock

and take the nutrients back into the plant.

The tips of the fungi can apply pressure

equivalent to the inside of a car tyre.

And this means they can physically penetrate

parts of the rock to extract nutrients.

These are the hidden helpers that allow oaks to get food

from the most inaccessible of places.

So, essentially,

what we've got here is an oak tree like any other oak tree

which is totally dependent on a vast army of microscopic fungal filaments,

without which it wouldn't survive and it's a win-win for each of them,

they're helping each other. They both require each other to survive.

If you stretched out the root system of a mature plant,

you would expect it to have about five miles of bare root system.

However, if you then stretched out the mycorrhiza network,

that actually would spread around the entire world. For a single tree?

For a single tree. The more I see this system...

Well, you know, what I thought of as a complex system is actually

probably 100 times more complex.

To see this root system in its full glory,

we are going to take our sapling out of the ground.

And put it on display in one of the outbuildings at East Malling.

This is what an oak tree in autumn really looks like.

At the top, we see leaves are being drained of their nutrients.

Below, a vast branching lattice of roots, evolved to keep

the tree standing and extract water and minerals from the soil.

It is here that our oak will store much of its food

over the winter months, but, laid out like this,

the roots are not just beautiful,

they also tell us a fascinating story.

Now, Peter, to the untrained eye, this just

looks like a tangle of roots, but you can tell a story about the tree now.

I think we can because what we can see is a root, as you can see,

going down here before it heads off out in that direction there

and this is almost certainly the root that was inside the acorn

and it's headed on down in this direction,

it's grown on down and then, in the place that this was growing,

we've got some rock underneath, some sandstone

and this root has hit that rock and you can see, it's branched,

it's sent out many branches to try and find

a way around the obstacle and it's sent this one off in that direction.

Beneath our oak at Wytham, the roots, like this sapling,

will be a kind of map, showing the structure

and composition of the earth in which they live.

Within the forest,

there's a very heterogeneous distribution of nutrients.

It's not uniform

and this particular tree has responded to that

by producing this plethora of roots,

this network of roots in this area, to fully exploit that resource.

And presumably once that particular patch of resource here

has been used up, it will just go away.

Yes, these roots are ephemeral, they'll die off fairly quickly

and the plant will invest its resources elsewhere

so it's a highly dynamic system. It's not just fixed and immobile.

No, absolutely not.

What we are looking at here is something which is

sensing its environment, responding to its environment

and utilising resources in a very dynamic way.

At 15 years old, this sapling has developed an amazing system of roots.

Our tree at Wytham will have roots many times thicker,

spreading out anywhere up to 30 metres from the trunk.

It may seem excessive, but our tree will need them -

not just for nutrients, but to keep itself standing.

As its last leaves are finally shed,

our oak is now fully prepared for the difficult conditions to come.

As winter begins, our oak now enters its most perilous season.

To survive, it has stripped itself of leaves,

revealing an otherworldly beauty.

Our tree needs to stay alive using almost no energy.

But in this dormant state, our oak will have to face

everything from gale force winds to sub-zero temperatures.

Well, it's now the depths of winter.

Our tree is bare and it's facing some of the harshest conditions

it will have to endure all year.

At night, the temperature's going to drop well below freezing

and out of the shelter of the forest,

the winds are going to be hitting the top of this tree at full force.

To get some idea of what the tree experiences,

I'm going to be sleeping - or trying to sleep -

40 feet up there.

While our tree looks lifeless in winter, oaks provide a home

to species ranging from spiders and woodlice to bats and owls.

They all utilise the great size

and stability of the oak to provide shelter.

A tree is not just a tree - it's a home.

I think you'll be warm enough going up there.

'I've always wanted to experience what it might be like

'to live in an oak tree and now I'm finally getting a chance,

'even if it is just for one night.

'Getting up to my perch is no mean feat,

'but it gives me a totally new perspective.'

Yes, it feels good!

Once I'm safely ensconced, it's time to try and get some sleep.

I'll have a look out.

'At this height, you get a sense of how big a space this really is.

'For a hibernating bat or nesting owl,

'our oak will provide everything they need to stay safe over the winter.

'But for me, sleep is not coming easily.'

OWL HOOTS

I did hear a couple of noises earlier,

which I thought might have been deer or...

I'm sure I heard a fox.

Well, it's about three o'clock in the morning and it's pretty cool.

It's just under three degrees and I'm toasty in my sleeping bag here.

I've got these layers of down and that really insulates me

pretty effectively from the cold and that is working pretty much

like the bark of the oak tree, which is an effective insulator.

The same principle that is keeping me warm is also keeping our oak

and its inhabitants warm.

Its thick bark is acting like a blanket.

But temperatures in winter can drop below minus ten

and, in those conditions, the bark is not enough.

CRUNCHING AND RUSTLING

Because water expands as it freezes,

if our oak were actually to freeze solid in winter,

it could cause catastrophic damage.

So the oak has an additional strategy.

In the lead up to winter, it withdraws some of the fluid

from its delicate living cells.

It dehydrates itself.

What liquid is left contains high concentrations of sugars

that act as a kind of antifreeze.

It is what allows our oak to survive not just one cold night,

but many tens of thousands of them.

Well, I came up at night, last night, in the dark.

And it is now apparent just quite how high I am off the ground.

Thankfully, for me, it was a pretty still night.

It's cold, but it's not windy.

And the view you get from here is certainly worth it.

But it does give me an absolutely unique experience

of life in an oak tree.

From up here you really begin to appreciate the scale of our tree.

It is a huge habitat.

In the winter, while our tree might look lifeless,

it is actually a vital part of the ecosystem at Wytham.

Our oak is crucial to the survival

of countless thousands of insects and other animals

over the inhospitable winter months.

It is now mid-January

and we are going to take a new and very different

digital scan of our tree.

By imaging the tree without its leaves in these still conditions

we should be able to get a much more accurate estimate

of the weight of our oak's wood, and this will be essential

to understanding how the tree changes over the year.

Dr Eric Casella from the Forestry Commission

is braving the cold for us.

And the model he is creating

will allow us to see our tree in a totally new way.

Eric's scan reveals the sheer complexity of our oak.

Using this model we can work out

that our tree is made up of almost 10 tons of wood.

But the scan also reveals more.

Its branches are distinctly clustered to one side of the tree.

Our oak has directed the growth of its branches

away from the side shaded by the forest

and towards the area that receives most sunlight.

It has uniquely optimised its shape to suit its position.

But this phenomenon is not just above ground.

An oak's root system adapts to help them stay standing in winter.

To see how this works, I want to try something

that has never been done before,

I want to simulate the effects of gale force winds

on an oak.

This is a Forestry Commission research site just outside Edinburgh

and here they are doing pioneering work

examining the strength and stability of many different tree species.

They have allowed me in for the afternoon

to come and watch one of their experiments.

Today, for the first time,

they are going to study how an oak tree behaves during a storm.

Some people might be a little bit shocked

that you are just about pull down a perfectly healthy oak tree.

What is the reason for doing it?

Well, one of the reasons that we do this is to assess

the stability of trees and forests,

without doing this we don't know

what happens when a storm hits.

You're trying to simulate the sort of forces

that that tree would experience in a high wind?

Exactly.

Before we pull it, the tree must be rigged with sensors

to monitor exactly how it behaves under stress.

Once everything is set it is time to get back to a safe distance

and begin the pull.

MACHINERY WHIRS

It is going.

Certainly going.

Beautiful.

With the tree down, Paul and his team can now analyse the results.

At what angle did the tree suddenly become sufficiently, you know,

tipped over, that it fell on its own?

Well, in this case it was only six degrees.

That is nothing, that is like that... That is correct, yeah.

So that tree, actually, it has got very shallow roots,

it is not very big.

No. That is exactly what it is.

You can see, when we looked at the roots, that it was very shallow.

While at first glance it may seem this oak came down quite easily,

it would have taken a force 10 storm

to produce the same effect as Paul's winch.

That size of storm can produce 12-metre waves at sea

and has gusts of wind anywhere up to 90mph.

This oak was, in fact, amazingly stable given its relatively shallow roots.

It is likely our oak has grown much deeper roots

and with its huge spread of branches

it is able to dissipate the force of the winds

much more effectively.

It means that our tree can withstand much harsher conditions.

That a large, heavy structure like our oak can remain standing

over 400 winters is a remarkable feat of evolutionary engineering.

And much of what has made it so successful

at surviving the cold and the storms of winter

has also made it useful to us.

For thousands of years, oak has been an essential building material.

By slicing and shaping trunks of oak into regular lengths

we are able to build all manner of shelters

to protect us from the elements.

In the harsh winter months, oak timbered houses

have kept us safe for centuries.

And thanks to the durability of the wood,

many of these incredibly old buildings still endure today.

But there is perhaps one building above any other

that showcases the extraordinary properties of oak timbers,

and just what they can help us create.

Built in the 13th century,

it remains one of the most imposing and impressive structures

in the British Isles.

And at its heart is oak.

This is Salisbury Cathedral.

It is one of the masterpieces of British medieval architecture.

Looking at its size and scale

it is hard to believe this building was created almost 800 years ago

and throughout its incredible structure,

everywhere you look, oak has been put to use.

During its construction,

an incredible 2,641 tonnes of oak

were employed to help build the cathedral.

But it is not until you ascend above the vaulted plaster ceilings

that you can really understand

how important this single species of tree has been.

This building has within it whole forests

reimagined and remoulded by human hands.

Now, these oak beams have been here for a very long time.

In fact these are among the oldest of the oak beams here.

Yes, the area we are in now is 13th century timber.

It has been tested, and it was felled in the spring of 1222.

The roof here can be dated so precisely

thanks to patterns in the wood.

As an oak grows it makes large amounts of new tissue in the spring,

followed by a much smaller amount of denser wood later in the year.

This rapid, then slow, growth gives the appearance of rings.

If the summer weather is good, a tree will grow a much wider ring

and that gives us a tantalising snapshot of the past and its climate.

By looking at similar patterns across many different samples

it is possible to date pieces of oak with extreme precision.

It is even possible to tell where an individual oak tree was growing.

And it turns out, to build this amazing roof

the local craftsmen used oak from as far afield as Ireland.

These two don't look quite the same, to me.

No, if you look at the rings, closely, this is Irish oak.

The tree rings are really tight together

because the summer and the winter almost blend into one another.

English oak, they have hotter summers,

so they have a better growth rate during the summer.

It makes it stronger, it is also slightly lighter as well.

Which, when you are putting thousands of tonnes of oak

into a roof structure, that helps.

It all adds up. Yeah.

With Salisbury's spectacular roof completed

its builders decided to add one extraordinary feature.

A monumental spire,

that must have filled the medieval population

in the surrounding area with absolute awe.

Today it still remains the largest spire in the UK.

And inside is an incredible lattice of oak timbers.

I tell you, if you didn't like heights,

this would be not much fun for somebody.

This is quite an amazing feat of engineering, really.

And it was essentially an afterthought,

after the cathedral was built,

and they have had to do this ingenious framework

to help them build it.

Yeah, a thousand people were working on it,

they were doing it to get closer to God, if you like,

so it was their vocation, their way of life, to be closer to heaven.

You just look up and marvel, and you almost think,

it is divine intervention, really.

It is probably one of the most amazing structures I have ever seen.

Well, you do get an amazing sense of the countryside from up here.

But don't forget, it is the oak forests growing down there,

the fact that they can withstand

all that the weather has to throw at them,

even in the harshest winter,

and the strength and durability and resilience

of the wood that they gave,

that made structures like this possible at all.

Oak is an incredible building material.

But even today we have yet to come anywhere close

to creating structures with the economy and beauty

of the oak tree in its natural form.

As the winter deepens and temperatures drop down below freezing,

our oak structure will really be put to the test.

I want to find out exactly how healthy our oak is

and how many more winters like this it might be able to endure.

Thanks to some ingenious new technology,

we now have the power to look inside it and find out.

This is very similar to the MRI scanner that we use of the body,

so it takes slices through the body,

we take slices through the tree,

and we're just trying to determine whether the wood is sound or not.

As electrical currents are passed through the tree

a map is created that will reveal the internal structure of our oak.

Well, there is the image. What does that show us?

Well, what it is showing us is

we have wet and dry areas, basically, George.

And the dry areas are in red,

some of them are around the outside of the stem,

the bulk of the stem in the middle, is blue,

but there are breaks in that,

and that suggests there's something wrong

with the inside of the stem, it is not a natural picture.

Clearly something has happened, we need to investigate that further.

This tiny gap between the roots of our oak may look unremarkable

but inside is a hidden world.

Let's get this into position so you can see.

That is quite a big hole, isn't it?

And we can see all that decayed wood.

So we have got a very, very large cavity

where the heart wood is missing,

and we can see fingers of wood hanging down,

where the fungus has rotted out the wood between it.

We call it the Eiffel Tower fungus.

It really only affects the lower part of the stem

and leaves the tree effectively standing

on its buttresses, like this,

a bit like the Eiffel Tower on its four legs...

Hence the name. We've got multiple legs, hence the name.

Even though that is quite a big hole

it is clearly not having a hugely harmful effect on the tree,

it is still here, it's still growing.

Absolutely, it has still got these feet in the ground,

if you like, it can still draw up nutrients and water,

and give it a firm footing in the ground,

even though the heart is gone,

and it could still be there in another 500 years.

It is likely our oak will still be standing here,

alive and growing in the landscape of the 26th century.

But after this vast span of time has passed,

the fungus eating away at our tree's inside,

and the age of its wood,

will mean it looks quite different.

Scattered across the UK are a select few oaks

that have survived over a thousand years.

And they give us clues about our tree's ultimate fate.

This is the Bowthorpe Oak in Lincolnshire.

And over the last one thousand years,

its insides have been almost entirely hollowed out by fungus.

Each valley, ridge, and peak in its wood

tells the story of the battles this tree has faced.

Ravaged by the bitter cold of a thousand winters,

its bark looks like the surface of an alien world.

Sculpted by huge passages of time.

The Bowthorpe Oak is a window into our tree's distant future.

But for now our tree is thriving.

It has endured everything the winter has thrown at it

and is ready and waiting to once again come to life.

As the temperature warms and the forest is bathed in sunlight,

the countless plants and animals in Wytham Woods come to life.

Once again, the forest is reborn with colour, movement, and life.

And for our oak, this will be the season of most dramatic growth.

After many months in a state of suspended animation,

our oak is beginning to come to life.

The buds are finally starting to burst

and our tree is about to undergo

one of the most dramatic changes of the year.

In the next few weeks, this oak is going to have an epic growth spurt.

To capture this transformation,

we are setting up two specially designed cameras.

Bolted to the spot, they will take over 100 pictures each day,

and allow us to compress this spectacular event

into a timescale we can appreciate.

Just like our tree, the cameras will be powered by the sun

and will capture images continuously for the next six months.

With everything set, the cameras are started.

As winter ends and spring begins,

over 700,000 individual leaves emerge across our oak.

It is a truly astonishing change.

This remarkable transformation needs huge amounts of water.

Hidden from the naked eye,

at its peak, our tree will be pumping 70kg of water each hour

out of the ground.

By looking at the oak wood just beneath the bark with a microscope,

we can see how this huge quantity of water gets moved around the tree.

These intricate pipes are known as the xylem vessels

and they run through a layer known as the cambium,

that carries water upwards, from the roots to the leaves.

And thanks to some ingenious technology,

we can now measure exactly how much fluid is moving through them.

With the help of Dr Lucy Rowland,

I'm going to set up an experiment that I hope will reveal

exactly how much water our tree is taking up

and how this changes over the spring.

This is a sap flow monitor.

And as water travels up the xylem tissue, these probes heat it up.

By measuring how quickly this heat is carried away,

the device can calculate how much water is flowing

through the trunk of the tree.

Over 24 hours of measurements,

we see our tree's water consumption varies dramatically.

This is at night when we don't have sap flowing up in the tree.

And this peak here, this is lunchtime-ish yesterday,

when we had maximum flow up through the stem of the tree.

And you can see here that we have got about 10kg of water per hour,

yesterday lunchtime, going up through the tree.

And that will increase as the leaf area of the tree increases?

Yes, so the more leaves that come out on this oak

over the next few weeks,

the bigger that this peak is going to be.

As we move through the next two weeks of spring,

our tree begins to consume ever more water in the middle of the day.

It reaches a peak of over 60kg of water an hour,

as more and more leaves emerge.

But leaves are not all our tree is now producing.

It is now late April

and for a precious few weeks, our oak grows these strange new structures.

Their role is to ensure the future of our tree,

and the continuing success of the oak.

These fragile little objects are known as catkins.

And they are oak's male flower,

and it is the appearance of these every spring

that signals the start of the oak's reproductive cycle.

And if you look carefully inside each of these little blobs,

you will find it is completely packed with grains of pollen.

But these pollen grains are only half the story.

Our oak will also produce a female flower,

but not until later in the spring.

It means that these pollen grains

will need to find a female oak flower on another tree,

if they want to pollinate.

And that means taking to the skies.

In spring, an oak tree like ours

can release up to two billion individual particles of pollen.

And inside each one of these tiny grains is the unique DNA of our tree.

Blown around by the wind, they can spread for miles,

but their mission is simple,

each grain is seeking a chance encounter with a female flower

of one of the other 5,000 oak trees in the surrounding woods.

Filling the air above the forest,

billions of our oak's individual pollen grains

are scattered by the spring breeze.

Up close, we can see how complex this tiny vessel really is.

A thick, warty shell protects the delicate genetic cargo inside,

as gusts of wind carry it for miles.

This is the target of our oak's pollen grains.

A female oak flower.

If the pollen is lucky enough to land here, it will fertilise the flower.

And over the next few months,

the female oak flower will combine its genetic material with the pollen

to create a tiny acorn.

A descendant of our oak.

The yearly act of pollination

is crucial for the long-term future of the oak.

But at Wytham, they have been using pollen

to open up a unique window into its past.

This is Marley Fen.

It is an area of Wytham Woods

that has remained largely unchanged for thousands of years.

And over that time,

as plants and trees reproduce every spring,

the air is filled with trillions and trillions of pollen grains

that eventually end up in this peat here.

As pollen settles on the surface of the fen,

plants, leaves, and other biological matter

gradually build up on top of it.

Over time, layer upon layer of pollen becomes preserved within the soil.

Inside this somewhat unremarkable looking mud,

an incredible story has been preserved,

one that records in detail

the ebb and flow of various trees and plants in the area

for the last 12,000 years.

But to uncover the story hidden in here, you have to dig down.

And that is what Dr Helen Walkington and her team

have been doing for the last ten years.

They use a long metal tube

to extract thin cylinders of peat from the fen.

This four metre long core

can tell scientists how the landscape and vegetation in Wytham Woods

has changed since the end of the last ice age.

This soil, from four metres down, was on the surface 12,000 years ago,

and shows Britain then was a cold and barren place.

So we have got here clay-rich material

with lots of iron and fragments of rock.

So, I don't know if you can see here,

but there are rock fragments within it,

so it tells us there was lots of erosion in this landscape,

and that's how we know that there was not much vegetation at the time.

Without plant roots to hold the soil in place,

the landscape of Britain after the last ice age

was prone to rapid changes.

But as we move along the core,

more and more pollen begins appearing

as plants of all kind take hold.

As the climate warmed, it meant oak was able to move north

and 9,000 years ago its pollen appeared

for the first time at Wytham.

This material would represent organic matter

that would have been moved into Marley Fen 9,000 years ago

and at the same time,

oak pollen would be blowing around in the atmosphere

and would settle out on the surface,

and gradually all the material in the rest of the core

would be on top and pushed down.

I find it incredible

that I can actually put my finger on that piece of core

and touch the exact part of the history of Wytham

where oaks came in.

9,000 years ago.

9,000 years ago, and I can actually physically connect with that.

And what are humans doing at this time?

At this time, we don't have humans at this point.

So this is it, this is pristine?

Once the humans do come into the landscape,

things start changing very quickly.

Moving through the core to nearly 2,000 years ago,

cereal grains begin to appear at Wytham,

and this signals a new type of human activity.

Cereal grains are brought in by the Romans,

and they need to completely clear the landscape

to make space for fields, to cultivate them.

The cereals, we don't know the exact type of cereal they were growing,

because the shape of the pollen grains does not unlock that for us

like it does for the trees,

which we can get down to the species level.

But certainly the Romans would be using this landscape to grow food,

and then as we progress up the core,

we find that oak becomes less dominant.

But it is still here.

It is still present, but it becomes less dominant.

And that is because humans have set about clearing these landscapes

on a much, much greater scale.

The oak tree that we are filming in Wytham Woods

is going to be going somewhere about here.

Yeah, it was probably an acorn around 0.7 metres,

something like that.

And so that represents the period of time

that your oak tree has been growing.

Well, at least it shows that things change over time.

And there have been huge, huge changes in 12,000 years,

which is a very short piece of earth's history.

Absolutely, and in 12,000 years

those changes have been natural and human induced.

There is a kind of interplay of those at this site.

And I am sure that in the next thousand years

that will be the case as well.

The oak's pollen offers us

a vivid glimpse of the challenges trees face over vast spans of time.

But, right now, our tree is gearing up to face

a much more imminent danger.

It's now late May, and our tree is in full leaf.

The oak boughs visibly droop with

the weight of the new material they have to support.

But this abundance of young, soft leaves are extremely vulnerable.

A great threat is now emerging

and our tree must react quickly if it wants to survive.

This is the lava of the winter moth.

It may not look very much,

but this is one of the oak's most fearsome enemies.

This little chap will eat an incredible amount of food

to become adult.

In fact, it will eat up to 27,000 times its own weight in young

oak leaves and, right now, there are countless thousands of these

caterpillars infesting our tree.

But our oak isn't powerless in the face of this attack.

After the oak's new leaves first emerge, for a short while,

the winter moth caterpillars, amongst others, will gorge themselves.

Unprotected from these attackers,

our oak would struggle to survive the summer, but, incredibly,

our tree is able to recognise exactly what's happening to it and respond.

Professor Sue Hartley has spent much of her career

looking at the ways plants defend themselves against insect attacks,

and was one of the first to recognise just how sophisticated

trees like our oak really are.

How does an oak tree know it's being attacked?

Well, that's really interesting. This is a winter moth,

and it's about to tuck in and you can see that when they eat

the leaf, they chew the edge, and they are really messy eaters.

Saliva's going all over the leaf.

There's lots of dew on the leaf surface and, within that saliva,

there are chemicals that the oak tree can recognise.

While we might see or hear approaching danger,

the oak senses it chemically.

It's hard to appreciate, as we have no analogous sense,

but it's an incredibly fine-tuned and refined system.

This chemical signalling is really sophisticated,

so our oak tree can tell whether it's a caterpillar

or whether it's a different kind of herbivore like a sap sucker,

or aphid that feeds in a different way, and it's even better than that.

The oak tree can tell the difference

between big caterpillars and small caterpillars.

The age of the caterpillar can be detected.

That is amazing.

Once our tree has sensed it's being attacked in one place,

it's actually able to signal to itself

to warn other parts of the attack.

It produces something called wound hormones,

and those hormones move all around the plant in the sap system and

that tells the plant to turn on its defences in other parts of the tree.

And they also cause airborne signals to be released that also

travel around the tree.

So the defences are ready all over the place.

So if one branch, if that little branch there

was suddenly attacked by lots of caterpillars,

the tree would know and it would protect all the rest of itself?

It would start to, yes. It talks to itself,

and there's a sort of chattering goes on across the whole canopy.

Once our tree knows it's being attacked, it begins

to produce poisons that will stop its attackers in their tracks.

The main defences of an oak

are chemicals called phenolics and tannins.

That's what you have in your teacup. That's what gives tea its taste.

Yes, tea contains a lot of tannin,

and it's tannin that produces that bitter flavour in tea because

the tannin binds with protein in your mouth,

the saliva, and gives it that sort of bitter taste.

And that's exactly what happens when the insects try and feed.

They find that the chemicals in the oak leaves will bind to

the proteins in their digestive system and stop them going so well.

So, it may look like the tree is just a big, green heap of food,

but eating it is not that easy.

It's a real challenge to eat plants.

They're full of defences and they're very clever,

and they're able to detect the things that attack them.

They've had millions of years to evolve to do that.

And they've got a very sophisticated armoury.

After keeping the insect hordes of early spring at bay,

our tree can continue its rapid growth.

But now, a new danger is emerging.

An outlandish group of insects that

have hijacked our oak's growth for their own ends.

They are, without doubt, the strangest

and most sophisticated foe our oak will face.

This is a gall wasp.

By laying its egg in a female oak flower,

it causes a profound change in the way our tree grows.

That produces a kind of tumour known as a gall

to grow in place of an acorn.

MUSIC: Piano Concerto No 21 by Mozart

Inside the gall, a grub develops, feeding on the nutritious

tissues within, while being given shelter from enemies.

This bizarre structure is the perfect nursery.

This particular structure is known as a knopper gall

and it's the product of just a single species of wasp.

These wasps always produce this type of gall.

But there are many other species of gall wasp

and they can induce very different shaped growths.

The remarkable thing about galls is their sheer diversity.

There are several hundred species of gall wasp

and each one makes a gall of a specific shape and size.

The goals are not just random overgrowth of the oak,

the gall wasps are actually using chemical signals in very subtle ways

to hijack the developmental machinery of the oak at an early stage.

The exact way each species of wasp manages to produce such

individual and unique galls is still somewhat of a mystery.

But it seems they may be actually altering the oak's DNA...

genetically engineering it to grow a home for their young.

The myriad of different types of structures these wasps create

for their offspring is simply staggering.

But, of all the weird and wonderful types of oak gall,

there's one that has a strange connection with the human race.

One type of oak gall has shaped our history.

That's because, for 1000 years,

it was the source of a special kind of ink

with which nearly all of our historical documents were written.

Crushed, mixed with water, iron sulphate and gum arabic,

the humble home of the andricus kollari wasp is transformed

into a cheap and extremely long-lasting ink.

This is the national archives at Kew.

In the vaults of this building are housed over 1,000 years

of British history, in the form of millions upon millions of documents.

Stored in these unassuming boxes is our past

and a huge amount of it is recorded in gall ink.

So, almost any document of any importance had to be written,

or was written using ink made from oak gall.

That's right.

It's the most important ink we have in Western history.

What made it so good? As an ink.

It's an indelible ink.

So it's very hard to remove.

And you can see in some of these documents here,

these are from the trial of Guy Fawkes.

Wow! The actual records?

Yep, these are the actual records of Guy Fawkes' trial.

And here, you can see a nice example of how indelible the ink is.

So here, the scribe has made a mistake

and, to correct his error,

he's actually had to scrape the surface of the parchment off,

remove the ink from the surface and then rewrite over it.

And you can see this dark patch here and the difference in the colour,

because this part of the ink was put on much later.

This is a really good illustration.

These kinds of legal documents had to be kept in ink that was

going to last, had to be written in ink that was going to be lasting.

So they're written on material parchment that is more durable

and they're written with an ink that is not going to

just vanish before your eyes.

But oak gall ink wasn't just used for official documents.

Everyone from poets, musicians and mathematicians to fine artists

used this ink to record their thoughts, feelings and ideas.

The whole of western civilisation between from about the end

of the Roman period to the 19th century,

our most important texts are in iron gall ink.

It seems just a bizarre twist of fate that all of this,

and there are how many thousands of documents here which are written

in this ink, began because a tiny wasp

laid an egg in oak buds that grew into a gall,

and that provided the basis for, essentially, our recorded history.

That's right.

What is surrounding us is just a small fragment

of all the documents that survive from those 1,400 years of history.

From wasp to gall to human hands.

This little quirk of evolution has shaped human history.

This incredible ink brought us the Magna Carta

and the American Declaration of Independence.

It has brought us the music of Mozart and Bach...

..and the drawings of Rembrandt and Leonardo da Vinci.

Thanks to gall ink, we have Isaac Newton's theories

and the letters of Charles Darwin.

Unwittingly, the oak tree has enabled us to record our past,

to express our most profound ideas and to share our deepest emotions.

In just three months, our tree has gone through

a radical transformation.

It has brought out its leaves, it has spread its pollen

for miles around, and it has repaired the damage sustained over winter.

Now, as the insect populations grow ever larger,

this mighty organism is finally ready to face its most challenging season.

It's now June and, under the intense sunlight,

trees and plants are working at full capacity.

For the countless life forms of the forest, it's a time of plenty.

And, at the centre of this frenetic activity is our oak.

Right now, it's literally being eaten alive.

There are hundreds of insects that depend on the oak for sustenance.

But I want to see the insects us humans do not normally come across -

the ones that live high up in the oak's canopy.

Well, it's now the height of summer and the tree is in full leaf.

There's even some acorns beginning to swell.

This is just an enormous, cathedral-like space.

What's very frustrating when you're on the ground is that you know there

are lots of fantastic insects and animals, but you can't reach them.

So, the only way to get to them is to climb.

HE GRUNTS AND GASPS WITH EFFORT

If I can just find a nice place to stand...

Oh! There we are.

Wow!

This is a very privileged view of an oak tree

and one that only an insect would have.

There are some insects up here that you never see from the ground.

If I can just shake the foliage, try and get some insects in the bag.

I'll bet there's lots of good stuff in here.

Now, the next bit of kit is the pooter.

That allows me to suck insects out of the net.

Without handling them, because lots of these things are very small.

So, let's see what we've got.

High up in our tree, there is a wealth of life.

This is where the good stuff will be.

Hmm!

Oh!

But, to get a sense of its diversity, and the unique

adaptations of creatures up here, we have to take a closer look.

And we can do that under the microscope.

Now, we've got quite a few insects in here.

I think we'll just empty them in there, and hope for the best. Great.

I'll just whack them in. I'm sure it will be fine.

A big earwig there, look at that!

What is absolutely amazing with this machine is the quality of that

image is just breathtaking.

Well, that is the head end of a cricket,

and she's having a preen here.

The very interesting thing about these insects is that they

have their ears on the knees of the front leg.

You will see a little opening there, and that is the opening of her

hearing organs, which are here and here.

And, by having their ears on their front legs, quite far apart,

they're able to triangulate and know exactly where that sound is from.

Now, let's see if we can see anything else here.

There are absolutely minute things in here.

A tiny little thing, a mite, absolutely minute.

And there are probably millions, tens of millions of these up a tree.

That animal is tinier than the claw on the hind foot of a cricket.

This spectacular variety of insects

are all at their most active in summer,

and many of them are specially adapted to eat our oak's leaves.

This is a plant hopper

and it's able to suck out sugary sap from individual plant cells.

When these sap suckers attack en masse,

it can be devastating to the delicate leaves of our tree.

There are many, many different insect species who call our tree home,

but there are a select few who have a special relationship.

Species that have evolved to specifically

take advantage of the oak.

This is one of our tree's infant acorns, finally beginning to emerge.

It's a beautiful, intricate structure.

Something here is not right.

This strange, black hole is a sign that this acorn

has been tampered with.

BELLS CLANG OMINOUSLY

The culprit is one of the most highly specialised

and bizarre species on the oak.

DISTORTED, CLANGER-LIKE NOISES

The acorn weevil.

Look at that! Ho-ho!

Is that not just the most beautiful thing?

This is an animal that's evolved specifically with oak trees.

It lays its eggs in acorns,

and it's got this enormously long beak that comes out of its head

and, at the end of that are a pair of tiny jaws,

and it drills deep into acorns to lay its eggs in the acorn,

and she has these peculiar antennae which are elbowed, hinged,

and, as she drills into the acorn,

she can fold them back along the side of the head.

Our weevil also has highly specialised bilobed feet

with which it's able to grip onto the smooth surface

of the oak's acorns.

Being able to see them this close brings you into their world.

You can understand the mechanics of what they have to do,

how they have to live.

It doesn't get any better than this, really.

That is just evolution at its most wonderful.

The acorn weevil is just one of many insects up our tree.

On one single branch, there's a beautiful and deadly lacewing.

Other insect predators, such as a damsel bug and a comb-footed spider.

And the tussock moth caterpillar, who can feast on our oak's leaves.

All of these insects have found ingenious ways to use the oak

for their own ends and extract food from it in some way or other.

And it's not just insects -

us humans also consume oak.

In fact, we can drink it.

To discover more about this, I'm going to the land of my forefathers.

Scotland.

This is the Scotch Whisky Experience in Edinburgh.

With 3.384 different bottles,

it's the world's largest whisky collection.

To be legally called a Scotch whisky,

the alcohol must be stored in oak barrels for at least three years.

Whisky is, in essence, oak-flavoured alcohol.

Does the growth of the oak tree effect what the whisky will

eventually be?

Yes, it absolutely can do.

Generally, what happens in quercus species is,

the tree lays down material in two distinct parts of the year,

springtime, it lays down early wood, which is like a sponge, very porous.

The rest of the year, late wood which is... hard and dense.

The early wood is more porous or spongy,

therefore it can give forth more flavour.

So, if you're really fussy about the type of barrel you want to use,

you will go for so-called tight-grained oak, typically,

12-16 growth rings per inch,

if you're going to get very specific about it!

By treating oak barrels in different ways, by charring them

and seasoning them with other wine and spirits, it's possible

to release multiple chemical compounds from the oak,

leading to an incredible diversity of whisky flavours.

So what we've got is actually a very complicated system.

All these compounds which give flavour to the whisky,

how many different flavourings are there, do you think?

I would say that there is probably between 50-100 different compounds

we can identify that have come out of the oak wood

that can influence the character and flavour of the whisky.

So, when you drink your mature whisky, all these lovely,

buttery flavours, the soft texture on the palate, the sweetness,

the vanilla, the coconut, the almond, all of these flavours

are drawn directly from the good-quality oak wood.

The multitude of flavours that whiskies possess

are testament to the complexity of the oak's wood.

From weevil to human, there are many hundreds of species that eat

or consume the oak in some way,

but what does our tree eat?

Where does it get its energy from?

The answer is, of course, the sun,

and at the height of summer, this process,

famously as photosynthesis, is at its peak.

To see how the tree does this,

we need to look at its leaves under the microscope.

These strange openings are called stomata.

And they suck carbon dioxide from the atmosphere into the leaves.

Then, powered by sunlight, this carbon dioxide is combined

with water and turned into sugars that our tree feeds on.

But, as they photosynthesise,

our oak leaves perform one final, magic trick.

Out of the many billions of stomata pours oxygen.

It is, perhaps, the single most important

process in the natural world.

At the height of summer, our oak, its magnificent structure

and its hundreds of thousands of leaves, are able

to bask in the sunlight and convert it into food.

In the process,

it pumps out the oxygen that we all rely on to stay alive.

In this single act,

our oak is performing a feat that we have yet to match.

As August begins, it's now been a year

since we made the first digital model of our tree.

Thanks to the detailed measurements we've taken over the years,

and the weather data from Wytham Woods, it's now possible to make

estimates that reveal the ways our tree has changed.

Despite its age, our tree has grown.

Over the last 12 months,

it has been extracting carbon dioxide from the atmosphere

through its leaves, and some of this has been refined into carbon

and forged into new wood.

While our oak's great size and age means that new growth

is extremely thinly spread, it has increased in size.

In fact, our tree has created 230 kg of new wood.

This much material has literally been plucked from thin air.

To help it grow and photosynthesise,

our tree has had to consume huge quantities of water.

Thanks to our sap flow data, we can see that, over the 71 days

we recorded it, the tree drank an incredible 58,822 litres of water.

But our oak tree hasn't just taken from the environment around it.

As it photosynthesises, its leaves produce oxygen.

Since we've been filming, our tree has released

an incredible 234,000 litres of oxygen into the atmosphere.

And that much oxygen is enough to keep me alive for a whole year.

By spending a year looking at this one tree, we have been

able to see just how dynamic and complex this organism really is.

We have seen how it can create 700,000 leaves and keep them safe.

We've seen how it can withstand the harsh winter conditions.

And we've seen how our tree sits at the centre of a vast,

interconnected web of life.

In the face of everything thrown at it, the wind, the rain,

freezing temperatures and constant attacks by insects and fungi,

our tree has thrived.

In the process, it provided a home

and a source of food for millions of individual organisms.

It's what makes this incredible species

such an important part of the British countryside.

The oak's endurance and longevity have woven it

into the lives of the thousands of creatures that rely on it.

And that includes us.

This colossus of the British Isles has permeated our culture.

Oaks have shielded us, protected us from danger.

They have allowed us to explore the seas.

They have brought us pleasure.

They have helped us express our most profound ideas.

Oaks have borne witness to our deepest sorrows

and our most joyful moments.

This plant, perhaps more than any other, has become part of us.