The Secret Life of Trees

3 trillion trees blanket our world.

From spectacular cherries in spring...

..to the golden maples of North America...

..and the strange-looking baobab trees of Madagascar.

They can bring us joy...

..and even keep us alive.

All these amazing trees around us give us the air that we breathe.

But what do we really know about their hidden world?

As you get closer to the tree,

it just gets more and more remarkable.

In this series, we'll follow their incredible life spans...

There are trees alive on Earth today

that are as old as the Great Pyramids.

..discover how trees communicate...

This whole wood is interconnected,

and that opens up so many possibilities.

..and learn how they survive in some of the toughest places on Earth.

So far, we've learnt

how even the mightiest trees begin life as tiny seeds.

And we followed them as they thrived through their mid-life years.

In this final episode,

we'll meet the surprisingly spritely elder statesman of the tree world.

As trees get old, they get really exciting.

When the old tree's had enough,

that newly-rooted branch can start to grow.

And we'll join them

through the final stage of their remarkable journey.

When a tree dies, you get to see its past life.

It may look dead

but actually, it's doing far more than the eye can see.

You'll never look at a tree in the same way again.

Trees are extraordinary for so many reasons.

But possibly, the most amazing to us is their incredibly long life span.

They say it takes an oak tree 300 years to grow.

300 years to live.

And 300 years to die.

But this third chapter in a tree's long life

is full of surprises.

Especially when some of the oldest trees on the planet

can live for nearly 5,000 years.

But how do you tell the age of a tree?

Professor Mary Gagen from Swansea University

investigates the life stories of ancient trees.

Trees are just absolutely wonderful things.

They're some of the biggest, most majestic natural things

that we see in our environment.

And they also grow with us.

A tree that's small when we're small,

will be a big, tall, majestic veteran as we age.

And that way that trees grow is really important

because it means that we can find out things

about that tree's life history from looking inside them.

So, we've got this lovely, healthy sycamore.

They're a really common tree in our woodland.

And looking at it, we can tell a few things about it,

its health, its history,

but if I want to know more about what it's experienced,

I need to look inside the tree trunk.

And we use this to take a small core from inside.

So, a little bit of hard work to get the core sample out.

For a tree this large,

having a core taken is just like us getting a blood test.

The tree will soon seal over the small hole.

I'm gonna pop this in to slide underneath the core.

Nice. A lovely core there.

And it's just really small. So what I can see

is what look like stripes inside this little piece of wood.

And they are the growth rings.

Trees don't grow in a haphazard way. They grow one ring every year.

And time is marked through this core

in these series of light and dark bands.

The faster growth of spring makes a lighter-coloured band.

Slower growth, in late summer and autumn, makes a darker band.

365 days of the tree gathering sunshine,

making its food, and growing bigger and stronger...

..all compressed and recorded in light and dark rings.

Counting the number of these rings tells us the exact age of a tree.

So that history is hidden inside its trunk -

everything that has gone on in its long life.

But tree rings don't just mark the passage of time,

they can also reveal something remarkable,

a tree's memories.

Because not every ring is the same size.

Where you see a wide ring,

something good has happened to this tree in that year.

Maybe it was very sunny, there was lots of water,

and the tree was able to grow a lovely, wide tree ring.

And that history of what's gone on in the woodland

is almost like a diary

that the tree records as it grows through time.

And we can have a look at that diary and read it.

Incredibly, tree rings can offer glimpses

into specific events in our history.

And reveal how the tree reacted to them.

This is a piece of oak from the southeast of the UK,

and this tree died in 1982.

And the outside rings are nice and wide.

We can see 1982, '81, 1980, '79,

1978, '77.

And then we reach, in 1976,

there is suddenly an incredibly narrow ring.

In 1976, the UK was hit by the worst drought for 250 years.

For the nation's trees, it was a battle for survival

as they competed for the scarce water that remained underground.

It was an incredibly dry spring and summer,

and thousands of trees didn't survive that drought.

But this tree is a real survivor, it managed to hang on,

even though it could only grow a really narrow ring.

This traumatic drought left its scar in the form of the narrow ring.

But what's really interesting is what the tree did next.

Its neighbours probably didn't do so well and died

because in the years after 1976,

this tree grows really wide rings

to take advantage of that space as it recovers from that drought.

After the drought, this stump's remarkable diary

has recorded how the tree responded to it,

with six years of healthy growth.

So, do trees have memory?

In some sense, the rings inside their trunks are their memory.

Trees are all individuals,

and this one was able to recover quickly and grow wide rings.

But not all trees do that.

Sometimes, those traumatic events stay in the tree's memory

for decades and decades,

and you see the memory of that recovery

in years and years of slower growth and narrower growth

as the tree gives itself time to get over that event.

Scientists are only just beginning to understand

how long tree memories can last.

And even if these lessons in life

could be passed on to the next generation.

One of the most useful things about looking at how a tree like this

has responded to a drought in the past

is that it gives us clues about how trees might respond

to droughts and to extreme events in the future

as our climate changes.

So, if trees could learn from events in their past,

what other secrets do they keep for an incredibly long life?

And which trees have survived

to become the oldest living things on Earth?

This is Traquair House...

..Scotland's oldest continually-inhabited home.

It's hosted passing kings and queens for over 900 years.

And in its grounds are Europe's longest-living trees.

Yews.

Yew trees can grow up to 20 metres tall

and their canopies of evergreen needle-like leaves

can spread across 300 square metres.

Expert Tony Kirkham

has come to uncover the tricks these trees use to reach a ripe old age.

I know I have many favourite trees but this is one of my favourites.

And many people think it's quite a boring tree, because it's green,

you know, it's just a bank of green...

It looks pretty bland from outside.

But actually, when you get inside,

I think you'll be in for a real surprise.

Oh, wow.

Wow.

It's cathedral-like.

You come inside, and it's like being inside a room of trees.

Yeah, it's quite special.

All you can hear is the wind coming through that upper canopy.

And the light, this shimmering light playing tricks on your eyes.

It's going to be good to explore this.

This yew tree is an estimated 1,000 years old.

It would have been a young sapling

when William the Conqueror invaded England in 1066.

As a mature tree, it's vast

and has grown a huge umbrella-like canopy,

consisting of millions of tiny leaves.

One of the interesting things about this tree

that allows it to live for so long

is that every part of the tree is toxic

to humans, animals and birds -

so the leaves, the branches, the bark, the wood.

The fact they're so poisonous

means yews have been linked with death for thousands of years.

It's why graveyards are home to many of the UK's oldest yews.

And because few creatures can harm them,

the trees just get on with growing bigger.

Over time, this tree's branches have entwinned with other yews.

At more than 900 years old,

they're only now considered mature trees.

And could live for another 2,000 years.

But they face a big problem.

As they grow taller and sprout more branches,

their trunks will grow wider to support the extra weight.

Eventually, the trunks will get so wide,

they will start to rot...

from the inside out.

Unfortunately, most of the ancient yews in the British Isles,

the centre is rotted out, so they are hollow.

But these trees already have a plan for extreme old age.

Yews are amazing trees,

and if you imagine that, you know, there's the trunk

and these long sweeping branches

are the arms, or the legs, of the yew.

Before its main trunk rots away,

this wise old tree has changed the way it grows its branches.

Rather than growing up,