Burnham Beeches Countryfile

Just a stone's throw from the River Thames, Burnham Beeches -

a woodland bought by the City of London in 1880

as a green lung, a place for city folk to escape the grime and smoke.

This is a pretty ideal day for me - blue skies, sunshine,

wandering through a forest surrounded by beautiful trees.

Some of these are more than 450 years old.

But it's not easy keeping these old girls alive, you know.

For hundreds of years, a combination of livestock grazing

and pollarding was used to keep them under control and safe.

Pollarding is basically coppicing or pruning,

but it's done at a high level to promote tree growth.

And it's done up there because if it was done at a low level,

the animals would have easy access to all the lovely shoots.

If that's not done, the trees can get top-heavy and topple over,

just like this one.

100 years of neglect here has taken its toll.

But 20 years ago, they brought back the cattle and resumed pollarding

to help save the trees for future generations.

I'm meeting head ranger Martin Hartup.

And I'm going to meet him by the invisible fence.

"Which invisible fence?" I hear you say.

That invisible fence. Hear it? CHIRRUPING

-Hello, Martin.

-Hello, Julia.

-Hi there.

So, what is all this jiggery-pokery, then?

Well, what you carried over that invisible fence there was a collar,

and this collar is worn by our cows when they're grazing this area.

Now, the fence itself is a cable that's buried under the ground,

only about four inches or so,

and it emits a radio signal which is picked up by the collar.

And when the cows get close to that buried cable, they hear that noise,

-which we heard as you walked over it.

-Yeah.

That tells them they're getting close to it.

Once they've learnt how the collars work,

they know that if they get any closer

or take another step onwards, they'll get an electric shock.

And you can see where we've used it in trial areas,

they graze it within about two metres of the line

without any problems at all.

-The advantage, of course, is no fences for us.

-No fences.

Nobody knows it's there, it doesn't stop anybody walking anywhere,

there's no interaction with anybody,

you can't touch like you would do a normal electric fence

and get a shock off it - it doesn't work like that at all.

It's not just the cows that need managing -

some of the other creatures here do, too.

Apparently, there are giant ants in this wood.

I'll believe that when I see it!

SHE SCREAMS

These are Formica rufa - wood ants to you and me -

and they're three times the size of normal garden ants.

They can be found at half a dozen locations in the UK,

but here they're in huge numbers.

Conservationist Dr Helen Reid has been keeping an eye on them

for more than 20 years.

OK, so, we've got a big nest here at the base of this dead tree.

-Uh-huh.

-And they're building their nests like this,

where there's bits of dead wood.

So they're building the nest over the dead wood

-and on the edge of the clearing.

-Just thousands and thousands!

How many ants do you think are here, Helen?

Well, it's been estimated that the biggest nests might have

a million ants in them, but I think probably most of them

are rather less than that -

probably up to half a million is more realistic.

-Oh, look, they're everywhere!

-Incredible, isn't it?

-They're very, very efficient workers, aren't they?

-Yes, yes.

I mean, they're spending a lot of time going up the trees,

a lot of time on the ground. So early in the year,

they're picking up nesting material to bring back to bolster the nest

and then they start feeding on aphids and honeydew

from the aphids up in the trees.

They're taking a sugar solution, and one nest like this might

bring back 46 kilos of sugar in a season from the aphids.

The ants are such top predators, they're eating other insects,

and even depriving birds of food.

The Rangers are well aware they could one day overrun the woodland.

So, Helen, how are you going to manage them?

Well, at the moment, the management we're doing on the trees,

some of the things you've seen this morning,

is really favouring wood ants,

because they like little sunny clearings around the trees

and they like it when we leave deadwood on the ground.

So there may be things that we could do in

our management to try and encourage the ants in certain areas

-but discourage them in other areas.

-Right.

Look at my foot.

Oh, I'm so itchy now.

They're all over us.

This is when you actually manage to get a hold of them.

Now they're all over us.

HELEN LAUGHS