This 400-year-old beech tree at the Ashridge Estate in southeast England is among those being tallied in a tree census. Experts hope the count will offer clues on how to maintain biodiversity in the transition to the next generation of trees.

RENEE MONTAGNE, host:

Naturalists say as many as 70 percent of the ancient trees in Europe may be in Britain. And now two British groups are working to identify every single one of them. Vicki Barker reports.

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VICKI BARKER: Out in the Chiltern Hills, Bob Davis strides through the woodland owned by Britain's conservation charity The National Trust.

Mr. BOB DAVIS (Head forester, Ashridge estate): We're walking through to probably the biggest tree in the woodland. It's a bit of a wow factor, this one.

BARKER: Davis is head forester of the trust's 700-year-old Ashridge estate, a job that stretches back to medieval times. He's a big, bearded, gentle man — think Hagrid in the Harry Potter books — but he's dwarfed by the gnarled and twisted giants around him.

Mr. DAVIS: Now tell me that's not a special tree.

BARKER: In a dim, dappled clearing, a tree almost as wide as it is tall -mossy, twisting, bulbous trunk; water pooling on its pocked bark, branches reaching out like arms semaphoring something just beyond human comprehension.

Mr. DAVIS: It's a beech tree that's been pollarded a number of years ago and that's just its growth pattern although the bark is grown and included and made these fantastic sort of whirls and it's just an incredible-looking tree.

BARKER: This tree is 400 years old. An English oak can live to 900. Britain's oldest tree, a yew, could be 3,000 years old. They are the last remnants of the forests which once covered all of Europe.

Talk about virgin forests, though, and naturalists will tell you there's been no such thing here for eons. The earliest Britons were already pollarding and coppicing, that is pruning young trees to harvest timber and fuel, 6,000 years ago, a millennium before Stonehenge.

Bob Davis is worried that trees like these are becoming increasingly isolated.

Mr. DAVIS: And there are some sort of insects that only live in the pools that are on trees. And then there are things that live on the things that live in the pools. So a lot of them haven't got a lot of distance in them when they're looking for a mate and to carry on. They need a similar habitat, either that tree or one very close by which is one of the problems with the modern world is we've fragmented these habitats.

BARKER: That fragmentation is one of the reasons the national trust has signed onto the five-year census of ancient trees organized by a sister charity, the Woodland Trust.

Nikki Williams is the project director. She says there's more at stake here than just the trees themselves.

Ms. NIKKI WILLIAMS (Project Director, the Woodland Trust): They are mini-ecosystems all in their own right, grown in our landscapes. If we lose these trees, then Great Britain will also lose a lot of its indigenous species.

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BARKER: Another national trust property, Osterley House. It's a neoclassical mansion set on 350 acres of parkland. It's also right under the approach path to London's Heathrow Airport.

Yet the same intricate ecological dance goes on here as at Ashridge. Brian Muelaner says, only when every ancient tree has been tallied, can scientists like him determine how best to help them and how to maintain that biodiversity in the transition to the next generation of trees.

Mr. BRIAN MUELANER (Scientist): What our survey will indicate is whether we have enough successional planting in place to ensure that we're preparing for that transition.

BARKER: Britain's ancient trees are messengers from the lost prime-evil forests which once covered all of Europe. Foresters say that if they're allowed to die isolated, lonely deaths, then all the myriad creatures that inhabit them could die, too.

For NPR News, I'm Vicki Barker in London.

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