Burnham Beeches is famous around the world for its huge gnarled beech trees, which are 450 to 500 years old. Their hollow trunks have girths of 25 feet or more and provide shelter for a wealth of rare wildlife. Here, beetles and spiders have bred undisturbed since the ice sheet, which once covered Britain, receded 10,000 years ago.
The ancient beeches used to be pollarded - every eight or ten years some of the new growth was taken to provide the Lord of the Manor or his retainers with fuel and building materials. But in the 18th and 19th centuries, when coal became readily available, the trees were left to grow undisturbed. Now the beeches have heavy branches which issue from the old pollarded crown some eight to ten feet from the ground.
Burnham Beeches is run by the Corporation of London and yesterday the Lord Mayor, Sir Francis McWilliams, planted some beech saplings to mark its new status. It is the first nature reserve run by a local authority to be given national recognition. The corporation spends about pounds 500,000 a year on the 540-acre reserve.
It plans to return Burnham Beeches to the condition it was in 200 years ago, when animals grazed among the trees. They have recently brought Berkshire pigs, British White cattle, and Exmoor ponies to forage in the woods. The pigs scavenge for acorns, beech nuts and blackberries - forage which was known in medieval times as pannage.
The animals keep down the young trees and brush wood which grew up between the pollarded beeches. After a year of grazing, the character of the wood is beginning to change: heathers, once shaded out by brushwood, are back, and wet areas with sphagnum moss are being re-established.
To restore the original character of the wood the beeches are being pollarded once again. There are no records of how this used to be done, so the corporation's wardens have had to experiment to find the best way.
Helen Reid, the resident ecologist at Burnham Beeches, said: 'We tried cutting different amounts off the top and found that the best way was to remove the oldest branches, always leaving some young growth that would keep the tree going. We have now repollarded 70 trees and only lost two.'
The ancient woodland contains some unusual trees: the wild service tree, which is only found in long established forests, and an unusual wild hybrid Sorbus, a cross between whitebeam and rowan. An unusual moss, Zygodon fosteri, found in only three places in Britain, lives on moist places on the trunks of the beeches.
'A very rare beetle, Oligella intermedia, has been found here. It is only a couple of millimetres long. I have seen one pickled in a bottle, but you have to be a real beetle expert to be able to find it and recognise it,' Ms Reid said.
And two weeks ago Ms Reid found two rare money spiders for the first time at Burnham Beeches. One of the spiders lives high up in the beech trees and another weaves its web in the empty husks of fallen beech nuts. They will join three species of fly, 13 species of beetle, a moth and a lichen on the list of species which are known at few if any other British sites.