'The trunk is hollow,' said Mr Green, who in the course of his work for English Nature has found more than 2,000 ancient trees in Windsor Park. 'All the old trees have hollow trunks. Foresters believe that a healthy tree must have a solid trunk. But a hollow trunk is nature's way of creating a stronger structure which will bend in the wind.'
He pointed to a bright orange bracket fungus growing out of an old beech.
'The fungus hollows out the old wood in the centre of the trunk leaving the healthy new wood on the outside,' said Mr Green. 'Fungi are the third kingdom. We can't exist without them.'
Mr Green was our guide in the south forest at Windsor which will be officially open to the public for the first time on monday. For Mr Green the forest was never closed - as a boy he lived nearby in Ascot and roamed freely among the trees diving for cover if he saw a ranger. He still knows every tree and has marked and annotated more than 7,000.
Ancient trunks stand rotting in the forest and large branches lie among the litter on the forest floor. Bracken fern and forest herbs grow in the massive bowl of an old oak trunk like a huge pot plant. This rotting wood is the home for some hundreds of species of strange insects and fungi, relics of Britain's primeval forest that have survived nowhere else.
Royalty first hunted the forest in 1086 And the biological link with the ancient flora and fauna of that time has never been broken. Some ancient trees were always left by loving foresters when the younger growth was cleared in two world wars. In the 1950s and 1960s much of Windsor forest was replanted with quick growing conifers such as Norway spruce and Scot's pine but some of the ancient trees were retained.
The 7,500 acres of Windsor Park is now recognised as the best site for wood fungi and forest insects in Northwest Europe and the Crown Estate was presented yesterday with a Centre of Excellence award for its work in maintaining the forest.
Mr Green has toured the whole of Europe in search of ancient trees and found very few in France, Germany or central Europe until he came to the wild forest on the borders of Poland and Russia.
'With the exception of Poland and Spain it seems that all the old trees of Europe have been cut down. Our trees were probably saved by coal. The railway brought coal to people all over England and they stopped gathering wood,' said Mr Green. 'Now Britain has 90 per cent of the ancient trees in Europe. They are just as much a part of our heritage as old buildings. We must not take them for granted.'
The South Forest in Windsor Park will be open to the public for the first time on bank holiday monday 30 June. Entrance by Forest Gate on the A332.