The oldest tree in Britain has started to change sex, it has been claimed.

The 5,000 year old Fortingall Yew, in Perthshire, Scotland, has for hundreds of years been recorded as 'male', meaning it produces pollen, whereas female yews produce red berries.

But in a 'rare and unusual phenomenon' baffling scientists, it has started to sprout red seeds, suggesting that part of the tree is changing gender.

Berry nice! The Fortingall Yew, in Perthshire, Scotland, has for hundreds of years been recorded as 'male', meaning it produces pollen, whereas female yews produce red berries

Max Coleman of Royal Botanic Garden Edinburgh, who spotted the 'three red berries', said that the phenomenon could be caused by 'environmental stress'.

He said: 'It's a rare occurrence... rare and unusual and not fully understood.

'It's thought that there's a shift in the balance of hormone-like compounds that will cause this sex-change.

'One of the things that might be triggering it is environmental stress.'

The tree is otherwise healthy, but Mr Coleman said it would be monitored closely for further changes.

Plump: But in a 'rare and unusual phenomenon' baffling scientists, it has started to sprout seeds, suggesting that part of the tree is changing gender (stock picture)

The gnarled tree is in an old church yard, and is difficult to age because its heartwood - the wood in the centre of the tree - has long rotted away.

THE TREES THAT CAN CHANGE SEX Although it may seem unusual, Dr Coleman said that yew trees have been observed to change sex. 'Odd as it may seem, yews, and many other conifers that have separate sexes, have been observed to switch sex. 'Normally this switch occurs on part of the crown rather than the entire tree changing sex. 'In the Fortingall Yew it seems that one small branch in the outer part of the crown has switched and now behaves as female.' It is difficult to calculate the age of ancient yews, as their annual growth rings rot away. A girth measurement in 1769 suggested the Fortingall Yew could be up to 5,000 years old. It has more recently been estimated at between 2,000 and 3,000 years old. Advertisement

Scientists have been able to estimate the tree's age by comparing its current size with measurements taken in the 1700s.

Mr Coleman wrote in his blog: 'Yews are normally either male or female and in autumn and winter sexing yews is generally easy.

'Males have small spherical structures that release clouds of pollen when they mature. Females hold bright red berries from autumn into winter.

'It was, therefore, quite a surprise to me to find a group of three ripe red berries on the Fortingal yew this October when the rest of the tree was clearly male.

'Odd as it may seem, yews, and many other conifers that have seperate sexes, have been observed to switch sex.

'Normally this switch occurs on part of the crown rather than the entire tree changing sex. In the Fortingall Yew it seems that one small branch in the outer part of the crown has switched and now behaves as female.'

He said the three seeds had been collected and would be included in a major project to 'conserve the genetic diversity of yew trees' by planting them out at the Botanic Garden.

Ancient: The gnarled tree is situated in an old church yard, and is difficult to age because its heartwood - the wood in the centre of the tree - has long rotted away