Britain’s oldest tree, a male, estimated to be between 3,000 and 5,000 years old may be undergoing a sex change, say researchers who found that part of it is turning female.

The Fortingall Yew in Perthshire, estimated to be around 5,000 years old, making it older than Stonehenge, is regarded as a male tree because it produces pollen – unlike female yews, which produce distinctive seed-bearing red berries.

However, botanists found three red berries on a branch of the yew this year, which indicated that at least part of the male tree is becoming female.

“Yews are normally either male or female and in autumn and winter sexing yews is generally easy,” said Max Coleman, of the Royal Botanic Garden Edinburgh, who spotted the berries. “Males have small spherical structures that release clouds of pollen when they mature. Females hold bright red berries from autumn into winter,” Coleman told The Telegraph.

“It was, therefore, quite a surprise to me to find a group of three ripe red berries on the Fortingall Yew this October when the rest of the tree was clearly male,” Coleman said.

Coleman said while it may seem “odd”, it was not unheard of for yews – and other conifers that have different sexes – to switch sex. “Normally this switch occurs on part of the crown rather than the entire tree changing sex,” he said.

“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 have been collected and will be included in a project to conserve the genetic diversity of yew trees across Europe, the Caucasus, Western Asia and North Africa where they grow, The Independent reported.

The project will involve hedges at Edinburgh’s Botanic Garden being replaced with a conservation yew hedge grown from cuttings and seed collections from wild populations and significant Yew Trees such as Fortingall’s yew.