The Fortingall Yew in Perthshire 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.
"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.
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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.