"Closer examination reveals the Fortingall Yew is a male tree. 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 Fortingall yew this October when the rest of the tree was clearly male. 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."
Yew trees, specifically the species Taxus baccata, are usually considered dioecious, meaning trees are either male or female only. But under rare circumstances they can become monoicous, having both pollen and egg-bearing structures on one plant. "It's a rare occurrence ... rare and unusual and not fully understood," Coleman said, according to Agence France-Presse. "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."
Coleman told the Guardian that the Fortingall yew might have undergone such a sex change before, but based on hundreds of years of records, he said it has been considered only male until now. "The sex change isn't the amazing bit in this case, it's the fact it's this particular tree," Coleman told the Guardian. Coleman went on to say that the three berries were collected for seeds that will be planted as part of an effort to "conserve the genetic diversity of yew trees across their geographic range including Europe, the Caucasus, Western Asia and North Africa.
The difficulty in more accurately estimating the age of the Fortingall yew is because the heartwood of the tree, which held some of the growth rings, has rotted away, Coleman wrote. The current estimates are based on historical measurements. This tree also has competition from other yews in Europe for the title of oldest. Last year, a yew tree in Wales was estimated to be over 5,000 years old.