Erin Paul Donovan / Alamy
Being female can be tough, even for trees. A study of the life cycle of striped maples – which can change sex from season to season – has revealed that healthy trees are more likely to be male, and most trees die while in the female flowering state.
“We had a suspicion they were changing sex, which is relatively rare among plants,” says Jennifer Blake-Mahmud of Princeton University. Between 2014 and 2017, she tracked the life cycles of striped maples ( Acer pensylvanicum) in state forests and park lands in New Jersey.
She visited 457 striped maples each spring, measuring their diameter, the condition of their leaves and branches, and recording whether they had female or male flowers.
“It was just me and my trusty field dog. I would go out and count all the flowers and then determine the sex of the different trees,” she says. She found that 54 per cent of the trees switched sex during the 4-year period, and a quarter of those switched sex at least twice.
A model based on the data she collected showed that, contrary to previous theories, healthy trees were more likely to be male and the size of a tree doesn’t influence its sex.
She found that the growth rate of trees that remained female for multiple years deteriorated, and 75 per cent of the dead trees had produced female flowers just before they died.
“It’s remarkable. When I see a tree that’s dead and I look back in my data sheet, it was almost always female the year before,” she says.
It’s not clear why this is the case. It could be that females need more nutrients because they produce seeds, and that’s so taxing the trees die, says Blake-Mahmud.
But it could also be that when a tree is dying, it switches to female as a last effort to create offspring and pass genes on to the next generation.
“These populations have a lot more male trees than female trees in general, so just by luck of the draw, a female has a better chance of being a parent of the next generation than a male,” she says. “If you’re going to die anyway, then being female is the way to go. That would make more evolutionary sense.”
Journal reference: Annals of Botany, DOI: 10.1093/aob/mcz037