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The Mystery of the Sex-Changing Striped Maple Trees

Yes, trees can be male or female. And sometimes they switch it up

Male flowers of the striped maple tree. (Jennifer Blake-Mahmud)
smithsonian.com

Scientists have known for some time that trees not only have a sex, but can sometimes switch between sexes. But they haven’t always known why. Now, as The Washington Post’s Amy Ellis Nutt reports, a new study suggests that for at least one species, the switch happens after injury.

During a biology field course, ​Jennifer Blake-Mahmud, a PhD student at Rutgers University, learned that the striped maple, Acer pensylvanicum, could switch from male to female, Nutt reports. But no one knew why​. So she and Lena Struwe, a professor of environmental and biological sciences at Rutgers, took a closer look at the sex of striped maples.

To determine the sex of trees, researchers turn to the flowers, which can take up to 11 months to start blooming, reports​ Nutt. While many trees have both male and female reproductive parts, others have just one or the other. So the duo cut branches of striped maple trees, which are common in the northeast United States and southeastern Canada, from various parts of New Jersy. They placed them in sugar water in a greenhouse and watched until they bloomed, publishing their results in the journal Trees: Structure and Function.

The first surprising find was in the development of the flowers. "[T]hese trees can finalize the development of flower bud parts within three weeks prior to flowering, compared to other species who do this many months in advance," Blake-Mahmud writes in an e-mail to Smithsonian.com. But even more unexpected was the sex of the branches. Although all of the trees they sampled from were male, all of the blooms were either female or female and male.

In a follow-up test, the researchers examined branches in a few different set ups, growing them both inside and outside. They also placed the branches in different solutions, dunking them in either in sugary water or plain water. The results were the same as before: branches turned female or female and male. Something about the branches, not the trees, were causing them to change sex.

The researchers concluded that the damage the branches experienced from being cut off a tree must have triggered the switch. 

Sex changes in trees happen more often than you might think. In 2015, experts at the Royal Botanic Garden in Edinburgh noticed that UK’s oldest tree, the 5,000-year-old Fortingall Yew in Scotland, was was undergoing a sex change, The Guardian reported at the time. The tree had begun sprouting berries on one of its branches — something only female trees do. Researchers had known yew trees could change sex but were surprised it happen for such an old tree.

And the changes don't just happen for trees that are male or female. Just last year, researchers discovered that gynodioecy plants — a breeding system in which female and hermaphroditic plants live together in a population — could change sex depending on how much light they receive.

As for the striped maple, Blake-Mahmud tells Nutt that everyday injuries, like a deer chewing on a branch, could prompt the change. “They don’t have an easy life, so it might make sense that there’s a 'damage' cue,” she says. “If the branch is going to die anyway, it might make sense to be female and produce seeds before dying.”

Blake-Mahmud’s research seems to build on a 2004 study which found that another species in the acer genus, Acer rufinerve, commonly found in Japan, would switch to female when the tree was in poor health and close to death.

According to Nutt, there might also be reason to suggest the sex changes could become more common due to climate change — if long-term changes in climate cause damage to trees, it might impact their reproduction. 

Editor's Note March 5, 2018: The headline and image of this article have been corrected to show that the study focused on the striped maple, not the sugar maple. The article has also been updated to correct and add a comment from the researcher regarding the timing of flower development. We regret the errors.

About Julissa Treviño

Julissa Treviño is a writer and journalist based in Texas. She has written for Columbia Journalism Review, BBC Future, The Dallas Morning News, Racked, CityLab and Pacific Standard.

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