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Why do some male trees turn female?

Male, left, and female buds of a striped maple. (Jennifer Blake-Mahmud.)

It's not often that a scientist reaches into fantasy literature for the perfect analogy.

“Usually trees take a long time to respond to their environment,” botanist Jennifer Blake-Mahmud said. “Remember the Ents in 'Lord of the Rings'? It takes them a long time to say anything in Entish!”

Some things — or people or Ents — just don't want to rush it. Most trees don't rush anything, especially when it comes time to bloom. A lot of trees have some type of flower, which contains their sexual organs. The showier plants, such as cherry, magnolia and dogwood, flaunt their sexuality. But for many trees, you can barely notice or see their blossoms. All this blossoming usually takes months to fully develop, sometimes as long as 11 months, but it's usually set in motion during the winter for their spring appearance.

Only when the trees bloom can you figure out their sex. Lots of trees are hermaphroditic — that is, their flowers contain both male and female reproductive parts. Other species have male trees and female trees, which you can tell apart by looking at their flowers: The male reproductive parts are the pollen-laden stamen; the female parts their egg-holding pistils.

Acer pensylvanicum, a striped maple found in the northeast United States and southeastern Canada, is that rarest of species: Not only can it take a mere three weeks to bloom (a nanosecond in arboreal terms), but an individual tree can switch sexes, from male to female. Blake-Mahmud announced the discovery, along with Lena Struwe, in the journal Trees: Structure and Function.

Why they switch sexes is of abiding interest to Blake-Mahmud, a PhD student at Rutgers University and an expert in plant reproduction. It all started six years ago when she attended a field course on the biology of sex at the Mountain Lake Biological Station in Virginia.

“We were traipsing through the forest and encountered the [striped maple]," Blake-Mahmud said. “They said they think it changes sex but are not sure why. I was like: 'Wow, what’s going on? This sounds crazy.' "

“Crazy” turned into recent laboratory experiments at Rutgers. Mahmud and her team cut branches from striped maples in various forested regions of New Jersey, then nurtured them in a greenhouse until the branches bloomed. They bloomed months earlier than expected, in just three weeks. When Mahmud checked the cut branches, which she'd placed in sugar water, they were either female or female and male. And yet the trees that the branches came from were male. What happened? Puzzled, Blake-Mahmud did a follow-up experiment, this time placing the cut branches in different environments — inside, outside, with sugar water, without. Once again, all the cut branches bloomed female or female and male.

“It was obvious that it was something about the branch, not the tree,” she said.

Virtually the only thing that was common to the branches — and different from the tree itself — is that they were all cut off for the experiment: “It seems damage is important as far as triggering a male tree to become female,” Blake-Mahmud said.

The Rutgers botanist may have, in effect, replicated what was going on in nature. More damage to the trees was resulting in more sex changes.

“There's a lot of sex going on in plants that people don't know about,” said John Kress, a research botanist and curator at the Smithsonian National Museum of Natural History.

And Kress thinks these sex switches might be happening more frequently because of climate change.

“If we start having these warm and cold periods that are out of the ordinary, are those injuries going to eventually affect the reproduction of maples? If they're all female they won't produce enough fruit. ... It’s a little bit of a wake-up call. Just like in rising sea levels and melting of polar ice caps. Only botanists would see this. Everyone else sees the water level rising. We’re just seeing [changes] that are more subtle.”

Even without the stress of climate change, there are everyday shocks, Blake-Mahmud says, which could contribute to sex changes.

“They go through a lot — antlers rubbing against them, deer chewing them, bigger trees falling on them. They don’t have an easy life, so it might make sense that there’s a 'damage' cue. ... If the branch is going to die anyway, it might make sense to be female and produce seeds before dying.”

The striped maple's swan song, if you will. Nature taking care of nature — and ensuring the survival of the species.

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