Striped maples are boring-looking understory trees in eastern forests, but appearances can be deceiving. These apparently milquetoast trees change sex they way some people change smartphones. Of course, changing sex may be more tempting when you have four choices, as striped maples do.
The ability to change sex is called environmental sex determination. As males and females make very different investments in offspring – even in plants – the ability to change one’s sex from year to year confers fitness benefits.
Some animals can change sex when they encounter certain environmental conditions, but the change is usually irreversible. Since plants habitually shed and regrow body parts, changing sex repeatedly is a considerably easier prospect for a maple than a turtle.
Some 198 animal and 250 plant species are suspected or confirmed to engage in environmental sex determination. But for many of the plants, scientists lack long term observations that would prove or disprove this status.
A team of scientists from Princeton and Rutgers decided to observe several hundred striped maples for four years to see what sex they chose annually. As flowering plants, they had four choices: male, female, non-reproductive, and monoecious, a state in which both male and female flowers were present on the same tree. This last status was chosen by no more than 5% of the population.
Although in any given two-year period, trees were most likely to choose the same sex, 54% changed sex during the four years, and 26% switched sex at least twice. A fickle 1% changed sex every year. The trees that chose “both” consistently could change their relative sex expression by as much as 95%.
But trees did not change sex without apparent reason. To the contrary, their condition exerted a strong influence. The trees most likely to be asexual in any given year were, unsurprisingly, young ones. The trees most likely to produce flowers of both sexes were big ones.
In the eastern forests where they live, white-tailed deer often browse striped maple leaves or rub their antlers on its bark. Overstory trees may also fall or drop limbs onto them. Such damage harms maples' health.
Scientists had expected that healthier trees were more likely to choose female and sicker trees male, since females make a much larger investment in their offspring by creating and provisioning fruit and seeds. This is the case in several other plant species.
What they actually found was the opposite: sick trees were much more likely to be female, and healthy trees male. Only 1% of male trees died in any given year. The mortality rate for females ranged between 13 and 39%.
Why choose female when in failing health? At least one other species of maple – Acer rufinerve in Japan – also chooses more often to be female when ill health strikes.
The answer is unclear. At least one model suggests it is more advantageous for plants to express the lower mortality and higher growth-rate sex first, when young. That may give the tree time to reach a larger size before in invests in a sex whose costs may very well kill it.
Blake-Mahmud, Jennifer, and Lena Struwe. "Time for a change: patterns of sex expression, health and mortality in a sex-changing tree." Annals of Botany (2019).