Jennifer Blake-Mahmud often thinks of plants and trees as being somewhat like Lego building blocks. Sometimes what they become looks nothing like the picture on the box.
For instance, they might change gender.
A postdoctoral fellow researching plant reproduction at Colgate University, Blake-Mahmud conducted a study in New Jersey that found that striped maple trees can change gender from year to year. She also discovered that when they are male, they grow faster, and when they are female, they are much more likely to die.
“Most people are pretty shocked to learn that plants even have a gender, much less that they can switch it,” Blake-Mahmud said, noting that 90 percent of flowering plant species combine both sexes in one plant. “There are multiple levels of puzzles going on with striped maples that are just so fascinating.”
Blake-Mahmud tracked several hundred striped maple trees by identifying their gender each year and collecting data about their size and health. Over a four-year period, she found that 54 percent of the trees changed gender, with many switching two or more times. Overall, male trees usually outnumbered female trees by more than three to one.
“Being female is really difficult because female trees have to put all their energy and nutrients into producing seeds,” she explained. “If you’re a female plant, you not only have to make flowers like males do, but you also have to make berries and seeds, and berries aren’t free. It takes a heavy toll on individual plants to make them.”
That may explain, in part, why 75 percent of the trees that died during the study were female.
Previous research suggested that larger tree specimens are more likely to be female. But size was shown not to be a determinant of gender in striped maples. What does matter is the tree’s health. “If it had a lot of bark infections, was missing branches, and overall looked pitiful, then it was more likely to be a female tree,” Blake-Mahmud said.
But it is still unclear why the trees switch gender.
“Maybe the tree is getting a signal that things aren’t going well, and that triggers a change in sex expression,” she said.
What this means for forest managers may be surprising.
“If you’re trying to manage a woodlot for the economically viable species, you probably don’t want a lot of striped maples in the understory,” said Blake-Mahmud. “But foresters fail to appreciate their [own] role in boosting striped maple numbers.” If striped maples are likely to become female after they’ve been injured, and logging equipment operating on a site injures small striped maples in the understory, “you’re setting yourself up to have a bumper crop of striped maples in the future,” she explained.