To get by in the waterlogged, low-nutrient soil of the Quebrada Chorro forest in western Panama, a species of tree fern repurposes its dead fronds, turning them into roots.
The discovery “was completely serendipitous,” says James Dalling, a tropical forest ecologist at the University of Illinois in Urbana-Champaign. In 2019, Dalling and colleagues were on an expedition to study how the roots of the tropical conifer Podocarpus acquire nutrients when the team came across a dense brush of Cyathea rojasiana tree ferns. The researchers tried to move dead fern fronds out of the way and found that the leaves were rooted in the ground. “We would have missed it if we hadn’t actually been digging around in the soil,” Dalling says.
Digging up the stubborn leaves revealed that the tips of their veins had sprouted rootlets, the team reports in the Jan. 18 Ecology.
Cyathea tree ferns are known to keep skirts of dead leaves around them, which may fend off climbing vines or insulate the ferns from frigid temperatures. Because C. rojasiana is relatively short compared with other tree ferns, when its leaves die and droop, they touch the ground. Dalling suspects that the fern uses its languishing leaves to seek out pockets of nutrients instead of investing resources in building new roots from scratch.
Dalling’s team dug up some of the leafy roots and, back in the lab, planted them in pots with a stable form of nitrogen called nitrogen-15. Later dissections of the rootlets showed that they pulled the nutrient up into the plants, indicating that the structures function as roots, the researchers report.
Plenty of other plants do odd things with their leaves or roots. The walking fern (Asplenium rhizophyllum) of the eastern United States, for instance, can reproduce by sprouting new plants from the tips of its leaves. And Dalling has seen roots in the Quebrada Chorro forest, like those of shrubby Clusia plants, climb up other plants. But C. rojasiana is the only plant in the world known to bring its dead leaves back to life as roots.
“I’ve never heard of any other fern doing this, or for that matter, any other land plant,” says botanist Erin Sigel of the University of New Hampshire in Durham.
Neotropical fern expert Robbin Moran is similarly stunned. “I have not previously heard of this in ferns or other plants,” says Moran, now retired from New York Botanical Garden in the Bronx. “It’s weird.”
Dalling plans to team up with a plant anatomist to better understand the structure of the rootlets and wants to probe them for mycorrhizal fungi, which play a role in how the roots of some plants absorb nutrients. “I’m really curious now,” he says. “Are there fungi in these rootlets?”