University of Illinois Urbana-Champaign plant biology professor James Dalling and his colleagues discovered that some tree ferns recycle their dead fronds into roots. The researchers call these repurposed fronds “zombie leaves." Credit: Fred Zwicky
Key points:
- A tree fern, Cyathea rojasiana, that grows only in Panama can reanimate its own dead leaf fronds.
- The weird phenomenon occurs only after the leaves die and droop to the ground, which is most likely why other plant biologists have not noticed before.
- Researchers attribute the adaptation to the nutrient-poor volcanic soils of the Jurassic period, which is when the Cyathea rojasiana first appeared.
A tree fern that grows only in Panama can reanimate its own dead leaf fronds, converting them into root structures that feed the mother plant. The fern, Cyathea rojasiana, reconfigures these “zombie leaves,” reversing the flow of water to draw nutrients back into the plant, according to a new study.
University of Illinois Urbana-Champaign plant biology professor James Dalling and his team were studying a different plant in a Panamanian forest reserve when he noticed something strange. Dalling saw that the fronds were strongly embedded in the soil and had sprouted a network of rootlets. Subsequent laboratory tests revealed that the zombie leaves were drawing nitrogen out of the soil.
“This is a truly novel repurposing of tissue. And it’s distinct from what we know other ferns do,” said Dalling. “Other plants, including some ferns, send out leaves or shoots that touch the ground and sprout roots to sustain a new plant. But reconfiguring dead tissue to feed the original plant has never been reported.”
Even after they are converted into roots, the wilted fronds look like decayed plant matter, which is probably why generations of plant biologists failed to notice that they were performing a life-sustaining task.
C. rojasiana belongs to an ancient lineage of tree ferns dating back to the Jurassic period. The team hypothesizes that the zombie leaves are most likely an adaptation to the nutrient-poor volcanic soils of that time.
“Panama is a land bridge between North and South America that coalesced 7 million years ago out of an archipelago of islands, and those islands are the result of volcanic activity in the past,” said Dalling. “In one site we discovered, a layer of volcanic ash several meters deep that looks like sand you would dig up on a sandy beach. The plants that grow there are distinct from those that we find elsewhere in that forest reserve.”
The patchiness of the vegetation means soil nutrients also are unevenly distributed.
“And so, the tree ferns seem to be putting out tentacles to sample the surrounding soils,” Dalling said. “They’re able to sample a greater range of nutrient environments for the same amount of investment of rootlets than if they just sent out a single rooting structure all around the fern. I think it’s all about the economics of how they use resources in a patchy environment.”