by Mona Patterson
Our predictions of how well wetlands can withstand climate change may not be as accurate as once believed. Research has often overlooked one critical factor: plant evolution. This spring, a research team with two Smithsonian biologists received the Ecological Society of America’s George Mercer Award for their findings on the century-long evolution of a wetland plant and its potential impact on wetland survival.
“There has been this long-standing idea that evolution is really slow and happens at magnitudes that are not impactful at the ecosystem level,” said Megan Vahsen, lead author of the study. “And I think what this paper shows is that’s not necessarily true.”
The George Mercer award recognizes an “outstanding” ecological research paper published within the past two years, with an early-career scientist as lead author. Vahsen, the early-career scientist, did the study as part of her dissertation work at the University of Notre Dame. Jim Holmquist and Pat Megonigal, two ecologists at the Smithsonian Environmental Research Center, joined Vahsen as co-authors on the discovery.
The researchers used a unique approach called “resurrection ecology.” The team “resurrected” plant seeds from the years 1931 to 1973, taken from four nearby marshes in the Chesapeake Bay. The seeds belonged to the marsh sedge Schoenoplectus americanus—a common North American plant that plays a crucial role helping wetlands resist rising seas. The research team planted the resurrected older seeds and compared their traits to more modern plants from 1994 to 2016. The experiment, coupled with an ecosystem modeling approach, gave them insights into how plant evolution can create ecosystem-level change.
“Will evolution be an important mechanism by which plants are responding to environmental change?” asked Vahsen.
Jim Holmquist (left) and Pat Megonigal, Smithsonian researchers and co-recipients of the George Mercer Award. (Photos: Lauren Brown and Tom Mozdzer)
After growing and measuring the plants, Holmquist and Vahsen worked with Kathe Todd-Brown and Jim Morris to create a free, open-source version of an existing marsh ecosystem model. This model predicts how tidal wetlands will respond to changes like sea level rise. It also assesses their ability to store carbon. Overall, the research suggests today’s wetland plants store less carbon—and make their wetlands more vulnerable to sea level rise—compared to plants from the mid-20th century.
The root of the problem, the team discovered, was in the roots themselves. The plants’ roots became shallower over time, with more roots accumulating near the surface in modern plants.
“The thing that I was surprised about was how rapidly the root traits in these plants can change over time, and how much of an impact that can have on our forecasts,” said Holmquist.
Changes in root traits can directly affect a wetland’s elevation gain—and in turn, its ability to withstand rising seas. Plant roots can help wetlands build higher soils by retaining sediment from the water, adding organic matter and by holding the soil in place, resisting erosion. Less root growth means less elevation gain and more frequent flooding. And that makes wetlands more susceptible to sea level rise.
The team also found that marshes composed of the older plants stored 18% more carbon than marshes composed of the modern plants.
“I think [the model] tells us the data that we have in hand right now is the best thing we have at this time to make forecasts about the future,” said Holmquist. “But we have to keep in mind that a lot of these plant traits aren’t fixed. They’re moving targets. And we need to understand their potential for changing over time in response to climate change and global change.”
While this study didn’t directly search for a cause, the team suspects the shallower roots are a response to excess nitrogen from fertilizer or urban runoff. Nitrogen is essential for plants to perform crucial functions like photosynthesis, and to grow healthy tissues and cells. When nitrogen is scarce, plants use their roots to access this vital nutrient belowground. Now, however, with nitrogen flowing freely off the land, plants don’t need deep roots to reach it.
“I think that’s a huge hypothesis coming out of here, which needs more testing,” Holmquist said. “I think across systems and across wetlands, we need to be checking our assumptions about which of these plant traits…are the big levers for ecosystem outcomes.”
The team attributes the paper’s success not only to their findings, but to the collaborative nature of ecology today.
“I think if we hadn’t been on a team with evolutionary biologists, ecosystem ecologists and statisticians…we wouldn’t have come up with such a cool approach to the paper,” Vahsen said. “So I think that is something I want people to take away, is that working in interdisciplinary groups can be really fruitful.”
A copy of the study “Rapid plant trait evolution can alter coastal wetland resilience to sea level rise,” is available in the journal Science at https://www.science.org/doi/10.1126/science.abq0595
