‘Zombie Seeds’ Brought Back To Unlock Salt Marsh Mystery

EDGEWATER, Md. – When researchers at the Smithsonian Environmental Research Center wanted to investigate how salt marshes will cope with sea level rise, they looked to Charles Darwin for inspiration.

“A lot of scientists studying things at the ecosystem scale don’t really think of evolution,” said Megan Vahsen, who, as an evolutionary ecologist, does just that. “Generally, we think of evolution as this slow process.”

But she and a team of scientists conducting an experiment at the Smithsonian’s one-of-a-kind research facility in Edgewater, MD, discovered otherwise. “We found these plants can evolve in meaningful ways over the course of decades,” Vahsen said. “And that’s really fast.”

The effort blended several disciplines, incorporating ecosystem computer modeling, classic field experiments and “resurrection ecology.” If that sounds like something out of Jurassic Park, you’re not far off. In this case, it involved reviving marsh plant seeds that had been entombed in mud for nearly 100 years.

The resulting scientific paper was published in January 2023 in Science, one of the world’s top academic journals. Earlier this year, the Ecological Society of America awarded the researchers with its George Mercer Award, which recognizes “outstanding” recent scholarship in the field.

“I’d say the reason this is in Science is because there’s been so little work done in this area,” said Pat Megonigal, a Smithsonian ecologist and one of the study’s co-authors. “What we’ve been able to do is say, ‘Hey, look, [evolution] matters.’”

Regarding the Chesapeake Bay’s life-enabling marshes, it mattered in a detrimental way, the study suggests. To understand how that could be, some further explanation is called for.

Salt marshes are one of the Chesapeake’s most important ecological engines, providing habitat for birds and young fish. They also serve as storm barriers for coastal communities and help filter nutrients and sediment before such pollution can harm the Bay.

Human-caused climate change, though, has brought about 1 foot of sea level rise to the Bay over the past century and is expected to bring as much as 5.2 feet of additional water over the next, experts say. They fear that the accelerating rate of sea level rise could wipe out the region’s 280,000 acres of tidal wetlands.

Across the globe, researchers are scrambling to shed light on how low-lying marshes can keep pace with rising seas. Ecologists are pinning their hopes on marsh plants’ natural ability to build up land elevation, a feat they accomplish by trapping sediment in their roots, among other processes.

Previous research has shown that several factors will likely play a role in whether coastal wetlands can survive, from the abundance of available sediment to how quickly seas rise.

To determine whether evolution could also be part of that dynamic, Vahsen, the study’s lead author, turned to the Smithsonian’s Global Change Research Wetland, a 170-acre brackish marsh that scientists have been monitoring for sea level rise impacts since the 1980s. The marsh borders the Rhode River, a Chesapeake tributary about 10 miles south of Annapolis.

A happy accident made it possible for the group to answer their evolutionary questions. About two decades ago, a Duke University graduate student named Colin Saunders had collected sediment samples from the marsh. He bored deep into the muck to obtain vertical profiles with the “younger” materials at the top and the “older” stuff toward the bottom.

Despite being kept in a cold, dark room, some of the marsh plant seeds buried in the soil began to sprout within a few months. That included ones far down the length of the profile.

“In other words,” Megonigal said, “these had to be fairly old seeds.” For her part, Vahsen likes to call them “zombie seeds.”

Flash forward to the recent past: The researchers selected the old seeds — believed to have originated between 1931 and 1973, gathered from four locations around the Smithsonian’s soggy property — and planted them in a tiny creek within the marsh. For comparison, they also started growing plants from seeds dating from 1994 to 2016.

The plant at the center of the effort was one of the marsh’s primary wetland species: a common reed called Schoenoplectus americanus, also known as three-square.

What they found didn’t bode well for the marsh’s future. While the younger and older reeds didn’t exhibit many differences above ground, the differences were stark beneath the soil: The modern specimens put out shallower, sparser roots.

“I would have loved to have found out that with sea level rising more quickly, it’s selecting for genotypes that will help the marsh build faster,” Megonigal said.

Instead, they found the opposite to be true.

“We think of evolution as being this adaptive and often positive thing. It’s typical survival of the fittest,” said Vahsen, who conducted the study as part of her dissertation at the University of Notre Dame. She is currently a postdoctoral fellow at Utah State University. “But what was going on for the plant turned out to be actually bad for the ecosystem and making it less resilient to sea level rise.”

The study wasn’t designed to ascertain why the three-square plants changed their growing behavior. But the scientists theorize that increasing nitrogen from the region’s farms and urban landscapes may have played a big role. With no need to reach far for nitrogen, the roots over time stayed closer to the surface, they said.

The computer modeling suggested that the modern-day plants are likely to build elevation 8% slower than their older counterparts. They also accumulated and stored 18% less carbon, one of the most pernicious greenhouse gases.

The study, Vahsen said, could have implications for preserving and restoring coastal marshes. For one, it highlights the importance of maintaining genetic diversity, so that the more adaptive types are available for the future. And it could lead to a day when plantings used for marsh restoration projects are sourced from older “resurrected” seed stocks to make sure they’re better suited to resist rising seas.