The Wetlands Are Drowning
Schoenoplectus americanus, or the chairmaker’s bulrush, is a common wetland plant in the Americas, and it has an existential problem. It has chosen to live in a place where it is always at risk of being drowned.
Like all plants, the bulrush requires oxygen to produce energy. One solution is obvious: Send shoots skyward like straws to suck down oxygen to the roots. But the bulrush also employs a more unusual strategy: raising the ground on which it grows. The plant builds its roots near the surface, where they trap the sediment and organic muck that flows into the marsh. Eventually, the whole ecosystem stands a little taller, and the bulrush isn’t smothered.
“We often call them ecosystem engineers,” says Pat Megonigal, an ecologist who directs the Smithsonian’s Global Change Research Wetland and studies the plants. “If the water gets deep, they have the ability to raise themselves up. And, in fact, right here at this marsh they’ve been doing it for 4,000 years.”
For a long while, wetland researchers have wondered whether that skill could help the plants build their way out of climate change. As sea levels rise, bringing fiercer and more frequent storm surges, so does the risk that the plants will drown. But increasing levels of carbon dioxide in the atmosphere are also a boon to the plants’ basement construction project, providing more fuel for photosynthesis and helping them build bigger roots. For 30 years, Megonigal and his predecessors have been watching this marathon unfold in a single marsh in Maryland on the Chesapeake Bay. It’s a duel between sea rise and plant growth, two forces with a common origin—humans burning fossil fuels, adding more CO2 to the air—and at this point, the result is becoming clear: The wetlands are losing.
Those findings, which were published last week in Science Advances, are upending some of the more optimistic assumptions about how coastal areas might adapt to rising seas. Wetlands are important ecosystems in their own right, and they mediate the flow of nutrients between land and sea. They also punch above their weight in terms of carbon storage, packing it away in dense peaty soils at concentrations that exceed those found in tropical forests. But the fate of those areas is uncertain in the face of climate change. By the end of the century, estimates suggest that climate-induced changes may cause 20 to 50 percent of those ecosystems to be lost. The ability of wetlands to raise themselves above rising waters is a key factor that will determine whether they can persist where they are or will need to migrate inland.
“Wow. We always thought elevated CO2 would help stabilize marshes, and this work really challenges that idea,” says Matthew Kirwan, an ecologist at the Virginia Institute of Marine Science who studies how coastal landscapes evolve. “Thirty-year experiments are almost unheard of, and in this case fundamentally changes how we understand marshland ecosystems.”