Researchers studied the effect of large-scale removal of vegetation in tidal marshes in the Schelde estuary, Belgium and southwest Netherlands. (Credit: https://beeldbank.rws.nl, Rijkswaterstaat/Joop van Houdt)

When the plants go, the whole marsh falls apart.

That’s what researchers have found in an innovative experiment in Belgium in which acres of reeds were literally mowed down, enabling the team to observe the consequences of extensive plant loss, which were more severe than expected.

“Once you lose the vegetation, the conditions for plant growth get worse and worse and worse – toward permanent loss of the vegetation,” said Stijn Temmerman, a geomorphologist with the University of Antwerp in Belgium, who led the research team.

Observations of this sort might help land managers in areas such as the Mississippi Delta, where tidal marshes are in peril, he added. Without marshes, coastal areas can become more vulnerable to storm surges, and lose a key natural water filter.

In tidal wetland ecosystems, vegetation ordinarily roots the soil in place and helps trap sediments, building up the marsh. Past computer models and studies in small research plots of a few square meters or yards have shown that plant die-off deprives marshes of new mud that elevates the land. The new study examines what happens in nature when the vegetation disappears over 4 hectares (about 10 acres) of a tidal wetland. Once the plants were lost, the new research shows, the marsh changed shape, creating difficult conditions for new vegetation to take root.

The study was published last week in Geophysical Research Letters, a journal of the American Geophysical Union (AGU).

It’s the first time researchers have been able to directly investigate the effects of removing the plants from a large tidal marsh, Temmerman said. And the findings reflect what could happen in the coming years, as rising sea levels flood coastal wetlands with more water than they can handle.

“The wetland plants that are growing there, they are stressed by this increase. We’ve seen in several places around the world that this leads to die-offs of the vegetation,” Temmerman said. Those places include Venice’s lagoons and the Mississippi Delta, where huge sections of marshes are shrinking.

At the Belgian marsh where the experiment took place, land managers routinely mow all the reeds, which helps the plants come back healthier the next year. Before and after that grooming, the scientists took measurements of water depth, velocity, and direction at several places across the marsh, including in the channels that drain the flats.

They found that when the tides came in, without the plants as a speed trap, the water flowed two to four times faster over the plain – and at those speeds, the sediment didn’t settle out to help build up the marsh. The water in the channels, however, slowed down to a third of its normal speed. And there, the sediments did drop out of the flow, causing channels to fill up.

This second finding was an unexpected result, Temmerman said. Blocked channels led to less drainage of the previously vegetated marsh, which created unfavorably soggy growing conditions. So without new marsh-building mud on the plains, and with blocked up drainage channels, the tidal marsh was swamped, making it more difficult for plants to grow back. Rising sea levels are expected increase the odds that a marsh will go under and no longer support new plant growth.

For land managers and others tasked with keeping wetlands healthy, the study is a warning to work as hard as possible to ensure the vegetation doesn’t disappear, Temmerman said. In places like the Mississippi basin, where managers direct water into surrounding wetlands to build up sediments, those actions need to be taken as soon as plants start dying. If vegetation is lost for a couple years, he said, it’s harder for the sediments to stick on the marsh plains, and rebuilding the wetland gets tricky.

“Once the marshes die off,” Temmerman said, “it’s very, very difficult to restore them.”

Temmerman, S., Moonen, P., Schoelynck, J., Govers, G., & Bouma, T. (2012). Impact of vegetation die-off on spatial flow patterns over a tidal marsh Geophysical Research Letters, 39 (3) DOI: 10.1029/2011GL050502

– Kate Ramsayer, AGU science writer