Plants in New England’s tidal marshes may not be able to spread inland fast enough to keep up with the pace of rising sea levels over the coming decades, concludes a recent study at the journal Biological Conservation.
“In southern New England, most of the area where tidal marshes would have to move into is forested,” said Christopher Field, a post-doctoral researcher at the University of Connecticut (UConn) who led the study. “We don’t know how quickly coastal forest is going to die back, but we know that right now, it’s happening at a much lower rate than marshes are responding.”
Marshes and other wetlands play a vital ecological role, acting as carbon sinks and holding more carbon per anchor than tropical forests do, said Field. They also provide a buffer zone in coastal areas that helps prevent flooding, can remove pollution from runoff, and provide a habitat for plant and animal species that can’t live anywhere else. Nature-lovers, birdwatchers, hunters, hikers and boaters would all be affected by disappearing marshes.
Field and his co-authors surveyed the wetlands and their surrounding forests in southern Connecticut. Their review determined that, as sea levels rise to flood wetlands, plants that traditionally grow closer to the ocean are spreading inland. However, the grasses and other vegetation being overtaken by these flood-resistant plants are not similarly spreading.
This suggests that, as sea levels rise due to climate change, marshes in southern New England will not be able to spread inland and could disappear entirely if the water line reaches the edges of existing forests.
The threat to marshes is particularly glaring in New England, where increases to sea level rise over the past thirty years have been three to four times the national average, according to a 2012 paper published in Nature Climate Change.
New England Coastal Areas More Vulnerable
Field, who graduated from UConn in the spring with a doctorate in ecology and evolutionary biology, designed the study and wrote the paper as part of his dissertation. He and his team studied the change in vegetation in tidal marshes along Connecticut’s Long Island Sound.
Previous studies had suggested that increasing rates of tree mortality and loss of tree canopy were likely to occur at the boundary between tidal marshes and forests. Field’s research, which included a survey of vegetation and an examination of the thickness of tree canopies surrounding tidal marshes, turned up no evidence of marsh plants expanding outward.
Scientists have expected that tidal marshes would be threatened by sea level rise, but the long-standing belief has been that, as long as there wasn’t too much human development in the areas surrounding typical wetland habitats, they could survive by moving landward, replacing surrounding ecosystems.
“The data from the tidal marsh plants, which are pretty good indicators of the flooding regime of tidal marshes, really is starting to strongly suggest that at least the higher-elevation areas of marshes are going to be disappearing and they’ve been disappearing for a few decades now,” Field said. “It doesn’t look like those areas are going to be able to keep pace [with sea level rise].”
Tidal marsh plants in other areas such as the Delaware Bay have managed to spread into surrounding ecosystems, but at a slower rate than sea-level rise. When he designed the study, Field expected to find a similar situation in the Connecticut marshes. He was surprised to find no evidence that salt marsh rush, a grass that grows at higher-elevation areas of wetlands, was spreading into the surrounding forest.
Field hypothesized that variation between mid-Atlantic and New England coastal areas relative to soil types, the slope between the marsh and forest, and the types of trees might account for differences in wetland expansion.
“I think probably the biggest factor is just differences in slope in the marsh edge in southern New England compared to the Mid-Atlantic,” he said. “There might be other factors at play too. We have a lot of hardwood trees here and they seem to be pretty resistant to stress and pretty resilient. They can also re-sprout and grow back after storms, so that could be playing into it.”
Field and his team at UConn want to return in a decade to re-examine the tidal marshes and investigate whether any changes have occurred. Scientists need to understand the issue more deeply, he said, by examining marsh habitats in other areas of the country and by studying how a forested habitat could change into a wetland.
“I think a lot of people are already doing the research that needs to be done, which is figuring out if marshes themselves can keep pace,” he said. “In southern New England, it’s just one piece of eastern North America’s tidal marshes, so we need data from a lot more areas to really quantify how much they’re responding to sea level rise and if this pattern that seems to be emerging is consistent.”
— Elise Harmon is a senior journalism student at Northeastern University. She is the managing editor of Northeastern’s independent student newspaper, The Huntington News, and has worked for The Boston Globe and BostInno.
Photo credit: Salt March in Ledyard, Conntecticut. Doug Kerr — Flickr.