Some water-loving plants may be out of a home as temperatures rise

By Beth Daley, Globe Staff

Chances are you haven’t heard of pannes – super soggy, low-oxygen depressions - but they can comprise as much as 40 percent of northern New England salt marshes.

The waterlogged areas support more than a dozen species of plants known collective as forbs: The succulent pickleweed with its hard to see tiny yellow flowers to the delicate pale purple blooms of sea lavender. Canada geese, ducks and other waterfowl munch on other kinds of forbs.

Aerial view of Nag Creek Marsh in Rhode Island

Now, new research from Brown University notes that pannes may be losing their mucky niche as the earth continues to warm. In a series of experiments, Keryn Gedan, a Brown University graduate student, showed warming temperatures initially allow the plants to grow more vigorously but then, rapidly die off. The research was published this month in Ecology Letters. Gedan is presenting her work today at the Ecological Society of America conference in Albuquerque.

Gedan, along with other Brown researchers found that warming temperatures allow forbs to take in more water. The results are drier pannes that are perfect for saltmarsh hay to move in and push out the forbs.

“The forbs basically engineer themselves out of their habitat by making it more favorable for
their competitor,” said Gedan, lead author of the paper.

Gedan and her advisor, Mark Bertness, chair of the Ecology and Evolutionary Biology Department at Brown, studied three pannes in Rhode Island and Maine between the summers of 2004 and 2007, subjecting them to temperatures as much as 6 degrees Fahrenheit warmer than the surrounding area.

At two sites — Nag Creek in Prudence Island, Rhode Island, and Little River in Maine — the forbs declined in coverage from 50 percent to 10 percent between 2004 and 2006. At the third site in Drakes Island, Maine, forb cover decreased from 50 percent of the plot to 44 percent in the summer of 2007 alone.

It’s not the end of salt marsh pannes (although Gedan says they could all but disappear in southern New England where they are already rare). And Gedan noted rising sea levels may allow forbs to survive in other water-inundated areas. But the experiments do show the sensitivity these key pieces of salt marshes have to warming temperatures – and the complicated fate of its vegetation.

“How all these things interact, we don’t really know,” Gedan said. “But we know that with [higher] temperatures, these changes happen rapidly.”