Vegetation Dynamics in Rhode Island Salt Marshes During a Period of Accelerating Sea Level Rise and Extreme Sea Level Events

Sea level rise is a major stressor on many salt marshes, and its impacts include creek widening, ponding, vegetation dieback, and drowning. Marsh vegetation changes have been associated with sea level rise across southern New England, but most of these studies pre-date the current period of rapidly accelerating sea level rise coupled with episodic events of extreme increases in water levels. Here, we combine data from two salt marsh monitoring and assessment programs in Rhode Island that were designed to assess marsh responses to sea level rise and use these data to document temporal and spatial patterns in marsh vegetation during the current period of extreme water level increases. Vegetation monitoring at two Narragansett Bay salt marshes confirms the ongoing decline of the salt meadow species Spartina patens during this period as it becomes replaced by Spartina alterniflora. Bare ground resulting from vegetation dieback was significantly related to mean high water levels and led to the rapid conversion of mixed Spartina assemblages to S. alterniflora monocultures. A broader spatial assessment of RI marshes shows that S. alterniflora dominance increases at lower elevation marshes toward the mouth of Narraganset Bay. Our data provide additional evidence that S. patens continues to decline in southern New England marshes and show that losses can accelerate during periods of extreme high water levels. Unless adaptive management actions are taken, we predict that marshes throughout RI will continue to lose salt meadow habitat and eventually resemble lower elevation marshes that are already dominated by S. alterniflora monocultures.