The Effect of Marsh Age on Ecosystem Function in a Rapidly Transgressing Marsh

Sea-level rise is leading to the migration of marshes into coastal forests throughout North America. Marsh migration represents a primary mechanism for marsh survival in the face of sea-level rise and leads to a fundamental reorganization of vegetation communities. Yet, the ecological implications of these changes remain unknown. To evaluate the effect of marsh migration on ecosystem function, we compared habitat, primary production, vertical accretion, nutrient cycling, and carbon accumulation between new and old salt marsh on Goodwin Island (Virginia, USA) where salt marsh is migrating landward into rapidly retreating coastal forest. Using historical imagery and radioisotopic dating of sediment cores, we determined marsh age (< 3 to approximately 616 years) across the landscape. We found that salt marsh functions generally depended more on elevation and/or landscape position than marsh age. Primary production and nutrient cycling (%C and %N soil content) did not vary significantly with marsh age. Accretion and carbon accumulation rates varied predictably with elevation in old marsh but not in new marsh. Instead, trends in soil formation were controlled by the colonization of Phragmites australis, a dominant plant in migrating marsh-forest boundaries along the Atlantic Coast. Vegetation zonation patterns were more clearly defined in old marsh, indicating that habitat types take time to develop. However, these vegetation differences did not translate to consistently different ecological functions. These observations suggest that marsh migration does not lead to permanent differences in ecological functions between new and old marsh, rather ecological functions will converge as new marsh ages.