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

Sea level rise and saltwater intrusion are 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 conducted a field-based study on Goodwin Island (Virginia, USA) in which we compared vertical accretion, primary production, nutrient cycling, carbon accumulation, and habitat type between old and young salt marsh in a coastal landscape where salt marsh is migrating landward into rapidly retreating coastal forest. We used aerial photography, historical maps, and radioisotopic dating of sediment cores to determine marsh age (<5 to >350 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 (soil C:N ratios) did not vary significantly with marsh age. Accretion and carbon accumulation rates varied predictably with elevation in old marsh but not in young marsh, where soil formation was instead largely driven by the presence of Phragmites australis, which is dominant at the modern marsh-forest boundary. Phragmites australis transitioned to native high marsh plants at a lower elevation in old marsh than young marsh, suggesting P. australis in young marsh will migrate into lower elevations over time. Interestingly, 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. Together these observations suggest that marsh migration does not lead to permanent changes in ecological function, and that ecological functions will converge as the marsh ages.