Surface Elevation Trends in Natural and Restored Coastal Forested Wetlands Reveal Vulnerability to Saltwater Intrusion and Sea Level Rise

Abstract Accelerating sea level rise and increasing frequency of storms are impacting coastal wetlands. Similar to salt marshes, coastal freshwater wetlands provide important flood protection and storm abatement services, but their capacity to keep up with sea level rise and associated saltwater intrusion remains unclear. Long-term monitoring of wetland soil surface elevation using surface elevation table and marker horizons (SET-MH) has been conducted in salt marshes, while forested wetlands have not received as much attention. In 2015, we installed 18 SET-MHs in one restored and two mature forested wetlands on the coastal plain of North Carolina, all of which have experienced saltwater intrusion in recent years. We hypothesized that the restored wetland would have higher surface elevation gains, and areas protected from saltwater intrusion within all three sites would also have higher surface elevation gains. From 2016 to 2022 we measured surface elevation change and used marker horizons to measure vertical accretion. Rates of surface elevation change ranged from -4.25 to 4.79 mm/year, and vertical accretion rates ranged from -0.27 to 4.16 mm/year. All sites are vulnerable to future inundation, as even the highest rates of surface elevation gain were less than the observed 50-year rate of local sea level rise. Areas that experienced higher salinity exhibited higher rates of shallow subsidence. Our results support previous evidence that many coastal forested wetlands in the southeastern US are lagging behind sea level rise and, if elevation change rates do not accelerate substantially, they will experience ecological transformations in the coming decades.