How tidal freshwater forested wetland response to salinization affects carbon balance and soil surface elevation

Background/Question/Methods .  With rising sea levels, river flow rate modifications, and land use change along major river corridors of the southeastern United States, tidal freshwater forested wetlands (TFFW) will migrate inland, change to marsh, or disappear. These habitat shifts affect the balance of carbon (C) storage and conveyance in coastal landscapes. Here, we measured aspects of the C cycle along downriver salinity gradients, from TFFW to high oligohaline marsh, on both the Savannah River (Georgia) and Waccamaw River (South Carolina). We quantified C inputs [from vegetation (wood, herbaceous, fine root production) and sedimentation] and C outputs [from decomposition (litter and root) and associated greenhouse gas emissions] to describe the ecosystem consequences of salinization on soil surface elevation, soil C sequestration, and nutrient cycling.

Results/Conclusions . Along both salinity gradients, aboveground forest productivity from wood and litterfall decreased while aboveground herbaceous and belowground fine root production increased as forested wetlands transitioned to high oligohaline marsh. Site-specific variation in contemporary (< 3 years) C sedimentation was large, but suggested greater C sedimentation in high oligohaline marshes compared to TFFW. Decomposition of root biomass was greater in high oligohaline marsh and TFFW with prevailing freshwater (<0.5 ppt) conditions, but lower in salt-impacted forest. When decomposition was modelled assuming different deliveries of salinity (drought, pulsed, sea-level rise induced), we found different fates of root material that differentially influence surface elevation maintenance. Soil CO₂ losses tended to be lowest from salt-impacted forests due to altered hydrology and decreased belowground productivity, while CH₄ fluxes were static. Along one river (Waccamaw), surface elevation increased in both TFFW and moderately salt-impacted forests, decreased in heavily salt-impacted forests, and increased in the high oligohaline marsh. Long-term C accumulation in soils (decades) were sometimes opposite to the contemporary C sedimentation patterns, with significantly less C sequestration in high oligohaline marsh. Acute pulses of salinity versus long-term persistence of salinity differentially alter the C balance along transects requiring separate treatment in models.