Modeling Soil Porewater Salinity Response to Drought in Tidal Freshwater Forested Wetlands

There is a growing concern about the adverse effects of saltwater intrusion via tidal rivers, streams, and creeks into tidal freshwater forested wetlands (TFFW) due to sea level rise (SLR) and intense and extended drought events. However, the magnitude and duration of porewater salinity in exceedance of plant salinity stress threshold (2 practical salinity units, psu) and the controlling factors remain unclear. In this study, we developed a TFFW soil porewater salinity model, in which the feedback mechanisms between soil salinity and evapotranspiration and hydraulic conductivity were incorporated. We selected sites (upper, middle, lower tidal freshwater forest sites, and oligohaline marsh site) along the coastal floodplains of two rivers, the Waccamaw River (SC, USA) and the Savannah River (GA and SC, USA), that represent landscape salinity gradients from tidal influence of the Atlantic Ocean. The model results agreed well with field measurements and revealed that with drought-induced saltwater intrusion, the mean annual soil porewater salinity and duration of elevated soil porewater salinity (>2 psu) increased significantly compared to the normal (nondrought) condition, posing a threat to the health and ecosystem services of TFFW even in the absence of SLR. Model results also showed more severe salinity stress under drought for the lower forest sites along the two rivers, where soil salinity values have already been at or in exceedance of the 2 psu threshold. Plain Language Summary Tidal freshwater forested wetlands (TFFW) provide valuable ecosystem services to society including, but not limited to, carbon sequestration, water quality improvement, and habitat for critical wildlife species. However, TFFW are vulnerable to saltwater intrusion due to climate change-induced sea level rise (SLR) and extreme drought events. Salinity stress to TFFW is often reflected in elevated soil porewater salinity in exceedance of plant salinity stress threshold (2 psu). Soil porewater salinity magnitude and dynamics at sites along two coastal floodplains, the Waccamaw River (SC, USA) and the Savannah River (GA and SC, USA) exposed to drought-induced saltwater intrusion during 2008-2016 were studied using a physical and ecological process-driven soil salinity model. The mean annual soil porewater salinity and duration of elevated soil porewater salinity at TFFW sites increased significantly with drought-induced saltwater intrusion, even without SLR, compared to the normal (nondrought) condition, thus posing a threat to the health and ecosystem services in TFFW. Soil porewater salinity is an effective ecological indicator and the soil porewater salinity model can be used for predicting changes in the timing, magnitude, frequency, and duration of salinity stress to TFFW sites under future climate change. Key Points Drought-induced saltwater intrusion significantly increases soil porewater salinity in soils of tidal freshwater forested wetlands Tidal freshwater forest sites located near the tidal marsh boundary will suffer more severe salinity stress with climate change Soil porewater salinity is an effective ecological indicator for tidal freshwater forested wetland condition