Tidal Freshwater Marsh Plant Responses to Low Level Salinity Increases

In a tidal freshwater marsh (TFM) in lower Chesapeake Bay, salt marsh cordgrass (Spartina alterniflora) recently has become established where TFM vegetation previously grew. To determine the potential contribution of saltwater intrusion to the observed species transition, we conducted a mesocosm study subjecting Spartina and two TFM perennials (Peltandra virginica and Leersia oryzoides) to sub-lethal levels of salinity (0, 2, 4 and 6). We measured plant performance as carbon dioxide flux over the leaf stomata as a proxy for net photosynthesis (Anet), aboveground and belowground biomass and tissue carbon (C), nitrogen (N) and phosphorus. For Spartina, all metrics were unchanged across the range of tested salinity. The TFM species, however, had lower Anet than Spartina and decreased with added salinity. TFM biomass was reduced up to 86 % in both the aboveground and belowground compartments, and aboveground nutrient allocation was altered: N increased in Leersia and P increased in Peltandra with increasing salinity. Under this simulation of increased salt intrusion associated with climate change and sea level rise, TFM species responded with decreased biomass and decreased tissue C:N. Although other factors must contribute, the abiotic salt stressor leads to plant responses consistent with the observed replacement of TFM species by Spartina.