With sea level rise and alterations to river discharge, coastal wetlands are experiencing elevated salinities, either intermittently or for extended periods. While long-term salinization is known to trigger immigration of salt-tolerant species into freshwater wetlands and thereby cause habitat shifts along estuaries, freshwater plant communities may be resilient to short pulses of low salinity water. Our goal was to understand how tidal freshwater marsh plant communities respond to different durations of saline water exposure using mesocosm communities. We collected the most common species in Atlantic Coast freshwater marshes along Altamaha River, GA, and treated them with different salinity levels (3, 5, and 10 psu) and exposure durations (5, 10, 15, 20 and 30 days per month), relative to a freshwater control (n=3 per treatment combination). After three-months of treatments (May – Aug, 2014), plants were watered with freshwater only to allow recovery from September, 2014 to June, 2015.
The responses and recovery of the freshwater vegetation depended on the salinization pattern (salinity and duration), and the salt tolerance of each species. Mild salinization caused a shift towards a salt-tolerant community. However, salt-intolerant species regrew during the recovery period in the mild salinization treatments, and the final community composition remained largely unaffected. With heavy salinization, all freshwater species decreased in abundance during the treatment period, and the community composition remained different from the controls after the recovery period. Heavy salinization did not cause a significant reduction in total aboveground biomass (all species combined) at the end of the recovery period, but species richness declined with increased duration of saline exposure. This suggested that species composition shifts in tidal freshwater marsh communities may prevent significant reductions in plant production following salinization, despite a loss of species diversity. Overall, our results indicated that tidal freshwater marsh plant communities are resilient to mild saline intrusion that may occur due to future climate change and human disturbances. In contrast, prolonged high salinities may cause significant changes in community composition without markedly altering ecosystem productivity.