Can elevated CO₂ modify regeneration from seed banks of floating freshwater marshes subjected to rising sea-level?

Higher atmospheric concentrations of CO₂ can offset the negative effects of flooding or salinity on plant species, but previous studies have focused on mature, rather than regenerating vegetation. This study examined how interacting environments of CO₂, water regime, and salinity affect seed germination and seedling biomass of floating freshwater marshes in the Mississippi River Delta, which are dominated by C₃ grasses, sedges, and forbs. Germination density and seedling growth of the dominant species depended on multifactor interactions of CO₂ (385 and 720 μl l^-1) with flooding (drained, +8-cm depth, +8-cm depth-gradual) and salinity (0, 6% seawater) levels. Of the three factors tested, salinity was the most important determinant of seedling response patterns. Species richness (total = 19) was insensitive to CO₂. Our findings suggest that for freshwater marsh communities, seedling response to CO₂ is species-specific and secondary to salinity and flooding effects. Elevated CO₂ did not ameliorate flooding or salinity stress. Consequently, climate-related changes in sea level or human-caused alterations in hydrology may override atmospheric CO₂ concentrations in driving shifts in this plant community. The results of this study suggest caution in making extrapolations from species-specific responses to community-level predictions without detailed attention to the nuances of multifactor responses.