Salinity and sea level mediate elevated CO₂ effects on C₃–C₄ plant interactions and tissue nitrogen in a Chesapeake Bay tidal wetland

Abstract Elevated atmospheric carbon dioxide concentrations ([CO 2 ]) generally increase plant photosynthesis in C 3 species, but not in C 4 species, and reduce stomatal conductance in both C 3 and C 4 plants. In addition, tissue nitrogen concentration ([N]) often fails to keep pace with enhanced carbon gain under elevated CO 2 , particularly in C 3 species. While these responses are well documented in many species, implications for plant growth and nutrient cycling in native ecosystems are not clear. Here we present data on 18 years of measurement of above and belowground biomass, tissue [N] and total standing crop of N for a Scirpus olneyi ‐dominated (C 3 sedge) community, a Spartina patens ‐dominated (C 4 grass) community and a C 3 –C 4 ‐mixed species community exposed to ambient and elevated (ambient +340 ppm) atmospheric [CO 2 ] in natural salinity and sea level conditions of a Chesapeake Bay wetland. Increased biomass production (shoots plus roots) under elevated [CO 2 ] in the S. olneyi ‐dominated community was sustained throughout the study, averaging approximately 35%, while no significant effect of elevated [CO 2 ] was found for total biomass in the C 4 ‐dominated community. We found a significant decline in C 4 biomass (correlated with rising sea level) and a concomitant increase in C 3 biomass in the mixed community. This shift from C 4 to C 3 was accelerated by the elevated [CO 2 ] treatment. The elevated [CO 2 ] stimulation of total biomass accumulation was greatest during rainy, low salinity years: the average increase above the ambient treatment during the three wettest years (1994, 1996, 2003) was 2.9 t ha −1 but in the three driest years (1995, 1999, 2002), it was 1.2 t ha −1 . Elevated [CO 2 ] depressed tissue [N] in both species, but especially in the S. olneyi where the relative depression was positively correlated with salinity and negatively related with the relative enhancement of total biomass production. Thus, the greatest amount of carbon was added to the S. olneyi ‐dominated community during years when shoot [N] was reduced the most, suggesting that the availability of N was not the most or even the main limitation to elevated [CO 2 ] stimulation of carbon accumulation in this ecosystem.