Plants alter biomass allocation to optimize resource capture. Plant strategy for resource capture may have important implications in intertidal marshes, where soil nitrogen (N) levels and atmospheric carbon dioxide (CO 2 ) are changing. We conducted a factorial manipulation of atmospheric CO 2 (ambient and ambient+340 ppm) and soil N (ambient and ambient+25 gm −2 year −1 ) in an intertidal marsh composed of common North Atlantic C 3 and C 4 species. Estimation of C 3 stem turnover was used to adjust aboveground C 3 productivity, and fine root productivity was partitioned into C 3 -C 4 functional groups by isotopic analysis. The results suggest that the plants follow resource capture theory. The C 3 species increased aboveground productivity under the added N and elevated CO 2 treatment (P< 0.0001), but did not under either added N or elevated CO 2 alone. C 3 fine root production decreased with added N (P< 0.0001), but fine roots increased under elevated CO 2 (P0 0.0481). The C 4 species increased growth under high N availability both above-and belowground, but that stimulation was diminished under elevated CO 2 . The results suggest that the marsh vegetation allocates biomass according to resource capture at the individual plant level rather than for optimal ecosystem viability in regards to biomass influence over the processes that maintain soil surface elevation in equilibrium with sea level.