Elevated atmospheric CO2 is known to stimulate photosynthesis and growth of plants with the C3 pathway but less of plants with the C4 pathway. An increase in the CO2 concentration can therefore be expected to change the competitive interactions between C3 and C4 species. The effect of long term exposure to elevated CO2 (ambient CO2 concentration + 340 #mol CO2 tool-1) on a salt marsh vegetation with both C3 and C4 species was investigated. Elevated CO2 increased the biomass of the C3 sedge Scirpus olneyi growing in a pure stand, while the biomass of the C4 grass Spartinapatens in a monospecific community was not affected. In the mixed C3/C4 community the C3 sedge showed a very large relative increase in biomass in elevated CO2 while the biomass of the C4 species declined.
The C4 grass Spartina patens dominated the higher areas of the salt marsh, while the C3 sedge Scirpus olneyi was most abundant at the lower elevations, and the mixed community occupied intermediate el- evations. Scirpus growth may have been restricted by drought and salt stress at the higher elevations, while Spartina growth at the lower elevations may be affected by the higher frequency of flooding. Elevated CO2 may affect the species distribution in the salt marsh if it allows Scirpus to grow at higher elevations where it in turn may affect the growth of Spartina.