Plants utilizing the C4 photosynthetic pathway show higher potential efficiencies in the use of intercepted radiation, water, and nitrogen for the production of dry matter than do other photosynthetic types. C4 species are most numerous in tropical and warm, temperature semi—arid zones, where their greater water—use efficiency appears to be a key selective advantage. This fails to explain the dominance of a few C4 grasses in floodplain communities of the wet tropics. One such species, Echinochloa polystachya, occurs along the fertile white waters of the Amazon region. In this habitat, where neither water nor nitrogen supply are likely to be limiting to production or survival, a C4 species may realize its high productivity potential. This possibility was examined by monthly measurements of stand demography, biomass, and radiation interception at a site in the central Amazon. The maximum standing crop of 80 Mg/ha and annual net primary production of 99 Mg/ha of dry matter are among the highest values recorded for natural vegetation. The mean rate of dry—matter production per unit of solar radiation intercepted by the stand was 2.3g/MJ, an efficiency close to the considered maximum for C4 species. The significance of this high productivity to the carbon budget of the region is discussed. Despite the investment of most of its high productivity into stem material, growth of E. polystachya was only just sufficient to keep pace with the 8 m rise in water level that occurs as part of the annual cycle in river discharge. In this unusual environment, the high productivity potential conferred by C4 photosynthesis may be essential, in allowing this rooted macrophyte to produce a stem which is of sufficient rigidity and length to hold the canopy above the rising flood waters.

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