Mangroves, woody halophytes restricted to protected tropical coasts, form some of the most productive ecosystems in the world, but their capacity to act as a carbon source or sink under climate change is unknown. Their ability to adjust growth or to function as potential carbon sinks under conditions of rising atmospheric CO2 during global change may affect global carbon cycling, but as yet has not been investigated experimentally. Halophyte responses to CO2 doubling may be constrained by the need to use carbon conservatively under water-limited conditions, but data are lacking to issue general predictions. We describe the growth, architecture, biomass allocation, anatomy, and photosynthetic physiology of the predominant neotropical mangrove tree, Rhizophora mangle L., grown solitarily in ambient (350 μll(-1)) and double-ambient (700 μll(-1)) CO2 concentrations for over 1 year. Mangrove seedlings exhibited significantly increased biomass, total stem length, branching activity, and total leaf area in elevated CO2. Enhanced total plant biomass under high CO2 was associated with higher root:shoot ratios, relative growth rates, and net assimilation rates, but few allometric shifts were attributable to CO2 treatment independent of plant size. Maximal photosynthetic rates were enhanced among high-CO2 plants while stomatal conductances were lower, but the magnitude of the treatment difference declined over time, and high-CO2 seedlings showed a lower Pmax at 700 μll(-1) CO2 than low-CO2 plants transferred to 700 μll(-1) CO2: possible evidence of downregulation. The relative thicknesses of leaf cell layers were not affected by treatment. Stomatal density decreased as epidermal cells enlarged in elevated CO2. Foliar chlorophyll, nitrogen, and sodium concentrations were lower in high CO2. Mangroves grown in high CO2 were reproductive after only 1 year of growth (fully 2 years before they typically reproduce in the field), produced aerial roots, and showed extensive lignification of the main stem; hence, elevated CO2 appeared to accelerate maturation as well as growth. Data from this long-term study suggest that certain mangrove growth characters will change flexibly as atmospheric CO2 increases, and accord with responses previously shown in Rhizophora apiculata. Such results must be integrated with data from sea-level rise studies to yield predictions of mangrove performance under changing climate.