Elevated CO alters anatomy, physiology, growth, and reproduction of red mangrove (Rhizophora mangle L.).

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 CO during global change may affect global carbon cycling, but as yet has not been investigated experimentally. Halophyte responses to CO 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) and double-ambient (700 μll) CO concentrations for over 1 year. Mangrove seedlings exhibited significantly increased biomass, total stem length, branching activity, and total leaf area in elevated CO. Enhanced total plant biomass under high CO was associated with higher root:shoot ratios, relative growth rates, and net assimilation rates, but few allometric shifts were attributable to CO treatment independent of plant size. Maximal photosynthetic rates were enhanced among high-CO plants while stomatal conductances were lower, but the magnitude of the treatment difference declined over time, and high-CO seedlings showed a lower P at 700 μll CO than low-CO plants transferred to 700 μll CO: 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 CO. Foliar chlorophyll, nitrogen, and sodium concentrations were lower in high CO. Mangroves grown in high CO 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 CO 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 CO 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.