RESPONSES OF COMMUNITIES OF TROPICAL TREE SPECIES TO ELEVATED CO2 IN A FOREST CLEARING

Communities of ten species of tropical forest tree seedlings from thre e successional classes were grown at ambient and elevated CO2 in large open-top chambers on the edge of a forest in Panama. Communities grew from 20 cm to approximately 2 m in height in 6 months. No enhancement s in plant biomass accumulation occurred under elevated CO2 either in the whole communities or in growth of individual species. Reductions i n leaf area index under elevated CO2 were observed, as were decreases in leaf nitrogen concentrations and increases in the C:N ratio of leaf tissue. Species tended to respond individualistically to elevated CO2 , but some generalizations of how successional groupings responded cou ld be made. Early and mid-successional species generally showed greate r responses to elevated CO2 than late-successional species, particular ly with respect to increases in photosynthetic rates and leaf starch c oncentrations, and reductions in leaf area ratio. Late-successional sp ecies showed greater increases in C:N ratios in response to elevated C O2 than did other species. Our results indicate that there may not be an increase in the growth of regenerating tropical forest under elevat ed CO2, but that there could be changes in soil nutrient availability because of reductions in leaf tissue quality, particularly in late-suc cessional species.