Marked growth response of communities of two tropical tree species to elevated CO2 when soil nutrient limitation is removed

As part of an ongoing project to understand the effects of elevated atmospheric CO2 on plants in complex, tropical communities, we studied biomass accumulation in a simplified model seedling community consisting of two species of tropical trees (Ficus insipida, a fast growing pioneer species, and Virola surinamensis, a slow-growing, shade-tolerant late successional species). The plants were grown at ambient and elevated (about two times ambient) CO2 concentrations using open-top chambers at a field site in Panama. Communities grew in heavily fertilized soil. Compared to a previous experiment with model communities of F. insipia and V. surinamensis grown on unfertilized soil (WINTER et al., Flora [2000] 195, 289) application of soil fertilizer markedly accelerated community growth rates at ambient CO2, and biomass accumulation was enhanced by an additional 52% at elevated CO2. In contrast, elevated CO2 had no significant effect on biomass accumulation in unfertilized communities. Communities growing on fertilized soil showed greater biomass allocation into leaves, i.e. higher leaf weight ratios (LWRs) than did communities on unfertilized soil. Higher LWRs were related to lower root: shoot ratios and together with greater specific leaf areas (area per unit leaf mass), largely a consequence of lower leaf starch contents, resulted in higher leaf area ratios (LARs). While elevated CO2 caused the relatively low LARs in unfertilized communities to decrease further, by strongly increasing leaf starch levels and decreasing specific leaf areas, these leaf characteristics changed only slightly in fertilized communities exposed to elevated CO2. Thus, by maintaining relatively high LARs at elevated CO2, fertilized plants were able to effectively use enhanced CO2 concentrations for increased carbon gain and growth. Leaves of plants on fertilized soil were characterized by relatively low C: N ratios which were largely unaffected by CO2 concentration. In contrast, C: N ratios in leaves of unfertilized plants were higher than those of fertilized plants and increased in response to elevated CO2.