CO₂ Enrichment Experiments

Atmospheric carbon dioxide (CO2 ) concentrations have increased from 280 ppm in the nineteenth century to 420 ppm today and continue to rise at more than 2 ppm per year. CO2 is the substrate of photosynthesis, which plants convert into organic matter to grow. This chapter highlights experiments at the Smithsonian Tropical Research Institute, especially those on Barro Colorado Island, that examined plant responses to experimental CO2 enrichment. CO2 augmentation stimulated growth in understory plants in situ under low light conditions and in tree seedlings on nutrient-rich but not nutrient-poor soil under medium to high light intensities, except for nodulated legumes, which responded positively even in nutrient-poor soil. CO2 augmentation consistently reduced water use per unit dry mass accumulated or CO2 fixed. Because plant phosphorus content increased with cumulative transpiration, lower rates of transpiration at increased CO2 concentrations may impede phosphorus uptake. This chapter discusses the implications of these observations for stand-level CO2 responses of tropical forests and emphasizes the need for large-scale CO2 enrichment studies in the tropics.