Growth response of three plantation species of the tropics exposed to elevated CO2 levels

Abstract

The response of forest trees, the largest carbon sinks on the earth, to continuing rise in atmospheric carbon levels is unknown. Reports state that increasing levels of atmospheric CO2 will stimulate photosynthesis and productivity in most ecosystems. However, the duration and magnitude of this stimulation, particularly in the tropics, remains a question. To investigate the effects of CO2 fertilization on plant growth, seedlings of three common plantation species, Casuarina equisetifolia, Ailanthus excelsa and Tectona grandis were grown in closed chambers enriched with CO2. After 180 days of treatment, morphological traits of seedling height, biomass of root and shoot and root-shoot allometric co-efficient were measured. The activity of carbonic anhydrase and contents of chlorophylls, total carbohydrates and soluble proteins were determined. In Tectona grandis, significant effects of CO2 supply were found on chlorophylls, root-shoot allometric ratio and seedling quality index. Ailanthus excelsa showed significant effect on only the shoot characteristics on exposure to elevated CO2 but the root characteristics and concentrations of chlorophylls were not significantly different. Casuarina equisetifolia also showed significant effects on exposure to elevated CO2 in terms of shoot characteristics and concentrations of chlorophylls. Total sugars, the major photosynthates, did not show any significant variation to elevated CO2 in any of the three species. Carbonic anhydrase, the key enzyme responsible for transfer of CO2 into the tissues significantly increased in all three species. Overall, all the variables responded to elevated CO2, reflecting the positive effects of one parameter of climate change conditions on seedling quality. A positive response of these three plantation species to elevated CO2 content is a good indication for their future existence in potentially changed climatic conditions.