VARIATIONS OF ECOSYSTEM GAS-EXCHANGE IN THE RAIN-FOREST MESOCOSM AT BIOSPHERE-2 IN RESPONSE TO ELEVATED CO2

The effects of elevated CO2 on tropical ecosystems were studied in the artificial rain forest mesocosm at Biosphere 2, a large-scale and eco logically diverse experimental facility located in Oracle, Arizona. Th e ecosystem responses were assessed by comparing the whole-system net gas exchange (NEE) upon changing CO2 levels from 900 to 450 ppmV. The day-NEE was significantly higher in the elevated CO2 treatment. In bot h experiments, the NEE rates were similar to values observed in natura l analogue systems. Variations in night-NEE, reflecting both soil CO2 efflux and plants respiration, covaried with temperature but showed no clear correlation with atmospheric CO2 levels. After correcting for c hanges in CO2 efflux we show that the rain forest net photosynthesis i ncreased in response to increasing atmospheric CO2. The photosynthetic enhancement was expressed in higher quantum yields, maximum assimilat ion rates and radiation use efficiency. The results suggest that photo synthesis in large tropical trees is CO2 sensitive, at least following short exposures of days to weeks. Taken at face value, the data sugge st that as a result of anthropogenic emissions of CO2, tropical rain f orests may shift out of steady state, and become a carbon sink at leas t for short periods. However, a better understanding of the unique con ditions and phenomena in Biosphere 2 is necessary before these results are broadly useful.