An Investigation of the Afternoon Decline in Tropical Forest Photosynthesis

The recent use of eddy covariance to measure the net exchange of CO2 between the atmosphere and a tropical forest in Santarem Para, Brazil, has indicated a substantial decline in forest photosynthesis in the afternoon, even after taking light differences into account. Afternoon declines in leaf-level gas exchange have been reported for many ecosystems, including tropical forests. Potential causes for this decline include stomatal responses to VPD, low leaf water potential, changes in biochemistry due to elevated temperature, photoinhibition, photorespiration, or intrinsic circadian rhythm. This study attempts to understand this decline by using leaf gas exchange to measure photosynthesis while canopy leaves are kept under constant light, humidity, and temperature conditions for 24 to 36 hours hours.At constant light levels of 100 PAR æmol~m^-2s^-1, 7 of 12 species tested showed a substantial decline photosynthesis at night, and a subsequent recovery in photosynthesis during the next day. In most cases, the internal CO2 of these plants increased at night, indicating that this diel cycle was not simply an effect of a circadian rhythm in stomatal conductance. Rather, the photosynthesis of a significant fraction of the plants surveyed appears to be under the direct control of a circadian oscillator.