Carbon dioxide exchange and nocturnal processes over a mixed deciduous forest

Abstract

This paper reports the results of the analysis of CO2 exchange from a one-month experiment conducted at a mixed deciduous forest, Camp Borden (80°65′W, 44°19′ N), Canada, in the summer of 1993. The mid-day CO2 flux from the forest under clear sky conditions was around −1.0 mg m−2 s−1, the average light and water use efficiencies 13 mmol CO2(mol photon)−1 and 7.95 mg CO2(gH2O)−1 , and the average nocturnal respiration rate 0.21 mg CO2m−2s−1. We observed different flow features at heights of 34.5 (14.5 m above the canopy) and 22.4 m at night. Wavelike structures were frequently encountered at z = 34.5 m. Depending on the phase angle between the vertical velocity and CO2 concentration time series, they could act to enhance the co-gradient (upward) flux or to create counter-gradient (downward) flux of CO2. We speculate that the wave events were limited to isolated regions in the upwind direction. Near the tree-tops (z = 22.4 m), the strong wind shear was able to maintain turbulence. Inverse temperature ramp structures were very common and flux of sensible heat was well behaved (directed downward).