Carbon dioxide exchange and nocturnal processes over a mixed deciduous forest

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 degrees 65'W, 44 degrees 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 CO3(mol photon)(-1) and 7.95 mg CO2(g H2O)(-1), and the average nocturnal respiration rate 0.21 mg CO2 m(-2) s(-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).