Gross primary production and net ecosystem exchange of a cool-temperate deciduous forest estimated by the eddy covariance method

An eddy covariance measurement system was installed to measure long-term turbulent fluxes of heat, water vapor, and CO2 over a cool-temperate deciduous forest in the central part of Japan. Previous to a long-term measurement, a comparison of flux measurements using open- and closed-path type infrared gas analyzers was conducted for CO2 and water vapor. The closure of the energy budget over the forest was also examined to test the validity of the flux measurements. Using a whole year data from 25 July 1998 to 24 July 1999, the relation between the night-time air temperature and the night-time CO2 flux over the forest (ecosystem respiration) was investigated. The results suggest that the night-time CO2 flux was significantly influenced by the vertical air temperature gradient over the canopy. The night-time CO2 flux increased exponentially with the temperature when the stability was nearly neutral, while it was insensitive to the temperature under stable conditions. Daily values of the net ecosystem exchange (NEE) and the gross primary production (GPP) of the forest were estimated, and they were parameterized as a function of the air temperature and the absorbed photosynthetic active radiation (APAR). The results show a clear seasonal change in GPP caused by changes in the light-use efficiency and the maximum rate of carbon fixation of the canopy with LAI. The estimated NEE based on the parameterization agrees well with the observed NEE during the observational period.