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

Turbulent fluxes of heat, water vapor, and CO2 were measured using the eddy covariance method over a cool-temperate deciduous forest at a mountainous region in the central part of Japan. The flux measurement system was based on a three-dimensional ultrasonic anemometer-thermometer and a closed-path type infrared gas analyzer. Previous to a long-term measurement, the closure of the energy budget and the frequency response losses of the measurement system were examined with relation to the wind direction.

Using a whole year data from 25 July 1998 to 24 July 1999, the relation between the nighttime air temperature and the nighttime CO2 flux over the forest was investigated. The results showed that the nighttime CO2 flux was significantly influenced by the vertical air temperature gradient over the canopy. The nighttime CO2 flux increased exponentially with the temperature when the stability was nearly neutral, while it was insensitive to the temperature under stable conditions.

Daily net ecosystem exchange (NEE) and 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 showed a clear seasonal change in GPP was caused by changes in the light-use efficiency and the maximum rate of carbon fixation of the canopy with Leaf Area Index.