Inter-annual variations of GPP and NEP above a cool-temperate deciduous forest at Takayama (AsiaFlux) in the last decade

Broadleaf deciduous forest is one of the major vegetative constituents of the main island of Japan. Natural ecosystem dominated by oak is widespread in Japan, Korean Peninsula, northeastern China, and eastern Mongolia. Studying the environmental controls on gross primary production (GPP) and net ecosystem exchange (NEE) of the ecosystem is, then, one of the main subjects for understanding the carbon cycle of terrestrial ecosystems in eastern Asia. The study site is about 15 km east of Takayama City, in the central part of Japan. Flux of CO₂ has been measured since September 1993 by the aerodynamic method using the vertical gradient of CO₂ concentration and diffusion coefficient (Yamamoto et al., 1999). Fluxes of sensible heat, water vapor, and CO₂ have been measured continuously since July 1998 by the eddy covariance method (Saigusa et al., 2002) using a three-dimensional ultrasonic anemometer and a closed-path infrared gas analyzer. The nighttime NEE was estimated with and without correction, depending on the atmospheric stability. The annual carbon uptake for 1999, 2000, and 2001 were estimated as 198, 309, 290 gC m^-2 year^-1 with the correction, and 251, 376, and 342 gC m^-2 year^-1 without the correction, respectively. The nocturnal correction is responsible for the uncertainty in annual NEE in this site of about 52-67 gC m^-2 year^-1. The NEP has been calculated for almost ten years, and it is the longest record at any particular site in Asia as a result of the flux measurements. The annual NEP was 224 +/- 82 gC m^-2 year^-1 (mean +/- SD; 1994-2001). A large year-to-year variability of up to 158 gC m^-2 year^-1 was observed in NEP. The annual NEP was highest in 1998, mainly caused by high CO₂ uptake observed during the first half of the growing period. The air temperature was significantly higher in April 1998 than in a usual year. The result in the present study suggests that the increase in CO₂ uptake due to a long growing period in 1998 was more remarkable than the enhancement of the respiration. References Saigusa et al., Agric. For. Met., 112, 203-215, 2002. Yamamoto et al., Tellus 51B, 402-413, 1999.