Inter-annual variability of carbon budget components in an AsiaFlux forest site estimated by long-term flux measurement

CO₂ flux has been measured by the aerodynamic method since 1993, and by the eddy covariance method since 1998, over a cool-temperate deciduous forest in Japan, which is one of the AsiaFlux forest sites. In the aerodynamic method, CO₂ flux is estimated using the vertical gradient of CO₂ concentration and the diffusion coefficient over the canopy. The eddy fluxes of sensible heat, water vapor, and CO₂ are measured using an ultrasonic anemometer and a closed-path infrared gas analyzer. The CO₂ flux determined by the aerodynamic method was validated by comparison with that estimated by the eddy covariance method.

The nighttime net ecosystem CO₂ exchange (NEE) was highly sensitive to atmospheric stability in the site. The nighttime NEE was estimated with and without correction, depending on friction velocity. The annual net ecosystem production (NEP) values for 1999, 2000, and 2001 were estimated to be 198, 309 and 290 gC m^-2 year^-1 with correction, and 251, 376 and 342 gC m^-2 year^-1 without correction, respectively. The nighttime correction is responsible for an uncertainty of approximately 50-70 gC m^-2 year^-1 in the annual NEP at this site.

The NEP has been calculated for nine years, which is the longest record at any particular site in Asia, as a consequence of the flux measurements. The annual NEP was estimated to be 256±68 gC m^-2 year^-1 (mean±SD) from 1994 to 2002 with a large year-to-year variability of up to 175 gC m^-2 year^-1. A high annual NEP was observed in 1998 (329 gC m^-2 year^-1) and in 2002 (346 gC m^-2 year^-1), mainly due to the high CO₂ uptake observed during the first half of the growth period in both years. The results of the present study suggest that the high gross primary production in 1998 and 2002 was caused by early leaf emergence related to an unusually warm spring under the influence of the El Niño phenomenon. The results also suggest that the increase in CO₂ uptake due to the long growth periods in 1998 and 2002 was more marked than the enhancement of respiration.