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

Abstract

The CO 2 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, the CO 2 flux is estimated using the vertical gradient of the CO 2 concentration and the diffusion coefficient over the canopy. The eddy fluxes of sensible heat, water vapor, and CO 2 are measured using a sonic anemometer and a closed-path infrared gas analyzer. The CO 2 flux estimated by the eddy covariance method was mainly used to analyze observational errors and uncertainties caused by measurement and data processing. The CO 2 flux determined by the aerodynamic method was validated by comparison with that estimated by the eddy covariance method. The nighttime net ecosystem CO 2 exchange (NEE) was highly sensitive to atmospheric stability because of the complex terrain around the site. The nighttime NEE was estimated with and without correction, depending on the friction velocity. The annual net ecosystem production (NEP) values for 1999, 2000, and 2001 were estimated to be 198, 309, and 290 g C m −2 year −1 with correction and 251, 376, and 342 g C m −2 year −1 without correction, respectively. The nighttime correction is responsible for an uncertainty of approximately 50–70 g C m −2 year −1 in the annual NEP at this site. The NEP has been calculated for 9 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 237 ± 92 g C m −2 year −1 (mean ± S.D.) from 1994 to 2002 with a large year-to-year variability. The annual NEP had a similar year-to-year variation to that for gross primary production (GPP), and the carbon budget components representing productivity had more remarkable year-to-year variability than the respiration (ecosystem respiration and soil respiration). A high annual NEP was observed in 1998 (329 g C m −2 year −1) and in 2002 (346 g C m −2 year −1), mainly due to the high CO 2 uptake observed during the first half of the growth period in both years. The results of the present study suggest that the high GPP in 1998 and 2002 was caused by early leaf emergence related to an unusually warm spring under the influence of the El Niño. The results also suggest that the increase in CO 2 uptake due to the long growth periods in 1998 and 2002 was more marked than the enhancement of respiration in the cool-temperate forest.