Seasonal Variation of CO2 flux over a broadleaf deciduous forest.

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Long-term measurement of CO2 flux has been continued since April 1997 over a temperate deciduous forest. The flux is directly measured by the eddy correlation method using a closed-path CO2 analyzer. Seasonal variation of the CO2 flux was clearly correlated with that of the leaf area index. When the foliation began in April, the CO2 flux changed its direction from upward to downward and increased rapidly as leaves grew. The downward CO2 flux reached its maximum in June when the leaf area index became maximum. The flux was then decreased gradually as leaves senesced, followed by rapid reduction during the leaf-fall. The direction of flux finally turned upward at the middle of November. Day-by-day variations were also observed in the CO2 flux during the growing season, which were mainly caused by the variations of photosynthetic photon flux density. The net annual carbon uptake was estimated by integrating flux data and interpolating missing data using the relations between measured fluxes and environmental factors. The net carbon flux was -357gCm-2y-1 during the 12 months from June 1997 to May 1998.
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