Modeling of gross and net carbon dioxide exchange over a cool-temperate deciduous broad-leaved forest in Japan: Analysis of seasonal and interannual change

A process-based model was developed to simulate daily gross carbon dioxide (CO2) fluxes by photosynthesis and respiration, and net ecosystem CO2 exchange of a cool-temperate deciduous broad-leaved forest in Takayama, Japan, one of the AsiaFlux sites. The model was derived from a simple carbon cycle model (called Sim-CYCLE), with several modifications to capture seasonal and interannual variability in the CO2 fluxes, which are regulated at the physiological level. The model for the Takayama site is composed of 12 carbon pools of canopy trees (deciduous), floor plants (evergreen), and soil organic carbon (litter and mineral soil), while the net ecosystem exchange (FNEE) is obtained from the difference between gross primary production (FGPP) and autotrophic (FAR) and heterotrophic (FHR) respiration. FGPP is estimated by using the Monsi–Saeki model, including regulation of the photosynthetic capacity by temperature, CO2, and soil water content. FAR is composed of growth and maintenance components, each of which is regulated independently. FHR is a function of temperature and soil water content. Leaf phenology (display in spring and shedding in autumn) of the canopy deciduous trees is empirically determined by the cumulative temperature. The model simulation was conducted for the period from 1948 to 2002, and the results for 1991–2002 were analyzed. Daily FNEE simulated by the model were quantitatively consistent with observations by the eddy covariance method, implying model feasibility to capture the temporal variation in the carbon cycle: +1 g C m−2 day−1 (source) in late autumn to −4 g C m−2 day−1 (sink) in early summer. Because of a past disturbance and environmental change, the Takayama site was evaluated to be a net sink of atmospheric CO2 (1991–2002 average, −206 g C m−2 yr−1). Leaf phenology of the canopy trees brought about a drastic seasonal change in the light environment and CO2 budget of the floor plants. A sensitivity analysis indicated that the critical ecophysiological parameters for the net ecosystem exchange of Takayama site were the maximum photosynthetic rate, minimum temperature of photosynthesis, specific leaf maintenance respiration rate, and temperature dependence of maintenance respiration. Finally, several possible applications of the model are presented, and further model modifications are discussed.