We quantified the sensitivity of estimated carbon budgets in Japanese evergreen coniferous forest (ECF) and deciduous broad-leaved forest (DBF) using NCAR/LSM simulations. To improve the model's performance for both forests, we modified parameters such as biomass and plant area index (PAI) based on measured values and calibrated the model using field-measured tower flux and biometric data at two AsiaFlux sites near Takayama City, Japan. The seasonal patterns and annual cumulative values of gross primary production (GPP), ecosystem respiration (RE), and net ecosystem production (NEP) predicted by the model agreed well with field measurements at the two sites. Our sensitivity analysis of the impact of growing period length on the carbon budget in the DBF showed that GPP and NEP increased by 12.7% and 48.0%, respectively, when we considered the temperature dependency of the growing period length. The different seasonal patterns of NEP between the two forest types were determined primarily by differences in GPP that resulted from differences in PAI from April to June. The annual values of GPP and RE were clearly greater in the ECF than in the DBF. Our simulation results suggest that the ECF has higher metabolic activity than the DBF in this region due to its larger biomass.