Carbon fixation capacity of forest ecosystem would differ from year to year depending on climatic conditions. However, inter-annual variations of carbon cycle and its difference among the different forest types at same region still remain uncertain. This study aimed to clarify the inter-annual variations of carbon cycle in two types of cool-temperate forests. The carbon cycles were assessed for three years by biometric method in adjacent Quercus serrate forest (Q) and Pinus densiflora forest (P) at Karuizawa field.

Averaged net primary productions in Q and P were 5.3±0.43 and 5.3±1.21 MgC ha^-1 yr^-1 respectively, and the variability was higher in Q. On the other hand, averaged heterotrophic respirations in Q and P were 3.9±0.27 and 3.6±0.15 MgC ha^-1 yr^-1 respectively, and the variability was higher in P. Consequently, the net ecosystem productions were estimated 1.5±0.56 and 1.7±1.36 MgC ha^-1 yr^-1. These results suggest that the sensitivity of CO₂ fixation by tree and decomposition of soil organic carbon by microbe to same environmental changes differ among the forest types. Therefore, carbon cycle and balance of various forest ecosystems would differently respond to predicted climate changes.