Carbon balance in a cool–temperate deciduous forest in northern Japan: seasonal and interannual variations, and environmental controls of its annual balance

We monitored variation in seasonal and annual net ecosystem production (NEP), gross primary production (GPP), and ecosystem respiration (R E) based on 7-year eddy covariance measurements above a cool–temperate deciduous broad-leaved forest (Japanese beech forest). The 7-year means (±SD) of annual NEP, GPP, and R E were 312 ± 64, 1250 ± 62, and 938 ± 36 g C m−2 year−1, respectively. Variation in NEP was much larger than variation in GPP and R E. During the growing season, the main factor controlling carbon balance was air temperature; variation in seasonal integrated NEP was regulated by accumulated air temperature (degree-day) with a significant negative correlation, whereas the seasonal ratio of R E to GPP was correlated positively with accumulated air temperature. Because the deviation of seasonal NEP was also significantly correlated with seasonal R E/GPP, NEP was controlled by R E/GPP, depending on air temperature during the growing season. Seasonal R E in the defoliation and snow seasons was also important for evaluating the annual carbon balance, because the total number of days in the two seasons was quite large owing to a long snowy winter. In the defoliation and snow seasons, we found defoliation season length was a major factor determining seasonal integrated R E, illustrating the positive correlation between R E and defoliation season length. The major factors controlling interannual variations in forest carbon balance are discussed.