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 seasonaland annual net ecosystem production (NEP), gross primary production (GPP), and ecosystem respiration(R E) based on 7-year eddy covariancemeasurements 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 seasonalintegrated NEP was regulated by accumulated air temperature ( degree-day) with a significant negative correlation, whereas the seasonalratio of R E to GPP was correlated positively with accumulated air temperature. Because the deviation of seasonalNEP was also significantly correlated with seasonalR E/GPP, NEP was controlled by R E/GPP, depending on air temperature during the growing season. SeasonalR E in the defoliationand snow seasonswas also important for evaluating the annual carbon balance, because the total number of days in the two seasonswas quite large owing to a long snowy winter. In the defoliationand snow seasons, we found defoliation seasonlength was a major factor determining seasonalintegrated R E, illustrating the positive correlation between R E and defoliation seasonlength. The major factors controlling interannual variations in forest carbon balance are discussed.