Interannual variability of net ecosystem production for a broadleaf deciduous forest in Sapporo, northern Japan

To estimate net ecosystem production (NEP), ecosystem respiration (RE), and gross primary production (GPP), and to elucidate the interannual variability of NEP in a cool temperate broadleaf deciduous forest in Sapporo, northern Japan, we measured net ecosystem exchange (NEE) using an eddy covariance technique with a closed-path infrared gas analyzer from 2000 to 2003. NEP, RE, and GPP were derived from NEE, and data gaps were filled using empirical regression models with meteorological variables such as photosynthetic active radiation and soil temperature. In general, NEP was positive (CO2 uptake) from May to September, either positive or negative in October, and negative (CO2 release) from November to the following April. NEP rapidly increased during leaf expansion in May and reached its maximum in June or July. The four-year averages (± standard deviation) of annual NEP, GPP, and RE were 443 ± 45, 1,374 ± 39, and 931 ± 11…−2 year−1, respectively. The lower annual NEP and GPP in 2000 may have been caused by lower solar radiation in the foliated season. During the foliated season, monthly GPP varied from year to year more than monthly RE. Variations in the amount of incoming solar radiation may have caused the interannual variations in the monthly GPP. Additionally, in May, the timing of leaf expansion had a large impact on GPP. Variations in GPP affected the interannual variation in NEP at our site. Thus, interannual variation in NEP was affected by the incoming solar radiation and the timing of leaf expansion.