Net ecosystem CO2 exchange over a larch forest in Hokkaido, Japan

Larch forests are distributed extensively in the east Eurasian continent and are expected to play a significant role in the terrestrial ecosystem carbon cycling process. In view of the fact that studies on carbon exchange for this important biome have been very limited, we have initiated a long-term flux observation in a larch forest ecosystem in Hokkaido in northern Japan since 2000. The net ecosystem CO2 exchange (NEE) showed large seasonal and diurnal variation. Generally, the larch forest ecosystem released CO2 in nighttime and assimilated CO2 in daytime during the growing season from May to October. The ecosystem started to become a net carbon sink in May, reaching a maximum carbon uptake as high as 186 g C m−2 month−1 in June. With the yellowing, senescing and leaf fall, the ecosystem turned into a carbon source in November. During the non-growing season, the larch forest ecosystem became a net source of CO2, releasing an average of 16.7 g C m−2 month−1. Overall, the ecosystem sequestered 141–240 g C m−2 yr−1 in 2001. The NEE was significantly influenced by environmental factors. Respiration of the ecosystem, for example, was exponentially dependent on air temperature, while photosynthesis was related to the incident PAR in a manner consistent with the Michaelis–Menten model. Although the vapor pressure deficit (VPD) was scarcely higher than 15 hPa, the CO2 uptake rate was also depressed when VPD surpassed 10 hPa.