Seasonal variations of atmospheric CO2, δ13C, and δ18O at a cool temperate deciduous forest in Japan: Influence of Asian monsoon

Atmospheric CO2 concentration and its isotopes have been measured at the Takayama site in central Japan since 1994, together with delta O-18 in precipitation since 2002, to examine their temporal variations in a cool-temperate deciduous forest environment strongly influenced by the Asian monsoon. The CO2 concentration and delta C-13 at the observational site show prominent seasonal variations accompanied by a secular trend, suggesting that the average seasonal cycle is caused mainly by a seasonal-dependent CO2 exchange with C-3 plants. However, the variation in summer is found to be related to CO2 exchange with a significantly heavier delta C-13 signal compared to the other seasons, indicating some influence at the Takayama site of atmospheric transport of CO2 impacted by exchange with C-4 plants from upstream regions. Compared with CO2 and delta C-13, delta O-18 measurements in atmospheric CO2 are scattered, showing no clear seasonal variation. From late spring to early fall, significant year-to-year differences in the variation of delta O-18 are observed, with low and high values associated with rainy and sunny summer conditions, respectively. Comparisons with data from other northern midlatitude sites suggest that the rapid decrease of delta O-18 seen at Takayama in the early summer of a rainy summer year is characteristic of the Asian monsoon region. On the other hand, the characteristic increase in delta O-18 observed from winter to spring is likely caused by long-range transport of air with enriched delta O-18.