Seasonal variations of atmospheric CO 2 , δ 13 C, and δ 18 O at a cool temperate deciduous forest in Japan: Influence of Asian monsoon

Atmospheric CO 2 concentration and its isotopes have been measured at the Takayama site in central Japan since 1994, together with δ 18 O in precipitation since 2002, to examine their temporal variations in a cool‐temperate deciduous forest environment strongly influenced by the Asian monsoon. The CO 2 concentration and δ 13 C 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 CO 2 exchange with C 3 plants. However, the variation in summer is found to be related to CO 2 exchange with a significantly heavier δ 13 C signal compared to the other seasons, indicating some influence at the Takayama site of atmospheric transport of CO 2 impacted by exchange with C 4 plants from upstream regions. Compared with CO 2 and δ 13 C, δ 18 O measurements in atmospheric CO 2 are scattered, showing no clear seasonal variation. From late spring to early fall, significant year‐to‐year differences in the variation of δ 18 O 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 δ 18 O 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 δ 18 O observed from winter to spring is likely caused by long‐range transport of air with enriched δ 18 O.