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

摘要

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.