Ecosystem-scale carbon isotope ratio of respired CO₂ in cool-temperate deciduous forests under Asian monsoon climate

The carbon isotopic ratio for ecosystem respiration (δ¹³C_R) is an important parameter in isotopic mass balance models for the global carbon budget. Recent studies in North American and European temperate forests showed that δ¹³C_R was controlled by stomatal regulation of gas exchange and associated changes in photosynthetic carbon isotope discrimination. In this model, δ¹³C_R depends on the vapor pressure deficit (vpd) and precipitation (i.e., plant water availability). We investigated the monthly and annual variation in δ¹³C_R and its controlling factors in two cool-temperate deciduous forests in the monsoon climate area of Japan. The Keeling plot approach was used to evaluate variation in δ¹³C_R. Overall, δ¹³C_R varied from -24.1‰ to -29.7‰ and was most negative in late summer and autumn. The amplitude of the variation was 2.0‰ to 2.2‰ within a forest. In 1996, which had unusually little precipitation (63% of normal precipitation), δ¹³C_R was correlated with vpd measured 7 d before sampling and monthly precipitation, consistent with predictions based on the stomatal regulation model. In contrast, δ¹³C_R in 1995 was insensitive to vpd and precipitation, but was significantly correlated with air temperature, suggesting temperature control associated with variabilities in the decomposition of labile and recalcitrant pools of soil carbon and in the availability of recent photosynthate-derived carbon for respiration. Variation in heterotrophic respiration rather than photosynthetic discrimination appeared to be the dominant factor governing δ¹³C_R in our forests at near-normal levels of precipitation. The results of our study can be used to better constrain future model estimates using δ¹³C_R and ecosystem-level discrimination in a temperate forest in the Asian monsoon region.