Forested ecosystems cover a large expanse of the terrestrial area in the world, which represent a large global sink of CO_２, and strongly influence the dynamics of the global carbon cycle. In light of the current increase in atmospheric CO_２ and warming climate, it is crucial to understand the mechanisms that control some ecological progresses in the forest ecosystems. Studies of forest CO_２ uptake have increased rapidly, especially after tower flux networks were established. However, the perspectives on net CO_２ exchange at ecosystem-level are relatively new to plant ecophysiology.
In this study, based on eddy covariance system on tower, we continuously measured CO_２ flux in the mixed forest of broad-leaved and Korean-pine in Changbai Mountain in 2003.The relationship between CO_２ flux and temperature at ecosystem-level during the growing season was analyzed. The results were as follows:
(1) When air temperature was below 20℃ in the definite range of photosynthetic activity radiation (PAR), the higher air temperature, the higher net ecosystem CO２ exchange (NEE). In contrast, when it was over 20℃, the higher air temperature, the lower NEE. This effect was significantly different amongst seasons. It was mainly controlled by plant physiology and leaf area index.
(2) NEE includes photosynthesis and respiration of ecosystem. Maximum photosynthetic rate Pmax and ecosystem respiration Re were calculated by Michaelis-Menten equation. The relationship between Pmax and air temperature exhibited S shaped curve, while Re was exponential to air temperature.
(3) The effects of canopy PAR, air temperature and vapor pressure deficit (VPD) on NEE were estimated respectively by partial correlation analysis and of difference amongst seasons. Partial correlation coefficients of temperature on NEE were higher in early and late growing period than that in the middle of growing seasons. It suggested that temperature in early and late growing seasons had more significant effects on NEE than that in the middle of growing seasons. Thus, we could identify the dynamic role that temperature played in controlling ecosystem fluxes during early and late seasons.