The eddy-covariance system monitored the carbon flux of Populus forest ecosystem in the middle and lower reaches of Yangtze River in Yueyang,Hunan Province.Daily and monthly variation of carbon flux between forest ecosystem and the relationship of carbon flux and environmental factors were analyzed.The results showed that the regulated variation and process of day and month of carbon flux of forest ecosystem.In daytime,the ecosystem began to absorb CO2 at about 7:30 am,and reached to the max.at about 12:00,then went down slowly,at about 18:00,the ecosystem began to release CO2.At the period of the end of March and the early of April,the Populus began to unfold leaves,and the forest ecosystem started to absorb CO2,namely carbon sink.At the period of the end of October to the early of November,the photosynthesis of leaves stopped by defoliation,at the same time,the forest ecosystem became carbon source.The paper also analyzed the relationships of carbon flux of forest ecosystem and environmental factors.The results were as follows:when the PAR>1 μmol·m-2s-1,the relationship of carbon flux and PAR was different in different range of temperature.When the PAR>1 μmol·m-2s-1,air temperature and carbon flux had quadratic equation.Namely when the air temperature arose,the carbon flux of nighttime was augmented.The nighttime CO2 flux and soil temperature in 5 cm deep had exponential correlation.choosing a day from different month and ridding off the carbon flux for u<0.2 m·s-1,the 30 min average carbon flux in nighttime and soil heat flux in 2 cm deep that had the relationship of exponential correlation.
