Forest plays an important role in carbon cycle and global climate change. To estimate accurately net ecosystem carbon exchange (NEE) between forest ecosystem and the atmosphere and understand the ecophysical factors influencing NEE, it is necessary to study the variation characteristics of CO₂ storage flux. In this study, the eddy covariance method and the profile method were used to measure CO₂ flux and CO₂ concentration over a warm-temperate mixed plantation in the Xiaolangdi area in 2008, respectively. The temporal and spatial variations of CO₂ concentration under sunny and cloudy sky conditions above the forest canopy were analyzed. CO₂ storage flux obtained by the eddy covariance was compared with those derived from the profile method and the diurnal and seasonal variations of CO₂ storage flux were investigated. The results showed that the diurnal and seasonal variations in CO₂ concentration were obvious. In the sunny days of the growing season, CO₂ concentrations above the plantation canopy decreased continuously and the minimal CO₂ concentration appeared at 12:00. After 12:30, CO₂ concentration increased slowly. However, in the cloudy days, the maximal and minimal CO₂ concentrations occurred in the early morning and at about 14:30, respectively. At the annual scale, CO₂ concentration peaked in March, with a value of 370 μmol/mol. The minimal CO₂ concentration occurred in August, with a value of 347 μmol/mol. CO₂ storage flux estimated by the eddy covariance method was 9% lower than that obtained by the profile method. During the growing season of 2008, monthly mean CO₂ storage flux was -0.0004 mg CO₂ m^-2 s^-1 and NEE was -0.091 mg CO₂ m^-2 s^-1. At the daily scale, the proportion of CO₂ storage flux to NEE was only 0.4%. In 2008, CO₂ storage flux and NEE were -46.1 and -1133 g CO₂ m^-2 a^-1, respectively. On an annual scale, CO₂ storage flux only accounted for 4.1% of NEE. Therefore, at the longer time (daily or annual) scale, CO₂ storage flux could be neglected when NEE is estimated in the mixed plantation of the Xiaolangdi area.