Abstract: This research employed off-axis integrated cavity output spectroscopy technique to observe CO₂ concentration and δ¹³C values of planted Platycladus orientalis in Beijing mountainous area. We compared the variation between CO₂ concentration and δ¹³C values in the plantation at different heights observed by every 0.5 h to explore how the CO₂ concentration and δ¹³C values responded to the meteorological factors. The results showed that the CO₂ concentration showed the pattern of “first decreased and then increased” after sunrise. The lowest value (352.5 μmol·mol^-1) appeared at 16:00-16:30, while the maximum value (402.0 μmol·mol^-1) was observed at about 5:00. However, the change of δ¹³C value was not significant and regular, which increased firstly and then decreased in the surface layer while opposite for the canopy. Atmospheric CO₂ concentration decreased with the increasing height. In the height of 0, 2, 5, 8, 12.5 and 18 m, the average daily value was 386.5, 369.9, 368.2, 367.8, 367.9 and 367.9 μmol·mol^-1, respectively. In contrast, the δ¹³C values tended to rise correspondingly with height with the average daily value being -16.0‰, -13.7‰, -13.5‰, -13.5‰, -13.1‰ and -13.3% at 0, 2, 5, 8, 12.5 and 18 m, respectively. The stepwise regression analysis showed that temperature and humidity were the main factors for the changes of atmospheric CO₂ concentration and δ¹³C values. The saturated vapor pressure difference (VPD) affected the concentration of CO₂ in the forest and wind speed could affect it on the canopy. However, soil moisture, soil electric conductivity and net solar radiation affected the CO₂ concentration and δ¹³C values in surface layer. All these environmental factors influenced CO₂ concentration and δ¹³C values through their influences on the photosynthesis and respiration.