[Variations and determinants of CO 2 concentration and δ 13 C in Platycladus orientalis plantation in Beijing mountainous area, China].

This research employed off-axis integrated cavity output spectroscopy technique to observe CO ₂ concentration and δ ¹³ C values of planted Platycladus orientalis in Beijing mountainous area. We compared the variation between CO ₂ concentration and δ ¹³ C values in the plantation at different heights observed by every 0.5 h to explore how the CO ₂ concentration and δ ¹³ C values responded to the meteorological factors. The results showed that the CO ₂ 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 δ ¹³ C value was not significant and regular, which increased firstly and then decreased in the surface layer while opposite for the canopy. Atmospheric CO ₂ 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 δ ¹³ C 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 CO ₂ concentration and δ ¹³ C values. The saturated vapor pressure difference (VPD) affected the concentration of CO ₂ in the forest and wind speed could affect it on the canopy. However, soil moisture, soil electric conductivity and net solar radiation affected the CO ₂ concentration and δ ¹³ C values in surface layer. All these environmental factors influenced CO ₂ concentration and δ ¹³ C values through their influences on the photosynthesis and respiration.