[Variations and determinants of CO2 concentration and δ13C in Platycladus orientalis plantation in Beijing mountainous area, China].

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.