图1. 野外环境下簇状叶室与激光同位素仪相连测量蒸腾水汽同位素信号(δ_T)(引自Wang et al., 2012, 有改动)。实际测量中, 空气(水汽浓度为q_A, 同位素信号为δ_A)以一定流速从叶室的进气口(5)进入叶室, 其与从叶室内叶片蒸腾出的水汽(T, δ_T)混合后(q_M, δ_M)以同样的流速从叶室出气口(6)排出。进入叶室的空气以及排出叶室的混合气在经由电磁阀控制的多路控制系统(8)后按设定的测量周期(如4 min一个测量周期)交替通往水汽激光同位素仪(9)进行测量。在q_A、δ_A、q_M、δ_M以及气流流速都被测定的情况下, 通过进出叶室的水汽质量守恒可以计算出蒸腾通量, 再通过同位素质量守恒方程即可计算出δ_T。详细推导过程及注意事项参考Wang et al. (2012)。

Fig. 1. Measurement of transpiration vapor isotopic signal (δ_T) by connecting conifer chamber with isotope ratio laser spectrometer in the field (cited from Wang et al., 2012 with change). In the measurement, the airs (vapor concentration q_A and isotopic signal δ_A) enter the clustered chamber at a velocity from the inlet (5), mixing with the transpiration vapor from leaves (T, δ_T) in chamber, then (q_M, δ_M) eject from the outlet (6) at the same flow rate. Passing by a solenoid valve controlled multi-channel system (8), the mixed airs are alternately connected to the isotope ratio laser spectrometer (9) in a set measurement period (e.g., a measurement period of 4 min). As q_A, δ_A, q_M, δ_A and flow rate are measured, the transpired vapor flux can be calculated by mass balance between vapor entering and outgoing the chamber, then δ_T of transpiration vapor can be calculated by isotopic mass balance equation. More details and derivational processes reference to Wang et al. (2012).