*2.3. Canopy CO2 Concentration and Carbon Isotope Composition Profile*

In this study, the canopy CO2 and δ13Cair were measured using a wavelength-scanned cavity ring down spectroscopy (WS-CRDS) analyzer (G2201-i, Picarro Inc., Santa Clara, CA, USA). This instrument performed well in measuring the δ13C of CO2 with a 5 min precision of 0.11‰ and a maximum drift of 0.25‰ day−1. Data were collected at about 1 Hz and expressed on the VPDB (Vienna Pee Dee Belemnite) scale.

A profile control system (PRI-8500, PRI-ECO Inc., Beijing, China) was used as a peripheral of the WS-CRDS analyzer to automatically change ports in the δ13C profile measurements (Figure 1a). Air inlets were installed at the seven heights on the tower. Different levels of air were simultaneously pumped from the inlets by a vacuum pump (R410, Rocker Scientific Co., Ltd., Taiwan, China) through 1/4-inch Teflon tubes followed by flow control valves (ASC200-08, AirTAC, Taiwan, China). Each of the inlets was selected and then scanned for 5 min one after another from lower to higher levels through a 16-port rotation valve (Valco Instruments Co. Inc., Houston, TX, USA). The air was pumped into the WS-CRDS analyzer at a flow rate of 30 mL/min using a Picarro external pump. The first minute of each measurement was discarded to remove the data of the response time after the port was switched.

**Figure 1.** Schematic of the atmospheric profile and respiration chamber system (**a**) and photographs (**b**) and diagrammatic representations (**c**) of the custom-made chambers. L1 and L2, leaf chambers; T1 and T2, trunk chambers; S1, soil chamber; Std1 and Std2, standard gas tanks; FC, flow controller; AF, air filter; P, pump.

Two standard gases (Std1: 370.7 ppm for CO2 and <sup>−</sup>20.8709‰ for <sup>δ</sup>13C; Std2: 799.0 ppm for CO2 and <sup>−</sup>21.5685‰ for <sup>δ</sup>13C) were scanned for 15 min once a day to conduct two-point gain and offset calibrations for CO2 measurements and single-point delta value offset calibrations for δ13C measurements [30]. The dependence of δ13C on CO2 for G2201-i was 0.09‰ per 100 ppm in the range of 368.1–550.1 ppm, as tested by Pang et al. [31]. Thus, it can be presumed that the CO2 dependence of δ13C is linear within the small CO2 range (about 400–495 ppm for the air profile measurement and 400–600 ppm for the respiration measurements; see Section 2.4) in the present study. The first 5 min and last 1 min of each standard gas measurement were discarded to remove biased data introduced by port switching.
