## 2 Materials and Methods
### Measurement of Whole Crown CO\({}_{2}\) Exchange  
The fully automated observation system used in this study offers a means of continuous measurements at short intervals (18 times per day at 1.5 h interval), allowing not only day and night-time high-frequency measurements, but also measurements in special weather conditions (e.g., hot and raining days). The system consists of eight whole-plant chambers, a high precision Picarro G2508 analyzer (Picarro, Inc., Santa Clara, CA, USA) based on cavity ring-down spectroscopy (CRDS) [33] (Picarro Inc., 2013), a multiplex system (PRI-8160, Beijing Pri-eco Technology Co., Ltd., Beijing, China), an air pump A2000 (Picarro Inc., Santa Clara, CA, USA), and a computer for data acquisition. The multiplex system controlled the closure of different chambers and the switching of air channels, as well as temperature and humidity sensors. Driven by the air pump, the air in the closed chamber passed through the multiplex system air inlet port, analyzer, pump, and the multiplex system air outlet port and then returned to the chamber, forming a closed-circuit system. A schematic overview is shown in Figure 1a. The Picarro G2508 Greenhouse Gas Analyzer was used to measure concentration changes of CO\({}_{2}\) and H\({}_{2}\)O in the chamber. Each chamber consisted of a cuboidal aluminum frame and transparent PVC board (2 mm), with light transmission in the visible range of 90%. The chamber was a square cube with a base area of 90 \(\times\) 70 cm, which was divided into two sections by transparent acrylic plate; the top section was 100 cm high, the bottom section was 60 cm. Likewise, the tree crown (in top section) was separated from the roots and soil (in the bottom section) by transparent acrylic plate, through which the tree trunk passes, allowing gas exchange measurements of the whole crown while excluding CO\({}_{2}\) released from the root respiration. Electric fans were running continuously to promote air mixing within the chamber. Each chamber had four windows (two for both the top and bottom sections) that were open except when monitoring _Ne_ to ensure air circulation and reduce the heating effect in the chamber under the light. A schematic overview is shown in Figure 1b. All raw data were recorded every 1-2 s and displayed in real time on a monitor, collected through a custom LabVIEW application, and automatically saved in user-determined forms. Data were continuous except for short periods when power was lost (about 170 h). In this paper, only the observation results of the crown (top-section of the chamber) were analyzed. Each chamber measured _Ne_ for 6 min each hour, and 15 data points were collected each day. The chambers were cleaned at least once a week, and the Picarro high-precision analyzer was calibrated to 380 ppm CO\({}_{2}\) once a month. In order to avoid the influence of gas path pollution by the air from the previous sample remaining in the flow path, invalid data were eliminated in the first 80 s and end 10 s of each 6 min sampling. During measurement, the CO\({}_{2}\) concentration in the top-section chamber was allowed to draw down in the daytime due to photosynthesis or build up in the night-time due to respiration over time, from which net crown carbon exchange rate was determined from the time series of the concentration as follows:  
\[F=\alpha\times\frac{dc}{dt}\times\frac{V}{A}\times\frac{P}{P_{0}}\times\frac{T_ {0}}{T_{air}+T_{0}} \tag{1}\]  
where \(F\) is the value of instantaneous flux and the unit is g Cm\({}^{-2}\) s\({}^{-1}\), \(P_{0}\) and \(T_{0}\) are the pressure (101.325 kPa) and absolute air temperature (273.15 K) under standard condition respectively, \(P\) and \(T_{air}\) are the pressure and absolute temperature values at the time of  
Figure 1: Schematic diagram of the observation process of CO\({}_{2}\)/H\({}_{2}\)O exchange in sapling crown by chamber method. (**a**) Schematic overview of the automated multichannel gas-exchange measuring chamber system.