## Materials and Methods
### Tower-Based Measurements
#### 2.1.2 Solar-Induced Fluorescence Measurements  
An automatically long-term SIF observation system (AutoSIF) was designed and deployed on the platforms of the towers at the two sites for long time series SIF measurements (Figure 2). This system was included in the ChinaSpec network [35], which links in situ spectral measurements with point EC observations for a better understanding of the atmosphere-biosphere interaction. The AutoSIF operating height and time window are listed in Table 1. This system was designed similarly to state-of-the-art systems around the world (e.g., the FluoSpec [21] and Piccolo systems [36]), which are all based on a dual field-of-view (FOV) and hemispherical-conical configuration (for a review, see Reference [37]). The main components integrated in AutoSIF include a customized Ocean Optics QE65 PRO spectrometer (Ocean Optics, Dunedin, FL, USA), with a spectral resolution of ~0.3 nm and a signal-to-noise ratio (SNR) higher than 1000 in the range of 645-805 nm, a bifurcated optical fiber (CPATCH, Ocean Optics, Inc., Dunedin, FL, USA), two optical shutters (FOS-1X1-TTL, Ocean Optics, Inc., Dunedin, FL, USA), an automatic refrigeration system, and a controller PC for data collection and storage. The spectrometer was connected to a bifurcated optical fiber with two optical shutters to split the optical signal into two channels: One channel pointing at the zenith, with a cosine corrector (CC3-3-UV-S, Ocean Optics, Inc., Dunedin, FL, USA) to capture downwelling incident radiance from a 180\({}^{\circ}\) FOV, and the downlooking channel, which points at nadir or off-nadir with a conical fore-optic (bare fiber) to capture upwelling radiance from a smaller FOV (such as 25\({}^{\circ}\)). The two optical shutters separately switched between open and closed to measure the upwelling, downwelling and the systems' dark current quasi-synchronously. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Site**} & \multirow{2}{*}{**Position**} & \multirow{2}{*}{**V Vegetation Type**} & \multicolumn{2}{c}{**Measured Height (m)**} & \multicolumn{3}{c}{**Time Window**} \\ \cline{3-8}  & & & **SIF** & **Flux** & **Meteorology** & **SIF** & **Flux** & **Meteorology** \\ \hline XTS & 116.44\% E 40.18\% N & Wheat, C3 & 3 & 3 & 3 & 2017: 4/30-5/28 & & 2017 \\  & & & & & & 2018: 4/10-5/21 & / & 2018 \\ \hline DM & 100.37\% E 38.86\% N & Maize, C4 & 20 & 5 & 5 & 2017: 6/9-9/20 & 2017 & 2017 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Site descriptions and details of the measured height and time window. Figure 2: AutoSIF observation systems in use at the XTS (**a**) and Daman (DM) (**b**) sites, in which the downwelling measurements were hemispherical, and the upwelling measurements were conical. Downwelling cosine corrected fore-optics (1), upwelling conical fore-optics (2) and the instrument housing box (3) are labeled in their respective positions. The measuring mode was a'sandwich' type: First, collect the solar irradiance by opening the shutter of the downwelling channel; second, collect the canopy reflected radiance by opening the shutter of the upwelling channel; finally, measure the solar irradiance again. With immediate channel switching, the system presents a relatively low time delay between up- and downwelling measurements (several seconds). We calculated the average of the two solar irradiance measurements to decrease the effects of weather changes caused by the time delay. Before each of the three single measurements, the integrated time (0.7-6 s) was optimized depending on the light intensity, and the corresponding dark current was measured and finally corrected from the total signal. Generally, the system completed a sandwich measurement cycle from 15 s (midday) to 2 min (sunrise or sunset). The measurement was carried out at a time interval of 2 min.