## Materials and Methods
### Instrumentations and Datasets  
Canopy-level SIF is measured by using a spectrometer (HR2000+, OceanOptics, Inc., Dunedin, FL, USA) mounted 5 m above the forest top. This spectrometer has a spectral resolution of 0.13 nm over the range from 679.6-775.6 nm and it had a field of view (FOV) of 5 m on top of the canopy. The spectrometer is linked with two fiber optics with a shutter (FOS-2x2-TTL, OceanOptics, Inc.). One fiber collects downwelling incident solar irradiance (_I_) and the other collects canopy upwelling radiance (_L_). \(I\) and \(L\) were measured every 5 min during the growing season (May-September). Note that upwelling radiance contains contributions from both reflected solar energy and SIF emission. The previous studies [26; 34] showed that these two components can be described by some simple mathematical functions at absorption lines. In this study, we used the three absorption lines including O\({}_{2}\)-B at 687 nm, water vapor at 720 nm, and O\({}_{2}\)-A at 761 nm. Reflectance and SIF emission at these three lines are expressed by the second order Taylor polynomials. Theoretically, it requires at least six measurements of \(L\) and \(I\) to retrieve a single instantaneous SIF emission. Please see Supplementary Materials for the details. Instantaneous SIF measurements in the selected bands are then aggregated to hourly values. SIF emission at the three absorption lines are referred to as SIF\({}_{687}\), SIF\({}_{720}\), and SIF\({}_{761}\), respectively.  
Turbulent fluxes of latent heat (\(\lambda\)_E_) were measured using the eddy covariance (EC) method [35] at the Environmental Measurement Station (EMS) site, which is close (about 1.4 km) to the SIF measurements site. The original \(\lambda\)_E_ was converted into hourly ET (kg H\({}_{2}\)O/m\({}^{2}\)/hour). Hourly GPP (g C/m\({}^{2}\)/hour) was obtained from the EMS tower in 2014 via the Harvard Forest Long-Term Ecological Research program. LAI was collected by using a LAI-2000 Plant Canopy Analyzer (LI-COR, Lincoln, NE, USA). We measured LAI daily during spring season (May and June) and autumn senescence (September and October) periods in 2014, and monthly in the mature period (July-August). Other meteorological variables, including VPD (kPa), rain fall (_Prcp_, mm/hour), friction velocity (_UST_, m/s), net radiation (_Ru_, W/m\({}^{2}\)), air temperature (_Tair_, Celsius degree \({}^{\circ}\)C), wind speed (_U_, m/s), and air pressure (_P_, kPa), were also measured at an hourly time step. Because forest at the site has a dense canopy, we assume ET is dominated by transpiration during the growing season [36]. To reduce the influence of evaporation from the intercepted rainfall, we excluded data when precipitation exceeded 1 mm/hour and 72 h after these events. We also removed measurements with sun zenith angle larger than 60 degree [37]. From hourly ET, GPP, and VPD data, we calculated ecosystem-level hourly WUE, IWUE, and uWUE. To estimate hourly WUE\({}_{i}\), Gs was calculated by inverting the P-M equation. Daily daytime SIF emission, WUE, WUE\({}_{i}\), IWUE, uWUE, and other related meteorological variables were aggregated into daily values by averaging half-hourly or hourly values measured between 7:00 a.m.  
and 5:00 p.m. local time. The daily values were excluded from the analysis if more than 4 hourly values were missed in one day. We used the moving average approach to remove outliers in the related time series. Specifically, values that deviated from the mean of a 5-point moving window by more than 25% were excluded and missing values were replaced by an inverse distance weighting method using adjacent \"clean\" points in time series.