## Materials and Methods
### SiF and Spectral Indexes  
A number of methods are reported for the extraction of SiF using the FLD method and variants on this approach [57] The approach taken in this paper utilised two band FLD approaches due to the relatively coarse resolution of the spectral data (1 nm resolution), obtaining the exact position of the two shoulders of the absorption feature accurately was not possible, and thus the more popular 3 FLD approach was not used. SiF values were extracted from the spectral data using [16] applied to the O\({}_{2}\)-A absorption region of the electromagnetic spectrum (\(\sim\)755-765 nm). [PERSON] [57] reported that all FLD approaches were similarly affected when coarser resolution data was utilised, and the two wavelength approaches tended to overestimate SiF. Accordingly, because the absolute SiF values reported in the present study may not be free from overestimation, we focus the analysis on the patterns of variation rather than absolute levels.  
Wavelength matching between pairs of temporally concomitant spectra, a requirement of the FLD method, was achieved by interpolating down-welling irradiance spectra to the same wavelengths as those of reflected radiance spectra using locally-weighted polynomial regression. Then, for each pair of measurements, wavelengths of local minima between 758 nm and 762 nm were identified in both irradiance and radiance spectra and were used as absorption wavelengths for retrieving energy fluxes. FLD shoulder wavelengths were identified by visual inspection of irradiance and radiance spectra; a constant value at 757.5 nm was used for both spectra and all retrievals.  
To account for energy re-absorption by molecular oxygen along the photon path between the sensor and the canopy, we applied the correction proposed in Reference [58]. Atmospheric transmittance values in the O\({}_{2}\)-A region were calculated for each SiF observation using the HITRAN molecular spectroscopic database [59; 60] along with air temperature and atmospheric pressure values measured at the flux tower. All data presented in this paper have therefore been corrected for atmospheric effects.  
Last but not least, because the amount of energy re-emitted by plants as fluorescence is directly proportional to the amount of incoming radiation, the functional interpretation of the SiF signal (eitherin terms of Fluorescence Yield or in terms of photosynthetic energy absorption or APAR) is not straightforward. Signal normalization by incoming PAR or indeed APAR is commonly a challenge because downwelling PAR radiation registered over the canopy does not necessarily reflect the actual PAR experienced by the foliage within the upwelling sensor FOV, due for example to diurnal changes in sun/shade fractions. In an attempt to control for variation in PAR directly within the sensors IFOV, we calculated a relative SiF yield (\(S\)\(iF\)\(L_{650}^{-1}\)), which is adimensional, by normalizing SiF observations by the reflected radiance measured in the red region of the spectrum (650 nm). In other words, diurnal and seasonal variations in \(L_{650}\) were used as proxy of average incoming PAR within the scene. The usage of reflected radiance (\(L_{\lambda}\)) as a proxy of variations in incoming PAR within the IFOV is based on two assumptions (See Equation (1) below): (i) Constancy in canopy reflectance in wavelength \(\lambda\) (\(\rho_{\lambda}\)), and (ii) proportionality between irradiance in the PAR region (\(E_{PAR}\)) compared to that at wavelength \(\lambda\) (\(E_{\lambda}\)).  
\[L_{\lambda}=E_{\lambda}\;\rho_{\lambda}\text{= }\;k\;E_{PAR}\;\rho_{\lambda} \tag{1}\]  
We preliminarily selected 650 nm band, but other wavelengths within PAR spectral domain could be equally useful. Given the common use however of a normalisation of SiF using APAR, we further computed SiF/APAR for purposes of comparison with the above normalisation scheme used in this study. APAR is described and calculated according to Equation (6).