## 2 Materials and Methods
### Reflectance Measurements and Vegetation Indices
#### 2.2.2 SRS-PRI Sensor Cross-Calibration  
A cross-calibration procedure is required to account for differences in sensor response and fore-optics [51]. The cross-calibration process allows for the normalization of irradiance and radiance outputs under varying light conditions. [PERSON] et al. [44] outlined a procedure for both midday and diurnal cross-calibration, noting that diurnal calibrations best represent xanthophyll cycle epoxidation states. Due to the remoteness of our study site and the manual nature of cross-calibration, both the midday and diurnal cross-calibration procedures were done prior and subsequently to field deployment, following the general principles outlined by [PERSON] et al. [44].  
SRS-PRI sensors were placed at the height of 30 cm over a 99% reflective white standard panel (Spectralon, Labsphere Inc., North Sutton, NH, USA), ensuring that the downward-looking sensor's FOV covered the standard. The upward-looking sensor simultaneously monitored the sky's irradiance conditions. The sensor pair measured continuously over a 23-day period. A pyranometer (SP-110, Apogee Instruments Inc., Logan, UT, USA) logged on a wireless sensor node (ENV-LINK-MINL, LORD Microstrain@Sensing Systems, Williston, VT, USA) was used to monitor incident radiation, sampled at 1-minute intervals. This radiation data was used to characterize the various illumination conditions over the experimental period, which varied from overcast to clear and sunny conditions. A solar radiation calculator (SolRad), developed by the Washington State Department of Ecology, provided modeled global radiation on a horizontal surface using the Ryan-Stolzenbach model [52]. Percent illumination was derived by comparing modeled to measured radiation data throughout the experiment. The 23-day experimental period allowed us to capture a nearly complete spectrum of illumination conditions (15-100%) over a full set of diurnal solar elevations (0\({}^{\circ}\)-56\({}^{\circ}\)). Solar elevations,defined as the angle between the horizon and the sun, were determined using NOAA's solar position calculator ([[https://www.esrl.noaa.gov/gmd/grad/solcalc/](https://www.esrl.noaa.gov/gmd/grad/solcalc/)]([https://www.esrl.noaa.gov/gmd/grad/solcalc/](https://www.esrl.noaa.gov/gmd/grad/solcalc/))).  
Cross-calibration responses as a function of illumination were calculated for every 3\({}^{\circ}\) solar elevations bin throughout the diurnal range. Three-degree bins were chosen as they represented approximately 30 min time intervals during the morning and evening periods where solar elevation changed most rapidly. A set of cross-calibration functions were derived for each sensor waveband. Percent illumination and solar elevations were calculated for onsite data collected during the 2015 season. Cross-calibration ratios derived from the set of empirical calibration functions were used to calculate corrected reflectance (\(\rho_{corrected}\)) as follows:  
\[\begin{array}{ll}\rho_{corrected}&=\frac{\rho_{\text{target}}}{\text{Cross- calibration}\ \text{ratio}^{\prime}}\\ &=\frac{\text{target}/\text{i}_{\text{sky}}}{\text{r}_{\text{standard}}/\text{ i}_{\text{sky}}}\end{array} \tag{5}\]  
where r\({}_{\text{target}}\) represents radiance from the canopy, r\({}_{\text{standard}}\) represents radiance from the white standard, and i\({}_{\text{sky}}\) represents irradiance from sky conditions. Midday cross-correlation functions were also derived for each sensor waveband and used to calculate midday corrected reflectance following Equation (5). Diurnal and midday corrected reflectance were used to calculated (diurnal and midday) corrected PRI following Equation (3). Corrected PRI values were averaged over 30 min to match eddy covariance data.