## 4 Discussion  
As suggested in other studies [70; 71; 72; 73], our results show that a cross-calibration must be performed to properly calibrate automatic optical sensors. Uncalibrated SRS-PRI values did not have good agreement with spectrometer PRI values. Unlike results from [PERSON] et al. [44], both diurnal and midday calibrations noticeably improved the ability to resolve diurnal PRI patterns. Although we do not know the reason for this difference in calibration outcomes, one possibility may be that our aspen stand could represent more a heterogeneous structure and illumination conditions. Wind conditions that are common at our calibration site could help randomize the leaf orientation so that both the spectrometer and SRS sensors measure similar sunlit and shaded leaf proportions. Our contrasting results advocate for the importance of characterizing site-specific light fields that can vary in complexity  
Figure 11: Time series (top panels) and regressions (bottom panels) of (**a**) absorbed photosynthetically active radiation (APAR) and gross primary productivity (GPP), (**b**) light use efficiency (LUE) model driven by apparent quantum yield (\(\alpha\)) and GPP, and (c) light use efficiency (LUE) model driven by scaled photochemical reflectance index from SRS sensors (sPRI) and GPP. between different canopy structures and may affect the ability to resolve diurnal dynamics accurately. A lack of ability to accurately resolve diurnal PRI patterns becomes problematic when diurnal patterns of the xanthophyll cycle and light use efficiency is wanted [39; 44; 74]. Although cross-calibrations provided more accurate PRI values, errors remain. In general, the main discrepancy between the SRS-PRI sensors and spectrometer measurements occurred as we moved away from midday. This suggests that the factory SRS sensors sensitivity cannot resolve the full range of reflectance changes. Uncalibrated SRS sensors result in an overestimation of PRI and LUE and modeled GPP values if used within a LUE model. To prevent erroneous PRI values that can lead to the mischaracterization of light use efficiency, we recommend regular calibrations and corrections of SRS sensors [71; 73]. Diurnal cross-calibration procedures confirmed that an offline calibration is possible and can be used to correct the effects of illumination and solar elevation on radiance and irradiance measurements. However, it is important to note that our procedure did not account for changes in solar azimuth that lead to changes in directional reflectance [75; 76]. Solar azimuth changes will have a meaningful impact in high latitude ecosystems as angles will vary significantly from start to peak growing season. Real-time calibrations would account from changes in solar azimuth. However, given the constant manual intervention needed during real-time cross-calibrations, we believe our procedure to be a good compromise. The offline diurnal calibration was able to resolve diurnal PRI pattern (and xanthophyll cycle changes) more accurately than midday calibrations. The diurnal cross-calibrations results corroborate previous findings showing the effect of sun-canopy-sensor geometry on PRI values [33; 43; 77; 78]. The strong correlation between SRS-sPRI and LUE (\(R^{2}=0.62\)) show that automated SRS-PRI sensors can be used as a proxy for light use efficiency. It is important to note the observed relationship between SRS-sPRI and LUE was stronger than the mean of 27 broadleaf forests reported correlations (\(R^{2}=0.59\)) reviewed by [PERSON] et al. [24]. Comparison between SRS-sPRI and apparent canopy quantum yield showed an even stronger correlation with an \(R^{2}=0.72\) that represents the upper third quartile boundary of the broadleaf forest PRI-LUE correlation meta-analysis. Although quantum yield is not equivalent to light use efficiency, the strong correlation between \(\alpha\) and LUE (\(R^{2}=0.88\)) show that \(\alpha\) can be used as an adequate proxy of LUE when PPFD is the primary driving variable of productivity.