## 4 Discussion
### The Fluorescence Ratio and LUE  
A valuable finding was that the red/far-red Fratio was successful for describing diurnal changes in canopy LUE at the mature stand where the NC2 tower was situated (\(r^{2}\) = 0.84 overall), with separate curves for midday (\(r^{2}\) = 0.58) vs. morning/afternoon (\(r^{2}\) = 0.91). The midday observations, when both canopy LUE and the Fratio changed rapidly, produced a steep slope with a separate cluster of midday observations (Figure 13A). Therefore, this was obscuring the underlying relationships when the midday observations were included with the combined morning/afternoon set. With the midday observations separated from the combined morning/afternoon datasets, a very strong relationship between the Fratio and canopy LUE was obtained (\(r^{2}\) = 0.96, one outlier removed). The red/far-red Fratio has been successfully related to canopy LUE in field campaigns previously [32; 33; 35] and has been used to improve forward modeling of GPP [54]. We investigated whether the midday LUE plateau observed in October (Figure 5F) also occurred during the summer 2013 peak production period, and substantiated that the midday LUE plateau was also often characteristic of midsummer diurnal responses (Figure 15A-C). In retrospect, this is expected given the hysteresis behavior of all light and temperature driven vegetation physiological responses (e.g., GEP, water fluxes, solar-induced fluorescence) that increase during mornings and decline during afternoons, as they pivot around the maximum achieved midday value. Therefore, it is apparent that a midday _instantaneous_ collection (by itself) may not be the optimal time period for accurate estimation of LUE from airborne remote sensing using fluorescence (Figure 13A). Here, the Fratio increased drastically (\(\sim\)0.4 units) over a small increase in LUE (\(\sim\)0.03 \(\upmu\)mol CO\({}_{2}\upmu\)mol\({}^{-1}\) photons). It was also not possible to estimate GEP from these (or any) spectral variables at midday, given the small range of GEP observed during the solar noon time period, and since fluxes were reported at half-hourly intervals. This new understanding may help explain the different results reported in the literature for experiments and campaigns conducted primarily in midday. Midday (cloudless) observations have been preferred as the ideal remote sensing measurements for the practical reasons of increasing SNR for passive optical instruments. These daily maxima are more accurate for estimating diurnal (i.e., total) carbon/water/thermal fluxes, as well as providing superior land cover classification maps from orbital sensors. However, in practice, and because field measurements are time-consuming, \"midday\" measurements often actually span a wider set of conditions on either side of solar noon, possibly blurring interpretations. Because midday observations (by themselves) display little variation, the observations acquired on either side of daily maxima (i.e., the upsides and downsides of these diurnal responses; cf. Figures 3 and 14) provide critical information about the functionality of the system, and are helping us to understand dynamical physiologically driven responses, including fluorescence. Figure 15: The AmeriFlux NC2 flux tower data at Parker Tract showing half hour flux values for six summer days, 13–19 June 2013. (**A**) GEP (\(\mu\)mol CO\({}_{2}\) m\({}^{-2}\) s\({}^{-1}\)); (**B**) Incident PAR (\(\mu\)mol PARi m\({}^{-2}\) s\({}^{-1}\)); and (**C**) Light Use Efficiency, LUE (\(\mu\)mol CO\({}_{2}\)\(\mu\)mol\({}^{-1}\) photons). We note that most of the satellite observations to be collected from the future FLEX mission, which will have an equatorial overpass time of ~10:00 am, will not be collected at/near solar noon. On each orbit for FLEX, there will also be a range of SZAs and PARi associated with the observations, ranging from near solar noon at high northern latitudes to early morning at high southern latitudes, the result of an oblique descending polar orbit.