## Results
### Aggregating Data to 49 m\({}^{2}\)
#### 3.2.2 Combining Data Types  
The PRI, commonly assumed to be associated with a subset of the carotenoid pigment pool, the xanthophylls (e.g., [37; 38; 39; 40; 41; 42; 43; 44]), was strongly correlated during early senescence with canopy TChl content estimated with MTCI (Figure 11A, \(r^{2}\) = 0.96). Age class did affect this relationship: the lowest values for both the TChl and PRI (indicating lower TChl and higher stress) were obtained from the youngest age group, with slightly offset linear relationships for the two intermediate age groups (12-27 year). In addition, the PRI relationship to F685 was clearly different for young stands vs. older stands (Figure 11B), producing an overall highly significant inverse non-linear relationship (\(r^{2}\) = 0.90). Figure 9: The canopy solar-induced fluorescence emissions (means \(\pm\) SE) acquired by the _HyPlant_ FLUO at each of the four diurnal measurement periods (morning, mid-day, early afternoon, and mid-afternoon) acquired on 26 and 27 October are shown for: (**A**) fluorescence (W m\({}^{-2}\) sr\({}^{-1}\) nm\({}^{-1}\)) averaged over all stands (means \(\pm\) SE) at each time period, for the red (F685, red bars) and far-red (F740, blue bars); (**B**) F685 for the four forest stand age groups (3–5 year (red); 12 year (aqua); 20–23 year (lime), and 27–30 year (navy)) within each of the four diurnal measurement periods; and (**C**) F740, same as (**B**). Figure 10: The canopy fluorescence yields (F/PARi, means \(\pm\) SE, J umol\({}^{-1}\) PAR sr\({}^{-1}\) nm\({}^{-1}\)) and two pigment-related indices are shown for four forest stand age groups, at each of the four diurnal measurement periods: (**A**) the F685 yield; (**B**) the F740 yield; (**C**) the Fratio (F685/F740); (**D**) the MTCI estimate for relative TChI content; and (**E**) the PRI. The four age groups are: 3–5 year (age 1); 12 year (age 2); 20–23 year (age 3), and 27–30 year (age 4). Bar colors show responses per age group at the 4 diurnal collections: AM @ 60\({}^{\circ}\) SZA (red); mid-day @ 49\({}^{\circ}\) SZA (aqua); early afternoon @ 52\({}^{\circ}\) SZA (lime); and afternoon @ 60\({}^{\circ}\) SZA (navy). The PRI was also strongly and positively affected by temperature difference (air _minus_ canopy, \({}^{\circ}\)C) (Figure 12A) (\(r\) = 0.98), also producing separate curves for young vs. older stands. Consistent with these results, we observed an overall inverse correlation (\(r\) = 0.80) of F685 over the diurnal cycle with this temperature variable Figure 12B). Notice that the hysteresis of the daily temperature gradient/cycle (shown with arrows) is noticeably amplified in the young open stands that have less foliage, lower TChl, and more surface background influences than older stands, and is compatible with trends given in Figure 9B. However, no consistent relationship for F740 with this temperature variable was obtained (Figure 12C). In examining solar-induced fluorescence and LiDAR variables together, the only notable but weakly expressed relationship obtained at this spatial scale was for F740 as a function of the canopy Vertical Distribution Ratio (VDR), which had an overall \(r^{2}\) = 0.36 (\(p\) < 0.000, \(n\) = 139). One of the successful results for this campaign--for one of the older mature forest stands--was the confirmation that the Fratio and the tower-based LUE (GEP/PARi, \(\mu\)mol CO\({}_{2}\)\(\mu\)mol\({}^{-1}\) photons) were strongly and linearly related (\(r^{2}\) = 0.84, all data with two time period categories). However, the Fratio vs. LUE relationship differed markedly for the midday collection when LUE response was flat and very low (cf., Figure 3F) vs. the combined morning/afternoon collection which spanned a large range of tower-acquired LUE conditions (Figure 13A). Therefore, the combined midmorning/afternoon set (blue curve in Figure 13A, \(n\) = 19) produced an even higher dependence of the Fratio on tower LUE (\(r^{2}\) = 0.91; SEM = 0.062), where the Fratio (AM, PM) = \(-\)0.500 + 33.75 \(\times\) LUE.