## Results
### Aggregating Data to 49 m\({}^{2}\)
#### 3.2.2 Combining Data Types
In contrast, only a moderate dependence of the Fratio on LUE was obtained at midday (\(r^{2}\) = 0.58, SEM = 0.10; Fratio (midday) = \(-\)7.52 + 372.51 + LUE. The influence of the canopy temperature difference (canopy-air, \({}^{\circ}\)C) on canopy LUE was more complex (Figure 13B), being strongly expressed in the morning when the temperature difference was greatest (1-2 \({}^{\circ}\)C) and less so in the two afternoon collections (0-1 \({}^{\circ}\)C) which displayed different slopes, but not strongly expressed at mid-day when the canopy and air temperatures were essentially equal (0 \(\pm\) 0.4 \({}^{\circ}\)C). Given this context, the Fratio vs. LUE relationship was simpler (Figure 13A) and adequate for capturing the diurnal dynamics. For this combined midmorning/afternoon observation set, GEP could also be estimated in a multivariate analysis with three remote sensing variables (Figure 13C): PRI (\(p\) < 0.000), NDVI (\(p\) < 0.000), and F685 (\(p\) = 0.004) which produced an adjusted coefficient of determination, \(r^{2}\) = 0.75 (SEM, 1.303 \(\mu\)mol CO\({}_{2}\) m\({}^{-2}\) s\({}^{-1}\), \(n\) = 34). Figure 11: The relationships of the PRI to two canopy-scale spectral variables for the four age groups across all LP stands (ROIs, 49 m\({}^{2}\)): (**A**) TChl (MTCI) for the four age class groups produced an overall linear relationship (overall: \(r^{2}\) = 0.96, \(p\) \(\leq\) 0.003, \(n\) = 143; age class, \(p\) \(\leq\) 0.000); and (**B**) the F685 (\(r^{2}\) = 0.90, \(p\) \(\leq\) 0.000, \(n\) = 138) with two age class groups. Age groups: 3–5 year (red circles), 12 year (aqua X), 21–23 year (line +), and 27–31 year (navy triangles). We further investigated the spectral data from the NC2 tower site, and confirmed that the red fluorescence was strongly and inversely related to the PRI (Figure 14A) as discussed earlier. The diurnal hysteresis with this pairing is readily apparent, increasing from the lowest F685 coupled with the highest PRI values during the morning collection to the highest F685 coupled with the lowest PRI values at midday. These responses reversed during the afternoon, finally returning to values similar to those in the morning set. Therefore, we reexamined this behavior for all stands, as previously shown in Figure 11B. Here, we separate the stands into young and older groups (and reverse the axes) but now the hysteresis for the combined stress response cycle expressed by the pairing of PRI with F685 is even more evident (Figure 14B). For the data cluster associated with the older stands (right cluster, Figure 14B), the highest PRI (\(\sim\)\(-\)0.01) was observed in mid-morning, then moved lower to \(-\)0.04 by noon, recovered to between \(-\)0.02 and \(-\)0.03 in early afternoon, and finally achieved values near zero  
Figure 12: The relationships of three canopy spectral indices to the relative canopy temperature (canopy-air, \({}^{\circ}\)C) at four times of day (\(n=135\)): (**A**) the overall linear relationship for PRI vs. temperature difference (\(r=0.98\), \(p\leq 0.000\)); (**B**) F685, tracking the fluorescence change per time period, showing an overall inverse response (\(r=0.80\)) but different diurnal hysteresis temperature trends across the four time periods, for the two age groups (age 1, 2–12 year; age 2, 15–30 year), where the responses of the two age groups are indicated with the arrows and (**C**) the non-significant results for F740. The legends for (**B**) and (**C**) are the same. by late afternoon. Comparable F685 (mW m\({}^{-2}\) sr\({}^{-1}\) nm\({}^{-1}\)) responses were: morning, 0.22 to 0.4; noon, 0.4 to 0.52; with recovery initiated in early afternoon, 0.28 to 0.45; and lowest values in late afternoon, 0.18 to 0.34. For the young stands (left cluster, Figure 14B), the PRI values are much lower (\(-\)0.04 to \(-\)0.08) and the F685 values are higher at midday, reaching 0.74 mW m\({}^{-2}\) sr\({}^{-1}\) nm\({}^{-1}\).