## Results
### Aggregating Data to 49 m\({}^{2}\)  
#### 3.2.1 Fluorescence Group Means for Age Classes and Time Periods  
The products described in Section 3.1 were aggregated within the ROI cutouts to a coarser spatial resolution (49 m\({}^{2}\)) to minimize the across track effects due to illumination changes that occurred during the 45 min long collections. This aggregation also minimized sensor noise/artifacts to enhance SNR, allowing us to explore statistically relevant relationships among variables. We found that stand heights derived from LiDAR CHM data at this coarser spatial resolution (more suitable for satellite validations) were highly correlated (\(r\) = 0.97) with age class across the 33 year span of the ROI stands selected from the LP chronosequence at this site. At this spatial resolution, the fluorescence emissions for F685 decreased with LP stand age (Figure 8A) between 2 and 10 year old stands to a lower, steady average (0.3-0.4 mW m\({}^{2}\) sr\({}^{-1}\) nm\({}^{-1}\)) for the next 17 year stand age span. The far-red F740 was more variable across age classes (0.35-0.68 mW m\({}^{2}\) sr\({}^{-1}\) nm\({}^{-1}\)), and exhibited higher values associated with stand ages \(\leq\)23 year, than for the oldest stands. Consequently, the red/far-red Fratio (F685/F740) was bowl-shaped, expressing a minimum value (\(\sim\)0.4) between 10 and 22 years (Figure 8B). Figure 7: (**A**) Spectra from the _HyPlant_ DUAL VSWIR spectrometer, as processed with the ATCOR-4 routine, describe ten LP stand age groups (LP 2 to LP 31 years.) and a mixed pine/deciduous forest group (brown spectrum), collected near noon on 26 October 2017. The young to mature LP stand ages are indicated by spectra of different colors: 2 year (purple), 3–4 year (navy, bright blue), intermediate age classes of 12, 14, 21, 23, and 27 year (dark-green, light-blue. aqua. bright-green, yellow), and the oldest age classes of 29 year (orange) and 31 year (red); (**B**) A forest classification map of the Parker Tract forest was obtained by merging the mosaics of _these HyPlant_ DUAL VSWIR spectra with G-LiHT LiDAR CHM. Black areas indicate falllow fields. The impact of the illumination changes associated with the four diurnal collections during the 2-day experiment, which occurred at SZAs of 60\({}^{\circ}\) in the morning, 49\({}^{\circ}\) around solar noon, and 52\({}^{\circ}\) and 60\({}^{\circ}\) in the afternoon, was evaluated by averaging the fluorescence values obtained for all stands at each time period (Figure 9A). These fluorescence responses show that the average F740 > F685 at each time period, with the highest daily mean fluorescence values at midday for both red and far-red fluorescence, 0.75 and 0.48 mW m\({}^{2}\) sr\({}^{-1}\) nm\({}^{-1}\), respectively. These data were further examined within the four time periods for four age group ranges (2-5 year, 12 year, 20-23 year, and 27-33 year) (Figure 9B,C). It is obvious that although F685 (0.25-0.80 mW m\({}^{2}\) sr\({}^{-1}\) nm\({}^{-1}\)) was typically lower (with exceptions) than F740 (0.3-1.3 mW m\({}^{2}\) sr\({}^{-1}\) nm\({}^{-1}\)), the highest F685 was produced from the youngest age group (red bars) across all four observation time periods. The F685 radiance was also less variable across age classes than F740 (Figures 8A,B and 9B,C). In addition, we observe that the youngest group (2-5 year) maintained higher F685 yields throughout the diurnal cycle than the older stands (Figure 10A,B), while the opposite trend was apparent for MTCI across age groups, with the lowest Tchl estimate obtained for the youngest age class (Figure 10D). In contrast to F685 yields, the F740 yields (Figure 10B) were more diurnally variable than F685 yields. The far red fluorescence by itself has been related to photosynthesis or GPP (e.g., [31; 32; 33; 35; 36]). The Fratio, which has been shown elsewhere to be inversely related to photosynthetic efficiency [32; 33; 35], but also being greatly influenced by canopy structure, varied across the stand age groups Figure 10C.