## 3 Results
### Relationships between SiF, SiF \(L_{650}{}^{-1}\), GPP\({}_{30}\) and Lulie\({}_{30}\)  
To explore the relationships between _SiF_, _SiF_\(L_{650}{}^{-1}\) with \(GPP_{30}\) and _Lulie\({}_{30}\)_, three steps were undertaken. Firstly all data were included in the analysis; these also included cloudy and clear observations as well as all solar angles (and shown in grey circles in Figure 5A-H). In a second step, data points were filtered to remove all observations that were not acquired on completely clear days (dark circles in Figure 5A,C,E,G). Finally, data points were filtered to also remove SZA values larger than 50\({}^{\circ}\), thus reducing spectral data variations due to changing illuminated/shadowed canopy fractions (Figure 5B,D,F,H). The relationships between _SiF_, _SiF_\(L_{650}{}^{-1}\) with \(GPP_{30}\) and _Lulie\({}_{30}\)_, when considering data collected at all SZAs showed scattered and non-significant relationships (Figure 5A,C,E,G). There was marginal improvement in the relationships when data were filtered to include only higher SZAs (>50\({}^{\circ}\)), as shown in Figure 5B,D,F,H. Linear regression analyses applied to these data showed only weak correlations between the analyzed variables (R\({}^{2}\) 0.24-29, \(p<0.0001\)) (Table 1). Figure 4: Seasonal patterns of a midday average (10–12 UTC) of: GPP\({}_{30}\) from flux tower data (**A**), SiF (**B**) and _SiF_\(L_{650}{}^{-1}\) (**C**) from canopy level measurements from the Ocean optics spectrometers, SIF/APAR (**D**), PRI, (**E**) NDVI (**F**) PRI, and \(\Delta F/F_{m}{}^{\prime}\) (**G**) from the PAM system; LUE (**H**), (**I**) _SiF_\(L_{650-1}\) to SiF/APAR. Rain and snow days were removed. Clear and cloudy observations are presented. Figure 5: Relationships between SiF and _SiF_\(L_{\rm 60}-\)1, and _GPP\({}_{\rm 20}\)_ and _LUE_\({}_{\rm 20}\) (10-12 UTC). Grey circles include data acquired on clear and cloudy days as well as all solar angles. Black circles represent data acquired during clear days (**A**,**C**,**E**,**G**), and also when solar zenith angle was smaller than 50\({}^{\circ}\) (n = 118 for **B**,**D**, 116 for **F**,**H**) on clear days (**B**,**D**,**F**,**H**). Interestingly, the relationship between SiF and GPP was better than between _SiF_\(L_{650}-^{1}\) and GPP, which could be expected since both SIF and GPP are strongly controlled by PAR, whereas _SiF_\(L_{650}-^{1}\) is readily normalized by PAR via _SiF_\(L_{650}-^{1}\). In contrast, LUE was slightly better related to _SiF_\(L_{650}-^{1}\) than to SiF, which was also expected since both LUE and _SiF_\(L_{650}-^{1}\) undergo normalization by PAR. We selected one clear day from each month from April through September and explored the relationships between SiF, GPP, PPFD, and SZA over a diurnal time course (Figure 7). GPP typically followed PPFD throughout the day in all days across the months, although its magnitude was smaller in the spring (April and May) than later during the growing season. During the early hours following sunrise, SiF increased following GPP and PPFD and peeked around 6 UTC. After this point, SiF decreased throughout the day, while GPP remained relatively stable or continued to increase until it peaked around solar noon. This pattern in SiF was consistent across clear days, but showed more variations on days when PPFD was less stable (July and September). Maximum SiF values occurred in May and July. Note that the time in Figure 7 is UTC, with solar noon occurring at 10 AM. Using the same clear days used in Figure 7, the diurnal relationships between leaf-level effective quantum yield from the PAM data, _SiF_\(L_{650}-^{1}\) and SZA were also explored (Figure 6). During clear summer days, _SiF_\(L_{650}-^{1}\) consistently showed a small peak around 4-5 UTC (Figure 6B-E). It then remained constant until around noon, when it started to increase until reaching a second peak of the same magnitude as that observed in the morning and which occurred from late- to mid-afternoon as the summer progressed.