## 4 Discussion  
### Measurement of Fluorescence in the O\({}_{2}\)B Band  
In this study, we examined the relationship between fluorescence at 760 nm quantified using a 3 FLD method from spectra with nanometer resolution (i.e., 1 nm FWHM) and fluorescence at 687 nm estimated from the same spectral measurements with the same resolution in adjacent spectral wavebands. According to [PERSON] et al. [12], resolution of the spectrometer of at least 1 nm is required for estimating fluorescence in O\({}_{2}\)A band, and for estimating fluorescence in the O\({}_{2}\)B band, a finer resolution of at least 0.5 nm is required to achieve a similar degree of precision in measurements. However, our data showed that 1 nm resolution is sufficient for estimating fluorescence in the O\({}_{2}\)B band: the estimated SIFB showed a clear signal, and the signal yielded similarities to the SIFA signal both seasonally (Figure 5) and diurmally (Figures 7 and 8). Our measurements showed that the signal of O\({}_{2}\)B fluorescence emission provided a meaningful trend across the season and  
Figure 11: Dependency between SIFA and GPP at examined forest sites is shown in the upper panels. Dependency between SIFB and GPP is shown in the bottom panels. Black points refer to seasonal values of parameters averaged across 2 h long intervals on days with clear sky conditions. Grey points show relationship between data on selected days in 30 min temporal intervals between 10:00 and 14:00 h. The asterisk *** indicates statistically significant result at \(p<0.001\). Figure 12: Seasonal variations in the ratio between fluorescence emission in the O\({}_{2}\)A and in the O\({}_{2}\)B band. Black points highlight average seasonal variation in SIFA-SIFB ratio estimated across 2 h long interval in the midday. Grey points show dynamics in 30 min intervals, temporal intervals are equal to those displayed in Figure 5.  
the changes agree with the pattern of NDVI. Fluorescence in O\({}_{2}\)A and O\({}_{2}\)B bands shared similar sensitivity to PAR and showed sensitivity to NDVI. Inversion in the relationship between O\({}_{2}\)B fluorescence and NDVI (Figure 10) suggested that the signal is reversibly scaled with NDVI; thereby we presume that SIFB emissions are highly dependent on chlorophyll concentration, as shown by [PERSON] et al. [10]. The yield of fluorescence emission at O\({}_{2}\)B band from the daily observations showed a decrease in the afternoon. The observed trend may be considered as a response to a general physiological decline in the afternoon hours, typically occurring in photosynthesis in environments with excessive PAR [35]. The measured SIFA was dependent on scattering and the amount of leaves in the field of view (FOV), whereas SIFB appeared to be a parameter linked more directly to physiology and less to shading effects in the forest canopies (Figures 7 and 8). Our results suggested that SIFB may be suitable for measuring GPP changes in deciduous plants, despite the negative slope across the season. The relationship between GPP and SIFB was largely driven by changes in NDVI (Figure 9). The dynamics of the SIF parameters at the start of vegetative period estimated for the beech forest (Figure 5) were similar to those observed by [PERSON] et al. [36] in a sorghum stand. These authors reported that fluorescence at 687 nm increased rapidly during early growth and then became saturated, even as SIFA continued to increase. The trend of decreasing SIFA emission and increasing SIFB towards the end of season was similarly matched by [PERSON] et al. [22], who conducted measurements of fluorescence at both oxygen bands together with NDVI in a soybean field. The observed trend towards the end of soybean growing cycle was matched with a decrease in NDVI, similar to the findings of our study. The dependence of SIFB on APAR in the beech forest was quite low (Figure 6), which suggests that the dependency between SIFB and GPP is strongly affected by different factors.