## 4 Discussion
### Relationship of Fluorescence Emissions to GPP  
[PERSON] et al. [37] found that far-red SIF (SIFA) in grasslands increases in spring, peaks in early summer, and then decreases in late summer. These changes are attributed primarily to NDVI of the canopy and to the PAR intensity. A similar trend was also observed by [PERSON] et al. [38] in a field of corn (_Zea mays_). Forests are quite different in structure from graminaceous vegetation, but we found comparable SIFA changes in both forests, with SIFA more strongly correlated with GPP across the season in the spruce forest (Figure 11). This dependency may have developed on the basis of a lower APAR later in the season, but the dispersion in the data also depended on the contribution of other factors, including perhaps pigment contents or the activity of the xanthophyll cycle. A large number of studies demonstrate a nonlinear relationship between fluorescence and GPP at fine spatial and temporal scales or under stress [8; 38; 39; 40; 41]. Unlike reflectance or vegetation indices, which are relatively stable over short timespans, SIF fluctuates instantaneously with PAR and dynamic regulations of photosynthesis and NPQ [21]. The competing influences of NPQ and photosynthesis on SIF yield under stress [7; 41] may account for the observed poor relationship between fluorescence emissions and seasonal GPP. Diurnal changes in SIFA may represent photosynthetic activity because both chlorophyll fluorescence and the xanthophyll cycle are usually involved in photoproduction under conditions of excessive radiation [42; 43]. The link between SIF and photosynthesis depends on the rate of NPQ and is influenced by stress levels because both radiative and non-radiative pathways of dissipation are sensitive to stress [44]. High PAR intensity and a high vapor-pressure deficit are considered to be critical factors for dynamics observed at midday [45; 46]. We observed high diurnal fluctuations of SIF in the spruce forest (Figure 5), which could indicate an enhanced NPQ later in the growing season. Spruce species are especially known to have a high capacity for dynamic NPQ [47]. Other studies have also found that decreases in SIF due to plant stress, especially under limited water supply, were linked to higher NPQ values [48; 49]. Part of the observed dynamics can be attributed to canopy scattering, which may increase reflected radiance in the far-red part of the spectrum in dried foliage and may, thus, increase the detected fluorescence. This may apply particularly where the detected signal is concomitant with any signal from the woody parts of the forest [50]. Therefore, a quite high SIF emission was detected in spruce forest before mid-July (Figure 6), which was an extremely dry period [51] ending with heavy rainfall that led to an increase in foliar NDVI (Figure 5). The dominance of APAR in the SIF signal at seasonal scales has been previously demonstrated [32; 52; 53]. The dependence of SIFA on APAR in our study is strong because morning periods with lower APAR were included in the analysis. SIFA, thus, likely contains information regarding both APAR and LUE, variables that determine GPP (GPP = APAR \(\times\) LUE). Fluorescence can thereby estimate changes in photosynthetic efficiency, as indicated in [PERSON] et al. [54], [PERSON] et al. [55], and [PERSON] et al. [56]. Dominance of this relationship was shown from the dependency of the fluorescence emission on APAR (Figure 9). The relationship between SIFA and GPP in the beech forest (Figure 10) may have been accentuated with regulation in GPP by physiological factors, e.g., with stomata [41; 43]. The dispersion in the data points can be effectively lowered with spatial resolution of the measurements by making measurements from a larger distance at a larger area [39; 57]. Some studies have provided inconclusive results regarding the SIFB-GPP dependency analysis, e.g., [PERSON] et al. [39]. [PERSON] et al.