## 4 Discussion
### PRI as an Indicator of Photosynthetic Activity and Phenology  
The initial testing of the ability of the different PRIs to describe variations in photosynthetic activity and LUE revealed mixed results. While previous studies have found significant relationships between MODIS PRI and LUE [26; 28], none of the PRIs correlated well with LUE in this case (Figure 6). A number of factors could contribute in obscuring the relationships observed between satellite observations and flux tower measurements. At the ecosystem scale, variations in canopy structure (e.g., leaf orientation), background effects and discrepancies in the spatial footprint of flux measuring station and the MODIS sensor can affect the PRI signal significantly [33]. The estimation of seasonal phenological dates from optical remote sensing observations of VIs in boreal regions may also be attributed to the high number of missing observations due to cloud cover, and the occurrence of snow at the same time as the vegetation is actually active. An important source of uncertainty also lies in the calculation of LUE, which was based on ground-measured GPP and PAR. While a number of different definitions of LUE have been expressed in the literature, the original model of LUE defined by [PERSON] [49] utilizes the amount of incident photosynthetically active light absorbed by the canopy (APAR). However, lack of data on the fraction of radiation absorbed by green vegetation meant that APAR could not be included. [PERSON] and [PERSON] [65] investigated the implications of using different definitions of LUE. They found that the definition of LUE based on either PAR, total absorbed PAR and PAR absorbed by photosynthetically active radiation had an impact on the estimations of seasonal vegetation productivity as well as the temporal behavior of LUE. Using APAR, rather than simply incident radiation could thus potentially produce different PRI-LUE relationships than observed here. While no strong correlation was found between PRI and LUE, sPRI\({}_{1}\), which uses the broad red band as a reference band, yielded the highest R\({}^{2}\). Results from studies by [PERSON] et al. [29] and [PERSON] et al. [26] indicate that red wavelengths (MODIS bands 1, 12 and 13) generally tend to be useful as reference bands for estimating vegetation LUE from MODIS PRI. sPRI\({}_{1}\) was also the only version of PRI, which correlated strongly with ground-measured GPP. It is noteworthy that sPRI\({}_{1}\) had a stronger relationship with GPP (R\({}^{2}\) = 0.63) compared with NDVI (R\({}^{2}\) = 0.47), which is more commonly used to observe vegetation productivity. Further other studies have found that NDVI and similar VIs that are related to leaf biomass do not capture the seasonal variation in LUE and photosynthetic activity as well as PRI in evergreen forests [66; 67; 68]. The strong relationships observed between SGS derived from NEE and sPRI\({}_{1}\) (when applying Gaussian and double logistic functions) suggest that PRI could serve as an effective indicator of seasonal transitions. This is in agreement with the findings of [PERSON] and [PERSON] [14] who tested the use of PRI for estimating spring phenology in conifers at the leaf and canopy scale. [PERSON] and [PERSON] [14] found that the seasonal increase in PRI was nearly coincident with spring activation of photosynthetic activity with only a couple of days lag. The authors could conclude that the main physiological process accounting for the response of PRI observed was the seasonal change in pigment pool sizes. While the highest \(R^{2}\) in this study was found for sPRI\({}_{1}\), the NDVI\({}_{\text{maiac}}\) predictions of SGS exhibited more consistently high correlations with the ground estimates (\(R^{2}>0.6\) in all cases). The results can thus not confirm that PRI is more useful for predicting phenological events in conifers compared to NDVI.