## 6 Conclusions  
This study analyzes the robustness of the satellite derived Plant Phenology Index (PPI) for estimating start of season (SOS) in boreal forests of the Northern Hemisphere (2000-2014), in relation to two other widely applied indices NDVI and EVI. These satellite-based observations were evaluated against GPP-retrieved SOS derived from a network of Flux tower observations. Results revealed that PPI performs significantly better in boreal regions as compared to NDVI and EVI, where very poor relations to ground observations were found. The main challenge related to SOS retrieval by usingNDVI or EVI is their sensitivity to changes in snow-cover rather than vegetation canopy changes, and lower response to variations in vegetation growth dynamics. It was shown that NDVI and EVI based SOS occurred up to 20-40 days earlier than PPI in the southern part of the boreal region driven by an increased temporal decoupling between the date of snow disappearance and the date of onset of photosynthetic activity. At higher latitudes, the convergence of snow disappearance and onset of photosynthetic activity results in a larger similarity between PPI and NDVI/EVI SOS. The SOS trends observed in this study have shown an overall advance in PPI SOS at the rate of 0.28 (days/year) as compared to \(-\)0.23 for EVI and \(-\)0.26 for NDVI, but with considerably larger differences between PPI and EVI/NDVI for specific land cover types and boreal regions. The most pronounced patterns of significant trends of advancing SOS when considering all three indices were observed for the land cover class of deciduous needleleaf forest, where PPI, however, revealed less dramatic changes as compared to EVI/NDVI (0.47 days/year as compared to 0.62 and 0.74). On the other hand, advancing PPI SOS trends were found to be even higher for deciduous broadleaf forests and savannas (0.54 and 0.56 days/year) not equally reflected in EVI (0.34/0.37) and NDVI (0.18/0.41). The largest patterns of significant PPI trends towards earlier SOS were observed in the Eastern and Western Taiga Shield in Canada (0.45 and 0.50 days/year) followed by the East and Northeast Siberian Taiga in Russia (both 0.35 days/year). Taken together, the findings of this study support the suggested improved performance of PPI over NDVI and EVI in SOS retrieval for areas of seasonal snow cover. The considerable difference in the spatial patterns of SOS trends between PPI and NDVI/EVI over the 15-year study period suggests that precautions must be taken when using and interpreting NDVI/EVI derived SOS results from northern latitudes and improved accuracy in assessing spatio-temporal patterns of SOS can be gained by applying PPI. **Supplementary Materials:** The following are available online at www.mdpi.com/2072-4292/9/5/485/s1. Table S1. Descriptive statistics from the evaluation of vegetation index based start of season (SOS) against gross primary production (GPP) estimates of SOS for the different land cover classes; Figure S1. Vegetation index start of season (SOS) evaluation against GPP SOS derived from flux tower data separated into its different land cover classes for (1) PPI; (2) EVI; and (3) NDVI. The different land cover classes are: (A) deciduous needle leaf forest; (B) Evergreen needle leaf forest (ENF); (C) mixed forest (MF); (D) open shrubland (OSH); and (E) wetland (WET). Which exact sites and years that are included in the different subplots can been seen in Table A1. **Acknowledgments:** The project was funded by the Danish Council for the Independent Research (DFF) Sapere Aude programme. [PERSON] received support from the Swedish National Space Board. The authors would like to thank [PERSON] and [PERSON] for providing the online access to the TIMESAT software. This work used eddy covariance data acquired and shared by the FLUXNET community, including these networks: AmeriFlux, AfriFlux, AsiaFlux, CarboAfrica, CarboEuropeIP, Carboltaly, CarboMont, ChinaFlux, Fluxnet-Canada, GreenGrass, ICOS, KoFlux, LBA, NECC, OzFlux-TERN, TCOS-Siberia, and USCCC.