## 5 Conclusions  
This study analyzed the reliability of various MODIS-derived indices and SIF for estimating phenological transition dates in pan-Arctic regions, and reported a high level of uncertainty regarding phenology extraction at these latitudes. The results indicated that all indices were influenced by snow cover and soil moisture during the transition dates, and although VI-based and EC-based estimates of phenological transition dates were related, the overall R\({}^{2}\) values were low. Available methods for satellite-derived time series in pan-Arctic regions were mainly constrained by constant snow cover, solar geometry, site-specific conditions, the short length of the growing season, and the relatively low frequency of data sampling. Of the tested VIs, PPI-estimated metrics showed seasonal and inter-annual patterns that were mostly closely related to the EC-based estimates. SIF was also shown to be a suitable indicator of the SOS and EOS in pan-Arctic regions, and may play an increasingly important role in line with advances in instrumentation.  
The results of this study challenge the utility of satellite-derived VIs for assessing phenology and its changes in high-latitude regions, given that estimates of phenological transition dates based on optical remote sensing are likely to coincide with the disappearance of snow, unless careful data screening and pre-processing are carried out to avoid this impact. Precautions must therefore be taken when interpreting spatio-temporal patterns of SOS derived from these indices. Significantly, careful validation with ground data are required for critical regions to enable the reliable detection of changes and to further our understanding of ecosystems to the level needed to allow accurate predictions to be made.  
Figure 8: Correlation between SOS and EOS extracted from tower-based GPP and satellite-based SIF. Dashed lines represent the 1:1 line and continuous lines are simple regression fit function.  
**Author Contributions:** [PERSON]. ([PERSON]) collected and processed the data; [PERSON] carried out the analyses and wrote the paper; [PERSON] supervised the work; [PERSON]. ([PERSON]) contributed with significant advice, and manuscript revision; LM. provided the flux data of FI-Hyy and helped with the revision; [PERSON] helped with the revision.  
**Funding:** This research was supported by the National Key R&D Program of China (No. 2017 YFF0205306) and by the Chinese Arctic and Antarctic Administration, National Natural Science Foundation of China, the Chinese Polar Environment Comprehensive Investigation, Assessment Program (Grant No. 41676176 and No. 41676182). This research was also supported by the Fundamental Research Funds for the Central Universities (Grant No. 312231103).  
**Acknowledgments:** We would like to thank Chinese Arctic and Antarctic Administration for providing financial support during our research. Eddy based canopy measurements are from Fluxnet ([[http://fluxnet.fluxdata.org/](http[PERSON]fluxnet.fluxdata.org/)]([http://fluxnet.fluxdata.org/](http://fluxnet.fluxdata.org/))) and European Fluxes Database Cluster ([[http://gaia.agaria.unitus.it/](http[PERSON]gaia.agaria.unitus.it/)]([http://gaia.agaria.unitus.it/](http://gaia.agaria.unitus.it/))); we would like to show our respects for the PIs and staff that are working on these sites. Also, we would like to thank the anonymous reviewers who have contributed to making the manuscript clearer and more coherent.  
**Conflicts of Interest:** The authors declare no conflict of interest.