## 5 Conclusions  
In this study, we assessed the consistencies and discrepancies of phenology estimated from reflectance-based and SIF observations and explored their advantages and disadvantages. We found: (1) duration of canopy greenness has a relatively weak correlation with the carbon uptake period in the study site such that a systematic later bias exists between the phenology markers derived from reflectance-based data and those from GPP time series, which is especially true in the autumn season; (2) SIF has a more direct connection with the seasonality of photosynthesis and it has a stronger ability to track photosynthetic capacity than the reflectance-based time series, particularly for the phenological markers associated with the autumn season; and (3) both physiological properties and meteorological variables could be the direct factors controlling phenological stages; the good performance of SIF emission in estimating the phenological metrics relies on its ability to track both of them. Our results show that there is a need to combine site-level and satellite-based SIF observations in future phenology studies, especially when satellite-based data with better resolutions FLEX are available. Furthermore, the lag in phenology derived from the reflectance-based data revealed in this study can also be used to rectify related phenology estimates or re-calibrate models that predict phenology based on reflectance-based data.  
end of spring, respectively. A1, A2 and A3 stand for the start of autumn, middle of autumn and end of autumn, respectively. Table S4: The phenological metrics derived from the ground-based SIF (SIF\({}_{\text{Ground}}\)) at the Harvard Forest site in 2014 and 2015. S1, S2 and S3 represent the start of spring, middle of spring and end of spring, respectively. A1, A2 and A3 stand for the start of autumn, middle of autumn and end of autumn, respectively. Table S5: The phenological metrics derived from the GOME-2 SIF (SIF\({}_{\text{GOME-2}}\)) at the Harvard Forest site during 2007-2015. S1, S2 and S3 represent the start of spring, middle of spring and end of spring, respectively. A1, A2 and A3 stand for the start of autumn, middle of autumn and end of autumn, respectively. Table S6: The phenological metrics derived from gross primary production (GPP) at the Harvard Forest site during 2007-2015. S1, S2 and S3 represent the start of spring, middle of spring and end of spring, respectively. A1, A2 and A3 stand for the start of autumn, middle of autumn and end of autumn, respectively.  
**Author Contributions:** [PERSON] and [PERSON] conceived and designed the study, and interpreted the results; [PERSON] carried out the analyses and wrote the paper; [PERSON], [PERSON], [PERSON] and [PERSON] contributed to the manuscript revision; and [PERSON] and [PERSON] made important contributions to the field experiments.  
**Funding:** This research was supported by U. S. Department of Energy Office of Biological and Environmental Research Grant DE-SC0006951, National Science Foundation Grants DBI 959333 and AGS-1005663, and the University of Chicago and the MBL Lillie Research Innovation Award to [PERSON] and China Scholarship Council No. 201506190095 to [PERSON]. [PERSON] was also supported by the open project grant (LBKF201701) of Key Laboratory of Land Surface Pattern and Simulation, Chinese Academy of Sciences.  
**Acknowledgments:** We would like to thank Harvard Forest for providing site and logistic support during our research. The water flux data were downloaded from the Harvard Forest data archive (PI: [PERSON]). We thank the National PhenoCam Network for managing the PhenoCam.  
**Conflicts of Interest:** The authors declare no conflict of interest.