## 5 Conclusions  
Our work is a global analysis investigating the relationship between SIFc and GPP at various spatial and temporal scales, which expands previous research on this topic, particularly in the following two areas. First, we used all GPP data in the FLUXNET2015 collection and two global SIFc products for the analysis, providing the most comprehensive coverage so far (208 flux towers and the longest study period from 2001 to 2014) to explore the SIFc-GPP relationship. Second, we used Major Axis regression to account for uncertainties in both SIFc and GPP estimates in the analysis of the SIFc-GPP relationship, which produced higher GPP/SIFc slopes than OLS. Our research expands several pioneering works which have reported the relationship between OCO-2 SIF and tower GPP at individual sites and a few biomes. We propose a two-slope scheme to differentiate ENF from the non-ENF biome and synopsize the GPP/SIFc slope variability across biomes and years. The relative biases were 7.14% and 11.06% in the estimated cumulative GPP across all EC towers, respectively, for GOSIF and CSIF, using the two-slope scheme. Nevertheless, our results suggested some major issues related to the SIFc-GPP relationship, including dataset dependency of the SIFc-GPP relationship, variability of the SIFc-GPP relationship across spatial and temporal scales, and a two-slope scheme that was distilled from the SIFc-GPP relationships across biomes. Thus, we call for more research on these abovementioned issues, and offer a few thoughts on the caveats and uncertainties of our research, as well as future research directions.  
The following are available online at [[https://www.mdpi.com/article/10.3390/rs13142824/s1](https://www.mdpi.com/article/10.3390/rs13142824/s1)]([https://www.mdpi.com/article/10.3390/rs13142824/s1](https://www.mdpi.com/article/10.3390/rs13142824/s1)).  
Conceptualization, S.L.; methodology, S.L. and H.G.; writing--original draft preparation, H.G.; writing--review and editing, S.L., W.L., A.R.S., W.Y., Z.W. ([PERSON]), X.P., R.V., L.X., Q.L., Y.F., M.M., T.P., J.L., Z.W. ([PERSON]), G.Z. and H.G.; supervision, S.L.; funding acquisition, S.L. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China (Grant No. 41971152); and the Hunan Innovative Talent Program (Grant No. 2019 RS1062) to [PERSON].  
This work used EC flux data acquired and shared by the FLUXNET community, including the following networks: AmeriFlux, AfriFlux, AsiaFlux, CarboAfrica, CarboEuropeIP, CarboMont, ChinaFlux, Fluxnet-Canada, GreenGrass, ICOS, KoFlux, LBA, NECC, OzFlux-TERN, TCOS-Siberia, and USCCC. The ERA-Interim reanalysis data are provided by ECMWF and processed by LSCE. The FLUXNET EC flux data processing and harmonization were carried out by the European Fluxes Database Cluster, AmeriFlux Management Project, and Fluxdata project of FLUXNET, with the support of CDIAC and the ICOS Ecosystem Thematic Center, and the OzFlux, ChinaFlux, and AsiaFlux offices. The CSIF dataset is provided by [PERSON], and the GOSIF dataset is provided by [PERSON] and [PERSON]. We thank the anonymous reviewers for their valuable comments.  
We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there are no professional or other personal interests of any nature or kind in any product, service, and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled \"Global analysis of the relationship between reconstructed solar induced chlorophyll fluorescence (SIF) and GPP.\"