## Discussion
### A Generic Two-Slope Scheme SIFc-GPP Relationship  
Our study found that at site-multi-yearly scale there was no significant difference between any biome pairs in GPP/SIFc slopes except a few pairs between ENF and others (Figure 3). Specifically, GPP/SIFc slopes in ENF biome were significantly higher than those in four biomes (DBF, EBF, GRA, and OSH) according to CSIF or five biomes (DBF, EBF, GRA, OSH, and MF) according to GOSIF, and the slopes between any other non-ENF biome pairs were not significantly different. To summarize these findings, we, therefore, propose a two-slope scheme to differentiate ENF from non-ENF and synopsize the GPP/SIFc slope variability across all biomes and years. It should be noted that the two-slope scheme is SIFc dataset-specific (Table 1), resulting from the systematic differences between these two SIFc datasets. To sift the statistics (mean or median slope) building the two-slope scheme, we compared the SIFc-derived GPP with flux tower GPP. It can be seen that the two-slope scheme derived from median values (median PB were 7.14% and 11.06% for GOSIF and CSIF, respectively) outperformed the one from the mean values (median PB were 31.65% and 20.67% for GOSIF and CSIF, respectively) in estimating GPP across all EC towers (Figure 9, Figure S8 and S9); probably, the median-based scheme effectively avoided the impacts of slope outliers. Thus, we used the median values of slopes to develop the two-slope scheme in this study. The median slopes for the GPP/CSIF were 43.21 (13.39) and 34.36 (11.66) with corresponding mean \(\pm\) SE as 42.75 \(\pm\) 2.04 and 38.41 \(\pm\) 1.74, respectively, \begin{table}
\begin{tabular}{c c c c c c} \hline
**Biome** & **N** & **CSIF** & **GOSIF** & **CSIF** & **GOSIF** \\  & & **Mean** & **Mean** & **Median** & **Median** \\ \hline CRO & 20 & 39.94 \(\pm\) 3.39 & 39.14 \(\pm\) 3.45 & 37.95 (15.45) & 34.68 (13.99) \\ CSH & 3 & 46.51 \(\pm\) 23.86 & 27.70 \(\pm\) 2.54 & 26.19 (10.25) & 30.09 (0.44) \\ DBF & 26 & 38.25 \(\pm\) 4.79 & 40.96 \(\pm\) 10.88 & 33.87 (6.27) & 30.75 (7.90) \\ DNF & 1 & 24.15 & 26.59 & 24.15 & 26.59 \\ EBF & 15 & 34.49 \(\pm\) 2.91 & 30.15 \(\pm\) 2.76 & 34.30 (9.85) & 30.48 (5.88) \\ GRA & 37 & 36.20 \(\pm\) 2.86 & 35.15 \(\pm\) 3.23 & 32.31 (14.43) & 30.38 (7.91) \\ MF & 9 & 42.36 \(\pm\) 7.60 & 35.83 \(\pm\) 8.91 & 37.18 (4.2) & 28.05 (4.24) \\ OSH & 14 & 30.69 \(\pm\) 4.47 & 28.64 \(\pm\) 3.99 & 28.05 (13.57) & 25.54 (15.12) \\ SAV & 8 & 56.90 \(\pm\) 17.40 & 42.49 \(\pm\) 7.77 & 38.63 (14.04) & 34.21 (15.87) \\ WET & 21 & 41.23 \(\pm\) 5.25 & 61.66 \(\pm\) 10.56 & 38.44 (22.35) & 33.89 (22.00) \\ WSA & 6 & 33.36 \(\pm\) 4.31 & 31.04 \(\pm\) 2.83 & 29.77 (5.78) & 29.04 (5.52) \\ \hline
**ENF** & **48** & **42.75 \(\pm\) 2.04** & **40.36 \(\pm\) 2.28** & **43.21 (13.39)** & **38.61 (10.65)** \\
**Non\_ENF** & **160** & **38.41 \(\pm\) 1.74** & **39.09 \(\pm\) 3.02** & **34.36 (11.66)** & **30.39 (9.65)** \\ \hline \end{tabular}
\end{table} 
Table 1: Two-slope scheme of the linear SIFc-GPP relationship based on GPP/SIFc slopes at a site-multi-yearly scale. The SIFc dataset-dependent scheme divides all sites into two groups: ENF and non-ENF (11 biomes), according to the significance of the GPP/SIFc slopes (see Figure 3). All slopes are represented as mean \(\pm\) SE or median (MAD). N is the number of sites included in the specific biome. for ENF and other biomes (Table 1), resulting in the following two-slope scheme for converting CSIF into GPP:  
\[\text{GPP}=\left\{ \begin{array}{ll}43.21\times\text{SIF},&\text{ENF~{}~{} biome}\\ 34.36\times\text{SIF},&\text{other~{}biomes}\end{array} \right. \tag{1}\]  
The median slopes for the GPP/GOSIF were 38.61 (10.65) and 30.39 (9.65) with corresponding mean \(\pm\) SE as 40.36 \(\pm\) 2.28 and 39.09 \(\pm\) 3.02, respectively, for ENF and other biomes, and the corresponding two-slope scheme was:  
\[\text{GPP}=\left\{ \begin{array}{ll}38.61\times\text{SIF},&\text{ENF~{} biome}\\ 30.39\times\text{SIF},&\text{other~{}biomes}\end{array} \right.