## 3 Results  
### Evaluating Different SIF-GPP Relationships  
We estimated GPP globally from SIF based on the fine-resolution GOSIF product [(31)] and a total of eight SIF-GPP relationships that we established (both universal and biome-specific; both non-zero and zero intercept; both site and grid cell level). The scatterplots of the SIF-GPP relationships at both site and grid cell level are shown in Figure 1, and the statistical measures (slope, intercept, and \({}^{2}\)) are provided in Table 1. The strength of the SIF-GPP relationship found at the site level (R\({}^{2}\) = 0.70-0.74, p \(<\) 0.001) was comparable to that at the grid cell level (R\({}^{2}\) = 0.72-0.73, p \(<\) 0.001). The slope of the grid cell level relationships for about half of the biomes (e.g., evergreen needleleaf forests, evergreen broadleaf forests, open shrulands) was higher than that of the site-level relationships, possibly due to the heterogeneity within the 0.05\({}^{\circ}\) grid cells in which some EC sites are located or the slight underestimation of our gridded SIF (GOSIF) for these biomes (Table 1). The universal SIF-GPP relationship at the ecosystem level (i.e., site level) estimated GPP fairly well at the site level, and other SIF-GPP relationships also estimated GPP fairly well (Figure 2). The performance was similar among the eight relationships with site-mean RMSE ranging from 1.60 to 1.77 g C m\({}^{-2}\) d\({}^{-1}\). The site-level relationships (i.e., the relationships based on SIF from OCO-2 soundings and tower GPP from 64 EC sites; boxplots 1-4 in Figure 2) had almost identical performance, suggesting that the SIF-based approach was relatively insensitive to the specific forms of the SIF-GPP relationship (universal or biome-specific, with or without intercept). The performance of the grid cell level relationships (i.e., the relationships derived based on 0.05\({}^{\circ}\) SIF and tower GPP; boxplots 5-8 in Figure 2) was slightly affected by the use of non-zero or zero intercept. At the grid cell level, the biome-specific relationships were slightly better than the universal ones. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Site Level**} & \multicolumn{3}{c}{**Non-Zero Intercept**} & \multicolumn{2}{c}{**Zero Intercept**} \\ \cline{2-6}  & **Slope** & **Intercept** & **R\({}^{2}\)** & **Slope** & **R\({}^{2}\)** \\ \hline All & 20.04 & 0.89 & 0.74 & 22.98 & 0.70 \\ \hline ENF & 19.24 & 1.38 & 0.70 & 24.01 & 0.62 \\ \hline EBF & 12.42 & 3.03 & 0.59 & 22.52 & 0.14 \\ \hline DBF & 22.21 & 0.98 & 0.88 & 24.55 & 0.87 \\ \hline MF & 20.33 & 1.67 & 0.77 & 24.08 & 0.72 \\ \hline OSH & 20.22 & 0.38 & 0.64 & 22.78 & 0.61 \\ \hline SAV & 17.51 & 1.54 & 0.63 & 25.17 & 0.46 \\ \hline GRA & 19.86 & 0.80 & 0.78 & 23.41 & 0.66 \\ \hline CRO & 18.59 & 0.73 & 0.61 & 20.43 & 0.60 \\ \hline \multirow{2}{*}{**Grid Cell Level**} & \multicolumn{3}{c}{**Non-Zero Intercept**} & \multicolumn{2}{c}{**Zero Intercept**} \\ \cline{2-6}  & **Slope** & **Intercept** & **R\({}^{2}\)** & **Slope** & **R\({}^{2}\)** \\ \hline All & 24.08 & 0.53 & 0.73 & 26.07 & 0.72 \\ \hline ENF & 27.35 & 0.92 & 0.74 & 31.79 & 0.70 \\ \hline EBF & 17.87 & 3.70 & 0.51 & 30.67 & 0.19 \\ \hline DBF & 22.10 & -0.03 & 0.86 & 22.01 & 0.86 \\ \hline MF & 21.80 & 0.70 & 0.76 & 24.38 & 0.74 \\ \hline OSH & 32.23 & 0.13 & 0.78 & 33.62 & 0.77 \\ \hline SAV & 24.85 & 0.09 & 0.85 & 25.28 & 0.85 \\ \hline WSA & 24.41 & 0.28 & 0.70 & 26.02 & 0.69 \\ \hline GRA & 24.65 & -0.06 & 0.77 & 24.34 & 0.77 \\ \hline WET & 24.38 & 0.17 & 0.89 & 24.89 & 0.89 \\ \hline CRO & 26.16 & -0.59 & 0.63 & 24.04 & 0.62 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Statistical measures for the SIF-GPP relationships at both site and grid cell levels. The slope of the site-level relationship was slightly different from that in [17] because we retained negative SIF as negative values, not as zero.