## 2 Materials and Methods
### PFTs Map and Selection of Homogeneous Samples  
To better understand the response of the GPP/SIF ratio to temperature under different PFTs, we used a vegetation map including 14 vegetated PFTs (Figure 2a). The vegetation map was produced by the European Space Agency (ESA) Climate Change Initiative (CCI) Land-Cover (LC) products, Land-Use Harmonization (LUH2), and an additional partitioning C3/C4 vegetation map using the cross-walking approach [70,71].  
The selection of homogeneous samples was performed on the CCI PFIs map, with a spatial resolution of 0.25\({}^{\circ}\)\(\times\) 0.25\({}^{\circ}\) for each PFT. To ensure sufficient PFT homogeneity and sufficient temporal sampling of the pixels, the selected samples had to satisfy prescribed selection criteria. The clustering approach was used to select samples for each PFT, referring the study of [PERSON] et al. [8]. Only pixels with (1) a fraction of dominant PFT over 50% (Figure 2a), (2) distribution in the North Hemisphere, and (3) at least eight TROPOMI SIF observations for the year 2020 were selected. The reason why only samples distributed in the Northern Hemisphere were selected was to avoid the seasonal pattern differences between the Northern and Southern Hemispheres; additionally, the samples distributed in the Northern Hemisphere contained most vegetation types.  
In addition, as some PFTs were distributed in regions with different temperature conditions, some of the selected samples of each PFT experienced low-temperature conditions, while others grew in relatively warm conditions. Some studies have reported that most photosynthetic activities are limited at temperatures below the \"biological zero\", which is defined as the minimum development temperature and is generally recognized to be in the range of 5-10 \({}^{\circ}\)C, depending on the species and climate [72, 73, 74]. In this study, we recognized temperatures below 10 \({}^{\circ}\)C as low-temperature conditions. Therefore, the selected samples of each PFT were further classified into two groups: one group experienced low-temperature conditions (below 10 \({}^{\circ}\)C) and had more than 8 observations below 10 \({}^{\circ}\)C in the year 2020 (defined as group A) (Figure 3a); the other group grew under temperatures above 10 \({}^{\circ}\)C throughout the year 2020 (defined as group B) (Figure 3b). Finally, the spatial distribution of groups A and B for each PFT can be seen in Figure 3. The number of selected  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Abbreviation**} & \multirow{2}{*}{**Full Name**} & \multicolumn{2}{c}{**Number of Pixels**} \\ \cline{3-4}  & & **Group A** & **Group B** \\ \hline BoDNF & \multicolumn{1}{c}{Boreal deciduous needleleaf forest} & 515 & — \\ TeC3 GRA & \multicolumn{1}{c}{Temperate C3 grass} & 3348 & 134 \\ TrC3 GRA & \multicolumn{1}{c}{Tropical C3 grass} & — & 879 \\ BoC3 GRA & \multicolumn{1}{c}{Boreal C3 grass} & 7125 & — \\ C4 GRA & \multicolumn{1}{c}{C4 grass} & 133 & 1055 \\ C3 CRO & \multicolumn{1}{c}{C3 crops} & 404 & 1644 \\ C4 CRO & \multicolumn{1}{c}{C4 crops} & — & 199 \\ \hline \hline \end{tabular}
\end{table}
Table 1: _Cont._  
Figure 2: (**a**) Map of the dominant plant functional types with a fraction greater than 50%. The full names in the legend can be seen in Table 1; and (**b**) the number of observations (at 8-day temporal resolution) with temperature below 10 \({}^{\circ}\)C for the year 2020.  
homogeneous samples for each PFT can be found in Table 1; it should be noted that not all PFTs belonged to both group A and B.