## 4 Discussion  
### Why Does the GPP/SIF Ratio Decrease at Low Temperatures  
Using satellite and tower-based data, we observed strong seasonal patterns in the GPP/SIF ratio and obvious differences in the GPP/SIF ratio for different PFTs. Although the SIF had a consistently seasonal pattern with GPP across different vegetation types, we found that there were noticeable differences in the seasonal patterns of the GPP/SIF  
Figure 14: Seasonal dynamics of five-day running means of cumulative daily GPP (green lines) at XTS ((**a**) 2020–2021) and Niwot Ridge sites ((**b**) year 2017–2018); the corresponding seasonal variations in daily average SIF (blue lines) (**c**), XTS site, (**d**), Niwot Ridge site); and the variations in GPP/SIF ratio with air temperature (Ta) ((**e**), XTS site, (**f**), Niwot Ridge site). The gray dotted line is the 10 \({}^{\circ}\)C line. The 90 % confidence interval is indicated by the shaded area. ratios between the selected samples that experienced low-temperature stress (below 10 \({}^{\circ}\)C) and those that grew under relatively warm conditions (above 10 \({}^{\circ}\)C) (Figures 9 and 10). The GPP/SIF ratio was generally positively related to temperatures below 10 \({}^{\circ}\)C, but did not consistently respond to temperatures above 10 \({}^{\circ}\)C (Figure 8, Table 2). For regions where low-temperature conditions do not occur (e.g., tropical regions), temperature may not be a factor limiting the GPP/SIF ratio (Figures 5 and S1; [41; 77]). Due to the selected samples in group A generally experiencing low-growth-temperature conditions (e.g., boreal ecosystems), with decreasing temperature, the GPP decreased faster than the SIF, and, hence, the GPP/SIF ratio declined again (Figures 9 and S1; [42]). In addition, the tower-based measurements indicated that the response of the GPP/SIF ratio to high temperatures differed with different PFTs, which is inconsistent with the results obtained based on satellite data (Figures 11, 12 and 14e,f). However, the GPP/SIF ratio decreased at low temperatures (below 10 \({}^{\circ}\)C) for both C3 CRO and BoENF in the results based on both satellite and tower-based observations. Small seasonal changes in canopy structure and chlorophyll content in BoENF also provide reliable evidence that the GPP/SIF ratio is related to changes in physiological factors affected by low temperatures. The GPP/SIF ratio generally had a low value at low temperatures, suggesting that the SIF-GPP relationship should be not assumed to be constant across PFTs and climates [1; 57; 78]. The reason why the GPP/SIF ratio decreases under low-temperature conditions may be attributed to the physiological differences between SIF and GPP in response to low temperature [21; 79]. As SIF is closely related to light reactions, the differences between light and carbon reactions in response to environmental pressure may affect the SIF-GPP relationships [27; 35; 36]. Low temperatures impose thermodynamic constraints, slowing the enzymes related to the activities of the Calvin cycle. The decrease in photosynthetic enzyme activity with low temperature might explain the faster decrease in GPP than SIF, thus decreasing the GPP/SIF ratio [34]. In addition, the potential change of distribution in the absorbed energy among PQ, NPQ, and SIF may be another factor affecting the SIF-GPP relationship under low-temperature conditions [5]. Some studies have reported that the proportion of energy allocated to each energy-dissipated pathway is not constant under different environmental stresses [33]. Under low-temperature conditions, the light-harvesting complex (LHC) cannot completely utilize the absorbed light for CO\({}_{2}\) fixation [49]. NPQ is an important photoprotective mechanism that plays a role in protecting photosystem II (PSII) from damage under low-temperature conditions [16; 46].