## 2 Materials and Methods
### Downscaling from Canopy to Photosystem Levels  
Due to scattering and reabsorption effects, SIF observed at the canopy scale represents only a small portion of the total SIF that chlorophyll molecules emit. In addition, canopy SIF contains contributions from both PSI and PSII photosynthetic activities. Therefore, to better understand the SIF-GPP relationship, PSI SIF and PSII SIF should be partitioned, and canopy-level SIF should be downscaled to photosystem-level SIF. According to [PERSON] et al. [46], the linear combination of PSI and PSII fluorescence radiance can be used to describe canopy SIF at 760 nm (\(\text{SIF}_{\text{TOC\_760}}\), mW/m\({}^{2}\)/nm/sr). The proportionate contribution of PSII SIF at the canopy level should be equal to that at the photosystem level. First, we excluded the effects of PSI SIF and calculated the contribution of PSII to \(\text{SIF}_{\text{TOC\_760}}\)[21]:  
\[\text{SIF}_{\text{TOC\_760, PSII}}=\text{SIF}_{\text{TOC\_760}}\times\text{f}_ {\text{PSII}} \tag{1}\]\[\mathrm{f_{PSII}}=\frac{\mathrm{m_{2}\times\varepsilon}}{\mathrm{m_{1}+m_{2}\times \varepsilon}} \tag{2}\]  
where \(\mathrm{f_{PSII}}\) (%) indicates the contributions of PSII fluorescence to the \(\mathrm{SIF_{TOT\_760}}\); \(\mathrm{m_{1}}\) and \(\mathrm{m_{2}}\) were obtained using a large number of SCOPE (Soil Canopy Observation, Photochemistry, and Energy fluxes) simulations and were equivalent to 0.00561 and 0.00917; \(\varepsilon\) represents the ratio of PSII steady-state fluorescence to minimum fluorescence, which has a maximum percentage of open PSII reaction centers [21]. Second, canopy-directional \(\mathrm{SIF_{TOT\_760\_PSII}}\) should be divided from the escape probability at 760 nm (\(\mathrm{f_{esc\_P-C}}\)) to obtain photosystem-level \(\mathrm{SIF_{TOT\_760\_PSII}}\), \(\mathrm{mW/m^{2}/nm}\)):  
\[\mathrm{SIF_{TOT\_760\_PSII}}=\frac{\mathrm{SIF_{TOT\_760\_PSII}}}{\mathrm{f_{ esc\_P-C}}} \tag{3}\]  
According to [PERSON] et al. [15], the probability of \(\mathrm{SIF}\) photons escaping from a leaf surface to the canopy's top (\(\mathrm{f_{esc\_L-C}}\)) can be described as  
\[\mathrm{f_{esc\_L-C}}\approx\frac{\mathrm{\rho_{NIR}\times NDVI}}{\mathrm{ fPAR}} \tag{4}\]  
where \(\mathrm{\rho_{NIR}}\) represents reflectance at the near-infrared band (760 nm in this study); fPAR indicates the percentage of absorbed photosynthetic active radiation. The probability of \(\mathrm{SIF}\) photons escaping from the chlorophyll molecules to the leaf surface (\(\mathrm{f_{esc\_P-L}}\)) can be roughly predicted using the leaf albedo and has a value of 0.9 in the near-infrared region, which is rather stable [20]. Therefore, in the NIR band, \(\mathrm{f_{esc\_P-C}}\) could be approximately calculated as follows:  
\[\mathrm{f_{esc\_P-C}}=0.9\times\mathrm{f_{esc\_L-C}} \tag{5}\]  
Finally, total PSII \(\mathrm{SIF}\) within the 640-850 nm spectral range (\(\mathrm{SIF_{TOT\_FULL\_PSII}}\)) could be estimated from \(\mathrm{SIF_{TOT\_760\_PSII}}\). According to [PERSON] et al. [21], PSII \(\mathrm{SIF}\)'s spectral shape is mostly stable, despite the complex process of \(\mathrm{SIF}\) escaping throughout the leaf and canopy. Total PSII \(\mathrm{SIF}\) at a specific wavelength (\(\mathrm{SIF_{TOT\_\lambda\_PSII}}\)) can be estimated from \(\mathrm{SIF_{TOT\_760\_PSII}}\) by multiplying the first principal component (PC1) obtained from the relationship between \(\mathrm{SIF_{TOT\_\lambda\_PSII}}\) and \(\mathrm{SIF_{TOT\_760\_PSII}}\) (defined as the conversion ratio, \(\mathrm{f_{C}(\lambda)}\)).