## 3 Results
### Comparison between VIs and GPP  
At both the SHB1 and SHB2 sites, Figure 8 illustrates the linear relationships between the VI\({}_{\text{ECA}}\) and GPP, as well as between the VI\({}_{\text{ECW}}\) and GPP. There were significant differences in the VI-GPP relationships between the two sites. At the SHB1 site, all VI\({}_{\text{ECA}}\) and VI\({}_{\text{ECW}}\) exhibited strong linear relationships with the GPP (with the R\({}^{2}\) ranging between 0.75 and 0.85), and the relationship between the VI\({}_{\text{ECW}}\) and GPP (with the R\({}^{2}\) ranging from 0.76 to 0.85) showed improvement compared to the VI\({}_{\text{ECA}}\) and GPP (with the R\({}^{2}\) ranging from 0.75 to 0.82). Notably, the relationship between the EV\({}_{\text{ECW}}\) and GPP was the strongest, with an R\({}^{2}\) of 0.85. In contrast, at the SHB2 site, the linear relationships between the VIs (both VI\({}_{\text{ECW}}\) and VI\({}_{\text{ECA}}\)) and the GPP were generally weaker, with each VI performing worse than at the SHB1 site.  
Figure 7: Seasonal variations in GPP and SIF (0.05\({}^{\circ}\) SIF, SIF\({}_{\text{ECA}}\), SIF\({}_{\text{ECW}}\)) at different observational scales across two stations.  
At the SHB1 site, the R\({}^{2}\) values between the VI\({}_{\rm ECW}\) and GPP for all VIs were superior to those between the VI\({}_{\rm ECA}\) and GPP. However, at the SHB2 site, only the R\({}^{2}\) values between the VI\({}_{\rm ECW}\) and GPP for NIRv and EVI were higher than those between the VI\({}_{\rm ECA}\) and GPP. Furthermore, at the SHB1 site, the fitted lines of the VI\({}_{\rm ECW}\) and GPP closely approximate those of the VI\({}_{\rm ECA}\) and GPP, whereas at the SHB2 site, there is a noticeable distance between the fitted lines of the VI\({}_{\rm ECW}\) and GPP compared to those of the VI\({}_{\rm ECA}\) and GPP. These results are consistent with the results of the SHB1 and SHB2 sites, which are consistent with the results of the SHB1 and SHB2 sites.  
Figure 8: The linear relationships of the VI\({}_{\rm ECA}\) and VI\({}_{\rm ECW}\) with the GPP at the SHB1 and SHB2 sites: the blue points and lines represent the relationship between the VI\({}_{\rm ECW}\) and GPP, while the red points and lines represent the relationship between the VI\({}_{\rm ECA}\) and GPP. (**a–f**) are the relationships of SHB1 sites Clr, MTCI, CSI, EVI, NIRv, and NDAE1 with GPP, respectively, and (**g–l**) are the relationships of SHB1 sites Clr, MTCI, CSI, EVI, NIRv, and NDAE1 with GPP, respectively.  
results indicate that at the SHB1 site, the values of the VI\({}_{\rm ECW}\) were relatively close to those of the VI\({}_{\rm ECA}\), while at the SHB2 site, there was a significant disparity between the values of the VI\({}_{\rm ECW}\) and the VI\({}_{\rm ECA}\). This discrepancy is attributed to the more homogeneous underlying surface at the SHB1 site compared to the greater spatial heterogeneity at the SHB2 site. Additionally, these findings suggest that at flux sites with complex underlying landscapes, the relationship between the VIs and GPP is generally poor and unstable.  
Figure 9 illustrates the seasonal variations in the VI\({}_{\rm ECW}\) and GPP. At the SHB1 site, throughout the entire growing season, the trends of the VI\({}_{\rm ECW}\) and GPP exhibit a relative correspondence. However, in localized areas, such as during the decline in the GPP in the summer, the VI\({}_{\rm ECW}\) failed to decrease correspondingly with the GPP. At the SHB2 site, the seasonal variations in the VI\({}_{\rm ECW}\) and GPP showed a significant difference. Particularly in the spring and autumn, the relationship between the VI\({}_{\rm ECW}\) and GPP was evidently mismatched. Furthermore, during the summer period of GPP decline, the VI\({}_{\rm ECW}\) at the SHB2 site also did not exhibit a corresponding decrease. In conclusion, it is difficult for VIs to track the seasonal variations in GPP.