## 4 Discussion
### Uncertainties of SIF and GPP Relationship under Stressful Conditions  
The relationship between SIF and photosynthesis is complex, especially when vegetation is under stress, making it challenging to estimate GPP using SIF. Therefore, a better understanding of the relationships between vegetation photosynthesis, fluorescence, and non-photochemical quenching (NPQ) under stress conditions is necessary for a more accurate estimation of GPP using SIF. In this study, using the 2009 drought event in the Xlingol grasslands as an example, the ability of SIF to monitor GPP changes caused by drought was investigated. According to the light use efficiency (LUE) model, GPP can be expressed as follows:  
\[\text{GPP}=\text{fPAR}\times\text{PAR}\times\text{LUE}_{\text{p}}=\text{APAR} \times\text{LUE}_{\text{p}}, \tag{3}\]  
where LUEp is photosynthetic light use efficiency. Similarly, SIF can be expressed as follows:  
\[\text{SIF}=\text{fPAR}\times\text{PAR}\times\text{LUE}_{\text{f}}\times \text{f}_{\text{esc}}=\text{APAR}\times\text{LUE}_{\text{f}}\times\text{f}_{ \text{esc}}, \tag{4}\]  
where LUEf denotes the light use efficiency of SIF, and fsec is the fraction of SIF photons escaping the canopy. Using Equations (3) and (4), the SIF and GPP relationship can be shown as follows:  
\[\text{GPP}=\text{LUE}_{\text{p}}/(\text{LUE}_{\text{f}}\times\text{f}_{ \text{esc}})\times\text{SIF}, \tag{5}\]  
The relationship between SIF and GPP depends on factors such as the LUEf (light use efficiency of SIF), LUEp (the efficiency of light utilization for photosynthesis), and the fraction of SIF photons escaping the canopy. Major factors influencing LUE and fluorescence quantum yield include canopy structure, leaf morphology, photosynthetic pathway (C3 or C4), and environmental conditions, while the escape coefficient is sensitive to changes in canopy structure and Leaf Area Index (LAI) [51]. These factors collectively influence the SIF and GPP relationship. However, relationships observed at the leaf, canopy, and short-time scales may not necessarily apply at larger scales or over longer time scales. Interactions among these factors during the upscaling process introduce uncertainties in the impact of these factors on the SIF and GPP relationship. Considerable research has been conducted on the ability of SIF to estimate GPP under different stress conditions. Results indicate that under non-stress conditions, SIF and GPP exhibit a strong linear correlation, while under stress conditions (drought, heat, herbicide application, etc.) [62; 63; 64], the ability of SIF to track GPP changes varies. Our results show a significant linear correlation between SIF and GPP under non-drought stress conditions (R\({}^{2}\) = 0.95), with a slight decrease in correlation under drought conditions (R\({}^{2}\) = 0.90). In summary, both under non-drought and drought conditions, a strong correlation exists between SIF and GPP, consistent with many large-scale studies. However, our study differs from some experimental sites. For instance, the relationship between SIF and GPP at different stages after herbicide treatment differs. In the early stages following herbicide treatment, SIF is negatively correlated with GPP, while with time, a significant positive correlation between SIF and GPP emerges. Under drought conditions, the correlation between SIF and GPP significantly decreases. Under heat stress, the linear relationship between SIF and GPP breaks down, exhibiting a non-linear relationship. This is because drought, heat, or herbicide stress can slowly impact vegetation photosynthesis by inducing stomatal closure and reducing carbon assimilation rates. Therefore, drought, heat, or herbicide stress does not necessarily cause a significant or abrupt decrease in SIF. However, drought, heat, or herbicide stress often leads to a reduction in vegetation photosynthesis, resulting in an increase in NPQ [65].