## 4 Discussion
### Tidal Effects on SIF\({}_{760}\), GPP, and ET Modeling and Underlying Mechanisms
Notably, during tidal inundation, the simulations were optimal when both air and water temperatures were between 21 and 23 \({}^{\circ}\)C, which are the optimum temperatures for vegetation growth. When air temperatures were between 30 and 32 \({}^{\circ}\)C, the simulations performed better under tidal-flooded conditions. This is likely because the fact that the specific heat capacity of water is higher than that of air, resulting in a more suitable temperature and alleviating high-temperature stress on vegetation. Furthermore, tidal inundation can also mitigate the impact of drought on the growth of coastal wetland vegetation. Due to its capacity to increase stomatal conductance, tidal inundation can counteract, within a certain range, the reduction in stomatal conductance and photochemical quantum yield of vegetation caused by drought [58; 59; 60]. The above discussion indicates that accurately modeling of \(\text{SIF}_{760}\), GPP, and ET in coastal wetlands is challenging. Firstly, under water-salt stress, the model struggles to capture the physiological variations in vegetation effectively. Therefore, it is essential to properly represent the effects of water-salt stress factors, such as salinity and inundation depth, on the biochemical and physiological conditions of the vegetation in the GPP-ET-SIF model for coastal wetlands. Secondly, it is necessary to develop a parameterization that can realistically represent the SIF reabsorption and scattering and the light absorption processes between air, water, and soil, as tidal flooding can significantly affect soil-plant-atmosphere radiative transfer processes [25; 61]. Thirdly, the simulations of \(\text{SIF}_{760}\), GPP, and ET performed differently under tidal inundation and non-tidal conditions at different phenological stages. The model showed much better performance for GPP and ET at the peak growth vegetative stage, while in terms of SIF, the simulations were generally better performed at the early and rapid vegetation stage but worse under tidal inundation. Therefore, how to effectively represent the differences in tidal effects across phenological stages and improve the modeling accuracy of coastal marshes under tidal influence during currently underperforming phenological stages are yet to be studied. Implications for Using \(\text{SIF}_{760}\) Remote Sensing to Track GPP and ET Dynamics at Tidal Salt Marshes  
The coupling mechanism between SIF and GPP as well as ET forms the theoretical foundation for using SIF to track the dynamic changes in GPP and ET. Therefore, a comprehensive understanding of the variations in the SIF-GPP and SIF-ET under tidal influence is crucial for establishing methodologies to monitor GPP and ET in coastal wetlands using SIF. Tidal inundation intervenes in the relationships of the SIF-GPP and SIF-ET through  
Figure 12: Variations in half-hourly (**a**) VPD, (**b**) salinity, (**c**) tide height, and (**d**) air, water, and soil temperatures during 2019.  
two pathways: affecting the radiative transfer processes of canopy SIF and altering the physiological processes of vegetation. Firstly, tidal inundation influences the radiative transfer processes of SIF within vegetation and the canopy. The inundation of vegetation intensifies the scattering and reabsorption effects exerted by the water layer on chlorophyll fluorescence, which may result in a diminished canopy SIF being observed. This attenuation may weaken the linear relationships between SIF and GPP, as well as ET, thereby compromising the reliability of using SIF to accurately track dynamic changes in GPP and ET. Furthermore, tidal inundation alters the reflection, absorption, reabsorption, and scattering properties of leaf, vegetation, and soil surfaces with respect to solar radiation, further impacting the radiative transfer within the vegetation canopy. These changes must be adequately considered when utilizing SIF to estimate carbon and water fluxes in coastal salt marsh wetlands. Secondly, tidal inundation also influences the physiological processes of vegetation.