## 4 Discussion
### Groundwater Table as the Key Driver in Carbon Fluxes  
Our results identified GWT as a consistently stronger predictor of carbon fluxes than SLR directly, in particular GPP (R\({}^{2}\) = 0.29) and RE (R\({}^{2}\) = 0.32), although the NEE-GWT relationship was weak (R\({}^{2}\) = 0.05), reflecting GWT's indirect ecological effects on NEE. These findings are consistent with the hypothesis that GPP and RE are highly sensitive to soil water content and oxygen availability, both of which are directly regulated by shallow groundwater dynamics [55] during non-flooded conditions.  
The high predictive power of GWT for RE supports previous findings that prolonged high water tables promote anaerobic microbial respiration, which becomes the dominant pathway for carbon mineralization in waterlogged soils [53; 56]. Moreover, as GWT rises, the zone of active root respiration shifts closer to the surface, consuming available oxygen and intensifying the redox sensitivity of the system [53; 56]. These hydrological controls are especially critical in forested wetlands, where fluctuations in GWT can determine vegetation composition, productivity, and long-term carbon storage capacity. The weaker relationship between GWT and NEE, compared to its effect on GPP and RE, likely stems from the composite nature of NEE, which integrates opposing trends of carbon uptake and release. While GPP may respond positively to short-term soil water increases, RE often exhibits slight increases under saturated conditions relative to GPP, ultimately diminishing or reversing the net carbon sink capacity [57; 58]. This decoupling highlights the importance of analyzing GPP and RE separately, especially in systems experiencing hydrological extremes.  
Importantly, our analysis suggests that GWT serves as a proximal ecological control, mediating both biological activity and physical transport processes in the soil-plant-atmosphere continuum [59]. These effects are further modulated by factors such as vegetation type, microtopography, and antecedent hydrological conditions [59]. In contrast, SLR primarily acts through long-term shifts in baseline water levels and saltwater intrusion, which may only translate into ecological changes after sustained exposure or in topographically low-lying zones in proximity to sources of salinity [2; 4].  
Further, GWT functions as a proximate driver, that is, immediately and directly influencing tree mortality at the stand level. GWT directly controls root-zone saturation and oxygen availability, which critically affect physiological processes such as gas exchange, root respiration, and water uptake in wetland trees [3; 60]. However, GWT is modulated by broader-scale boundary drivers, particularly long-term environmental forces such as sea level rise (SLR) [13].  
Rising sea level can gradually alter hydrological regimes by increasing baseline groundwater levels, impeding the drainage of inland freshwater systems, and leading to persistently elevated groundwater tables, and in certain landscape settings, saltwater intrusion [2; 8]. These changes effectively shift the environmental baseline over which proximate stressors operate, thereby intensifying tree stress and mortality risk. As a result, future ecosystem transitions from forested wetlands to marshlands or the development of \"ghost forests\" are likely to arise from the cumulative effects of proximate stress acting under evolving boundary conditions [5; 6].  
Overall, these results establish GWT as the dominant hydrological driver of carbon fluxes at our study site. With climate change driving faster sea level rise and more frequent extreme rainfall events, tracking groundwater table (GWT) changes will be vital for forecasting ecosystem shifts and guiding adaptive management efforts.