## 5 Conclusions  
This study integrates long-term carbon flux observations, hydrological data, and tree mortality records to identify the key drivers, predictive indicators, and actionable thresholds of coastal forest decline under rising sea levels. From 2009 to 2015, the wetland ecosystem alternated between flooded and non-flooded conditions, with daily NEE fluctuating between net carbon uptake and release. Non-flooded periods saw significant increases in both GPP and RE. After 2016, non-flooded conditions became infrequent, and elevated RE, likely driven by enhanced microbial decomposition and root respiration during these brief non-flooded periods, consistently exceeded GPP. As a result, NEE variability increased, and the wetland transitioned to a persistent net carbon source. Our results also demonstrate that GWT dynamics, rather than SLR directly, is the dominant hydrological control shaping tree mortality risk and land--atmosphere C exchange. Through GAM, we found that GWT, LAI, NEE, and Rn together explained nearly all observed variations in tree mortality, underscoring the multi-dimensional nature of ecosystem collapse in water-logged environments. However, these findings should be interpreted with caution, as their applicability may vary without site-specific verification. Broader studies across diverse wetland ecosystems, including assessments of tree health, physiological function, and structural integrity, are needed to evaluate generalizability and identify key drivers across varying contexts. By applying ROC analysis, we established empirically derived thresholds for each variable, enabling the development of early warning systems to anticipate mortality transitions. While these thresholds may shift with climate change, plant succession, and land-use changes, they offer a crucial foundation for developing dynamic, data-driven early warning systems for forest vulnerability. These findings not only advance our mechanistic understanding of ghost forest formation but also provide practical tools for adaptive ecosystem management, such as hydrological buffering and remote canopy monitoring. Importantly, the framework developed here bridges the gap between process-based modeling and real-world application, offering a replicable approach for other vulnerable bottomland hardwood wetland forests. As climate change drives faster sea level rise, more extreme rainfall events, and altered hydrological regimes, the ability to anticipate and respond to ecological tipping points is critical for maintaining the resilience of coastal carbon sinks. Future research should prioritize refining these thresholds and testing their applicability across different spatial scales and vegetation types to inform sustainable coastal land use, guide wetland restoration, and improve carbon accounting practices. Conceptualization, M.A. and J.Z. ; methodology, M.A. and J.Z. ; software, M.A. and J.Z. ; validation, M.A. and M.L.B. ; formal analysis, M.A. and J.Z. ; investigation, M.A. and J.K.; resources, M.A. and J.K.; data curation, M.A. and J.Z. ; writing--original draft preparation, M.A., J.Z. and M.L.B. ; writing--review and editing, M.A., J.Z., M.L.B., G.S., S.M. and J.K.; visualization, J.Z., M.A. and M.L.B. ; supervision, G.S., S.M. and J.K.; project administration, M.A. and J.K.; funding acquisition, G.S., S.M. and J.K. All authors have read and agreed to the published version of the manuscript. This research was funded by the USDA NIFA (Multi-agency A.5 Carbon Cycle Science Program) award 2014-67003-22068. Additional funding was provided by the DOE NICCR award 08-SC-NICCR-1072, the USDA Forest Service award 13-JV-11330110-081, and the DOE LBNL award DE-AC02-05 CH11231.