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ABSTRACT Sea level rise and storm surges affect coastal forests along low‐lying shorelines. Salinization and flooding kill trees and favour the encroachment of salt‐tolerant marsh vegetation. The hydrology of this ecological transition is complex and requires a multidisciplinary approach. Sea level rise (press) and storms (pulses) act on different timescales, affecting the forest vegetation in different ways. Salinization can occur either by vertical infiltration during flooding or from the aquifer driven by tides and sea level rise. Here, we detail the ecohydrological processes acting in the critical zone of retreating coastal forests. An increase in sea level has a three‐pronged effect on flooding and salinization: It raises the maximum elevation of storm surges, shifts the freshwater‐saltwater interface inland, and elevates the water table, leading to surface flooding from below. Trees can modify their root systems and local soil hydrology to better withstand salinization. Hydrological stress from intermittent storm surges inhibits tree growth, as evidenced by tree ring analysis. Tree rings also reveal a lag between the time when tree growth significantly slows and when the tree ultimately dies. Tree dieback reduces transpiration, retaining more water in the soil and creating conditions more favourable for flooding. Sedimentation from storm waters combined to organic matter decomposition can change the landscape, affecting flooding and runoff. Our results indicate that only a multidisciplinary approach can fully capture the ecohydrology of retreating forests in a period of accelerated sea level rise. 

Abstract The interactions between the atmosphere, ocean, and beach in the swash zone are dynamic, influencing water flux and solute exchange across the land‐sea interface. This study employs groundwater simulations to examine the combined effects of waves and evaporation on subsurface flow and salinity dynamics in a shallow beach environment. Our simulations reveal that wave motion generates a saline plume beneath the swash zone, where evaporation induces hypersalinity near the sand surface. This leads to the formation of a hypersaline plume beneath the swash zone during periods of wave recession, which extends vertically downward to a maximum depth of 30 cm, driven by the resulting vertical density gradients. This hypersaline plume moves approximately 2 m landward to the top of the swash zone and down the beachface due to wave‐induced seawater infiltration and is subsequently diluted by the surrounding saline groundwater. Furthermore, swash motion increases near‐surface moisture, leading to an elevated evaporation rate, with dynamic fluctuations in both moisture and evaporation rate due to high‐frequency surface inundation caused by individual waves. Notably, the highest evaporation rates on the swash zone surface do not always correspond to the greatest elevations of salt concentration within the swash zone. This is because optimal moisture is also required—neither too low to impede evaporation nor too high to dilute accumulated salt near the surface. These insights are crucial for enhancing our understanding of coastal groundwater flow, biogeochemical conditions, and the subsequent nutrient cycling and contaminant transport in coastal zones. 

The impact of saltwater intrusion on coastal forests and farmland is typically understood as sea-level-driven inundation of a static terrestrial landscape, where ecosystems neither adapt to nor influence saltwater intrusion. Yet recent observations of tree mortality and reduced crop yields have inspired new process-based research into the hydrologic, geomorphic, biotic, and anthropogenic mechanisms involved. We review several negative feedbacks that help stabilize ecosystems in the early stages of salinity stress (e.g., reduced water use and resource competition in surviving trees, soil accretion, and farmland management). However, processes that reduce salinity are often accompanied by increases in hypoxia and other changes that may amplify saltwater intrusion and vegetation shifts after a threshold is exceeded (e.g., subsidence following tree root mortality). This conceptual framework helps explain observed rates of vegetation change that are less than predicted for a static landscape while recognizing the inevitability of large-scale change. 

Abstract Sea level rise and storm surges are salinizing coastal areas, transforming forests and agricultural fields into salt marshes. This study examines seasonal variations in soil and groundwater hydrology at the transition zone between salt marshes and upland areas along the Mid‐Atlantic coast of the USA. We apply Linear Discriminant Analysis to data sets of soil moisture, soil conductivity, soil temperature, water table depth, groundwater salinity, and groundwater temperature to identify and categorize patterns within hydrological data. We further used Canonical Correlation Analysis to determine relationships between vegetation characteristics (greenness and Basal Area (BA)), soil composition (fraction of clay and sand), and hydrological parameters (groundwater level, groundwater specific conductivity, soil water content, and soil conductivity). Our analysis indicates that storm surges increase spatial variations in seasonal subsurface hydrological conditions among sites, as each location responds differently to disturbance. This heightened inter‐site variability contrasts with previous findings of storm‐driven homogenization within individual sites. By summer, sites become more homogeneous as hydrological variability decreases, indicating that vegetation gradients along the marsh–forest ecotone are mainly governed by winter and spring conditions. Vegetation characteristics (BA and greenness) are more strongly associated with soil water than with groundwater characteristics. Vegetation greenness also correlates closely with soil water conditions in fall and winter, when site differences are more pronounced. Groundwater salinity and water table depth are related to clay content in the soil. This regional‐scale comparison highlights the hydrological variability of the marsh‐upland boundary, how it is influenced by soil characteristics, and its relation to vegetation cover, offering insights for effective adaptation strategies to mitigate salinization and preserve coastal biodiversity. 

Abstract Seawater intrusion (SWI) affects coastal landscapes worldwide. Here we describe the hydrologic pathways through which SWI occurs ‐ over land via storm surge or tidal flooding, under land via groundwater transport, and through watersheds via natural and artificial surface water channels—and how human modifications to those pathways alter patterns of SWI. We present an approach to advance understanding of spatiotemporal patterns of salinization that integrates these hydrologic pathways, their interactions, and how humans modify them. We use examples across the East Coast of the United States that exemplify mechanisms of salinization that have been reported around the planet to illustrate how hydrologic connectivity and human modifications alter patterns of SWI. Finally, we suggest a path for advancing SWI science that includes (a) deploying standardized and well‐distributed sensor networks at local to global scales that intentionally track SWI fronts, (b) employing remote sensing and geospatial imaging techniques targeted at integrating above and belowground patterns of SWI, and (c) continuing to develop data analysis and model‐data fusion techniques to measure the extent, understand the effects, and predict the future of coastal salinization.