Drivers of community composition and groundwater dynamics in salt marsh-forest transition zones

Andrew Robert Payne

doi:10.17918/00011255

Sea level rise is driving the landward migration of coastal salt marshes into upland forests, rapidly transforming coastal landscapes across the northeastern United States. Despite the increasing prevalence of this process, the ecological and hydrologic mechanisms that govern marsh migration and forest retreat remain poorly understood, particularly across sites that differ in slope, soils, and groundwater dynamics. This dissertation examines how environmental gradients, groundwater hydrology, and subsurface salinity structure plant communities and forest persistence across marsh-forest ecotones in New Jersey, New York, and Massachusetts. First, I assessed relationships between plant community composition and environmental variables across sites experiencing upslope marsh migration. Inundation time alone explained a limited proportion of variation in vegetation composition, whereas multivariate models incorporating salinity, flooding duration, redox potential, and light availability explained substantially greater variance. Machine learning models predicted native halophyte presence and mature tree presence with high accuracy (77-86% and 82%, respectively), identifying salinity as a dominant driver of community transitions. Threshold Indicator Taxa Analysis revealed salinity changepoints at 0.8 and 7.6 PSU, delineating upland forest, marsh-forest ecotone, and salt marsh communities. However, treeline position relative to the tidal frame varied among sites, indicating site-specific differences in tree tolerance to tidal flooding and salinity. Second, I investigated groundwater hydrology and salinity dynamics to identify mechanisms underlying forest retreat. Water table elevations were influenced by seasonality, storms, precipitation, and soil characteristics, with consistently higher water tables during the dormant season. Across all sites, water table elevation closely tracked mean high water, producing relatively low hydraulic gradients that left even steeper slopes vulnerable to saltwater intrusion. Sites with higher soil hydraulic conductivity exhibited lower water tables but enhanced tidal advection, suggesting that well-drained soils may simultaneously promote drainage and increase susceptibility to salinization. Flooding of the forest surface had inconsistent effects on groundwater salinity, potentially due to variation in antecedent conditions such as soil saturation. Finally, I used geophysical imaging to characterize spatial and vertical salinity patterns across the marsh-forest boundary. Electromagnetic induction surveys showed consistent increases in salinity with decreasing elevation and proximity to the marsh, with no evidence of hypersaline zones along the retreating forest edge. Electrical resistivity tomography revealed contrasting subsurface structures among sites, including the presence of deeper freshwater layers beneath saline surface soils at some locations. This configuration, distinct from classic estuarine salt wedges, suggests that trees may persist by accessing deeper freshwater reserves despite episodic saltwater inundation. Together, these results demonstrate that marsh migration and forest retreat are governed by interacting surface and subsurface processes rather than inundation alone. By integrating vegetation dynamics, groundwater hydrology, and subsurface salinity structure, this dissertation highlights the importance of local environmental context in shaping coastal ecosystem transitions and improves our understanding of the relative importance of various hydrological drivers of groundwater salinity.

Drivers of community composition and groundwater dynamics in salt marsh-forest transition zones

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Drivers of community composition and groundwater dynamics in salt marsh-forest transition zones

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