Precise Elevation Thresholds Associated with Salt Marsh–Upland Ecotones along the Mississippi Gulf Coast

Coastal marshes provide essential ecosystem services related to biodiversity, water quality, and protection from erosion. As increasing rates of relative sea-level rise affect many coastal marsh systems, a thorough understanding of marsh responses to sea-level change, particularly the migration of marsh–upland boundaries, becomes essential. The goal of this study was to determine precise elevation thresholds associated with coastal marsh, the marsh–upland ecotone, and upland plant communities along Mississippi’s Gulf of Mexico coast (diurnal, microtidal). Elevations (NAVD88) were measured using survey-grade Global Navigation Satellite System solutions integrated with high-precision leveling. Plant species were sampled at approximately 1-m intervals along each of thirty-three transects extending from intermediate marsh through the marsh–upland ecotone. Elevation thresholds associated with plant community change were determined based on relevant quartiles of the data. Probabilities of occurrence of each plant community type were computed for elevations at the centimeter scale. Results indicated transitions from marsh to ecotone and ecotone to upland at elevations of approximately 0.40 m and 0.60 m, respectively. Understanding the precise nature of these centimeter-scale dependencies of marsh vegetation on coastal elevation will facilitate spatial modeling of marsh transgression in response to sea-level rise, subsidence, changes in sediment flux, and land use change