High and dry: the persistence of diverse coastal marsh communities depends on available upland area and connection to surrounding waters

Sea‐level‐rise and alterations to natural hydrology have contributed to coastal land loss. To combat coastal land loss and maintain the ecosystem services provided by marshes, marsh creation has been broadly implemented. Marsh creation projects can be performed in conjunction with the construction of elevated terrestrial ridges that seek to mimic historic delta‐formed distributary ridges. Ridges and similar elevated structures, such as those surrounding created marshes, offer the opportunity to sample the ecotones between low marsh and upland areas within a small area. We sampled marsh assemblages on elevation gradients at eight sites in the Louisiana coastal zone. We identified nine marsh communities based on co‐occurrence of taxa and abiotic conditions (soil salinity, percent time inundated, soil percent moisture, soil bulk density, and soil pH) with a two‐step process of hierarchical clustering and random forest classification. Inundation time and salinity were identified as the most important conditions related to community composition. We used future estimates of coastal elevations based on local relative sea‐level‐rise estimates to make predictions of community change up to 50 years into the future. Our predictions, meant to serve as generalizations for the coastal region, indicate a near total loss of high‐elevation salt marsh communities at the sites sampled. Elevation variation supports diverse marsh communities when water is allowed to flow as in a natural marsh. Hydrology regulates inundation, and by extension, salinity. Evaluating hydrologic exchange can be incorporated into decisions regarding future marsh creation to support marsh health and persistence.