Plant thresholds and community composition of coastal marsh–forest ecotones in the <scp>US</scp> Northeast

Abstract Sea level rise is causing coastal salt marshes to migrate upslope into coastal forests and other terrestrial ecosystems. The factors that control marsh migration rates are not well understood, particularly in the US Northeast, where this phenomenon has received little attention. To determine the relationship between environmental variables and plant species composition in marsh migration zones, we examined plant coverage and environmental data for sites experiencing mars upslope migration in New Jersey, New York, and Massachusetts that varied in slope from 1% to 3%. We found that only 11% of variation in plant community composition was explained by inundation time, while models containing multiple predictor variables, including salinity, explained much greater variance. Random forest models predicting native halophyte presence had accuracies of 77%–86%, with salinity, flooding duration, and light availability as key predictors. The most accurate model for mature tree presence (82% accuracy) highlighted salinity and redox potential as the most important variables. Threshold Indicator Taxa ANalysis (TITAN) identified salinity changepoints at 0.8 and 7.6 PSU, which delineate the transition from upland forest (&lt;0.8 PSU) to salt marsh (&gt;7.6 PSU), with the marsh–forest ecotone occurring between these two thresholds. Treeline elevation relative to the tidal frame varied among sites, suggesting site‐level differences in mature tree tolerance to tidal flooding. These results highlight the importance of multiple environmental variables interacting to determine species distributions and community composition in marsh–forest ecotones.