<p>Sea-level rise is leading to the transgression of coastal ecosystems, including the retreat of barrier islands, marshes, and coastal forests. Along the Mid-Atlantic coast of the USA, rapid sea-level rise has converted approximately 240 km² of forest to salt marsh in recent decades. A primary cause of forest retreat is increased soil salinity; however, data informing salinity thresholds that trigger forest-to-marsh conversion are scarce. To better understand the dynamics of sea-level driven ecosystem turnover and identify salinity thresholds marking that turnover, we evaluated characteristics across vegetation zones at five sites along the Mid-Atlantic, including tree health, vegetation structure, and species composition. Vegetation zones spanned gradients in soil salinity and elevation, providing space-for-time substitutions that captured snapshots of local ecosystem response. We found that tree canopy cover declined and salt marsh cover increased abruptly with increasing soil salinity, at salinity thresholds from 1.5 to 4.2 ppt. Accompanying ecosystem transition, we found that species richness of trees decreased from high-elevation forest to the forest-to-marsh transition zone, generally along a gradient of increasing soil salinity and, although tree seedlings were frequently present in forest understory, few saplings were present, indicating a decline in forest regeneration. Together, these results identify important thresholds for state changes between forest and marsh in a regional sea-level rise hotspot. Our findings also suggest consistent structural and compositional changes in vegetation occurring in disparate coastal forests, highlighting sea-level rise as a dominant driver of coastal ecosystem reorganization.</p>