A casualty of climate change? Loss of freshwater forest islands on Florida's Gulf Coast

Sea level rise elicits short‐ and long‐term changes in coastal plant communities by altering the physical conditions that affect ecosystem processes and species distributions. While the effects of sea level rise on salt marshes and mangroves are well studied, we focus on its effects on coastal islands of freshwater forest in Florida's Big Bend region, extending a dataset initiated in 1992. In 2014–2015, we evaluated tree survival, regeneration, and understory composition in 13 previously established plots located along a tidal creek; 10 plots are on forest islands surrounded by salt marsh, and three are in continuous forest. Earlier studies found that salt stress from increased tidal flooding prevented tree regeneration in frequently flooded forest islands. Between 1992 and 2014, tidal flooding of forest islands increased by 22%–117%, corresponding with declines in tree species richness, regeneration, and survival of the dominant tree species, Sabal palmetto (cabbage palm) and Juniperus virginiana (southern red cedar). Rates of S. palmetto and J. virginiana mortality increased nonlinearly over time on the six most frequently flooded islands, while salt marsh herbs and shrubs replaced forest understory vegetation along a tidal flooding gradient. Frequencies of tidal flooding, rates of tree mortality, and understory composition in continuous forest stands remained relatively stable, but tree regeneration substantially declined. Long‐term trends identified in this study demonstrate the effect of sea level rise on spatial and temporal community reassembly trajectories that are dynamically re‐shaping the unique coastal landscape of the Big Bend. Through long‐term field observations, we investigated the replacement of coastal freshwater forest islands by salt‐tolerant vegetation in response to sea level rise. We found that in forest islands subjected to infrequent tidal flooding during a 22‐year period, live trees and an under story composed of forest vegetation persisted, though tree regeneration and species richness declined. In moderately flooding islands, tidal flooding increased 43%–117%; few live, nonregenerating trees remained by 2014, and under stories became dominated by halophytic shrubs. In frequently flooded islands, tidal flooding increased 22%–33%, and freshwater forest was completely replaced by herbaceous salt marsh. Our study demonstrates how climate change‐driven community reassembly is altering the unique coastal landscape of Florida.