Sea-level rise and coastal forest retreat on the west coast of Florida, USA

We investigated patterns, rates, and mechanisms of forest replacement by sa lt marsh in relation to sea-level rise on the west coast of Florida, USA. T he geomorphology of this region typifies that of low-lying, limestone coast lines considered highly susceptible to sea-level rise (e.g., much of the ea stern Gulf of Mexico, the Yucatan Peninsula, and low-lying limestone island s throughout the world). This coast is microtidal, shallowly sloping, and h as a rate of relative sea-level rise similar to that of eustatic rise. To d etermine patterns of forest change in relation to sea-level rise, we examin ed patterns of tree-species zonation, tree recruitment, and tree mortality in relation to site elevation and tidal-flooding frequency. To reconstruct histories of forest change in relation to sea-level rise, we estimated age distributions of Sabal palmetto, the most widely distributed tree species a t our site, relating age structures of stands to reconstructed histories of tidal flooding in the stands. Finally, to assess the relative roles of flo oding stress (hypoxia), salt exposure, and competition from encroaching sal t-marsh vegetation in the decline of forest stands, we examined patterns of soil redox potential, groundwater salinity, and density of halophytic vege tation among stands in different stages of decline. Zonation among tree species was related to tidal-flooding frequency. For mo st trees, seedlings were absent from the most frequently flooded stands in which the species occurred. Reconstructed flooding histories of stands and age estimates for S. palmetto suggest that many decades elapse between cess ation of regeneration and local elimination of a tree species. Even during the relatively short duration of the study (4 yr), however, composition of some stands changed in the direction predicted from species zonation and se a-level rise. Forest understory replacement by halophytic vegetation appear ed to follow, rather than cause, failure of tree regeneration. Tidal floodi ng rarely produced severe reducing conditions in soil, but groundwater sali nity was correlated with tidal-flooding frequency. Forest retreat in this s ystem, therefore, involves the development of relict (non-regenerating) sta nds of different tree species at different flooding frequencies. Exposure t o salt appears to be the major cause of tree regeneration failure, with flo oding stress and interference from marsh playing minor or negligible roles. These interactions differ somewhat from those on deltaic coasts or coasts with high freshwater outflows, where flooding stress may play a larger role in regeneration failure, and from sandy coasts, where erosion may play a l arger role in forest retreat. Regardless of the cause of tree regeneration failure, the development of relict stands may be a general forest response to sea-level rise.