Sea-Level Rise and Coastal Forest Retreat on the West Coast of Florida, Usa

Abstract

We investigated patterns, rates, and mechanisms of forest replacement by salt marsh in relation to sea‐level rise on the west coast of Florida, USA. The geomorphology of this region typifies that of low‐lying, limestone coastlines considered highly susceptible to sea‐level rise (e.g., much of the eastern Gulf of Mexico, the Yucatan Peninsula, and low‐lying limestone islands throughout the world). This coast is microtidal, shallowly sloping, and has a rate of relative sea‐level rise similar to that of eustatic rise. To determine patterns of forest change in relation to sea‐level rise, we examined 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 at our site, relating age structures of stands to reconstructed histories of tidal flooding in the stands. Finally, to assess the relative roles of flooding stress (hypoxia), salt exposure, and competition from encroaching salt‐marsh vegetation in the decline of forest stands, we examined patterns of soil redox potential, groundwater salinity, and density of halophytic vegetation among stands in different stages of decline. Zonation among tree species was related to tidal‐flooding frequency. For most 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 cessation 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 sea‐level rise. Forest understory replacement by halophytic vegetation appeared to follow, rather than cause, failure of tree regeneration. Tidal flooding rarely produced severe reducing conditions in soil, but groundwater salinity was correlated with tidal‐flooding frequency. Forest retreat in this system, therefore, involves the development of relict (non‐regenerating) stands of different tree species at different flooding frequencies. Exposure to salt appears to be the major cause of tree regeneration failure, with flooding stress and interference from marsh vegetation 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 larger 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.

Wetlands In a Changing Climate: Science, Policy and Management

“…Without regeneration, vegetation enters a relict state (Williams et al 1999). Worldwide examples of relict foundation species are growing, and such vegetation may be poised for abrupt decline if disturbance removes adult vegetation (Middleton et al 2017).…”

Nonlinear dynamics in ecosystem response to climatic change: Case studies and policy implications

“…Despite low rates of relative sea-level rise, the rapid loss of coastal forests in this region has been attributed to the effects of sea-level rise. During the course of one field study between 1992 and 1996, Williams et al (1999a) observed tree mortality in the frequently flooded forest islands. That study indicated that tree regeneration generally ceases before canopy trees are eliminated.…”

Florida Land Use and Land Cover Change in the Past 100 Years

“…The region's forests, salt marshes, and near coastal marine environments are legendary for their productivity due to a combination of factors including massive fresh water inputs from the Suwannee River and several first-magnitude springs. That said, it has certainly not been spared from human use, which is intensifying, while it is being rapidly inundated again by rising seas (Williams et al 1999;de Santis et al 2007;Raabe and Stumpf 2016).…”