COS 50-6 - Dynamics of vegetation composition and diversity during coastal transgression in the C111 watershed since 1995

Abstract: While not locally rich in plant species, coastal wetland landscapes in the Everglades provide a diverse habitat mixture that supports robust faunal communities. During coastal retreat driven by sea-level rise, zonally arranged wetland communities may migrate inland. However, the transgression process must eventually be slowed or prevented by canals or roads, resulting in coastal wetland loss that may be preceded by a reduction in diversity. In this case study, we asked whether vegetation transgression over a 21-year period in a fixed band midway within the C111 basin resulted in reduced plant diversity at local or sub-regional scales. We sampled plant species composition and potential environmental drivers at 26 coastal wetland locations in 1995-96 and 2016-18, defined vegetation types by cluster analysis, and examined the trajectories of individual sites and 1995-defined types along environmental gradients in non-metric multidimensional scaling ordination space. α-diversity was assessed via the inverse Simpson metric, and β-diversity was determined during each sampling event as the total variance of the community matrices within 1995-defined types and across the study area. The application of a temporal beta diversity index allowed analysis of changes in among-site variation between surveys. Two broad vegetation types occupied the area – an open brackish scrub community dominated by the mangrove Rhizophora mangle closest to the coast, and an interior freshwater marsh in which sawgrass was the leading species. The vegetation ordination indicated a significant shift toward halophyte dominance over time, signaled also by a transformation of 5 of 12 marsh sites to mangrove scrub. Loss of salt-sensitive species caused α-diversity to decline slightly in scrub sites while remaining stable in the interior, where small decreases in freshwater-dependent taxa were matched by rapid mangrove colonization. Similarly, β-diversity was largely unchanged within types and across the study area over time. Our results suggest that sea-level rise-driven taxonomic homogenization in coastal wetland plant communities at sub-regional scales may be slow to develop, lagging niche-based changes in community structure and composition. Nevertheless, diversity metrics are fundamental for coastal restoration aimed at counteracting sea-level rise effects over the long term.