Ongoing Range Shift of Mangrove Foundation Species: Avicennia germinans and Rhizophora mangle in Georgia, United States

Abstract Climate change is leading to shifting vegetation patterns in coastal areas worldwide. One such shift in vegetation is the ability of tropical species to establish and survive poleward of recent historical distributions. A well-studied example of this tropicalization is the expansion of mangroves into saltmarshes, creating a marsh-mangrove ecotone thought to be controlled by a decrease in extreme freeze events in the southeastern USA. However, accurate mangrove distribution data at their northern limit in this region are currently lacking. Here, we report information collected from targeted surveys along the Atlantic coasts of Florida and Georgia at the northern limit of mangroves in the USA. We document both Avicennia germinans and Rhizophora mangle individuals naturally occurring in Georgia, poleward of previous reports by more than 23 km ( A. germinans ) and by almost 80 km ( R. mangle ), respectively. Our results highlight the lack of distributional information in this under-sampled region and the need for additional study of species-specific physiology and tolerance, especially for R. mangle , which exhibited surprising resilience and responses to freeze events. These data will be crucial to validate models showing a northern shift of mangroves. Knowing where mangroves have been established and the associated conditions that allowed establishment will help inform models of tropicalization and ecotonal shifts in areas where foundation species are expanding, giving managers and researchers the needed insight as current landscapes change.