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

Abstract Ongoing 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 farther north than they would have in the past. A well-studied example of this tropicalization is the expansion of mangroves at the marsh-mangrove ecotone, thought to be controlled by a decrease in extreme freeze events in the SE USA. However, accurate mangrove distribution data at their poleward limit in this region is currently lacking. Here, we report information collected from targeted surveys along the Atlantic coasts of Florida and Georgia at the poleward limit of mangroves in the USA. We document both Avicennia germinans and Rhizophora mangle individuals naturally occuring in Georgia, USA, poleward of previous reports by more than 23 km (A. germinans) and by almost 80 km (R. mangle), respectively. 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 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 needed insight as current landscapes change.