Daniel A. Friess, Ena Suarez, Anna R. Armitage, Cheryl Doughty, Laura C. Feher, Ilka C. Feller, Alejandro Fierro, Kara R. Radabaugh, Lorae T. Simpson, William C. Vervaeke, Carlos Zamora-Tovar

The ‘Mangroves of the Warm Temperate Northwest Atlantic’ province is a regional ecosystem subgroup (level 4 unit of the IUCN Global Ecosystem Typology). It includes the marine ecoregions of Carolinian and Northern Gulf of Mexico. The biota is characterized by 3 species of mangroves: Avicennia germinans, Laguncularia racemosa, and Rhizophora mangle, and 1 mangrove associate Conocarpus erectus, though not all species are equally distributed throughout the province. Mangroves in this province cross the USA and Mexico and are quite unique, as they exist at the latitudinal range limit of the mangrove ecosystem. The majority of mangroves in this province are located in Florida and Louisiana (USA), though substantial mangrove patches can be found in Texas (USA) and Tamaulipas (Mexico). Small mangrove patches are present in Mississippi and have recently been found in Georgia (USA) at a latitude of 30.74°N, making these some of the northernmost mangroves in the world. Mangroves in this province also experience a longitudinal aridity gradient, with more arid conditions experienced in the west (Tamaulipas, Mexico; Texas, USA) and wetter conditions in the east (Louisiana, Florida, USA). Today, mangroves in the Warm Temperate Northwest Atlantic cover a minimum of 83.11 km2, though due to challenges in measuring the extent of patchy mangroves at their latitudinal range limits, we expect the actual extent to be higher, and we consider the extent presented here to be a substantial underestimate. Based on global datasets, mangrove net area change in this province has been -11% since 1996, with mangrove loss caused by a combination of anthropogenic (land use change, pollution) and climatic (freezes, drought) drivers. If this trend continues, an overall change of -55% is projected over the next 50 years. Furthermore, under a high sea level rise scenario (IPCC RCP8.5) ≈83% of mangroves in the Warm Temperate Northwest Atlantic would be at risk of submergence by 2060. Moreover, ≈3% of the province’s mangrove ecosystem is experiencing degradation, with the potential to increase to ≈8% within a 50-year period, based on a vegetation index decay analysis. Overall, mangroves in the Warm Temperate Northwest Atlantic province are assessed as Critically Endangered (CR). However, this conclusion should be interpreted with caution, due to challenges to mapping mangroves at this latitudinal range limit, as well as limitations to the sea-level rise modelling approaches used in this study. Important data gaps found in this study highlight a strong need for continued focused mangrove research in this province.