Areas of Angiosperm Endemism and Floristic Regions in the USA

Aim Insights into complex patterns of endemism are critical for reconstructing the historical assembly of a flora, guiding conservation strategies, and refining biogeographical regionalization. We analyzed a diverse dataset of mostly angiosperms (223,929 georeferenced records for 767 species in 93 families) to delineate areas of endemism and integrate them into a preliminary regionalization framework. Location Southern North America. Methods We applied a computationally intensive grid‐based endemicity analysis method across multiple spatial resolutions. It identifies areas of endemism, defined by optimal clusters of geographic cells maximizing species co‐occurrence and exclusivity. We compared the identified areas with the most recent regionalization framework by dissolving grid cells and merging them into polygons, quantifying their areas, and integrating five area classes with Hausdorff distance measurements to align the results with the Nearctic regionalization. Results We detected 31 areas of endemism and 12 nested sub‐areas in the Nearctic. These were mapped and integrated into a preliminary floristic regionalization, partitioning the region into three major areas: Northwestern and Eastern (Boreal Subkingdom) and Madrean (Madrean Subkingdom) of Takhtajan's Holarctic Kingdom. Across analyses, many areas with particularly high endemicity scores were recovered in the arid zones of the Madrean region, with a strong concentration of narrow endemics. The Madrean area comprised two major, nested or partially overlapping sub‐areas—the Southwestern and Southcentral super areas. The Sonora–Mojave arid center and Chihuahuan Desert, California Floristic Province, and the forested Gulf Coastal Plain and Southern Appalachians emerged as the most significant areas of endemism. Main Conclusions Resolving fine‐scale patterns of endemism is essential for delineation of provinces and subprovinces. Major U.S. floristic regions largely correspond to both classical and recent phylogenetically defined regionalization schemes. Phylogenetic evidence suggests that southern Nearctic harbours basally branching taxa across multiple lineages, offering key insights into North American floristic diversity and conservation priorities.