High vascular plant species richness in the Usumacinta River Basin: a comprehensive floristic checklist for a natural region in the Mesoamerican biodiversity hotspot

Background: Mesoamerica is one of the most important biodiversity hotspots on the planet. Despite significant efforts made over two centuries to contribute to the floristic knowledge of this region, our understanding of its flora is still scattered and uneven. Questions: What is the magnitude of the vascular plant species richness in the Usumacinta River Basin? Study site and dates: Usumacinta River Basin (Guatemala and Mexico), 1838-2018. Methods: We compiled the checklist by systematizing the floristic information acquired from various sources derived from numerous floristic and ecological studies. Results: We recorded 6,977 species, 1,892 genera and 274 families. The largest numbers of species (5,746) and records (58,859) correspond to the Mexican portion of the Usumacinta River Basin, compared to its Guatemalan counterpart (4,445 species and 19,952 records). The most species-rich families were Orchidaceae (598 species), Fabaceae (512), and Asteraceae (476). The prevalence of these and all other families with significant contributions to the flora varied among three elevation-defined sectors into which the Usumacinta River Basin was subdivided (lower, middle, and upper basin). Conclusions: The Usumacinta River Basin is a strategic region for plant biodiversity conservation as it hosts almost one third of all vascular plant species known for Mesoamerica and ca. 6 % of the entire flora in the Americas. Further botanical exploration should focus on those areas of the basin for which little or no information is available in order to gain a better appreciation of its flora.