An introduction is provided for the modern and fossil vascular plant flora of the contiguous protected areas of Organ Pipe Cactus National Monument, Cabeza Prieta National Wildlife Refuge, and the Tinajas Altas Region in southwestern Arizona—the heart of the Sonoran Desert. These three entities encompass 514,242 hectares (1,270,700 acres), approximately 5141 km2 (1985 mi2). Elevation and ecological diversity generally decreases from east (Organ Pipe) to west (Tinajas Altas) while aridity increases from east to west, both correlating with decreasing botanical diversity. The lower elevation portions area are within the Lower Colorado Valley subdivision of the Sonoran Desert—the most arid portion of this desert; the higher elevation portions are within the Arizona Upland subdivision of the Sonoran Desert, and the highest elevations of the Ajo Mountains in Organ Pipe have a unique dwarf woodland above the actual desert.
The modern flora includes 737 taxa (species, subspecies, varieties, and hybrids) in 423 genera and 93 families. Non-native species make up 10.9% (80 species) of the total flora, although only 6.9% (51 species) are established as reproducing populations. Twenty-six of all the non-natives are grasses. Seven non-natives have become invasive species that pose serious threats to the native ecosystems: red brome (Bromus rubens), buffelgrass (Cenchrus ciliaris), fountain grass (C. setaceus), Arabian and Mediterranean grasses (Schismus arabicus, S. barbatus), Sahara mustard (Brassica tournefortii), and tamarisk (Tamarix chinensis).
The fossil record includes more than 201 species of vascular plant species recovered from packrat (Neotoma spp.) middens spanning more than 43,000 years. The fossils include at least 171 species still present in the flora area, or 26% of the modern native flora of 656 taxa, as well as at least 30 species that are no longer present, or l4.6% of the native flora.
We anticipate multiple forthcoming articles in this journal for a comprehensive temporal flora of the three contiguous protected borderland areas in southwestern Arizona. These floristic treatments will include current taxonomy, original identification keys, brief descriptions, common names when available in English, Spanish, and the local O’odham language, local and global distributions, specimen citations for the modern flora as well as the fossils, and pertinent observations including natural history and literature. These contributions will also be posted open access on the website of the University of Arizona Herbarium (ARIZ).