Organ Pipe Cactus National Monument

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 km (1985 mi ). 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.