AJO PEAK TO TINAJAS ALTAS: FLORA OF SOUTHWESTERN ARIZONA: AN INTRODUCTION

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 hear t of the Sonoran Desert. These three entities encompass 514,242 hectares (1,270,700 acres), approximately 5141 km 2 (1985 mi 2 ). Elevation and ecological diversity generally decreases from eas t (Organ Pipe) to west (Tinajas Altas) while aridity increases from east to west, both correlat ing with decreasing botanical diversity. The lower elevation portions area are within the Lower Color ado Valley subdivision of the Sonoran Desert—the most arid portion of this desert; the higher elev ation portions are within the Arizona Upland subdivision of the Sonoran Desert, and the highest elevat ions of the Ajo Mountains in Organ Pipe have a unique dwarf woodland above the actual desert. The modern flora includes 737 taxa (species, subspecies, var ieties, and hybrids) in 423 genera and 93 families. Non-native species make up 10.9% (80 species) o f the total flora, although only 6.9% (51 species) are established as reproducing populations. T wenty-six of all the non-natives are grasses. Seven non-natives have become invasive species tha t 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 ).