Paleo Slide Set: Packrat Middens: Vegetation & Climate Variability in the Southwestern United States
Upper Sonoran Desert vegetation along the Apache Trail, central Arizona.

No glacial records of key Sonoran Desert plants such as the giant saguaro (Carnegiea giganteae) or 
paloverdes (Cercidium spp.) have been found north of the U.S.-Mexico border. One hypothesis to explain 
vegetation now encountered in the outskirts of El Paso, Phoenix, and Las Vegas is that the modern distribution 
of this vegetation is the product of plant migration during the past 10,000 years. 

Photo Credits:
Julio L. Betancourt 
U.S. Geological Survey