Cholla Forest, Lower Sonoran Desert, Arizona (Infrared) ©2019 James D Peterson. Chollas are among the gnarliest (and nastiest) cacti found anywhere. Members of the Opuntia genus, they are related to prickly pears and are found throughout the southwestern US. They sometimes occur in patches called "forests", which are packed with their rugged and contorted forms and which can be pretty risky to navigate.