Saguaro National Park

My guess is that it is the state tree of Arizona, Cercidium floridum, commonly known as Palo Verde (green stick) Retama, Jerusalem Thorn (a corruption of Girasol-turns toward the sun) and Lluvia de Oro (golden rain). The green stems and bark of Palo Verde make it easy to identify but more importantly allow it to survive in harsh desert environments. Palo Verde trees are drought deciduous and thus shed their leaves during extended dry spells. When no leaves are present the tree's green stems and branches carry on the essential energy-producing process of photosynthesis. The leaves of Palo Verde are so small that even during the short periods (mid July to late November) when they are present they still need assistance with photosynthesis. As Palo Verde trees age, the bases of their trunks trunks turn from green to a very distinct grey. Residents of the Arizona and the southwest in sprawling Sonoran Desert know the yellow flowers of the Palo Verde  tree as harbinger of Spring in the region.