Below the red-tile roofs of the Catalina Foothills, an affluent area on the north end of Tucson, Arizona, lays a blanket of desert green: spiky cacti, sword-shaped yucca leaves, and the spindly limbs of palo verde and mesquite trees. Head south into the city, and the vegetation thins. Trees are especially scarce on the south side of town, where shops and schools and housing complexes sprawl across a land encrusted in concrete.