In the Santa Cruz Flats in southern Arizona, the beige of sandy plains, dunes, and clay-filled basins alternate with green swaths of irrigated cotton fields. Save for a rare jackrabbit, the only mammals are underground. Bird-watchers come to see migrating hawks perched on power poles waiting for rodents to venture forth into one of the remaining cultivated maize fields or sod farms.
To call the Santa Cruz Flats a man-killing dust bowl is no exaggeration. Fierce dust storms known as haboobs blow across barren “playacitas”–little beaches of bare ground. When the Flats’ fine surface soils are swept away by dust storms, it’s so hard to see that scores of people have died in rear-end traffic accidents on Interstate 10. The annual rainfall is less than 9 inches, more than half of which falls during a monsoon season that may flood a normally dry Santa Cruz River to overflowing.
It may be hard to believe, but, until about 100 years ago, portions of the Flats supported a thriving, if fragile, grassland. In the summers, you could walk among grasses named Rothrock grama, Arizona cottontop, and tanglehead that grew up to 2 feet high. Ironwood and acacia trees regularly dotted the landscape and provided shade for mule deer and pronghorn antelope. Two species of jackrabbits were abundant, and, at the Flats’ southern extremities, you might catch a glimpse of a shy bobwhite quail. After a humid thunder-and-lightning storm passed, such diverse species as the green toad and the cheddar-yellow Arizona caltrop flower would wake up from their slumber as if by magic.
The Santa Cruz Flats were part of an ecosystem known as the Sonoran Savanna Grassland that once covered portions of lowland southern Arizona below elevations of 3,500 feet in places like Tubac, Altar Valley, Avra Valley, and Red Rock. This grassland received an annual average rainfall from 11 to 16 inches–a little more than nearby Tucson–with half or more of this falling during the July to September monsoon. The lower, drier parts of this grassland–such as the Santa Cruz Flats–benefited from flooding from the Santa Cruz River and from soil conditions with good moisture retention. There were no large grazing animals and fires ignited by lightning blazed occasionally across the landscape.
What used to be Sonoran Savanna Grassland has been swallowed up by the surrounding Sonoran Desert. Most sites are now so lacking in grass cover that some biologists have questioned whether they ever had grasses at all. Indeed, a century of farming and grazing appear to have made this change irreversible. Like the Sahara Desert’s relentless advance on the Sahel (the African transition zone in between the desert and the savanna), the retreat of Arizona’s savanna has been a one-way exodus. Only historical photographs and a few areas at the upper edges of Sonoran Savanna Grassland that retain their integrity give us a picture what has been lost.