The Sonoran Desert Heart

he heart of the Sonoran Desert, a land mass larger than Connecticut, sprawls across the southwestern corner of Arizona and into the northern reaches of Sonora, Mexico; this region encompasses Organ Pipe Cactus National Monument, Cabeza Prieta National Wildlife Refuge, the Barry M. Goldwater Gunnery Range, and the Sierra Pinacate Biosphere Reserve.

Summer heat is stunning. In mid-June, a thermometer perched six feet off the ground may stall between 110° and 120°F day after day, while at ground level readings reach 180°. Searing dryness dominates this place. In the Sonoran’s most arid regions where in prosperous rain-years three inches might fall, annual evaporation capacity can exceed 10 feet. Every natural feature – the contour of a saguaro cactus column, the color of a lizard, the gradient of a dry wash, the clarity of the night sky, the density of soil, the spoor of a serpent, the flowering of an ocotillo – is an evolutionary spinoff of this extreme heat and aridity.

Rainfall, when it does come, varies from an average of nine inches annually in Organ Pipe National Monument on the eastern boundary to scarcely three inches in the Yuma Desert on the western perimeter where two or three years may pass without measurable precipitation. Occasionally, after a violent summer thunderstorm, the central arroyos, Growler Wash and San Cristobal Wash, carry water ephemerally and ponds fill at Las Playas, a dry lake bed.

No perennial streams flow here, and springs that produce reliable water are fewer than the fingers on one hand. Often, the only water to drink is ephemeral rainwater trapped in natural rock catchments. Perhaps by caprice or by a kind of logic turned on end in a place where water is desperately sought, nearly half of all place names – Agua Dulce, Tinajas Altas, Quitobaquito Spring, Papago Tanks, Bates Well – scream "water" in boldface. Names aside, travelers through the region carry their own water – or they die.

Yet, this Sonoran Desert core is the most species-rich and biologically-diverse arid land on Earth – far more diverse in life forms, for example, than the three other North American deserts that lie partly within Arizona: the Great Basin, Mohave, and Chihuahuan. In the Sonoran, botanists count more than 600 plant species; saguaro, organ pipe, and senita cacti prosper here, along with velvet mesquite, blue palo verde, elephant tree, white-thorn acacia, and ironwood.

In years of ideal weather, myriad spring wildflowers -- including Ajo lily, Mexican poppy, desert lupine, dune evening primrose, sand verbena, and desert marigold -- carpet the desert. On the highest peaks, where elevations approach 5,000 feet, more than 140 rare "sky-island" plant species thrive, among them a Pleistocene-remnant juniper and Quercus ajoensis, an endemic oak.

The endangered Sonoran pronghorn and lesser long-nosed bat endure here, too, and desert bighorn sheep clamber steep rock faces. In the wildest recesses dwell animals whose habitat has never been trespassed by human tread, among them: badger, kit fox, mule deer, and mountain lion; Mojave rattler, desert tortoise, Gila monster, and rosy boa; raven, phainopepla, loggerhead shrike, Say’s phoebe, prairie falcon, and black-throated sparrow.

Two intertwined factors – location and climate – contribute to the unique biodiversity of the Sonoran Desert. Its location is subtropical. Consequently, the Sonoran is less susceptible to killing winter frosts that limit plant growth in the more northern Great Basin or the Chihuahuan Desert, which, although farther south, is higher and exposed to blasts of Arctic air.

Oddly, the other favorable climatic factor is precipitation. Two rainy seasons, winter and summer, give the Sonoran Desert more moisture than other deserts. In winter, gentle, long-lasting frontal systems move into the region from the Pacific Ocean. These rains are more nourishing than in higher, cooler deserts where plants go dormant and precipitation is mainly snow. Winter rains in the Sonoran Desert sometimes produce stunning spring wild flower displays.

The summer rainy season is a monsoon, a wind shift that funnels moist tropical air into the desert, producing sporadic and heavy, though localized, thunderstorms. In good years the advent of the monsoon is like a second spring.

Beautiful and alluring, teeming with life but fatal to those who ignore its harsh terms, the vast heart of the Sonoran Desert is one of the last great untrammeled places on Earth.

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