A Sagebrush Realm

Despite the hardships the desert imposed on travelers, the plants and animals that evolved to fit into its austere seasons and angular design demonstrate the adaptability of life, especially in the most unlikely places. The transition from the low, hot Sonoran Desert to the high, cold Great Basin Desert demonstrates the tradeoffs that plants and animals must make in response to conditions. The Sonoran Desert's saguaro can withstand months without rain, but cannot withstand frost and relies on summer monsoon rain. The frost-resistant Joshua tree of the Mojave Desert shrugs off temperatures that would kill a saguaro, but could not withstand the heat and intermittent droughts of the Sonoran. The creosote bush that so stubbornly resists heat and drought gives way to sagebrush in the face of freezing winters. The transition from the heat-adapted Mojave to the cold-adapted Great Basin Desert starts when the inconspicuous Blackbrush displaces the creosote. The Blackbrush, in turn, soon mingles with the sagebrush, the fragrant, tough, cold-adapted signature plant of the northern reaches of the Great Basin Desert.

Ecologist Ann Zwinger observed, “Boundaries like this one fascinate me. Where does the last saguaro become a mere armless post in the ground and finally give up its footing? Where does the kangaroo rat
pause, one well-adapted desert foot poised in the air, nose twitching toward a wetter, lusher existence, and not cross over? Where is the line beyond which the desert cockroach does not tunnel? Where is the barrier that keeps the sidewinder and the fringe-toed lizard at home on the hot sands? Where does the desert tortoise blink its slow eyes and turn back to the only home it knows? In trying to define where a desert is not, one learns where it is.”

The great, continuous realm of sagebrush that defines the Great Basin Desert constitutes the single largest range of any ecosystem in the western United States, covering some 300 million acres (1.2 million sq km) and half of 11 western states. The sagebrush ecosystem evolved quickly after the planet warmed and the glaciers retreated some 12,000 years ago. Gray-green, fragrant, and stubborn, the leaves of the sagebrush are covered with almost furry hairs that protect them from both heat and frost. Sagebrush has both a fibrous surface root system for drinking in even fleeting, scattered rains and a deep-reaching taproot to quest down for groundwater. Although sagebrush plains seem flat and easy to travel through, each bush creates its own little mound as it grows up through the sand that drops from the almost ceaseless winds riffling through its leaves. For the wagon trains of settlers in the 1800s, the mounds around the plants made the sagebrush plains a great misery of constant lurching and bumping.

A single, shoulder-high sage may live 150 years. Sage keeps its leaves all year long, weathering both 100-degree summers and freezing winters. It uses a different process for converting sunlight into energy, which makes it slow-growing but stubborn and drought resistant. Sagebrush did not truly take over the Great Basin Desert until people tried to make a profit from the desert by grazing cattle there. Once, the sagebrush alternated with grasslands over much of the area. But the cattle quickly chomped the grass. Once the grass was gone, the wildfires that used to control the sagebrush no longer had fuel to spread. But the cattle also couldn't control the sagebrush because the leaves have resins to protect against drought that give most animals indigestion. As a result, cattle will mostly not touch it. So after a few decades of eating most of the grass, the cattle grazing in a huge area ran out of things to eat. And because a single bush can spread a million seeds in a year, the sagebrush invaded the overgrazed grasslands, creating a vast monoculture of sagebrush.