There was a day when cows grazed grasslands at higher elevations, and before the cows could be seen herds of pronghorns, golden, graceful, large-eyed, with black horns and white rump patches. Imagine if you can how Tornillo Flat must have looked at the turn of the century with fine grasses bending under the wind and the pronghorns flashing their rump patches in sudden semaphore. Depending on speedy flight for protection, pronghorns seek wide-open places where they can see a long way off. Today, Tornillo Flat is a bald shrub desert. The pronghorns vanished long ago. The small band sometimes seen north of Tornillo Creek is the remnant of a herd reintroduced in the 1940s.
The grass that flourished on Tornillo Flat in its heyday was a very good clump-forming grass called tobosa. To the first settlers it looked as though there was more grass along Tornillo Creek than could ever be eaten off. Yet the tobosa soon disappeared, falling first to mowing machines and finally to the domestic herds that cropped it out of existence. Hay balers plied Tornillo Flat into the 1930s.
Grasses spread through seed germination, and in some cases by underground and aboveground stems. As a general rule, root depth equals the height of the grass, but half of the root system dies back each year. This is natural and even desirable since the dead roots create new soil and you still have a sound root system if the grass isn’t topped too much. But heavy overgrazing causes grass roots to die back even more and creates an abnormally shallow root system. This is doubly dangerous in desert country where heat and the need to search out water pose crucial problems for all plants. Nor is this all, because other good things come—and other good things go—with a healthy ground cover. As each year’s grasses die back they fertilize the soil and insulate seedlings and roots from the sun’s killing heat, but when this cover is reduced, good grasses disappear and poorer grasses take over. Surface conditions are finally so changed that neither grasses nor grassland forbs and shrubs can grow. Seedlings may burst into brief life with a rain but when the sun bears down again day after day the unprotected plants perish. With nothing to hold it the soft topsoil blows and washes away. Desert downpours soon gully the ground, changing drainage patterns and lowering the water table beyond the reach of short grass roots. Longer rooted desert shrubs move in, first the shortlived tarbush, and finally creosotebush. The result is seen on Tornillo Flat today: typical Chihuahuan shrub desert with sparsely scattered creosotebush and cactuses.