Plants, because they are immobile, have been forced to make ingenious adaptations in order to live in the desert. They must endure a land of little shade and pitiless, inescapable heat, where drought is the rule and rain the exception. Plants of the desert must learn three lessons: how to get water, how to conserve water and how to do without water.
To survive the long dry season plants must spend much time in a dormant state. Some persist as seeds to evade the dry season. Others resist the drought and live from one rainy season to another by locating, storing and reducing their need for water. Many, particularly cacti and other succulents, have evolved into bizarre shapes as drought resistant plants with water-saving and moisture-collecting modifications.The most successful plants anywhere, particularly in the desert, are easy to recognize because they are abundant and conspicuous, and adapt easily to a wide range of environmental conditions. The most efficient plant of and North America is Larrea divaricata, the creosote bush. It occupies more territory than any other single species on the North American deserts and is also common on the Argentine desert (Biologists are still speculating on which came first and how they colonized in such distant areas). Its upper limit is the northern boundary of the Mohave and Sonoran deserts; beyond the creosote bush is sagebrush territory.
What the creosote bush does best is grow where nothing else will. It is a wispy shrub with scraggly branches, blackish stems, small, shiny dark green leaves and yellow flowers. It seems to serve no purpose except to be there, tying down soil that would be harassed by eroding water and wind. Hummocks of sand that surround each bush provide home-building sites for numerous desert animals such as ground squirrels, kangaroo rats, pack rats, lizards, snakes and toads. Animals that find any part of the creosote edible are rare.
Despite its name, it is not a source for creosote, the wood preservative extracted from wood or coal tar. Its common name reflects the odor of creosote given off by its leaves, which is especially strong after a rain. Although it is a clean, refreshing smell, its common name in Mexico is hediondilla, meaning "little stinker." No matter, too much of a strong, clean, refreshing smell is still too much.
The creosote bush is a small, hardy, ubiquitous, thoroughly adapted shrub of the hot Southwestern deserts, usually 3 to 6 feet tall, but in optimum conditions it can grow to over 10 feet. Its tap root seems to serve more as support than as a water finder, but the rest of the root system is incredibly efficient. The roots grow laterally not far from the surface, where water would most likely be found. They can reach and exploit the last vestiges and the most minute quantities of water.
The creosote bush grows in pure stands over wide areas of the Mohave. As suggested above, it welcomes places that other plants consider inhospitable, and the creosote bush does little to throw out the welcome mat. Any moisture near a bush has already been commandeered by the existing creosotes, so their own seedlings cannot become established. As an added deterrent to invading plants, the roots of creosote bushes produce a toxin that reduces competition for water by killing nearby plants.
Because the roots compete for the small amount of moisture available in the soil, spacing of creosote bushes is strikingly uniform. They frequently appear to have been planted, because once each bush is established, no other plant can grow within several feet of it. Therefore, an individual bush tends to colonize nearby ground, not by brodacasting seeds and producing new individuals, but by sending out new stems at a distance from its base.
As the bush spreads outward, the stems in the middle die away, and the bush expands in the shape of a ring. Since competition is non-existent, the bush continues to grow outward, and the ring grows larger and larger. In the Mojave Desert, a large, circular creosote bush has been found that is over 70 feet in diameter. The central wood has died away, leaving a barren area surrounded by a ring of shrubs. Although the outer stems in the ring are not very old, the plant, considered as a single organism, has been estimated to have started from a seed nearly 12,000 years ago.