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Plantsin the Deserts Some cacti, like the saguaro, grow to tree size, but true trees needmore moisture than most desert environments can supply, so they are scarce ondeserts. Close to streambeds, cottonwoods can sometimes be found. Though thesestreams are dry most of the year, water flows there longest and is usuallyavailable fairly close to the surface. Elsewhere, trees must send taproots deepinto the hard baked desert soil to draw on underground water. Perhaps themost widespread family of trees on the worlds deserts is the acacia, whosetaproots drill down as far as 25 feet. The mesquite common onNorth American deserts in both tree and shrub forms, does not begin to growabove ground until its root system is completely developed, ensuring the planta supply of moisture. The roots of shrubs and trees help to hold the desertsoil in place. Their stalks and branches also act as screens to keep the windfrom sweeping great drifts of sand along the surface. These services are vitalif a desert is to support life. Scientists estimate that a desert needsyear round plant cover over 20 to 40 percent of its surface. If shrubs aretoo far apart - separated by a distance greater than five times their height -soil around them is likely to blow away. Without the shelter of establishedshrubs, new seedlings will have difficulty in getting a start. On the otherhand, plants that are too close together may compete for underground moisture.To protect themselves from this competition some shrubs give off a substancethat kills young plants that sprout too close to them. In addition to a fewvarieties of trees and tough shrubs, most deserts have grasses, herbs, andother annual plants. These do not compete for moisture with the longer lived growth.They spring up quickly after rains, when the surface is moist. Then, for abrief time, the desert can be literally carpeted with color. Almost as quicklyas they appeared, these small plants die away. But they have developed specialways of ensuring the life of another generation when rains come again. | 
