These patterns are due to variations in the environment. For the desert is hotter and drier in some places than in others. The gradual downslope changes in vegetation reflect the decrease in the amount of soil moisture available to plants—a condition caused by the decreasing size of soil particles and consequent shrinkage of water-holding space between particles. Desert washes encourage plant growth because they channel water and cold air. Strangely enough, night temperatures are often lower in a desert valley than farther up the slopes. This “inversion” is due to drainage of cold air down mountainsides, forming cold “pools” in valley bottoms, especially in winter. Cold air is heavier than warm, and (since air flows much like water) it is channeled down the draws—a fact that will strike you if you walk into a wash near the mountains at night or early in the morning. Add this phenomenon to the great amounts of moisture that lie beneath the surface of washes, and you can see that desert drainageways are really linear oases. More subtly, desert hills reflect in their vegetation the differences in soil moisture from north slopes to south slopes caused by increasing exposure to sunlight.
One controlling factor, then, is dominant in the desert: the scarcity of water. This results not only in the unusual forms and adaptations of desert plants, but also in a distinctive type of plant community. In wetter climes, plants compete mostly for sunlight. They can grow close together, as long as they receive adequate illumination. During the regrowth of forests, several distinct sets of plants appear, each succeeding group more tolerant of shade than the last, as the forest canopy closes. In the desert, there is no such succession. Clear a patch of desert vegetation, and the same species will reappear—spaced out, with bare ground between them as before. For here sunlight is abundant (we might say overabundant), but there is not enough water to allow plants to cover the ground.
Dr. Forrest Shreve, who was a botanist at the Carnegie Institution’s Desert Laboratory near Tucson and a master student of deserts, defined a desert as “an area in which deficient and uncertain rainfall ... has made a strong impression on the structure, functions, and behavior of living things.” The distinctive characteristics of desert plants and animals have evolved through millions of years, in a trial-and-error process in which only the better-fitted forms have survived. It would be enlightening to know how many of the species and varieties of plants that developed during the past 60 million years or so have failed to adapt to Sonoran Desert conditions. It is fascinating to study the hundreds of forms that have succeeded and to try to determine what structures they have perfected and what methods they have originated that enabled them to maintain themselves in such a harsh environment.
Desert plants can be classified as “escapers,” “evaders,” and “resisters,” according to their means of adaptation to water shortage. Escapers, such as the annuals, avoid the problem entirely by waiting out the dry periods as seeds, to sprout and reproduce only when the rains come. Evaders, such as the ocotillo, reduce their water loss during droughts by such methods as dropping their leaves or going into a state of dormancy. Resisters, however, “hang in there” all year, taking the desert’s worst. The cactuses, prime examples of this group, rapidly soak up water from each rain and store it for use during drought; the mesquite’s deep roots tap a more constant source of moisture.
Succulents
A large group of desert plants conserve water for use in periods of drought by storing it in specialized tissues during the wet season. Some of these “succulents,” principally the yuccas, eschevarias, and agaves, have developed water-storage tissues in their leaves. A few, notably the NIGHTBLOOMING CEREUS (see [appendix] for scientific names of plants), have slender stems but an enormous, carrot-shaped root in which the moisture-storage tissue is located. The GOURDS also have large, thick, moisture-retaining roots, as does the WILD-CUCUMBER.
The cactuses store water in their stems and thus are called stem succulents. In the course of its evolution the cactus has eliminated leaves, and their function has been taken over by the green outer covering of the stems. Thus the amount of transpiration (moisture loss through plant breathing pores) has been greatly reduced.
Saguaro buds.