Desert biomes occur in regions with less than 10 in, of annual rainfall, or sometimes in hot regions where there is more rainfall, but unevenly distributed in the annual cycle. Lack of rain in the mid – latitudes is ofter due to stable high pressure zones; deserts in temperate regions often lie in ‘’rain shadows,’’ that is, where high mountains block off moisture from the seas. Two types of north american deserts are a ‘’cool’’ desert in washingtom with sage brush, and a ‘’hot’’ desert in arizona, where creosote bushes and cacti are conspicuous. The characteristic spacing of deesert vegetaiton and the pssibility of birth control mechanisms were discussed in this post. North american deserts are not as extreme as those in other continents, such as the african sahara or the asian gobi. Some seasonal rain can be expected every year in U.S. deserts, but rainless periods in extreme deserts may span years.

      Four very distinctive life forms of plants are adapted to the desert ecosystem: (1) the annuals (such as cheat grass) which avoid drought by growing only when there is adequate moisture. (2) the desert shrub with numerous branches arising from a short basal trunk, and small, thick leaves that may be shed during dry periods; the desert shrub survies by its ability to become dormant before wilting occurs. In the coller deserts, the shrubs develop very deep root systems that tap moisture that remains. Available after the surface completely dries out. In such the leaves and stems may remain green and active throughout the summer. (3) the succulents, such as the cacti of the new world or the euphorbias of the old world, which store water in their tissues. (4) microflora, sich as mosses, lichens, and blue green algae that dormant in the soil but are able to respond quickly to cool or wet periods.

    Animals such as reptioles and some insects are ‘’preadapted’’ to deserts, for their impervious integuments and dry excretions enable them to get along on the small amount of water. Mammals as a group arre poorly adapted to deserts but some few species have become secondarily adapted. A few species of nocturnal rodents, for exampe, that excrete very concentrated urine and do not use ater for temperature regulation, can live in the desert without drinking water. Other animals such as camels must drink periodically but are physiologically adapte to withstand tissue dehydration for periods of time. For more on adaptations of desert animals.

      In the past mankind has developed remarkable culturess, includeing adapted domestic plants and animals for life in or along the edges of deserts. In fact, life in dry regions requires ingenuity and a conservation ethic, two attributes badly needed in more benign regions. Because watr is the dominant limiting factor, the productivity of a given desert region is almost a linear function of rainfall. In the california mohave desert a 100 mm annual rainfalp will result in about 600 kg dry matter/ha while 200 mm will increase net production to about 1000 kg/ha. Where evaporative losses are less in the cooler great basin deserts, a 200 mm rain produces 1500 – 2000 kg/ha.

   Where soils are suitable, irrigation can convert deserts into some of our most productive agricultural land. Whether productivity continues or is only a temporary ‘’bloom’’ depends on how well man is able to stabilize biogeochemical cycles and energy flow at the new increased rates. As the large volume of water passes through the irrigation system, salts may be left behind that will gradually accumulate over the years until they become limiting, unless means of avoiding this difficulty are devised. The water supply itself can fail if the watershed from which it comes is abused. The ruins of old irrigation systems, and civilizations they supported, in the deserts of the old world warn that the desert does not continue to bloom for man unless he understands the laws of the ecosystem and acts accordingly.