Q. why are plants generlly few and far between a desert?
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Deserts around the world are home to a wide variety of plants, all adapted to living in the harsh conditions of a hot, arid ecosystem. Deserts and areas that are nearly deserts make up about one-third of all the land areas on Earth.
Two characteristics define deserts: a lack of water and extreme temperatures. Deserts typically receive less than 10 inches (25 centimeters) of rain per year. This rain may come all at once in one storm or over the course of several months. Parts of the Sahara (the largest desert in the world) in Africa may go for years without a drop of water.
Desert temperatures vary from extremely hot during the day to freezing cold at night. In deserts that are considered "cold," precipitation falls mainly as snow. The Gobi in China and the Great Basin in the United States are two examples of "cold" deserts. In so-called "hot" deserts, such as the Sahara and the Mojave in the United States, precipitation falls in the form of rain.
There are many things that can cause an area to become dry enough to qualify as a desert. For instance, deserts sometimes form on the inland side of a mountain range—the so-called rain-shadow effect. As air rises to the top of the mountain range, it cools; as a result, it drops its moisture in the form of rain or snow. By the time the air reaches the top of the mountain range, it is very dry. Once the air heads back down the other side, it begins to grow warmer and regains its ability to hold moisture—making it less likely to rain or snow. Eventually, the lack of moisture causes a desert to form. Deserts are also likely to form in areas where there is little vegetation to absorb the heat of the Sun; where no lakes, streams, or other bodies of water exist to add moisture to the air; and in regions where persistently windy conditions cause any available water to evaporate quickly.
Scientists believe that the deserts on Earth today existed as far back as 3 million to 4 million years ago. As Earth's climate changed over the millennia, so, too, did the size and aridity (dryness) of the world's deserts.
Despite such unpromising conditions, plant (and animal) life flourishes in the desert. In fact, only tropical rain forests have a greater variety of life. In general, plants succeed in the desert by either devising ways to survive in the heat and dryness or avoiding those conditions as much as possible. Even though there is a wide variety of plant life in the deserts, the plants tend not to grow close together as they would, for instance, in a rain forest. Desert plants also typically grow close to the ground.
Some plants are able to live in the desert because they have evolved special structures to help them store water. Cacti—such as the prickly-pear, saguaro, and barrel cacti—are among the best-known desert plants. Cacti are a type of succulent, or water-storing plant. Instead of leaves, cacti have a thick, waxy cuticle, or outer layer, that protects against water loss. Cacti are also able to store water in their stems.
The spines (thorns) on a cactus can help collect water, too—while also providing a bit of shade for the growing plant and a degree of protection from thirsty animals. In addition, the stomata (pores on the underside of leaves that allow the plant to take in air) on a cactus are usually sunken and can close during the day to prevent water loss.
Cacti have shallow roots that are far-reaching enough to quickly take up any available water when it rains. Plants such as the agave and the euphorbia use many of the same or similar mechanisms for capturing and storing water.
Other types of plants, such as perennials—plants that grow for several years—also have special adaptations for surviving in the desert. For instance, sagebrush (Artemisia) have tiny leaves to cut down on the amount of water the plant loses through transpiration (the evaporation of water into the air). Other plants have evolved waxy leaves—the chaparral bush (Larrea tridentata) is a notable example. This bush has other features that help it flourish in the desert, including an unpleasant smell and taste that discourage hungry or thirsty animals from visiting it. The stomata on its leaves open only at night, minimizing the moisture lost to the daytime heat. Like cacti, the chaparral bush has widespread shallow roots for catching rainwater; it also has roots that grow deep into the ground, tapping water from the water table. These features help the chaparral bush survive long periods without precipitation.
Another desert plant that relies on deep roots for survival is the mesquite tree (Prosopis). Its roots can extend 30 to 100 feet (9 to 30 meters) in their search for underground water.
Some desert plants survive the harsh environment by entering dormancy—a cessation of growth during dry spells. Such plants come to life suddenly when it rains, often producing flowers and seeds in very short order. The ocotillo plant (Fouquieria splendens) sheds its leaves and stops growing during periods of dry weather, but grows leaves, sprouts flowers, and bears seeds in the weeks after a precipitation event. After the seeds fall off, the plant becomes dormant again until the next time it rains. Depending on the weather, the ocotillo can go through this process several times a year.
Desert plants that belong to the lily family enter dormancy during dry times, too, losing their leaves so that only the bulb of the plant remains in the ground, unseen. Some desert plants remain green the entire year. Schott's pygmy cedar (Peucephyllum schottii) stays green because it is able to absorb dew.
Annuals, or plants that live for only one growing season, also take advantage of the rains. Annuals in the desert can go through their life cycle—growth, flowering, and seed production—in a matter of weeks. For example, winter rains will generally spur such plants as the Mojave aster or the desert paintbrush to begin growing in the spring. Exactly when annuals begin to grow and flower depends on the rainfall, temperature, light, and elevation. Plants that grow at higher elevations tend to bloom later in the season.
Although annuals produce seeds quickly, the seeds may not germinate (begin to grow) for a year or even more. The seeds will begin the growth process only when the temperature and the amount of rainfall are just right.
The seeds of the paloverde tree (Cercidium) also need just the right combination of conditions to germinate. The very hard seed of this tree must be cracked open for the tree to germinate. This may happen during a rainstorm, when rushing water and debris strike the seed.
Botanists believe that the seeds of some desert plants may not germinate immediately because of a seedling-inhibiting substance that some mature plants produce. Such a substance would prevent young plants from growing near the mature plant, and therefore would eliminate a potential competitor for precious water.
In the desert's harsh environment, plants and animals often depend on each other for survival. Many plants rely on animals to pollinate them and to disperse their seed. The plants, in turn, offer the animals food, shelter from the sunlight, and protection.
Around the world, humans have been able to adapt to desert living. Nomadic tribes, for example, survive in the deserts by traveling from oasis to oasis for water. Such is the case in certain African, Asian, and Australian deserts.
Irrigation systems and, in the case of modern cities in the desert, air-conditioning and other technologies have made the desert livable for people accustomed to temperate climes. The changes that humans bring to the desert can cause problems, however. If irrigation is not implemented correctly, salt and alkali from the surface water and groundwater can render the soil sterile and unable to grow plant life. On semidesert lands that are cleared in order to plant crops, the exposed soil is likely to erode.
Desert plants are threatened by overgrazing livestock. People may carelessly trample or drive over desert plants, or they may cut the vegetation for use as fuel or to sell to gardeners and others eager to raise exotic plants.
All these actions threaten to make existing deserts and semidesert lands less able to support plant and animal life, a process called, ironically, desertification. Generally, as desertification spreads through an area, the groundwater tables decrease, the topsoil and water become more saline (salty), surface waters, such as streams and lakes, dry up; soil erosion increases; native plants disappear; and biological diversity—the wide array of plants and animals that an area such as a desert can sustain—is lost. Around the globe, desertification is a major problem, particularly in Africa. An estimated 10 billion acres (4 billion hectares), or approximately one-third of the world's land surface, are impacted by desertification, directly affecting more than 250 million people. The combination of high temperatures and lack of water make the desert environment one that is easily disturbed. Careful planning may be able to save these unique ecosystems, but for now the future seems uncertain.
How to cite this article:
MLA (Modern Language Association) style:
Pine, Devera. "Desert Plants." The New Book of Popular Science. Grolier Online, 2011