A desert is a geographical region in which the annual rainfall is less than 25 cm. Sometimes, in hot regions even if the rainfall exceeds 25 cm in a year, deserts can form if the rainfall is concentrated only for a few days. In the sub-tropics, a desert is formed due to the presence of high-pressure belt which restricts cloud formation and rainfall. In the temperate latitudes, a desert is formed in the rain shadow region such as an inter-montane plateau where high mountains block moisture-laden winds blow from the sea.
The type of desert, particularly its climate, is defined by latitude and altitude. In the tropics, close to the mean sea level hot deserts are formed (e.g., Thar desert), whereas at high altitudes cold deserts can be found (e.g., Ladakh). As the distance from the equator increases, cold deserts are found to occur particularly in the continental interiors (e.g., Gobi desert in China).
The common plant species which are found in deserts include the creosote bush, cactus, Ferocactus etc. In shallow depression with salt deposits Sarcobatus, geesewood, seepwood and salt grasses are the common vegetative species.
Water is the dominant limiting factor which determines the productivity of a desert ecosystem. If the soils are suitable, irrigation can convert the region into a productive agricultural land (e.g., parts of Rajasthan irrigated by the Indira Gandhi canal). The productivity of a desert ecosystem is mainly dependent on how the biogeochemical cycles and energy flow of the region are managed by man because of artificial irrigation. As the rate of irrigation increases, salts are left behind on desert soils, due to capillary action combined with excessive evaporation, which can limit their productivity in the long run.
Adaptation by plants in a desert
- Leaves are modified into thorns to reduce water loss. The leaves and the stem are succulent and water-storing.
- In some plants, the stem contains chlorophyll to facilitate photosynthesis in the absence of leaves.
- They have an extensively developed root system to absorb water from greater distances and depths.
- The seeds only germinate during the short rainy season. Similarly, the plants reproduce and bloom only when the water is available.
Adaptation by animals in a desert
- Many animals are fast movers/runners. They are nocturnal in order to escape the sun's heat during the day. They conserve water by excreting concentrated urine.
- Birds and animals usually have longer legs to keep their body away from the hot ground.
- Certain lizards in the desert can survive without water for several days.
- Herbivorous animals have developed the ability to extract water from the seeds they eat.
- Camel, known as the ship of the desert, can survive without drinking water for several days and can travel long distances in the desert.
Hot Desert in India
- It is known by the name, Thar desert.
- The climate of the region is characterized by continuous drought with rainfall being irregular and scanty. The monsoon winds do not bring any significant rainfall to this region since the adjoining Aravallis lie parallel to the rain-bearing winds. The winter rains of north India, brought by Western disturbances, do not penetrate into this region.
- The cold weather (winter) season begins from the middle of November and continues up to the middle of March. This season is characterized by extreme variations in temperatures. The temperatures often drop below the freezing point at nights.
- The rest of the year experiences hot weather. During April to June, the heat becomes intense with scorching winds prevailing over the region. These winds have a desiccating effect, removing all the moisture from the surrounding atmosphere. The relative humidity in the region is always low. Such hostile climate necessitates that the flora and fauna which survive in the region possess certain special adaptations.
- The adaptations of desert species are mainly for two reasons - to enable them to obtain water, and to retain the water thus obtained.
- Most of the desert vegetation consists of scrub variety species including perennial herbs and shrubs which are capable of resisting and surviving drought for a long period. There a few species of trees which can be found. These are usually stunted and thorny, adaptations which are necessary to survive the dry weather and to protect themselves from herbivores.
- The desert vegetation is mainly of two types viz., the one which is directly dependent on rains, and the other which is dependent on subterranean water.
- The plant species which are directly dependent on rain are subdivided into two types - ephemerals and rain perennials. The ephemerals are delicate species without any special (xerophilous) adaptations. They have slender stems, root-systems and also large flowers. They appear soon after the rains, grow and bloom with flowers and fruits for a very short period. They die out as soon as the soil moisture dries up. The rain perennials have an elaborate underground root system. Only during the rainy season, they become visible above the ground.
- The second variety of plant species is dependent on subsurface water. Most of these plants are capable of absorbing water from deep below the ground with the help of a well-developed root system. The main part of the root system is usually a slender, woody tap root of great length.
- Various other xerophilous adaptations by the plants in this region include reduced leaves, succulence, coatings of wax, thick hairy growth, protected stomata, thick cuticle etc. All of these become necessary to reduce water loss due to transpiration.
- Some endemic floral species of the Thar desert include Calligonum Polygonoides, Tecomella undulate, Prosopis cineraria, Sueda fruticosa, and Cenchrus biflorus etc.
- The important mammal species of the desert include chinkara, blackbuck, desert fox, wild ass, caracal, sandgrouse etc. These are found in the grasslands, open plains and also near saline depressions. the desert ecosystem and its grasslands are home to the Great Indian Bustard, India's largest flying bird. The Great Rann of Gujarat is known for the nesting grounds of flamingoes and the only known population of Asiatic wild ass. The Great Rann is along the way of migration of many cranes and flamingoes.
Cold Desert or Temperate Desert
The cold desert in India covers the areas of Ladakh, Leh, and Kargil in Jammu and Kashmir; Spiti Valley of Himachal Pradesh; parts of Uttarakhand and Sikkim. These areas remain arid because they do not come under the influence of monsoon winds over India. They lie in the rain shadow areas of the Himalayas.
The weather in this region is severe with extremely cold conditions during winters. The terrain is denuded of soil which can support the growth of plants, due to prolonged dry spells and fast blowing winds. Scattered and isolated shrubs can be found here but they are overgrazed. The grazing period is short, less than 4 months in duration. Important floral species such as oak, pine, birch, deodar, and rhododendron are found at the edges of the desert. Yaks, goats, and dwarf cows are the prominent animals found in this region. Other important species include the highly adaptative, rare and endangered fauna such as Asiatic Ibex, Tibetan Argali, Tibetan Antelope (chiru), Tibetan Gazelle, Snow Leopard, Brown Bear, Tibetan Wild Ass (Kiang), Ladakh Uriyal, Bharal, Tibetan Wolf, Wild Dog etc.
Prolonged dry spells characterize the region with mean annual precipitation being less than 40 cm. Much of it occurs as snowfall during the months of November through March. The temperatures remain close to the freezing point throughout the year and drop as low as -50 degrees Celsius in winters. The soils are sandy to loamy with neutral to alkaline in character. They are poor in organic content, have a low water retention capacity and prone to severe wind erosion.