DESERT AND SEMI-DESERT

DESERT AND SEMI-DESERT
Very few areas in the world, apart from ice-sheets, are absolutely vegetation-less and devoid of any form of life. But there are expansive regions where biomass and organic productivity are very low. The largest areas where this is the case are climatically conditioned, either by a lack of water as in hot deserts, or by extreme cold as in tundra regions (polar and cold deserts).

TROPICAL DESERTS There are very small areas totally free of vegetation. But where vegetation exists, it consists of plants adapted to conditions of severe drought. Mostly situated between 15°N and 300N and between 15"5 and 30"S, these deserts occur on the western sides of land masses except for Africa where they extend from coast to coast, joining the Asian deserts. The chief regions are Sahara (North Africa); Arabia; parts of Iran, Iraq, Syria, Jordan and Israel; parts of Pakistan; central Australia; Namib Desert (South-west Africa); Atacama (coastal Peru and North Chile); South California, North Mexico, and parts of Ari­zona (North America). The most common plants are cacti, thorn bushes and coarse grasses.

MID-LATITUDE DESERTS These are situated in the interior of continents of Asia and North America, 30° to 35° latitude. Aridity and a great annual temperature with extremes of wp1ter cold mark the region. In North America these deserti. are found in basins surrounded by the Rockies. In South America the Patagonia desert lying to the east, of the Andes is an example.

TUNDRA This type of vegetation is mainly to be found in the northern hemisphere, surrounding the Arctic Ocean in the continents Q..f Eurasia and North America and Greenland coast. The growing season is about two months, when the surface soil thaws but the subsoil remains frozen. The pattern of vegetation is influenced by the fact that water is on the surface, and the plant types mostly consist of mosses, lichens, sedges and a few small shrubs.

HIGH MOUNTAIN REGION
High mountains show a decrease in temperature with height. There is a variety of temperatures which corre­spondingly influence vegetation types. As one goes up, the vegetation zones roughly correspond to the zones one comes across on going poleward from lower to higher latitudes.