Dry desert is a formation class of xerophytic plants widely dispersed and providing almost negligible ground cover. In dry periods (which are the rule) the visible vegetation consists of small hardleaved or spiny shrubs, succulent plants (cacti), or hard grasses. Many species of small annuals may be present, but appear only after a rare but heavy rain has saturated the soil.
Desert floras differ greatly from one part of the world to another. Well known are the treelike saguaro cactus, the prickly-pear cactus, the ocotillo creosote bush, and smoke tree. Much of the desert of the southwestern United States is in fact scrub, thorn scrub, savanna, or steppe grassland. In the Sahara Desert (most of it very much drier than in the American desert) a typical plant is hard grass; another, found along the dry beds of watercourses, is tamarisk. The coastal desert of southwest Africa is known for the strange tumboa plant with strap-shaped leaves radiating from a central tap root, which penetrates deep into the ground.
Much of the area assigned to dry-desert vegetation has no plants of visible dimensions,. being composed of shifting dune sands, or almost sterile salt flats.
Distribution of dry deserts is shown in the world vegetation map, Plate 4, as Class H. As a study of climate will show, three variations of the deserts trend exist. The true tropical-continental deserts have not only extreme aridity but also extremely high air and soil temperatures; the middle-latitude deserts (30° to 35° lat.) have both aridity and a great annual temperature range including extremes of winter cold; the tropical west cost deserts have remarkable uniformity and relative coolness of temperature along with persistent coastal fog. A dominant pedogenic process of dry deserts is salinization, locally producing areas of salt crust where only salt-loving (halophytic) plants can survive. Calcification is conspicuous on well-drained uplands; encrustations and deposits of calcium carbonate (caliche)arc commonplace. Humus is lacking and soils are pale gray of red in colour, e.g., sierozems and red-desert soils.
Arctic fell field is the arctic equivalent of the dry desert, being found in the extremely cold tundra and icecap climates. The arctic fell field consists of rocky ground surfaces, produced by intense frost shattering and having only small patches of fine textured mineral soil. vegetation is very sparse and consists mostly of lichens, mosses, and a few small shrubs. For example, in northern Baffin Island, typical plants are a small polar willow purple saxifrage and woolly moss (Rhacomitrium lanuginosum.)
Arctic fell field can be found as far north as the extreme limits of land, namely up to 84° N latitude and southward to ice-free parts of Antarctica. Soil-forming processes consist of little more than rock disintegration by frost action, manifested in the formation of stone polygons in which finer particles are sorted from the coarse into patches.
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