Environmental Engineering Reference
In-Depth Information
discussed in Chapter 28 but the other areas are included here because of the importance
of the seasonal dryness.
DESERT
DESERT VEGETATION
Arid and semi-arid land covers almost one-third of the land surface of the globe. Almost
60 per cent of it is true desert. The remainder varies from steppe grassland to thorny
scrub. In all cases, however, potential evapotranspiration greatly exceeds rainfall. In these
areas of low and erratic mean annual precipitation, vegetation is sparse and the growing
season limited. The popular idea of desert areas as vast expanses of barren, shifting sand
is false for all but a small part of this biome. Most deserts and semideserts support
widespread, relatively sparse vegetation with a distinctive array of wildlife. Over time
nature has evolved a great variety of ways of coping with extreme conditions of dryness
and heat.
Plate 27.1
Small linear dunes in the Peruvian desert near Ica.
Photo: Peter Smithson.
Desert vegetation consists mainly of short perennial grasses and thorny scrub (Plate
27.2). Only in extreme cases, such as rocky hamadas and regs, and the shifting sand
dunes and sand seas, is vegetation absent. Even in those areas, locally developed lines of
vegetation occur along wadis, with lusher growth around oases (Plate 27.3). In all cases,
plants must be able to survive periods of drought, and thus xerophytic plants
