Geoscience Reference
In-Depth Information
Moving away from the core areas of the dry lands, we
encounter increased moisture availability. The areas
polewards and on the western side of the continents
have been included in
Chapter 25
on Mediterranean
environments. Eastwards towards continental interiors
precipitation stays low, as we are a long way from major
sources of moisture and cyclonic storms are infrequent.
Areas such as the Canadian prairies and the steppes of
central Russia definitely qualify as dry lands. Equator-
wards we gradually change from true desert, through
semi-arid environments to the savanna zone with decid-
uous woodland and eventually into the monsoonal and
tropical rain forests. In this direction, precipitation
becomes more frequent and abundant; the rainy season
lengthens and vegetation becomes more lush, especially
during the wet season. Eventually we expect to reach the
tropical rain forest, which will be discussed in
Chapter 27,
but the other areas are included here because of the
importance of the seasonal dryness.
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 evapotran-
spiration 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 as
a vast expanse of barren, shifting sand is false for all but
a small part of this biome. Most deserts and semi-deserts
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.
Desert vegetation consists mainly of short perennial
grasses and thorny scrub (
Plate 26.2
).
Only in extreme
Plate 26.2
Desert vegetation in the southern Kalahari. Small amounts of vegetation survive on moisture provided by the alluvial
fan emerging from the valley in the background. Mean annual rainfall is less than 150 mm.
Photo: Peter Smithson
