Geoscience Reference
In-Depth Information
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 26.3
).
In all cases, plants must be able to survive periods of
drought, and thus xerophytic plants predominate. The
adaptation of plants to desert conditions varies. For
example, the saguaro cactus develops a widely spreading
root system; the mesquite has roots that may reach depths
of over 50 m; many cacti and agaves store water in their
roots, stems and leaves. Some plants reduce water loss
through evaporation by controlling their stomata, while
others have long dormant periods, growing and flowering
briefly and irregularly when moisture is available
(
Plate 26.4
).
Some species of plants avoid excessive
exposure to the sun and drying winds by growing largely
underground.
Soils
The soils associated with desert conditions are typically
little weathered, and lacking in humus. In the most
extreme cases no true soil exists, but, even where sufficient
plant growth does occur to provide a surface accumula-
tion of plant debris and a food base for soil fauna, the lack
of leaching and chemical weathering leaves soils relatively
infertile. Salinity may be a problem where the rock type
produces saline ground water, or where salty sea water
seeps into
aquifers
, as it does in many coastal areas. Winds
blowing from the sea also may introduce salt. Constant
evaporation from the surface draws water from the lower
layers of the soil and leads to the accumulation of salts in
the upper horizons. If the parent material is rich in sodium
salts, solonetzic soils may develop. Practically no leaching
occurs, so, even though the salt is soluble, it accumulates
in the soil.
Plate 26.3
Oasis near Ica, Peru. Note the contrast between the albedo of sand and that of vegetation.
Photo: Peter Smithson
