6
Desert
North Africa is dominated by the
Sahara, a mixture of sand ergs,
gravel plains and vast rocky
massifs that rise high above the
desert floor. Much of Arabia is
desert land – including the great
sand sea of Rub al’Khali – that
comprises a mixture of sand,
gravel and rocky outcrops and
ranges. A considerable area of the
great central and eastern plateaux
of Iran is classified as desert, including the salt desert known as Dasht-e
Kavir. True desert is characterized by very low annual rainfall (less than
100 mm). There is often little or no rain for long periods. As a result
vegetation is very sparse and in some areas there may be none at all
for several years at a time.
Semi-desert
Semi-desert is not defined as
clearly as true desert, but includes
the Sahel belt to the south of
the Sahara and large areas in
the Middle East. Annual rainfall
is unpredictable: less than 500
mm (often considerably so),
but not usually below 100 mm.
Vegetation is more apparent than
in desert areas and may include
some trees and bushes, often
with strong seasonal growth of annual and perennial grasses and
herbaceous plants following rain. Wherever semi-desert is found, vast
tracts have often been severely degraded by livestock overgrazing.
Montane Areas
These areas include the Atlas
ranges of north-west Africa,
the Saharan massifs, the ranges
of western Saudi Arabia and
Yemen, those of northern Oman,
the Zagros and its extension
eastwards, the Makran range,
Alborz of Iran and the numerous
systems that rise up over much
of Turkey. Vegetation and rainfall
vary considerably between ranges
and at different altitudes. Many ranges attract differing amounts of
rainfall on one face or the other: one side may receive higher rainfall
and harbour good vegetation cover; the other may be dry and sparsely
vegetated. A number of ranges receive heavy snowfall in winter.
Many of these ranges have also been heavily grazed and browsed by
domestic livestock.