Geoscience Reference
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region, annual rainfall does not exceed 200 mm and in much of it, rainfall is less
than 100 mm (Zohary 1973 ). In the northwest corner of the region, precipitation is
highly variable from year to year, ranging from less than 50 mm to over 300 mm
and falling mostly as rain from November to May (UNEP-WCMC 1989 ). The driest
parts of Iran are found in the central and eastern parts of the plateau, with the Dasht-
e-Hut receiving only up to 50 mm per year. The low amount of rainfall is aggravated
by high evaporation rates.
The rivers descending into the central plateau from the surrounding mountain
ranges carry high levels of soluble salts, and the ground in the plateau tends to
be highly saline (Zohary 1973 ). On the margins of the plateau, and in a few
patches in the interior where the topography is such that the soil is less saline,
areas of piedmont fans and alluvial soils exist and can be farmed. Zohary ( 1973 )
differentiates the central plateau into a series of habitats, including poorly drained
flats inhabited by halophytic communities and better drained flats inhabited by a
variety of sagebrush ( Artemisia ) steppes. The flats are interspersed with sand dunes
and gravel deserts. True sand deserts, consisting of vast dune fields, exist in Iran
only in the central plateau and cover a surface area of about 183,000 km 2 ;theyare
found at altitudes of about 500-1,200 m.
According to Zohary ( 1973 ), the central Iranian sector hosts the most typical
vegetation of Iran's steppe and desert regions. Dwarf scrub vegetation is common
in large areas of the interior of Iran and is very diverse and rich in species; in non-
saline areas, a variant with many thorn-cushions (Zohary 1973 ) is formed. Under
extremely arid conditions, a very open variant of the dwarf shrublands appears,
also characteristic of large areas of the Iranian interior; the dominant species are
sagebrush ( Artemisia siberi , Astragalus gossypius ), and others (Frey and Probst
1986 ). In areas receiving over 100 mm of rain, other genera such as Pteropyrum ,
Zygophyllum and Amygdalus can also be found.
With regard to vegetation of the sand deserts in the interior regions of Iran,
among the more characteristic genera are Ephedra , Calligonum , Heliotropum ,and
others (Frey and Probst 1986 ). Endemic shrubs and perennials include Astragalus
( Ammodendron ) kavirensis, Heliotropum rudbaricum , and others. Many species
here are highly specialized as psammophytes; these sand-adapted species are
estimated to make up one third to one half of the total number of species in the sand
deserts of Iran and Afghanistan (Freitag 1986 ). Iranian deserts also have a striking
number of Ta m a r i x species; they have been reported to occur on the margins of the
more sandy and gravelly parts of the Dasht-e-Lut (Breckle 1983 ).
Halophytic communities of varying composition are found on the margins
of the undrained salt pans of the central Iranian region, such as the Dasht-e-
Kevir. In such areas, clays and sand soils have a high surface salt content due to
insufficient water and high summer evaporation. Characteristic genera and species
include Aellenia spp., Halocnemum strobilaceum , Haloxylon spp., Salsola spp.,
and others (Frey and Probst 1986 ). The inner parts of the salt pans have almost
no vegetation. Zohary ( 1973 ) points out that saline soils can harbor an impressive
number of plant communities, and he characterizes Iran as “outstanding in its rich
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