222
H. WALTER AND E. STADELMANN
retain the water (Walter, 1932, p. 498ff). These conclusions were verified
by measurements in the Negev Desert (Hillel and Tadmor, 1962, p. 41).
With the same annual rainfall the amount of water available for plants
was 35 mm in loess soil, 90 mm in sandy soils, 50 mm in rocky soils with
appreciable runoff, and 250-500 mm in dry valleys with high water inflow.
When sandy or rocky soils become moistened down to the water table
roots of plants can grow deep enough to reach the ground water.* The
water supply of the plant is then assured. An example of this was found
north of Basrah (Iraq). The water table in this region is 15 meters deep
and supplied with water from the Euphrates and Tigris rivers through
layers of gravel. An annual rainfall of 120 mm moistens only the uppermost layers of the soil and hence the roots of plants cannot reach the
ground water. Only a scanty ephemeral vegetation covers the soil after
the light winter rains. The natives dig wells and use the water to grow
vegetables. The plants are planted in furrows and watered 5 times a day
in hot summers. Maximum air temperatures in summer may reach 50°C.
Because the high evaporation causes salt accumulation on the soil surface
vegetables can only be cultivated for 1 year. However, Tamarix articulata
cuttings which root readily are already planted between the vegetables in
the first year.
The irrigated soil is moistened down to the level of the ground water,
and it remains moist during the following year, although it is no longer
irrigated. Hence, Tamarix roots grow deeper during the following years
and finally reach the ground water level and form larger trees. These trees
are cut every 25 years for fuel and suckers form again from the stump.
In this way all previously irrigated vegetable land of the desert is converted
to Tamarix forest.
All deserts of this type with deep ground water can be reforested if the
planted young trees are irrigated during the first year intensively enough
to moisten the soil down to the water table.
The better growing conditions for plants in sandy soil compared with
clay soil under arid climate are also demonstrated by the fact that the same
type of natural vegetation grows on sandy soil with less rainfall than on
clayish soil. For example, Smith (1949, p. lOff) found Acacia tortilis
in the Sudan in a semidesert area with a rainfall of 50-250 mm on sandy
soil; on clay soil this vegetation grew only in areas with a rainfall of at
least 400 mm/year. Correspondingly, the Acacia mellifera short grass
savanna developed on sandy soils with a rainfall of 250-400 mm/year and
on clay soils only when rainfall reached 400-600 mm/year.
* Roots of desert plants grow deep only in sandy or rocky soils. Normally, the
roots grow near the soil surface because only the upper layers of the soil are
moistened after rainfall.
H. WALTER AND E. STADELMANN
retain the water (Walter, 1932, p. 498ff). These conclusions were verified
by measurements in the Negev Desert (Hillel and Tadmor, 1962, p. 41).
With the same annual rainfall the amount of water available for plants
was 35 mm in loess soil, 90 mm in sandy soils, 50 mm in rocky soils with
appreciable runoff, and 250-500 mm in dry valleys with high water inflow.
When sandy or rocky soils become moistened down to the water table
roots of plants can grow deep enough to reach the ground water.* The
water supply of the plant is then assured. An example of this was found
north of Basrah (Iraq). The water table in this region is 15 meters deep
and supplied with water from the Euphrates and Tigris rivers through
layers of gravel. An annual rainfall of 120 mm moistens only the uppermost layers of the soil and hence the roots of plants cannot reach the
ground water. Only a scanty ephemeral vegetation covers the soil after
the light winter rains. The natives dig wells and use the water to grow
vegetables. The plants are planted in furrows and watered 5 times a day
in hot summers. Maximum air temperatures in summer may reach 50°C.
Because the high evaporation causes salt accumulation on the soil surface
vegetables can only be cultivated for 1 year. However, Tamarix articulata
cuttings which root readily are already planted between the vegetables in
the first year.
The irrigated soil is moistened down to the level of the ground water,
and it remains moist during the following year, although it is no longer
irrigated. Hence, Tamarix roots grow deeper during the following years
and finally reach the ground water level and form larger trees. These trees
are cut every 25 years for fuel and suckers form again from the stump.
In this way all previously irrigated vegetable land of the desert is converted
to Tamarix forest.
All deserts of this type with deep ground water can be reforested if the
planted young trees are irrigated during the first year intensively enough
to moisten the soil down to the water table.
The better growing conditions for plants in sandy soil compared with
clay soil under arid climate are also demonstrated by the fact that the same
type of natural vegetation grows on sandy soil with less rainfall than on
clayish soil. For example, Smith (1949, p. lOff) found Acacia tortilis
in the Sudan in a semidesert area with a rainfall of 50-250 mm on sandy
soil; on clay soil this vegetation grew only in areas with a rainfall of at
least 400 mm/year. Correspondingly, the Acacia mellifera short grass
savanna developed on sandy soils with a rainfall of 250-400 mm/year and
on clay soils only when rainfall reached 400-600 mm/year.
* Roots of desert plants grow deep only in sandy or rocky soils. Normally, the
roots grow near the soil surface because only the upper layers of the soil are
moistened after rainfall.