Adaptations to life in deserts - Climatic Change in Deserts - page 43

Figure 4.4. Influence of rainfall and soil texture on two acacia species, Sudan. (After
Smith, 1949 , and Williams et al., 1982 .)

available for plant growth than the water within sandy soils. The leafless saxaul tree
( Haloxylon persicum ) is native to the steppes and deserts of southern Russia and
central Asia and may be found on dunes extending from the southern Negev Desert
to the sand deserts of Inner Mongolia, where it provides shade and fodder for the
Bactrian camels and occasional timber and fuel for the Mongolian herders.
In a now classic investigation, Smith ( 1949 ) studied the distribution of tree species
in Sudan in relation to rainfall and soil texture along a south to north transect from
wet to dry and found that a given species of Acacia growing on sand needs only
two-thirds of the annual rainfall required by the same species when growing on clay
( Figure 4.4 ). Smith was careful to select level sites that neither received water from
run-off nor shed water to other sites.
However, the influence of rainfall and soil texture on plants in other semi-arid
and arid environments is often more complex than Smith ( 1949 ) had inferred more
than sixty years ago. Walter ( 1972 ) formulated the 'two-layer' hypothesis of desert
ecosystems (Ogle and Reynolds, 2004 ). Walter noted that in savanna ecosystems,
woody and herbaceous plants could coexist because they drew their water from
different layers in the soil. In three influential accounts of desert ecosystems,