Geoscience Reference
In-Depth Information
is now widely known as Goyder's Line. North of this limit rain-fed cultivation was
proscribed, but light sheep and cattle grazing was encouraged. The line coincides
roughly with the 250 mm (10 inch) rainfall isohyet. His recommendation was ignored
but renewed droughts in the 1870s brought home the wisdom of his advice, reflected in
many abandoned stone homesteads still very evident today. Other salt-tolerant plants
(or
halophytes
) are the Tamarix trees that can excrete salt from their leaves.
Succulents are very rare in the Sahara despite its mixed heritage of plants derived
over time from a variety of phytogeographic regions, including the Irano-Turanian,
Mediterranean, Saharo-Arabian and tropical African regions. Plants of the Saharo-
Arabian region are predominantly
xerophytes
, that is, plants able to resist drought.
Xerophytes fall into three broad groups: plants that can cope with desiccation, plants
that stay inactive during the dry season and plants that remain active during the dry
season.
Lichens are some of the more obvious organisms in deserts that are able to endure
desiccation, regenerate rapidly on contact with water, grow anew and, once the mois-
ture supply is used up, dry up and remain dormant until the next supply of rain or
dew. Lichens are unusual in that they are not one organism but two - a fungus and an
alga - that function together symbiotically. Xerophytic algae also form crusts on the
surface of dunes and rocks, and some can remain viable even after several years of
desiccation. Very few higher plants possess this ability to survive desiccation without
enduring severe damage to their tissues.
Among the xerophytes active during the dry season are shrubs and dwarf shrubs,
certain trees and bi-seasonal annuals. The shrubs and trees can reduce dry season tran-
spiration losses by shedding part of the plant, including branches. The disconcerting
(and sometimes fatal) impact of large branches falling from eucalypt trees during hot
dry spells is a prime example of this process in action. Other adaptations include the
replacement of large leaves by spines or needles, the latter sometimes jointed and able
to be shed in segments, the development of a waxy cuticle on the leaf surface and the
ability to modify the position of the stomata below the epidermal surface. Another
form of adaptation involves the root systems of desert plants. Depending on the hab-
itat, the roots may be shallow and of considerable lateral extent, tapping near-surface
water in the shallow wetting zone following light rain, or they may extend to depths
of many metres to tap the local water-table. In some instances the plants have adopted
both strategies, with a combination of deep roots and a considerable lateral spread at
shallow depth.
Although devoid of surface water, desert dunes are often reliable repositories
of shallow groundwater, because capillary rise of stored water is precluded by the
relatively coarse particle size of sand grains, so that water loss from evaporation is
minimal. Where sand dunes or sand sheets occur sporadically on alluvial clay plains
in semi-arid areas, trees and many grasses will grow preferentially on the sandy soils
and avoid the clays, where the soil water is held under tension and is therefore less
