Geoscience Reference
In-Depth Information
Plate 26.4
The desert in bloom. Winter rains in Namaqualand, Northern Cape Province, South Africa, have helped to germinate
the profusion of annual seeds that now carpet the desert with colour.
Photo: Peter Smithson
woodland in many cases, with widely spaced and rather
scrubby trees.
The formation of savanna is of considerable interest
and dispute. Climatic factors alone cannot account for the
character of these areas except in the drier parts. Although
they experience a distinct dry season during which many
plants become dormant, and although precipitation is
variable, towards the ecotone with the tropical rain forest
it appears that the climate could support a much more
luxuriant and diverse flora. One possible reason for the
disparity is that the savanna represents a form of
plagio-
climax
, one which has been severely curtailed by human
activities. Human-induced wildfires, in particular, have
played a major part in the development of savanna, and
many of the trees are fire-resistant. With the action of fire,
and the voracious appetite of termites, seeds rarely survive.
In response, trees produce enormous numbers of seeds
each year.
Acacia karoo
, for example, releases as many as
20,000, of which about 90 per cent are typically fertile.
SAVANNA
Vegetaton
As we move equatorwards from the more arid parts of
deserts, vegetation becomes more abundant. Initially
grasses become dominant to provide a suitable habitat for
grazing animals. Individual, and then small clumps of,
trees gradually appear as rainfall levels increase. The trees
are usually smaller than similar species in damper condi-
tions and often appear gnarled. Many have developed
protective mechanisms to stop grazing animals denuding
them. For example, some
Acacia
species possess sharp
thorns (
Plate 26.5
).
Savanna biomes cover approximately 11ยท6 per cent of
the land surface of Earth. They are most closely associated
with the southern continents, covering about 65 per cent
of Africa and 60 per cent of Australia. Although it is often
referred to as a grassland biome, the savanna is an open
