Geoscience Reference
In-Depth Information
Plant types
Most savanna grass species belong to the C
4
group of
photosynthetic plants, in contrast to the C
3
group, which
dominate in temperate latitudes. In the C
4
group the rate
of photosynthesis increases with the increase in intensity
of solar radiation rather than reaching a plateau of CO
2
uptake, as happens in most C
3
species. This feature makes
them very efficient photosynthesizers where light condi-
tions are strong, as in savanna regions. However, there is
some evidence that their ecological advantage may be less
strong in an atmosphere of higher CO
2
concentration.
With C
3
plants the elevated CO
2
levels of the enhanced
greenhouse effect atmosphere are expected to lead to
greater plant growth, but this is unlikely to occur with the
C
4
group, where solar radiation is more important. Species
of the C
4
type include major food crops such as maize and
sugar cane which, unlike the temperate cereal crops and
rice, would not be expected to benefit from increased
levels of carbon dioxide.
Wildlife
The number of animal species is relatively low in the
savannas, though their populations are large. Surprisingly,
interspecific competition seems to be limited, and the food
chains are short, with few secondary consumers. Most
carnivores prey directly on herbivores. For example, lions
attack mainly zebra, wildebeest, antelope and giraffe.
However, many scavengers and decomposers, including
mammals and insects, also feed on the lions' kills. Termites
are very abundant, and their mounds are a major feature
of the savanna landscape (
Plate 26.7
).
These insects
attack and macerate plant debris, making it more
readily available for decomposition by other organisms.
They also eat growing plants, especially during periods
of drought. As a result of these activities, methane is
released to add to the increase in greenhouse gases in the
atmosphere. Although the calculations are imprecise, the
contribution from termites is believed to be about 8 per
cent of the global production of methane.
Soils
DRY LANDS
The soils of the savanna are variable. They include
ferralsols, acrisols, vertisols and luvisols. Their distribution
is related to climatic, geological and geomorphological
conditions. Slope processes are active, for the vegetation
is often insufficient to prevent erosion and downwashing
of nutrients. Consequently, marked catena sequences
develop on the hill slopes, grading from shallow stony soils
to deeper, less well drained, base-rich alluvial soils (
Figure
formed of hardened iron oxides (laterite) and indicate old
erosion surfaces.
Water resources
The dry lands of the world present many problems of
development, most stemming from the constraints
determined by a climate with a lack of water. Depending
upon the degree of aridity, water may be available from:
1
Perennial rivers whose headwaters are in wetter areas,
such as the Nile, the Niger or the Indus.
Red non-swelling,
acidic kaolinitic soils
Lateritic
crust
Deep, moderately
nutrient rich soils
Black, nutrient-rich,
swelling,
montmorillonitic soils
Figure 26.5
Catena sequence in a savanna area.
