Geoscience Reference
In-Depth Information
course begs the question of just what 'desertic' means. However, and more usefully,
he also pointed out that desertification caused impoverishment of ecosystems, accel-
erated soil degradation, reduced plant and animal productivity, and impoverishment
of dependent human livelihood systems. When there was a combination of climatic
stress and land use pressure, the almost inevitable result was land degradation and,
in extreme cases, famine (Mabbutt,
1978
). Other workers have also emphasised the
outward signs of desertification in affected landscapes (Kassas,
1995a
; Williams and
Balling,
1996
; Williams,
2000
a; Williams,
2002b
; Williams,
2003
). The causes of
desertification will vary from region to region, as discussed in Sections 24.7 to 24.10.
When there is a combination of climatic stress and land use pressure, the almost inev-
itable result is desertification. Some of the more immediate signs of desertification
may include any or all of the following:
Accelerated erosion by wind and water;
A decline in soil structural stability, with an ensuing increase in surface crusting and surface
run-off, and a reduction in soil infiltration capacity and soil moisture storage;
An increase in the flow variability of dryland rivers and streams;
A decline in soil organic matter and nutrient status with an attendant decline in crop and
fodder yields and, in extreme cases, social disruption, famine, and human and livestock
migration;
Salt accumulation in the surface horizons of dryland soils;
An increase in the salt content of previously freshwater lakes, wetlands and rivers;
The replacement of forest or woodland by secondary savanna grassland or scrub;
A reduction in species diversity and plant biomass in dryland ecosystems.
An increase in dust particles;
An increase in carbon particles and trace gases from biomass burning.
24.7 Fire and the impact of biomass burning
Fire is often cited as a major cause of land degradation, and there is little doubt that
during drought years the impact of wildfires can be devastating, as witnessed in recent
years in California, northern Russia, China, the Mediterranean countries of Europe
and most parts of Australia. The impact of unregulated fires throughout the drier parts
of the world has now become so severe in terms of loss of life, livelihood and homes
that it merits our particular attention.
We saw in
Chapter 17
that our prehistoric ancestors may have begun to use fire
about a million years ago. Some of the evidence remains equivocal, but by Upper
Palaeolithic/Late Stone Age times, fire was widely used for hunting, for preserving
meat, for day to day cooking and for heating certain types of stone in order to make
them easier to flake and fashion into knives and other implements. The Aboriginal
inhabitants of Australia made frequent use of fire and contributed to substantial
changes in the plant cover as well as the possible extinctions of certain species of
