Geoscience Reference
In-Depth Information
most recent fans as a guide, they estimated that the impact of this rainfall event was
confined to an area no more than about 10-15 km in radius. The downpour caused
channel entrenchment above the fan apices and fan sedimentation below. Buried soils
exposed in the banks of the most recent gully showed repeated episodes of dune
dissection, fan deposition and soil development. The soils would have formed when
the fan surface was oncemore vegetated and stable. The youngest soil probably formed
during a regionally wetter interval in the Sahel dated to about 150-350 years ago.
Their overall conclusion was that the 1974 phase of dune dissection was no greater in
scale than previous Holocene phases and that there was every reason to assume that
recovery would again be possible. Needless to say, this conclusion would probably
apply to other parts of the Sahel, provided that grazing allowed re-establishment of
the plant cover.
24.3 Detecting desertification using changes in plant cover
One unresolved problem in detecting a climatic influence on desertification concerns
the high degree of variability in the annual plant cover along the desert margins.
Tucker et al. (
1991
) used remote sensing data for the 1980-1990 period in an attempt
to plot the expansion and contraction of the Sahara Desert during that decade and
found that there was such a high degree of variation in plant cover relative to both
longitude and latitude that they were forced to conclude that ten years were too few
for any meaningful trend to be discernible. The previous year, Nicholson et al. (
1990
)
had sought to use the Normalized Difference Vegetation Index (NDVI) to evaluate
the response to rainfall in the Sahel and East Africa. They discovered that there was
a linear correlation between NDVI and mean precipitation.
Tucker and Nicholson (
1999
) revisited this problem nearly a decade later and
claimed that the longer period of observation did enable them to detect significant
trends in the response of plant cover to decadal variations in rainfall. This type of
research is needed in all desert margin systems, given that the response of the NDVI
to annual rainfall may well vary from region to region, depending on the different
soil types and plant associations. For example, acacia species in the Sudan growing
on sandy soils require up to 35 per cent less precipitation than the same species in the
same latitude growing on clay soils (Smith,
1949
; see also
Chapter 4
).
One example will suffice to show how past estimates of changes in plant cover along
the southern edge of the Sahara have been taken out of context and misused. From 21
October to 10 November 1975, the ecologist Dr Hugh Lamprey completed an aerial
survey of the plant cover along the desert margin of northern Sudan (Lamprey,
1975
).
A comparison with the northern limit of semi-desert vegetation in the Sudan mapped
by Harrison and Jackson (
1958
) revealed a change for the worse, prompting Lamprey
to conclude that 'the desert boundary has shifted southward by an average of about 90-
100 kilometres in the last 17 years'. Given the high degree of interannual variability in
