The Full Picture

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have been possible without a long-term, regional satel-

lite data archive.

Distinguishing human-induced degradation from

drought

In arid and semi-arid regions vegetation production

varies between years caused primarily by inter-annual

rainfall variability. Southern Africa suffers periodic

droughts, some caused by the El Niño phase of the El

Niño-Southern Oscillation (ENSO) cycle. Extreme vari-

ability in vegetation production between years makes it

exceedingly difficult to distinguish long-term changes

caused by human-induced land degradation from the

effects of periodic droughts. One approach to monitor-

ing land degradation is to use both long-term AVHRR

NDVI data and rainfall surfaces to identify any negative

trends in vegetation production per unit rainfall through

time (1985–2003).

In the Limpopo Province negative trends were

largely associated with the degraded communal lands,

although some well-known degraded areas did not

show continued negative trends during the study

period (1985–2003). A distinction should be made

between degradation that occurred within or before

the start of the satellite time-series; some homelands

were created as early as 1913 so much of the degra-

dation could have occurred before and may not have

worsened since. The results of this analysis in north-

ern SA are validated by the National Report on Land

Degradation,

which shows the perceived rates of

energy used by plants for photosynthesis. Remotely sensed vegeta-

tion production may very well be the single most useful indicator of

land degradation at regional and decadal scales.

In the early 1990s NDVI data from the Advanced Very High

Resolution Radiometer (AVHRR) sensor on the NOAA series of

meteorological satellites were analysed and showed that the Sahara

desert was not marching southwards into the Sahel, but rather

expanded and contracted as a result of interannual variations in

rainfall.

Recently, a number of studies using 20 years of AVHRR

NDVI data have shown that the Sahel, which was once believed to

be suffering severe desertification, has in fact become greener

during the past ten years as a result of higher rainfall.

The occur-

rence of desertification in the Sahel and elsewhere has become

highly controversial, but the debate is being conducted in the

absence of objective, consistent data at the appropriate scales.

While repetitive, global remote sensing has been applied to

mapping and monitoring degradation, interpretation of the results

has not always been based on sound ecological principles. A criti-

cal aspect of a useful degradation mapping and monitoring system

is the ability to distinguish the impacts of human activities from

natural variability in climate and spatial variations in soils and land

cover types.

Land degradation in the former homelands of South Africa

In South Africa, large degraded areas have been found within the

former ‘homelands’ – self-governing areas created before and during

the Apartheid era.

Today these homelands, now called communal

areas, are characterized by high human and livestock populations,

overgrazing, soil erosion and sparse grass cover. Degradation contin-

ues to threaten the local resource base upon which rural communal

livelihoods depend. The underlying cause of degradation is a combi-

nation of unemployment, poverty and an absence or failure of land

use regulation. The degradation observed in the communal lands is

thus principally a consequence of high population densities caused

by the oppressive apartheid system rather than the outcome of tradi-

tional communal pasturalism.

These degraded rangelands have been subjectively mapped using

single date Landsat images as part of the National Land Cover (NLC)

map. The vast majority of the large degraded areas fall within the

communal areas. We investigated the long-term vegetation produc-

tion of these degraded areas with 18 years of 1km

resolution

AVHRR NDVI data by comparing them to non-degraded adjacent

areas with the same soils and climate. It was found that the non-

degraded areas had consistently higher vegetation production as

indicated by the seasonal total of NDVI obtained from satellite obser-

vations despite a six-fold variation of annual rainfall. This indicates

that the degraded areas produce less vegetation per unit of rainfall

and suggests that they may have changed to a different ecological

state. The results nevertheless indicate that these degraded areas are

functionally stable and resilient, but never reach the same vegeta-

tion productivity as the neighbouring non-degraded areas.

These

results are both alarming and encouraging: alarming because the

degraded state seems to be permanent; and encouraging because

the difference in productivity was approximately 20 per cent, far

from a complete loss of production as is sometimes suggested in

degraded areas. Whether the difference in total rangeland produc-

tion translates into economic loss remains to be determined. These

results have made a significant contribution to understanding the

ecological processes in degraded areas – research that would not

Former homelands, provinces and study area in South Africa

Source: K Wessels

Large degraded areas have been found in South Africa’s former homelands

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