Grasslands of the World.pdf - Disasters and Conflicts - UNEP

The changing face of pastoral systems in grass-dominated ecosystems of eastern Africa 55
(2) either long-term experiments or observational studies at the pasture scale
that assess the relative impacts of drought, livestock and other agents on ecosystem
dynamics . Most of the research on land degradation and desertification
in Africa has been focused on the Sahel , which has been intensively studied
since the first droughts in the 1970s. Some landscape-scale studies based on
a combination of fine-resolution satellite data and field measurements have
been carried out in different parts of Africa and demonstrated the existence
of local-scale degradation of rangelands in Ferlo, Senegal (Diouf and Lambin,
2001) and Turkana, Kenya, where highly affected areas covered only 5 percent
of the land surface (Reid and Ellis, 1995). The importance of studying land
degradation and desertification problems over a sufficiently long period is
illustrated by studies conducted in Burkina Faso by Lindqvist and Tengberg
(1993) and later by Rasmussen, Fog and Madsen (2001). The first group of
scientists studied the amount of woody vegetation cover in three sites in
northern Burkina Faso (1955–89). They found that important loss in woody
vegetation cover occurred during the first of a series of droughts that started
in the late 1960s, when large areas of bare soil developed. The authors found
little evidence of vegetation recovery until 1989, despite increasing rainfall since
1985. Rasmussen, Fog and Madsen (2001) revisited the area, adding 10 years of
satellite data. They found a decrease in albedo and thus increase in vegetation
cover over the period 1986–1996, which was confirmed by fieldwork, thus
showing the recovery of vegetation after drought. Interviews with local people
indicated that the species composition of the regenerating herbaceous vegetation
has changed considerably since the late 1970s. Land degradation caused by
heavy grazing pressure was mostly found in the proximity of important water
resources , which probably cover only a small proportion of the landscape in
total. Schlesinger and Gramenopoulos (1996) also used the amount of woody
vegetation cover as an indicator of desertification in western Sudan, but studied
sites that were devoid of human use over the period 1943–1994. They analysed
a time series of aerial photographs and Corona satellite images and did not find
a significant decline in woody vegetation for the study period, despite several
droughts having occurred during that period. Thus, at least in this area, they
showed that the Sahara is not expanding and that drought had little effect on
woody vegetation. In another part of the Sudan, the concentration of livestock
around water points and settlements led to local loss of vegetative cover and
accelerated erosion (Ayoub, 1998). In contrast, others have found that heavy
livestock grazing around pastoral settlements in arid areas had minor impacts
on woody vegetation and biodiversity , with impacts confined within the settlements
themselves (Sullivan, 1999). Woody vegetation can replace palatable
grass species in heavily grazed areas, caused by grazing pressure rather than
climate (Skarpe, 1990; Perkins, 1991).
At the pasture or field level, the picture is more complex. Generally, livestock
grazing , browsing and trampling causes loss of vegetation , competition