There is concern that the communal rangelands of Botswana are overstocked with
cattle, and that this has led to unwanted ecological changes. These changes are
assumed to be most prominent around boreholes. This study describes vegetation and
soil piospheres around boreholes in Eastern Botswana and investigates factors
associated with their development.
The classical range succession model and its modifications, the bush encroachment
theories and the soil-nutrient transfer model, have been applied here to explain
changes in vegetation and soil variables with distance from artificial water points.
Data on soil texture, soil nutrients, vegetation species composition and cover, dung
weight and bare ground were collected from boreholes of different ages, with different
numbers of cattle using them.
Piosphere patterns were observed. However, some of the patterns were not in
accordance with the classical range succession model and the bush encroachment theories. The frequent occurrence of palatable grasses (Panicum maximum, Setaria
verticiliata and Digitaria milanjiand) nearer to water points where the grazing
pressure is highest, and of unpalatable ones (Aristida congesta and Eragrostis
rigidior) further away, where the grazing pressure is minimal, contradicts the classical
range succession model. Total tree cover along the gradient did not show any changes,
which contradicts bush encroachment theory (two-layer soil moisture model). The
concentration of soil nutrients and dung weight close to boreholes was well predicted
by the soil-nutrient transfer model. The frequency of Acacia tortilis and Dicrostachys
cinerea and bare patches, increased towards boreholes. Grass cover, occurrence of
Terminalia sericea, Boscia albitrunca and Combretum apiculatum increased with
distance from water points.
The results suggest that the range succession model and its modification, the twolayer
soil moisture model, cannot satisfactorily explain vegetation changes with
distance from water points. Species composition at a point may be affected by the
growth, survival, recovery and reproductive strategies of different species, fire,
selectivity of browsers and nutrients status, as well as grazing disturbance. Therefore,
the 'state and transition' model is recommended, as it considers factors that are ignored
by the classical range succession model, and is appropriate for analysing vegetation
change under highly variable rainfall.
The adverse impacts of grazing around boreholes could be reduced by controlling
cattle distribution on the range as a whole.
| Identifer | oai:union.ndltd.org:ADTP/219219 |
| Date | January 1994 |
| Creators | Moleele, Nkobi Mpho, n/a |
| Publisher | University of Canberra. Resource, Environmental & Heritage Science |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | ), Copyright Nkobi Mpho Moleele |
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