Observations by local people in the Mier area, southern Kalahari, South Africa, indicated that
degraded rangeland does not recover within a time frame acceptable to landowners.
Pristine vegetation in this linear dune system consists largely of a herbaceous layer dominated by
perennial grasses. Woody vegetation is sparse on dunes and interdunes in good condition. The
dunes and interdunes react differently to disturbance, probably because of differences in
substrate stability, soil particle size distribution and consequent differences in soil nutrient
distribution and moisture content. Degraded dunes are devoid of any vegetation, except for
Stipagrostis amabilis, a rhizomatous grass which remain in small clumps, and the tree Acacia
haematoxylon. The latter increases in numbers probably due to the high moisture content in
degraded dunes. Moisture content in degraded dunes remain high even during prolonged dry
periods. Interdunes are more susceptible to degradation and are invaded by the long-lived shrub
Rhigozum trichotomum and the annual grass Schmidtia kalahariensis. Both these species
compete with perennial grasses for moisture. Additionally, depleted seed banks and increased
seed predation by ants (Messor capensis) may also affect the re-establishment of perennial
grasses in the interdunes. The main conclusion from this study is that degraded southern
Kalahari rangeland cannot recover spontaneously at the landscape scale because of a negative
feedback mechanism that prevents establishment and growth of seedlings. The hypothesis put
forward is that rangeland ceases to react to rainfall as an ecosystem driver as it becomes
degraded. Once degraded, wind controls the dynamics of the system and recovery, irrespective
of rainfall, is almost impossible. In contrast, detailed studies at the level of small isolated
populations of S. amabilis and S. ciliata on degraded dunes indicate high rates of population
growth. The scales at which these processes exist are contradictory and may not be
contradicting. Rangeland managers should take cognisance of the influence of the spatial and temporal scale at which they operate, and on which they base their decisions. / Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 2000.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/10247 |
| Date | 17 December 2013 |
| Creators | Van Rooyen, Andre F. |
| Contributors | O' Connor, Tim G. |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
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