- Vegetation change in the Karoo region
|Last update: April 2, 2012 12:36:37 PM|
Vegetation change in the Karoo region
From a re-evaluation of data and theories pertaining to vegetational change, Karoo and desert encroachment, climatic change and droughts I have come to the conclusion that desert encroachment, as a natural process, cannot be regarded as an important ecological factor operating in the Karoo Region. There is no positive evidence that desert, as such, is encroaching (except in localised areas) nor that the climate is effectively changing (except for short-term temporary largely seasonal changes) toward the more arid.
One is tempted to allocate the term desert encroachment to degenerative vegetational changes; especially in arid regions such as the Karoo. It does however not necessarily follow that the degeneration of vegetation ends in desert conditions; mostly, in South Africa, it does not.
The newly coined term, desertification, appears to fit, to a considerable extent, the vegetation situation in South Africa. It implies to make desert. One can also justifiably speak of the desertification of a desert. This term desertification is also equated with soil processes such as salinization and steppezation. An acceptable definition for desertification appears in the publication Desertification: Its causes and consequences by the United Nations Conference on Desertification in 1977. This definition implies that desertification arises from the interaction between a precariously balanced arid environment and its utilisation by man in order to make a living.
In spite of the relative paucity of data on vegetational change and related aspects, it appears that vegetational change in the Karoo Region took place in three board overlapping but recognizable phases. These phases can be seen as seral stages of desertification.
These chases are briefly:
This phase was characterised by the rapid breakdown of the pristine vegetation under increasing pressure by escalating numbers of domestic stock, especially sheep, and the decimation of indigenous fauna. This phase gained effective momentum during approximately the middle of the nineteenth century and continued well into the first quarter of the twentieth century. The destruction of valuable, productive and soil protective plants occurred largely during this phase.
The thinning of palatable vegetation (Phase One) under grazing by stock (primarily sheep) took place at a rate which could not be compensated for soon enough by less palatable and grazing resistant species. There was thus a considerable time lag (Phase Two) before the replacing species could build up sufficient momentum to offset the results of thinning out.
The result of this lag was that for many decades the general vegetation was very much sparser than it used to be or than could potentially be supported by the existing climate and soils. This phase lasted up to approximately the nineteen-sixties, gaining its peak from the nineteen-twenties well into the nineteen-fourties and early nineteen-fifties. This phase was characterised, as a result of the sparser vegetation, by increased run-off and high rates of erosion with an accompanying decrease in rainfall effectivity. This alarming phase spawned theories and investigations into droughts, climatic change and desert encroachment. During this Second Phase the greatest reduction in grazing capacity of the veld, as well as in stock numbers took place - especially during the late nineteen-thirties. The widespread development of bare patches (kaalkolle) was a conspicuous result of this phase. The development of these bare areas reached a maximum during the 'thirties and 'fourties.
During the whole of the Second Phase the Third Phase had begun its development with greatly varying and variable pace.
During this phase the replacement of the thinned-out. vegetation of the Second Phase had gained momentum. The replacing species were largely those not normally eaten by stock and those variously called invaders, unpalatable and undesirable species, encroaching species (large 