- 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 and small) and Karoo pioneers. Some of these species were ecologically so successful that they had, to a considerable extent, blurred the development of the second phase in many localities as for example in the Karoo mountains.
Under continuous and severe grazing stress the development of Phase Three can be inhibited.
A direct result of the Phase Three development was the near complete obliteration of bare patches in the Karoo Region (especially during the late nineteen-sixties and nineteen-seventies), and the general thickening-up of the vegetal cover. Vast areas were occupied and dominated by less desirable and less productive problem plant communities and species. This replacement has already reached such proportions that a relative state of stability exists. The position is now such that the general vegetal cover is nearly as dense, as, and, in some instances, is even denser than that of the vegetation in Phase One due to a more effective water economy.
The present situation is significant in three respects: namely, that it provides an excellent opportunity for the redevelopment of Phase One vegetation; that erosion and run-off has slowed down; and that loss of grazing capacity has occurred generally.
Inevitably, especially during Phase Two, considerable loss in the potential of soils to support the original vegetation had taken place. It is therefore at present quite impossible, even under ideal treatment, to revert to a Phase One type of vegetation. However, there undoubtedly exists in Phase Three a vast potential and a reclamation slack which should be exploited to the full in order to approach the Phase One situation as nearly as possible.
It should also be realized, that over the last two centuries many ecological processes have been disrupted, diverted and deflected, whereas others have gained unprecedented momentum and even in some instances, stability. To change such vast and stimulated ecological processes and such stabilized situations requires concerted and co-ordinated efforts especially in the scientific fields of plant ecology, and veld utilization and management.
Karoo vegetation has now reached a most critical stage in Phase Three which, if mismanaged, will inevitably develop into a Phase Four situation.
The active spreading and increase in so-called invading species, are positive indications of the progress toward Phase Four in which they could eventually become dominant.
This phase is where Phase Three, with continued development, has arrived at a point where productive and utilizable species have reached such a low level that the less desirable, semi-palatable species and even those considered unpalatable have become the main source of natural fodder. There already exist clear-cut examples of the Phase Four situation over large areas in the western Karoo and mountains. Subjecting Phase Four vegetation to increasing utilization, especially under pressure, would inevitably lead to the destruction of protective vegetation. The general consequences of this would undoubtedly be many times more severe than those encountered in the Phase Two situation. The development of a Phase Four situation should be prevented at all costs.
Phase Four vegetation consists basically of a stable and relatively permanent cover of unpalatable or inedible vegetation dominated by one or two species. The floor (i.e. open ground between the established species) will at time become covered by largely short-lived or annual herbaceous species; these include the lesser grass species which may be annual or bi-annual. The grazing capacity of Phase Four vegetation is on the average low and is subject to considerable fluctuation as it is practically entirely dependent on rain. The phase Four situation is thus an unpalatable shrub (or succulents or woody low trees) matrix filled in by a short-lived floor cover dependent on rain for its development. In this situation run-off and erosion rates are usually high, but not necessarily so. To reclaim Phase Four vegetation would be many times more difficult and would take much longer than that of the Phase Three situation. In fact, Phase Four would probably require artificial veld reclamation measures. The destruction of Phase Four vegetation the development of Phase Five appears to be a final stage; this is where vegetation is practically entirely destroyed and the soil completely exposed to erosive agents.
It must be borne in mind that the concept of vegetation change as set out above has bearing on the general situation in the Karoo Region. At any particular locality, depending on its grazing history and climate – especially average rainfall - the various phases may vary in duration and intensity and may overlap and merge considerably or may even be indistinguishable. The various phases should never be considered as distinct or discrete stages.
In the marginal grassveld regions the Phase One situation rapidly develops into Phase Three. Phase Two is usually of relatively short duration and is usually not severe. Also, as a result of the higher potential of grassveld areas there are far greater prospects of the reversal or halting of phases here than in arid areas, excluding possibly that of the western arid grasslands (Bushmangrassveld). It should be pointed out here that there are still vast tracts of veld - especially in the eastern dry grassveld and adjoining grassveld which are in the late stages of Phase One.
The ecological unravelling of these phase changes and those taking place within phases, afford some of the most demanding and complicated studies that can be undertaken by the plant ecological and pasture sciences. The identification of the phases as set out above can afford the basis for developing new philosophies and new approaches to the reclamation of vegetation.
It should be pointed out here that the geographical distribution of insects, birds, mammals and reptiles are also affected by the development of the phases.
In the Karoo Region there are deserts, semi-deserts and arid lands, such as they have been over the centuries. It cannot be disregarded that there is desert encroachment and there is climatic change. However, currently these processes are negligible as compared to the alteration of degradation of the vegetation under grazing by domestic animals. Grazing by stock is today the most active and overriding ecological factor operating in the South African grazing lands. It can, however, operate for better or for worse.
In the years that lie ahead the Karoo farmer will have to face the problem of vegetation utilization as well as reclamation. However, utilization and reclamation are well within reach as the numerous and excellent examples of veld reclamation show. In order to substitute for loss of grazing capacity and to decrease grazing pressure on the veld without forfeiting economy, it is imperative that additional sources of stock feed be created on the greater majority of farms. In this respect it is inevitable that in the nineteen-eighties the establishment of plantations of drought hardy fodder species and fodder reserves will gain new status and impetus. Such resources, coupled with sound veld and stock management, spells good for the future of extensive stock farming in the arid region of South Africa. There is every reason to believe in the economic success of extensive small stock farming.
Karoo Agric 1 (5), 15-16