- Interrelationships between Climate, Vegetation and Run-off in the Karoo
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Interrelationships between Climate, Vegetation and Run-off in the Karoo
In an arid region, such as the Karoo, water (rainfall, soil moisture, run-off and evaporation) is the most important natural variable environmental factor influencing vegetation. The grazing animal - primarily the sheep - can, on the other hand, undoubtedly be regarded as the major induced factor directly affecting vegetation.
Figure 1 illustrates, in the most elementary fashion, the main factors involved in the interrelationships between climate, fun-off and vegetation. Undoubtedly many more factors could be added to this figure; however, the figure illustrates those generally regarded as pertaining to the interrelationships under discussion. It should be pointed out that Farm Economy is not an environmental factor; it does, however, directly influence decisions on stock numbers, which in turn has a profound effect on the vegetation and practically everything associated with it. This is especially in respect of the composition, density, productivity and stability of the vegetation. From a hydrological point of view, the type of vegetation and its density has a direct effect, on run-off (thereby rate of erosion), microclimate and soil moisture status.
Generally, the sparser the vegetation the greater the rate of erosion and the higher the run-off and, consequently, other unproductive water losses.
2. Plant phenology, grazing and erosion (run-off)
Phenology is of the utmost importance in respect of effects of climate on vegetation, effects by grazing animals and effects on run-off and erosion. Figure 2 (Roux, 1968a) illustrates the natural plant phenological rhythms of the main components of Karoo vegetation in relation to season, rainfall and temperature.
2.1 Shrubs versus grass
Figure 3 illustrates the difference in soil-protecting ability between a shrub cover and a grass cover having the same canopy densities. The protection afforded to soil in terms of canopy, as illustrated, is in both cases (i.e. shrub and grass) the same. Protection against splash erosion, through canopy interception, is thus very similar. Direct protection against surface run-off is much more favourable under the grass than under the shrub cover. In the former case, the base to canopy ratio is small (favourable); under shrubs it is large (unfavourable). Grass also has the advantage of binding the soil, by means of its roots, more effectively than shrubs do. In the open veld, where grass has become largely replaced by shrubs as a result of grazing practices, especially overgrazing, soil erosion (by water and wind) is accelerated.
2.2 Effects of cover on boulder-strewn slopes
On boulder-strewn dolerite hill slopes, interesting information was obtained by measuring soil depths in two adjoining experimental paddocks, viz. one under continuous grazing by Merino wethers since 1942 and the other completely protected since the same date. The former had developed a sparse shrub cover and the latter a dense grass cover. The only significant difference between the soil depths of the two paddocks was the preponderance (20,5 %) of shallow (1-12 mm) layers of soil where grass was dominant as against the less frequent occurrence (9 %) where grass was absent. These results indicate that shallow soil layers are removed to a greater extent as a result of trampling by stock and run-off. These removed shallow layers, mostly covering boulder edges, probably contribute to a major extent to silt from slopes. The removal of these shallow layers causes the greater exposure of boulders, which then contribute to a greater run-off.
2.3 Vegetation, erosion-rates and micro topography
Figure 4 (Roux, 1973) shows the effects of fixed grazing treatments with Merino wethers on mixed (grass and shrubs) Karoo vegetation. It is clear from the figure that winter grazing promotes grass; summer grazing shrubs (Karoo bushes) and resting mixed grass and Shrubs.
These results were obtained over a period of 30 years. The reaction to the grazing treatments is as a result of seasonal selective grazing and plant phenological rhythms. Soil erosion was measured in the winter (grass) and summer (shrubs) grazed camps. The lowering of the surface, as measured over 16 years, was 4,9 mm and 20,7 mm respectively. Micro topography was also determined. The results are given in Table 1. On a Karoo flat the section line increased by 90,3 % under sparse grass and 175,5 % under Karoo bush over a period of 29 years. Surface erosion was 3,9 mm and 17,5 mm, respectively, over 16 years. It is clear from the results that a grass cover affords the greater protection against soil erosion and consequently also run-off. (It should be pointed out here that erosion cannot be ascribed to water alone but also to a considerable degree to wind). The promotion of grass through grazing strategies would decrease run-off.
3. Vegetation retrogression and run-off Figure 5 (Roux, 1968 b) illustrates the stages of conversion and retrogression of climax grassland to a sparse shrub cover as caused primarily by the impact of grazing. This situation is not uncommon; in fact, it is fairly general. Run-off, soil moisture and soil depth are also illustrated in the figure.
Table 2 gives an indication of the increase in run-off, velocity and moving power of water in a catchment area where the grass cover changes from dense to bare and eroded. Evaporation can also be expected to be high where the cover is sparse and soil exposed. Figure 5 and Table 2 illustrate the basic principles involved in the interaction between vegetation, run-off and erosion.
They also illustrate the importance of maintaining or developing as dense a cover as possible in the veld to ameliorate the impact of run-off in the drainage system. The erection of soil conservation works in channels is primarily an attempt to treat symptoms and not to eliminate the cause.
It should always be borne in mind that where run-off increases soil moisture decreases. Consequently a drier habitat results, with a corresponding change in the vegetation from mesic to less mesic and even to xerophytic. This is a very important mechanism in the retrogression of vegetation
Figure 6 illustrates an interesting aspect, namely drainage density, which is a result of erosion, and the occurrence of a climax grass species. The climax grass, Tetrachne dregei, occurs primarily between stream densities of 0,45 and 0,55 (Roux, 1968 b). If erosion should continue, a drainage density figure of 0,55 would eventually become 0,60 or higher, in which case the particular species would decrease in density, occur as a relict or become entirely non-existent. This example illustrates the principle of erosion and its effects on the vegetation redistribution. The primary stimulus for erosion, and consequently run-off, is undoubtedly to be sought in grazing practices. The sequence of events is usually the thinning of the vegetation, increased erosion and effects of erosion on the vegetation (see Fig 5). In the example on drainage density (Fig 6), the density could be primarily ascribed to geological erosion and secondarily to man-made effects, which, in this particular case, is by far the lesser effect, but nonetheless a most important one even over the short term. Drainage density is expressed in channel length per unit of area (Melton, 1958).
4. Instability and variability of Karoo vegetation Apart from the impact of the grazing factor, Karoo vegetation is by no means stable over the years and exhibits significant fluctuations primarily as a result of the short term shifts in seasonal rainfall (Roux, 1966) and its differential promotion of vegetation components according to their broad phenological rhythms. Figure 7 gives a clear illustration of the effects of shifts in summer and winter rainfall on the grass and shrub components of the vegetation. The situation in Fig 7 is not unique, but reflects the general pattern in many other situations, such as in continuously grazed vegetation, rotationally grazed vegetation, etc. It may be accepted that the changes, in the cover components will result in a corresponding fluctuation in the potential rates of erosion and run-off.
Figure 8 gives an example of the trends in summer rainfall for Aliwal North over a period of 109 years. The trends were obtained by calculating the cumulative departure (Roux, 1966) of summer rainfall from the long-term mean. An interesting aspect comes to the fore in Fig 8, namely that from 1868 to 1902 (34 years) very favourable falls of rain occurred; from 1903 to 1957 (54 years) largely unfavourable summer rainfall prevailed, which was resumed from 1964 to 1972 (8 years), interspersed only by 5 years of favourable rainfall. From 1973 to 1977 (5 years), rainfall was once again favourable. Thus from 1903 to 1977 there were 62 years of less favourable summer rainfall and only 10 favourable. From 1868 to 197744 were favourable as against 62 less favourable. Overall the average annual rainfall remains constant. However, as a result of the fluctuations described above, it is quite possible that the higher rainfall of the favourable years cannot erase the effects of the drier years. Consequently it may be speculated that there is a gradual natural drift to a drier type of vegetation or a sparser vegetation. This could result in a gradual but naturally induced increase in run-off. The dry periods, followed by spates of higher rainfall, are in all probability also responsible for the increase of deep and extensively rooted woody and so-called invading species, such as the Swarthaak (Acacia mellifera var. dentinens), Mesquite (Prosopis spp.), Acacia (Acacia karoo), Kriedoring (Lycium spp.), Driedoring (Rhigozum trichotomum) and many more. The rate at which these species spread or form denser stands is enhanced where the competition status of the vegetation has been reduced as a result of grazing practices. An increase in these woody species affects run-off. The increase in "woodiness" of the African vegetation is well documented by Shantz & Turner (1958).
The general situation regarding vegetational changes in the Karoo areas (see Fig 1in article by M. Vorster in this issue) is described by Roux (1981). Phase 2 (Roux, 1981) was probably a period of maximum run-off and erosion, whereas in Phase 3, the present dominant phase, a decrease is taking place as a result of a denser, albeit lower quality, vegetation.
From the foregoing, it should be clear that land use has the most far-reaching effects on run-off and erosion. It should ever be borne in mind that the manipulation of the grazing animal in veld utilization strategies has by far the greatest effect on vegetation, run-off and erosion than any other environmental factor.
MELTON, M. A., 1958. Geometric properties of mature drainage systems and their representation in an E4 phase space. J. Geol. 66, 35-37.
Roux, P. W., 1966. Die uitwerking van seisoenreënval en beweiding op Gemengde Karooveld. Hand. Weidingsveren. S. Afr. 1, 103-110.
Roux, P. W. I 968a. Principles of veld management in the Karoo and the adjacent dry Sweet-grass veld. Ch. XIX in: The Small Stock Industry in South Africa. Edited by W. J. Hugo, Pretoria: Government Printer.
Roux, P. W., 1968b. The autecology of Tetrachne dregei Nees. Ph.D. thesis, Univ. of Natal.
Roux, P. W., 1973. Hoe beweiding met kleinvee Karooveld beïnvloed. Boerderykeur 1973, 115-119. Johannesburg: Promedia Publikasies.
Roux, P. W., 1981. Vegetational change in the Karoo Region. Karoo Agric. 1 : 5, 15-16.
SHANTZ, H. L. & TURNER, B. L., 1958. Vegetational Changes in Africa. University of Arizona, Report 169, pp. 158.
Karoo Agric 2 (1), 4-8