Colophospermum mopane Shrubveld on Flat Concave Basalt Plains.
Location and Geomorphology
This landscape extending northwards from the Timbavati River, is intersected by the Olifants and Letaba Rivers, broadens to include the plains of tsende and Dzombo, is again intersected by the Shingwedzi River and extends further north to Klopperfontein. Structurally it consists of flat to concave plains with a number of drainage channels which can ultimately be classified as marshes or vleis. Good examples of such marshes are Shawu, Dzombo, Nkulumbeni, Boyela, Nwatwitsumbe and Hlamalala. The geological rock formations upon which this landscape developed is basalt, and Bristow (1980) classifies the basalts of this specific part of the KNP as Letaba Basalts. Darker coloured soils with vertic characteristics usually develop here as opposed to the Sabi River Basalts which occur further south and give rise to more red soil. Dolerite intrusions occur extensively in the basalts. The mopane shrubveld is situated between 300 and 400 metres a.s.l. and is the largest and most homogeneous landscape. It covers 1993 km/2 or 10.3 percent of the KNP.
The rainfall of this landscape varies between 450 and 500mm per annum (Gertenbach 1980). Letaba (462 mm) and Shingwedzi (472 mm) are comparable rainfall stations. Temperature data is also available for the two stations in Table 5 and 6. The summers are very hot and as a result of the flat topography there is relatively little difference in micro-climate.
The soils that occur in this landscape are darker in colour (melanic) and usually have a high clay content (20 to 50% clay). In some cases the A-horizon is thin (300mm) and overlay a thick layer of lime concretions. Such soils belong to the Milkwood, Mayo and Mispah Forms. These types of soil occur mainly on the middle-and footslopes. On the higher middleslopes the colour of this soils are usually red and the dominant soil Forms are Bonheim, Swartland and Mayo. Where the topography is flat or even concave, darker coloured soils with vertic characteristics occur.
The soils expand and contract with wetting and drying because of the presence of montmorillonite-type clay. Soils developing under such circumstances are Bonheim, Arcadia and Rensburg. These vertisols are inclined to granulate spontaneously on the surface. The soils on the dolerite intrusions are more shallow and are better drained with Mayo, Milkwood and Glenrosa as the dominant Forms. The spruits that drain the landscape are shallow marshes and the soils are dark in colour, sometimes having a gleyed clayey B-horizon. Dominant soil Forms are Willobrook, Bonheim and Inhoek.
The woody vegetation of this landscape is dominated by Multi-stemmed mopane shrubs, one to two metres in height. As many as 600 of these shrubs can occur per hectare. The absolute dominance of Colophospermum mopane results in other woody species being relatively scarce. Individual species that occur are the following: Combretum imberbe, Euphorbia guerichiana, Grewia bicolor, Commiphora glandulosa, Acacia exuvialis, Combretum apiculatum, Lannea stuhlmannii, Acacia nigrescens, Dalbergia melanoxylon, Lonchocarpus capassa, Sclerocarya caffra, Ozoroa engleri, Securinega virosa, Grewia villosa, Albizia harveyi, Acacia tortilis, Ehretia rigida, Combretum mossambicense, Maerua parvifolia, Combretum hereroense and Dichrostachys cinerea subsp. africana. The mopane-shrubveld can be divided into three variations on the basis of the composition of the field layer. The occurrence of the three variations corresponds to a large extent with the position in the topography. Gertenbach (1983 in prep.) refers to the three variations as the Bothriochloa radicans-variation, the Themeda triandra-variation and the Setaria woodii-variation.
The Bothriochloa radicans-variation (Fig. 53) occurs mainly on the lower-, middle- and footslopes on the Milkwood soils. The woody component of this variation is a typical open shrubveld, but the field layer is dominated by Bothriochloa radicans. Other important species that differentiate the variation are Indigofera heterotricha, Neuracanthus africanus, Brachiaria eruciformes, Dicoma tomentosa and Euphorbia guerichiana. Species in common with the Themeda triandra-variation are Clerodendrum ternatum, Rhynchosia totta, Enneapogon cenchroides, Aristida congesta subsp. congesta, Fingerhutia africana, Seddera capense, Indigofera schimperi, Tephrosia polystachya, Schmidtia pappophoroides, Heteropogon contortus, Cenchrus ciliaris, Urochloa mosambicensis, Panicum coloratum, P. maximum and Digitaria eriantha var. pentzii.
The Themeda triandra-variation (Fig. 54) occurs on the middleslopes and convex uplands. The soils are usually deeper (Bonheim, Swartland, Mayo) and the woody structure differs slightly from the Bothriochloa radicans-variation. Small Colophospermum mopane and Combretum imberbe trees are dispersed in between the mopane shrubs. The field layer is dominated by Themeda triandra and Panicum coloratum, but Bothriochloa radicans occurs especially under overgrazed conditions. Other grasses occurring in the Themeda triandra-variation are Setaria woodii, Eragrostis superba, Aristida congesta subsp. barbicollis, Schmidtia pappophoroides, Heteropogon contortus, Cenchrus ciliaris, Urochloa mosambicensis, Panicum maximum, Sorghum versicolor and Digitaria eriantha var. pentzii.
Fig. 53 Landscape 23. Bothriochloa radicans-variation, Mopane Shrubveld. – page 85
Fig. 54. Landscape 23. Themeda triandra-variation, Mopane Shrubveld.- page 85
Forbs are scarce in the veld because of the dense grass cover but the following species are common: Heliotropium steudneri, Clerodendrum ternatum, Rhynchosia totta, Seddera capense, Indigofera schimperi, Tephrosia polystachya, Vernonia fastigiata, Cassia mimosoides, Sericorema remotiflora, Tephrosia multijuga and Rhynchosia minima.
The Setaria woodii-variation (Fig.55) occurs on concave terrain where soils are very clayey and expands and contracts with wetting and drying (Arcadia, Rensburg and Bonheim). The woody structure differs from the former two variations in that Colophospermum mopane-shrubs are more sparse and other species such as Acacia nigrescens, Albizia harveyi and Lonchocarpus capassa become more dominant. In certain areas such as Tihongonyeni an almost homogeneous stand of Acacia nigrescens trees occurs. The field layer comprises all the species occurring in the former two variations, but they differ in dominance. Setaria woodii is the dominant grass with Cenchrus ciliaris, Urochloa mosambicensis, Panicum coloratum, Themeda triandra, Eragrostis superba, Panicum maximum and Sorghum versicolor as constant species. Forbs which occur are Solanum panduraeforme, Ipomoea obscura, Merremia kentrocaulos, Rhynchosia minima, Tephrosia multijuga and Vigna triloba. Where dolerite intrusions occur in the basalt, the soils are shallow and the internal drainage better. The grass cover is also sparser and according to Gertenbach & Potgieter (1979) small mopane trees are present. They explain this on the basis of a sparser grass cover and a lower intensity fire. Considering that the dolerite intrusions occur as reasonably straight dykes, the small trees tend to occur in straight lines. These dolerite intrusions give a good indication as to where to drill for water for game.
The drainage channels in the landscape are shallow marshes or vleis (Fig.56) with a dense grass cover from 1 to 1,5 metres high and practically no trees. Woody species that occur here are Acacia xanthophloea, Lonchocarpus capassa, Hyphaene natalensis, Albizia harveyi, Dalbergia melanoxylon and Croton megalobotrys. Sporobolus consimilis is the dominant grass with the following constant species: Cyperus sexangularis, Corchorus asplenifoius, Sutera bolusii, Ischaemum brachyatherum, Chloris gayana, Phragmites australis, Eustachys paspaloides, Corchorus trilocularis, Sesbania sesban, Sporobolus fimbriatus, Cynodon dactylon, Leptochloa uniflora and Typha latifolia.
This landscape is of major importance to the rare game species that occur in the KNP. Almost 85 percent of the roan antelope population of the KNP occurs in this landscape. Tsessebe, sable and eland occur in fair numbers. Zebra and buffalo are, however, the animals that are present in the largest numbers. Elephant bulls are common, while breeding herds occasionally move through to other landscapes. Waterbuck generally occur at permanent waters and at Shawu dam a population of approximately 150 animals can regularly be seen. In the shrub mopane-veld impala are very rare but steenbok and Sharp’s grysbok are quite common. Reedbuck occur widespread but concentrations are usually restricted to the marshy flats. Kudu and giraffe are very poorly represented and warthog occur near permanent water.
Fig. 55. Landscape 23. Setaria woodii-variation. Mopane Shrubveld. – page 87
Fig. 56. Landscape 23. Nwashitsumbe vlei. Mopane Shrubveld ����� page 87