Elephant Bulls, Zebra, Wildebeest Favour Thornveld on Gabbro Landscapes | Gettenbach 19
Thornveld on Gabbro
Gabbro is a coarse grained igneous or volcanic rock with composition similar to basalt. Very good examples (rocky koppies) can be seen in the Kruger National Park between Phalaborwa and Letaba and close to Pretoriuskop. This area is predominantly Granite so the intrusion makes for very interesting study. Learn more about the rocks of South Africa here.
Location and Geomorphology
The Kruger Park’s “Gabbro intrusion” (Brandt 1948: Gertenbach 1978: Schutte 1982) extends from Malelane in the south of the KNP to Phondaheuwels west of Shingwedzi.
The southernmost section of this intrusion in the vicinity of Orpen, is characterized by a thorn savanna with a dense grass cover. This landscape is a series of islands of gabbro origin, sometimes linked by narrow dykes. It extends from Malelane including koppies such as Ship Mountain and Sithlave, extending beyond the borders of the KNP at Mkhuhlu Station. A number of sub-units of this landscape occur between the Sabie and Nwawitsonto Rivers and at Orpen it forms extensive outcrops. Coetzee (1983) describes the geology of this landscape as dolerite, but evidence indicates that it is gabbro (Brandt 1948: Schutte 1974: Gertenbach 1978).
The landscape generally has a higher altitude than the surrounding granite (between 550 and 600 metres a.s.l.) and it is flat to slightly undulating with prominent koppies such as Ship Mountain and Sithlave. North of Orpen the thornveld on the gabbro is replaced by a shrub Colophospermum mopane community (Landscape 24). This landscape covers 685 km/2 or 3.5 percent of the KNP.
An interesting association exists between this landscape and the most southern distribution of the roan antelope. Gertenbach (1978) has already referred to this association and it would appear as if this applies to the whole landscape. The last group of roan antelope at Pretoriuskop occurs on the gabbro intrusion and until quite recently there were still some of these animals present at Orpen, associated with the gabbro. Other species of game occurring in this landscape are kudu, impala, giraffe, waterbuck, warthog, buffalo and elephant bulls. Zebra and wildebeest usually occur in the largest parts of this landscape shortly after a fire, but the Chloris virgata/Acacia nigrescens-shrubveld near Orpen is an area highly preferred by wildebeest.
The rainfall of this landscape varies considerably from south to north. At Malelane in the south an average rainfall of 620 mm prevails while that of Kingfisherspruit is 582 mm. Temperature data from Pretoriuskop (Table 1), Skukuza (Table 2) and Satara (Table 3) are applicable to this landscape.
The soils that develop from gabbro are usually dark in colour and clayey. Where the terrain is flat to slightly concave, the soil becomes deeper and the following soil Forms can be expected. Bonheim, Mayo, Shortlands, Arcadia and Swartland. On the middleslopes, Mayo, Milkwood, Glenrosa and even Hutton soil Forms can be expected. In the southern parts of this landscape the soils are darker in colour and the grass cover more dense. Loose rock is often present on the surface and there is little soil development on the koppies and it can be classified as lithosols. Where the gabbro and surrounding granite are in contact a mixed soil sometimes develops. It frequently happens that the A-horizon originates from gabbro that has spilled over the B-horison (which originated from granite).
In certain areas of the landscape the vegetation is dominated by stands of Acacia nigrescens trees which vary in height from three to seven metres. Where the knobthorn trees become dense, the trees are usually lower. Otherwise the landscape is characterized by an open savanna with a dense grass cover. Gertenbach (1978) divided the vegetation on gabbro, in the vicinity of Orpen, into two communities viz. Chloris virgata/Acacia nigrescens-shrubveld (Fig. 43) and a Sclerocarya caffra/Acacia nigrescens-savanna (Fig. 44) The first mentioned community occurs on shallow soil and normally has a sparser grass cover and is grazed more intensively.
Fig. 43. Landscape 19. Dwarf Acacia nigrescens Thornveld on Gabbro. – page 72
Fig. 44. Landscape 19. Sclerocarya caffra/Acacia nigrescens Savanna pn Gabbro. – page 72
It is a low tree veld to a shrub veld with Acacia nigrescens, Ziziphus mucronata, Acacia tortilis, Ormocarpum trichocarpum, Bolusanthus speciosus, Securinega virosa and Grewia bicolor, as the most important woody species with Chloris virgata, Cenchrus ciliaris, Sporobolus nitens, Enneapogon cenchroides, Schmidtia pappophoroides, Digitaria eriantha var. pentzii, Bothriochloa radicans, Eragrostis superba, Panicum maximum and Urochloa mosambicensis as the dominant grasses. Forbs present are Cyphocarpa angustifolia, Sidu rhombifolia, Solanum panduraeforme, Corchorus asplenifolius, Seddera suffruticosa, Heliotropium steudneri and Abutilon austro-africanum (Gertenbach 1978).
The Sclerocarya caffra/Acacia nigrescens-savanna occurs on deeper soils and has a dense grass cover which is not intensively grazed. It is an open tree savanna with Acacia nigrescens, Sclerocarya caffra, Acacia tortilis, Combretum apiculatum, Albizia harveyi, Dalbergia melanoxylon, Bolusanthus speciosus, Lannea stuhlmannii and Grewia bicolor as the dominant woody species. Themeda triandra, Digitaria eriantha var. pentzii, Bothriochloa radicans, Cymbopogon plurinodis, Panicum maximum and Urochloa mosambicensis, are the dominant grasses with a few forbs of which Heliotropium steudneri, Pavonia patens, Ipomoea crassipes, Lantana rugosa and Tephrosia polystachya are the most important.
Coetzee (1983) distinguishes another vegetation variation on gabbro between Skukuza and Tshokwane and he referred to it as a Lannea stuhlmannii/Pterocarpus rotundifolius/Themeda triandra dominant shrubby tree veld (Fig. 45).
Fig. 45. Landscape 19. Pterocarpus rotundifolius/Themeda triandra Savanna on Gabbro. – page 73
Fig. 46. Landscape 19. Ship Mountain, a gabbro outcrop. – page 74
Fig. 47. Landscape 19. Open thornveld on gabbro near Pretoriuskop. – page 74
The same woody species that occur in the Sclerocarya caffra/Acacia nigrescens-savanna described by Gertenbach (1978) also occur here, but woody species such as Lannea stuhlmannii, Pterocarpus rotundifolius and Combretum hereroense are prominent dominants. Themeda triandra, Digitaria eriantha var. pentzii, Panicum maximum, P. coloratum and Urochloa mosambicensis are the dominant grasses. Where the topography is very flat or concave Setaria woodii can be expected.
Where soils originating from gabbro and granite mix, a situation is encountered where the woody species correspond to the adjacent granite landscape, while the field layer corresponds very much to the gabbro landscape (Gertenbach 1978; Coetzee 1983).
On gabbro koppies such a s Ship Mountain and Sithlavu unique vegetation occurs (Fig. 46). Woody species worth mentioning are: Aloe marlothii, Ficus soldanella, Balanites maughamii, Spirotachys africana, Ozoroa paniculosa, Maytenus heterophylla, Cassine transvaalensis, Hippocratea longipetiolata, Grewia subspathulata, Sterculia rogersii, S. murex, Vangueria infausta, Erythrina lysistemon, E.latissima, Urera tenax and Cussonia natalensis.
Coetzee (1983) distinguished nine different vegetation variations on gabbro in the Central District of the KNP. This includes the two variations of Gertenbach (1978). In the vicinity south of the Sabie River where there is a slight increase in the rainfall a further variation of the landscape occurs. This is an open shrub savanna with a dense field layer (Fig. 47). Between Skukuza and Pretoriuskop the tarred road crosses this variation and the common woody species are Dichrostachys cinerea subsp. nyassana, Dalbergia melanoxylon, Albizia harveyi, Lannea discolor, Acacia nigrescens, Pterocarpus rotundifolius, Combretum collinum subsp. suluense, Acacia gerrardii and Bolusanthus speciosus. Dominant grasses are Themeda triandra, Cymbopogon plurinodis, Urochloa mosambicensis and Heteropogon contortus.