Acacia welwitschii Thickets on Karoo Sediments Gertenbach Landscape 13
Acacia welwitschii Thickets on Karoo Sediments … Location and Geomorphology
Geologically the KNP can roughly be divided into granite and gneiss in the western half and basalt in the eastern half (Shutte 1974, 1982). Karoo sediments occur where the granite and basalt make contact and this forms a strip that extends from north to south throughout the KNP. The Karoo sediments consist of Cave Sandstone, Red Beds and Ecca-shales. This landscape is limited to the Ecca-shales. The terrain is concave, low lying and reasonably flat with slight slopes. It extends as a narrow strip from Crocodile Bridge northwards to the vicinity of the Timbavati picnic area. The thickets are seldom broader than four kilometres.
The general orientation of the spruits and rivers in the KNP are from west to east. Considering that the shales are prone to weathering and erosion it often happens that the spruits running from west to east through granite, turns north or south when reaching the shales. Examples of such spruits in this landscape are the Vurhami, Salitji, Nwaswitsonto and Nsemani. The soils have a strong structure and poor internal drainage with the result that small pans commonly occur. Examples are Leeupan, Nkayapan and Ngumula pan. The landscape is low lying (between 260 and 320 metres above sea level) and occupies 520 km/2 or 2.7 percent of the KNP. Good examples of this landscape can be seen along the tourist roads at Gomandwane and Leeupan.
According to Gertenbach (1980) this landscape receives a wide range of rainfall (500 to 600 mm). In the vicinity of Crocodile Bridge the annual rainfall is 599 mm and it declines to as low as 548 mm at Satara. The temperature is high during the summer and there is little possibility of frost during winter. Table 3 indicates the daily maximum and minimum temperatures for Satara which could possibly be the same for this landscape.
The Ecca-shales were deposited under warm humid conditions and the soils that develop from these shales are rich in sodium. The presence of large quantities of sodium results in the development of soils with a very strong prismatic structure in the B-horizons. A gleyed horizon sometimes develops on the prismacutanic B-horizon, but usually this horizon as well as the A-horizon have been washed away. The common soil Forms in this landscape are thus Sterkspruit, Swartland and Escourt. Where the material from weathered shales and Cave Sandstone mix. Valsrivier and Oakleaf soils sometimes occur. The soils in this landscape are generally very susceptible to erosion and therefore any disturbance should be kept to a minimum.
The vegetation of this landscape is described by Van Wyk (1973) as Delagoa Thorn Thickets, while Pienaar (1963) describes it as dense thorny bush thickets. The landscape is unique considering that the vegetation is dominated by Acacia welwitschii subsp. Delagoensis, a taxon only found in the Republic of South Africa, apart from certain stands in Swaziland and in the south of Zimbabwe.
According to Gertenbach (in prep.) this landscape is differentiated by the following woody species: Euclea divinorum, Acacia welwitschii subsp. Delagoensis, Teclea pilosa, Capparis tomentosa, Boscia mosambicensis, Grewia bicolor, Maerua parvifolia, Dichrostachys cinerea subsp. Africana, Spirostachys africana and Rhus spinescens (Fig. 30) Other woody species that do occur are Ehretia rigida, Zanthoxylum humilis, Securinega virosa, Acacia tortilis, Bolusanthus speciosus and Acacia gerrardii.
The Acacia welwitshii-thickets can be divided into two variations on the basis of the field layer. The Senecio longiflorus-variation is characterized by the occurrence of Senecio longiflorus, Enteropogon macrostachyus, Oropetium capense, Sporobolus smutsii and Hibiscus palmatus while the Urochloa mosambicensis-variation does not include these species. Dominant grass species in the field layer of both the variations are Sporobolus nitens, Dactyloctenium aegypteum, Chloris virgata, Panicum coloratum, Chloris roxburghiana, Aristida congesta subsp. Barbicollis, Tragus berteronianus, Panicum maximum, Urochloa mosambicensis, and Bothriochloa radicans. Forbs present are Abutilon austro-africana, Ruellia patula, Cyphocarpa angustifolia, Justicia flava, Blepharis intergrifolia, Pupalia lappacea, Tragia dioica,
Solanum coccineum, Achyranthes aspera, Amaranthus thunbergii, Ocimum americanum, Gisekia africana, Cyathula crispa, Commelina bengalensis, Phyllanthus asperulatus, Seddera suffruticosa, Heliotropium steudneri, Solanum panduraeforme, Neuracanthus africanus and a large variety of other species. The combination of grasses and the variety of forbs show that this landscape is heavily grazed. The grass cover is therefore usually less dense (between 30 and 60% crown cover) and sometimes disappears altogether with the advent of the dry season. The structure of the woody component is a moderate tree savanna with tall shrubs and sparse low shrubs. The following structural analysis describes the landscape fairly accurately.
|2 – 4 metres||20|
|0.5 – 2
Where the soils originating from shales and Cave Sandstone mix a complex of plant communities occur that consist of a combination of one of the varieties of the Acacia welwitschii community and a Albizia petersiana subsp. Evansii community. The latter community usually contains dense tall shrubs with a better grass cover than the Acacia welwitschii community (Fig.31). Other woody species that occur with Albizia petersiana are Dichrostachys cinerea subsp. africana, Acacia nigrescens, Euclea divinorum, Acacia tortilis and Lonchocarpus capassa.
This landscape carries what is possibly the largest biomass of game in the KNP. As a result of the palatable short grazing and open low shrub layer, a large number of impala, wildebeest and zebra are present. Acacia welwitschii is a good fodder tree and therefore giraffe, kudu, steenbok and duiker are all present. Elephant breeding herds prefer the dense tree veld and buffalo are continuously on the move through the thickets. White rhino have a strong association with this landscape and waterbuck are often encountered at the pans between the trees. As a result of the high density of prey species, lion and hyaena are plentiful while cheetah are regularly seen.