Thickets of the Sabie and Crocodile Rivers Gertenbach Landscapes 4
Location and Geomorphology … see map below
As the name indicates this landscape consists of the low lying areas along the two rivers and is underlain by archian granite and gneiss intersected by dolerite intrusions. The landscape is horseshoe-shaped, starting at the Sabie River with the Mtshawuspruit as the western boundary, along the Sabie eastwards to Lubyelubye. Then southwards across the watershed to the Crocodile River and then westwards following the banks to the vicinity of the Malelane restcamp. The topography is concave to relatively flat but is intersected by numerous spruits that flow into the two rivers. Spruits worth mentioning are the Nwaswitshaka, Nwatimwambo, Nwatimhiri and Lubyelubye that flow into the Sabie River and the lower Mbyamide, Bume and Mlambane that flow into the Crocodile River. A few granite koppies occur in the landscape of which Shirimanthanga. Renoster Koppies, Thekwane, Mlaleni, Siyalo and Sihehleni are the most important.
The altitude varies between 200 and 350 metres and the landscape occupies 1242 km/2 or 6.2 percent of the KNP, which makes it one of the largest landscapes in the southern district.
The climate of this low-lying landscape shows greater extremes than the adjacent landscapes. As far as temperature is concerned a great variation between day and night-time temperatures is experienced. The average daily maximum temperature is above 31 o/c for the months of November to March (Table 2) while sporadic frost occurs in the winter in the bottomlands. The rainfall varies between 500 and 550 mm per year with an annual average of 546 mm for Skukuza (Gertenbach 1980).
Table 2 … Temperature data for Skukuza … (Data collected since 1965)
Temperature in degrees C
|Absolute Maximum||Average Daily
The soils in this landscape are normally shallow and where it is deeper it is usually saturated with sodium. It developed mainly as a result of the accumulation of clay and mineral elements in the low lying areas. Harmse & Van Wyk (1972) identified two groups of soils in this landscape, namely Mispah and Glenrosa soils on the uplands and Sterkspruit, Escourt and Valsrivier soils in the bottomlands. Dundee, Oakleaf and Inhoek Forms of soil are usually found on the banks of spruits and rivers. The soils present in the vicinity of dolerite intrusions are usually darker in colour and Forms that can be expected are Mayo, Milkwood and Swartland. As a rule it can be said that the soils of this landscape are usually shallow and show no signs of a recurrent pattern.
This landscape is characterized by a dense woody vegetation which can basically be referred to as an Acacia nigrescens/ Combretum apiculatum association and it corresponds to a large extent with the bottomland vegetation in Landscape 3 and 5. Van Wyk (1973) refers to this landscape as “…thorny thickets on brackish granite flats…” while Coetzee (1983) calls it “…spiny arid bushveld”. Pienaar (1963) refers to it as “…dense thornbush thickets”.
The differentiating species of the landscape are Acacia nigrescens, Combretum apiculatum, Grewia bicolor, G. flavescens, Dichrostachys cinerea subsp. Africana, Euclea divinorum, Terminalia prunioides, Spirostachys Africana and Acacia grandicornuta. Two variations of the vegetation can be identified viz. Combretum apiculatum-dominated uplands (Fig.10) and Acacia grandicornuta-dominated bottomlands (Fig.11). Both variations of vegetation are dense and according to Joubert (1976) the relative crown cover of woody species are as follows:
|> 4 metres||7.4|
|3 – 4 metres||10.7|
|2 – 3 metres||11.0|
|1 – 2 metres||70.9|
The soils of the uplands are shallow and stony and dense stands of the following woody plant species are present: Combretum apiculatum, Acacia nigrescens, A. exuvialis, Terminalia prunioides, Dichostachys cinerea subsp. Africana, Grewia bicolor, G. flavescens, Combretum hereroense, Lannea stuhlmannii, Ziziphus mucronata, Sclerocarya caffra, Lonchocarpus capassa and Acacia tortilis. The field layer is sparse and is dominated by Aristida congesta subsp. Barbicollis, Pogonarthria squarrosa, Rhynchelytrum repens, Panicum maximum, Urochloa mosambicensis, Schmidtia pappophoroides, Digitaria eriantha var. pentzii and Eragrotis rigidior. Forbs normally present are Waltheria indica, Tephrosia polystachya, Clerodendrum ternatum, Evolvulus alsinoides, Heliotropium steudneri, Aptosimum lincare, Kohautia virgata and Agathisanthemum bojeri.
Fig.10. Landscape 4. Combretum apiculatum-variation –page 23
Fig.11. Landscape 4. Acacia grandicornuta-variation -page24
On the brackish soils in the bottomlands Combretum apiculatum is less common and the lower shrub layer less dense. Sometimes large bare patches occur with only single Acacia grandicornuta trees. Dominant woody species are: Acacia grandicornuta, Acacia nigrescens, A. exuvialis, Terminalia prunioides, Spirostachys africana, Dichrostachys cinerea subsp. Africana, Grewia bicolor, G.flavescens, Xanthocercis zambesiaca, Euclea divinorum, Acacia tortilis, A. nilotica, Ormocarpum trichocarpum, Schotia brachypetala and Ehretia rigida. Species which are relatively rare in other landscapes are constantly occurring in this landscape. Such species are: Ptaeroxylon obliquum, Balanites maughamii, Croton gratissimus, Zanthoxylum humilis, Gardenia spatulifolia, Adenium obesum, Pavetta cataphylla and Rhigozum zambesiacum.
The field layer of this variation is once again sparse and for the most part in an over-utilised condition. Even under very favourable conditions no good, stable grass cover develops. As a result of overgrazing, fires occur less frequently in this landscape and thus the reason why it is usually densely overgrown with woody species. Grasses found in the brackish bottomlands include the following: Sporobolus nitens, Urochloa mosambicensis, Chloris virgata, Aristida congesta subsp. Barbicollis, Bothriochloa radicans, Schmidtia pappophoroides, Digitaria eriantha var. pentzii and Eragrostis trichophora. Under conditions of lower utilization grasses such as Themeda triandra, Panicum maximum, Heteropogon contours, Sporobolus smutsii and Cymbopogon plurinodis sometimes increase. Forbs found on brackish spots include the following: Dyschoriste rogersii, Abutilon austro-africanum, Crossandra mucronata, Justicia flava, Cyphocarpa angustifolia, Blepharis integrifolia, Pupalea lappacea, Euphorbia neopolycnemoides, Sansevieria hyacinthoides and Achyranthus aspera.
Some of the woody species that occur on the koppies in the landscape, strangely enough often concurs with vegetation expected on brackish soils. Such plants are: Croton gratissimus, Schotia brachypetala, Sclerocarya caffra, Acacia nigrescens, Spirostachys africana and Lannea stuhlmannii. Other woody species that occur on the koppies are: Combretum hereroense, Ozoroa paniculosa, Ficus soldanella, Pterocarpus rotundifolius, Iboza riparia, Diospyros mespiliformis, Lannea discolor, Tricalysia allenii, Maytenus tenuispina and Grewia hexamita. Dominant grasses are Panicum maximum and Digitaria eriantha var. pentzii. The soil is more clayey where dolerite intrusions occur and Acacia nigrescens is the dominant woody species. The grass cover is usually denser with grasses such as Themeda triandra and Cymbopogon plurinodis as the dominants.
The banks of the two large rivers in the landscape (Fig.12) are densely overgrown with woody species and the following are the most common: Ficus sycomorus, Breonadia microcephala, Nuxia oppositifolia, Combretum erythrophyllum, Diospyros mespiliformis, Acacia robusta, Trichilia emetica, Kigelia africana, Berchemia discolor and Ekebergia capensis, while rare species such as Anthocleista grandiflora are also encountered here. The field layer is usually absent, but when present it is dominated by Panicum maximum.
Fig. 12. Landscape 4. Sabie River, Riverine Vegetation – page 25
This landscape accommodates what is probably the largest impala population in the whole of the KNP. Other common game species present are kudu, duiker, steenbok, bushbuck and giraffe (Giraffa camelopardalis). During 1974 a number of red duiker (Cephalophus natalensis) were released in the dense riparian vegetation, and in 1981 a number of nyalas (Tragelaphus angasii) from Natal were also released here. Elephants are frequently found in this landscape especially during the dry winter months and a herd of 80 are regularly found in the Nwatimhiri bush. Lion (Panthera leo), leopard (Panthera pardus), wild dog and spotted hyaena are the most important predators, especially the former two species are relatively abundant in this landscape. Buffalo bulls are sometimes present in the reeds of the river beds, but breeding herds only visit this landscape on route to water. Hippo(Hippopotamus amphibious) are plentiful in the rivers and contribute largely towards keeping the grass short.