Ботанический журнал, 2019, T. 104, № 6, стр. 943-966

Geology

Almost the entire area of Sehlabathebe National Park is on Cave Sandstone of Clarens formation. Here the formation reaches the highest altitude known in southern Africa. Where Cave Sandstone protrudes, it has been eroded, forming deep pools, caves, arches and other structures.

The geological formations represent geological processes spanning around a period in excess of 200 million years, i.e. from the Triassic period to the present. The park as a whole can be subdivided into three distinct geological formations.

1. Sedimentary materials as represented by a variety of sandstones, mudstones and shales. These form a major portion of the substrata of the plateau and the material from which are carved the park’s sandstone rock pools, arches and caves.

2. Igneous materials are represented by the Drakensberg basalt flows. This material covers much of the northern half of Lesotho and is exhibited in the park in the form of Thaba Ntšo and the Tree Bushmen Mountains.

3. Intrusive igneous rocks as represented by the numerous dolerite and sills, which dissect the park, and by a number of volcanic breccias. These harder rocks tend to be the controlling factors influencing the topography in that they constitute most of the parent materials from which are formed ridges and hills and the Tsoelikana Waterfall.

Soils

Two major units were surveyed. One of these coincided with the actual surface area of 7244 ha of the Park, as it is now gazetted and fenced. The other unit, consisting of 373 ha, borders the Park and incorporates the southern slopes of Thaba Ntšo. This smaller unit has been proposed for annexation to the Park in an effort to contain the entire upper watershed of the Tseolikana River within Sehlabathebe National Park boundaries. This is believed to be of importance to the protection and management of the park for several reasons. First and foremost was the need to protect the soils, water quality, and vegetation of the entire watershed. At present, the southern slopes of Thaba Ntšo, which are extremely steep, are being overgrazed. Consequently, while the eroded areas within the park have been stabilizing and regenerating, the Thaba Ntšo slopes are continually impacted. This situation, therefore, places the entire watershed in jeopardy. Rainfall on the upper, unprotected slopes wash down the hillside and effects gully and sheet erosion, which in turn fridges the protected portion below. This affects water quality of streams and rivers within the present park proper. Some content that this impacts on water quality by increasing the level of sediment and, therefore, forms a threat to the survival of the endangered Maloti Minnow Pseudobarbus quathlambi, a fish species that inhabits the Tsoelikana River.

Major soil types in Sehlabathebe National Park include: Lekholong-Rock outcrop complex (6744 acres, i.e. 37.7% of the park surface), Lekholong loams (11.6%), Rockland sandstone (10.5%), Lekholong-Tsenola loams (10.4%), Sani loams (7.5%), Popa Matsana loams (6.5%), Tsenola loams (5.2%), Thabana clay loams (3.2%), Popa loams (2.2%) and others (5.2%).

By combining approximately 97% of the park’s land of all soil types into slope categories, it was calculated that approximately 2.2% of the land consisted of 0–3% slope, 8.0% of 3–6% slope, 14.7% of 6–12% slope, 15.3% of 12–20% slope, 34.0% of 20–35% slope and 22.7% of over 35% slope. Consequently, approximately half (56.7%) of the parkland consists of soil types exceeding 20% slope and nearly a quarter (22.7%) exceeds a 35% slope. The high percentage of land with relatively steep slope has implications not only for erosion control, watershed management, and vegetation manipulation, but also places constrains on the types and methods of development and construction projects permissible in the park.

Hydrology

Sehlabathebe National Park has: a) rivers, streams and their tributaries; b) oxbow ponds; c) marshes and bogs; d) rock pools; e) man-made dams. The park consists two watersheds: Leqooa River and Tsoelikane River. The Leqooa flows from the north to the south through the Sehlabathebe Valley. The Tsoelikane has its headwaters in the Park and near Moshebi it merges to form the Tsoelike River, which empties into the Senqu near Qacha’s Nek.

The Tsoelikane flows in SW direction and descends from 2410 m to 2220 m above sea level on its 12 km course through the park. Its overall pattern is dendritic forming numerous valleys and canyons. The Tsoelikane Valley is a mature alluvial flood plain, marked by numerous meanders and oxbow ponds. Many tributaries of both Tsoelikane and Leqooa contain water only during the wet season.

Associated with the sandstone outcroppings in Sehlabathebe National Park are the rock pools, which contain rainwater. While a few of these pools are quite large (up to 5 m deep and 10 m in diameter), the majority are of much smaller size. The larger rock pools are concentrated in several places. The highest concentration is located in the NE edge of the park.

The rock pools are of extremely significance to the Park. They provide a habitat for highly endemic and endangered Aponogeton ranunculiflorus, which is known to exist nowhere else in the world but in the Park and its close vicinity. Out of 70 pools, in which the plant was recorded during the years 1970–1978, 66 were located within Sehlabathebe National Park. All, except one, of these pools are confined to an area of approximately 2.5 km² in the rook pool area near the NE edge of the park. A. ranunculiflorus appears to favour those rock pools, which have clear, still water and coarse sandy/gravely bottom. The plant inhabits not only pools, which contain water all year round, but also pools that are dry in winter.

During the years 1970–1974, eight dams were built near the headwaters of the Tsoelikane. All of them were formed by excavation of stream valleys and construction of earthen dams. Four of the reservoirs, representing a waterway chain of impoundments, were built in NE corner of the park, and one dam, which today constitutes a marshland, was built about 6 km further South. As the dams are continually flushed by new supplies of fresh stream water, the water of these reservoirs is usually clear and relatively cold. Most of the dams were built to provide a habitat, which would support trout fishing. All the dams serve also as wildlife habitat.

Climate

The climate of Sehlabathebe National Park is a mixture of climate effecting Lesotho and Natal Drakensberg as a whole. While the main Lesotho climatological patter flows across the highlands in SE direction, that from KwaZulu-Natal often flows in W direction, bringing moist and subtropical air currents inland from the Indian Ocean. In fact, the seemingly dynamic and unpredictable weather which characterize here can be viewed as the product of the forces of these two main climatological patterns mixing and collecting the Drakensberg interface.

During much of summer, the dynamic and unpredictable weather is often exemplified by the following sequence of weather patterns, all spanning a single day: dense mist from the east; a light, easterly wind and clear sky; increasing cloud cover; a thunderstorm approaching from the east; heavy rain; clearing sky; dense mist from the Drakensberg Escarpment.

Grassland

Themeda triandra is the dominant grassland species. In spite of the Acocks (1988) name: Themeda-Festuca, Festuca caprina is not a dominant species in the park. The park is predominantly Themeda grassland, frequently associated are Trachypogon spicatus and Helictotrichon spp. In valleys and where slopes are slight, Stiburus spp. is often as much in evidence as Themeda. This may be due to the slightly higher soil moisture content.

The steep south-facing slopes are almost exclusively Pentaschistis spp. grassland. Woody sub-shrubs also appear to be invading the grassland in a few places. In the area east of the lodge, Cliffortia nitidula has begun to appear. At least in two other places, heather Erica spp. appears to be invading. Small to large grey-green patches of Helichrysum aureonitens are also scattered through the grassland. It is not clear whether these patches represent areas recovering from heavy past utilization of the veld or whether they represent a stage of degradation.

Wetlands

Schwabe (unpubl. rep.) distinguished three types of wetlands within the SNP: tarns (depressions in the sandstones filled with water), riparian marshes and inland freshwater marshes.

Wetlands occupy the flood plain of the Tsoelikana River, the banks of some of the tributaries, and other areas where standing water is present at the surface or where the soil is saturated with water.

Marshes are characterised by standing water and boggy, wet soils along streams. They are characterised by sedges and/or grasses, such as Scirpus, Carex, Pennisetum, Fingerhuthia, Anthoxanthum, Phalaris, Fuirena, etc.

In the grassland, where streams have cut down into the ground, Danthonia macowanii is a common and readily recognized species. Sand and gravel bars in rivers and streams are colonized by Juncus spp. and Moraea huttonii. On steep, rocky slopes, particularly on Thaba Ntšo, Peucedanum caffrum and Berkheya multijuga commonly occur.

Bogs are scattered all over the park. Haplocarpha nervosa is generally the dominant species in the bogs, but Limosella sp. and Ranunculus meyeri are also characteristic of these habitats. Ponds, i.e. pools of water with muddy bottoms, occur on the Tsoelikana flood plain (oxbows), in the rock pools area, N of the lodge, and in Maal Cof. Pond margins are characterised by a gradation inward from emergent aquatics (e.g. Juncus spp., Eleocharis sp., Scirpus spp.) to floating aquatics (e.g. Potamogeton thunbergii, Crassula natans, Scirpus fluitans) in deeper water.

Rock pools, found in five areas (north of the lodge, Leqooa drainage, Maal Cof, the ridge from Thaba Moea to the falls, and the KwaZulu-Natal border ridge NE of Mofoqoi Valley), are shallow to deep pools scoured out of sandstone outcrops. These pools generally have a sand and gravel bottom and support floating and submerged aquatic species, such as Limosella capensis, Crassula sp., Ilysanthes conferta, Isoetes sp. and the rare and endangered Aponogeton ranunculiflorus, which is restricted to 2 of the 5 rock pool areas.

Rocklands

Sandstone outcrops and cliffs are important for the growth of woody plants. Typical species of outcrops are Relhania acerosa, Felicia filifolia, Euryops tysonii and Passerina montana. Species such as Rhus divaricata and Printzia laxa are common in crevices on cliffs. Shady crevices and shady spots beneath sandstone overhangs are typical habitats for fern species and some flowering plants. Some of the dolorite dykes are quite barren and rocky, and constitute a habitat of species seldom encountered elsewhere in the park. Ridge tops and some slopes are quite rocky and have extremely thin soil layer. Eragrostis capensis, Elionurus argenteus and Sporobolus centrifuges are characteristic species.

Disturbed lands

Areas which have been disturbed by man, have quite distinctive vegetation, with introduced weeds and pioneer species. Kraal areas, used in the past as summer cattle posts by Basotho herders, are characterized by dense stands of Bromus willdenowii. Also common in these areas are Cirsium vulgare and Urtica urens. Abandoned cattle tracks, where soil has been removed down to the bedrock, or unconsolidated rocks are dominated by sub-shrubs (e.g. Helichrysum infaustum) and pioneer grasses Eragrostis spp.

Construction of roads produced altered habitat along the roads. In many areas, particularly in valleys, Eragrostis curvula and Eragrostis caesia form dense stands. Other areas, particularly along the road up and over the neck between Tsenola and Khubetsuana, have Chrysocoma tenuifolia and Senecio inaequidens lining the roadside.

The area in the Leqooa drainage, around the Rest Hut, was cultivated and grazed in the past and is still heavily overgrazed. With the grasses eaten down, field weeds such as Raphanus raphanistrum, Cineraria lyrata, Hibiscus trionum, Spergula arvensis and Monsonia angustifolia are readily noticed. Some of these exotics were probably introduced by itinerant travellers and were mixed with the seed brought in for planting in the field.