Types of Vegetation, Distribution and Migration of the South African Flora:

Types of Vegetation

There are two main floral vegetation types in South Africa: the tropical and subtropical elements (northern vegetation); and the temperate element (southern vegetation).

The Karoo flora is a mixture of tropical and temperate vegetation types.

Tropical and subtropical elements:

This vegetation is widely distributed and covers the eastern, central, and northwestern parts of South Africa. There is considerable mixing of this vegetation with the temperate element e.g., Themeda spp. occurring in fynbos areas and boekenhout (Faurea saligna) (belonging to the Proteaceae family) found in thorn veld.

The typical tree form is forest, occurring along the eastern areas, as far down as Knysna, wherever there is protection, usually in frost-free areas and moist localities.

The savanna (bushveld) i.e., a mixing of trees and grass, is found in drier areas.

 

 

The grasslands are very extensive and cover most of the summer rainfall area, except the Karoo and central Cape.

The karroid vegetation is confined to the central and northwestern Cape and the southwestern Free State. Grasses are found but these become less with increased aridity. This vegetation is highly evolved and represents the final result of adaptation to arid conditions. It is thought to consist of a mixture of tropical and temperate floras.

Temperate vegetation (fynbos or macchia):

It is a very uniform vegetation that is completely dominant in the winter rainfall region. The leaves are hard, needle-like but often flat.

If the leaves are open or flat, adaptations to aridity, like woolliness or hairiness occur. The vegetation is shrub-like and woody.

 

 

Karoo flora:

Most of the large shrubs, many grasses and some of the succulents are of tropical origin. However, the succulent habit is not peculiar to any one vegetation type but is rather a reaction to habitat, in particular to a permanent scarcity of moisture. Succulents are represented in all vegetation types of South Africa. Those of the Karoo, therefore, can be derived from both the temperate and tropical floras.

 

The other important constituent, the non-succulent Karoo bush, originates from the temperate flora. It appears, too, that in its undisturbed state, the Karoo would be a grass savanna, particularly in the eastern area. It is the fynbos-derived elements of the Karoo which play a leading part in the Karoo invasion.

The grasses are still surprisingly plentiful, despite the largely soil-less condition of the Karoo. They are always eaten flat, so one must conclude that the most palatable grazing plants in the Karoo are still grasses and that the Karoo bushes are valuable mainly as reserves for winter and droughts when there is no green leaf matter left on the grass tufts. The great bulk of the Karoo bushes are unpalatable.

The grasses would be of tropical origin, of the drier bushveld savanna, the most important being Themeda, Setaria, Panicum, Eragrostis, Enneapogon and Aristida spp., all of which are still to be found in protected places.

 

Distribution and Migrations

Different vegetation has migrated across South Africa

 

Tropical and subtropical elements:

The tropical regions are regarded as the reservoir from which South African vegetation was derived.

The general migration of vegetation southwards has, however, not been over the whole continent due to temperature, rainfall, and topographic barriers.

 

On the western coast, colder temperatures and lower rainfall have tended to retard the southward advance of vegetation from the tropics.

On the east coast, the generally warmer conditions as well as topographic characteristics, have favoured the migration.

Various plants have taken different migration routes, the most important being the following:

• The east coast has been a favourable route due to the moderating influence of the warm sea current and the absence of frost. This belt of migration is still fairly broad in Zululand but narrows progressively towards the south. The number of different species that have migrated southwards also decreases progressively.
• River Valley migration – the penetration inland from the coast belt migration, occurs along the river valleys. These valleys are usually warmer and present a range of microclimates e.g., warm, dry, northwestern aspects, frost-free areas and areas sheltered from wind. Due to the ruggedness of many valleys, there is protection from fire and animals. Towards the mountains, the river valleys are not so deep, allowing for the migration southwards from one river valley to the next.
• Mountain range migration especially the Drakensberg and the Highveld. This migration consisted mainly of grasses, most of them belonging to the Panicum spp. and the Andropogon spp.

The sea coasts are under the moderating influence of the sea and are more or less frost-free, even though they may be arid: the mountains usually receive better rainfall than the plains, while the valleys are usually warmer than either the plains or the mountains. The mountains and valleys provide a wide variety of climatic conditions: warm, dry, northern, and western aspects, cold southern and eastern aspects; frost-free areas resulting from peculiarities of air drainage; areas sheltered from the severity of winds; areas receiving winter moisture in the form of snow, and so on. They also protect against widespread fires and are less accessible to grazing animals. Further, during major climatic fluctuations, these routes remain open far longer than do flat country, which provides no harbours for refuge for the many plants that have little power of adapting themselves to changing conditions.

Temperate vegetation:

South African temperate grasslands

 

It is thought that the temperature flora originated in the southwestern Cape, and it is only the remnant of a larger temperate flora. The theory of continental drift assumes that the Antarctic, Australia, India, and South America were centred around Africa. Amongst others, the fact that this temperate flora shows similarities with the vegetation in other temperate areas of the Southern Hemisphere, e.g., Proteaceae occurring only in the Cape and Australia, seems to support this assumption.

There is, however, the other view that the vegetation originated in the Northern Hemisphere. It is suggested that the vegetation migrated southwards along the mountain ranges of Africa, down to the Western Province, where new genera and species evolved. To support this theory, it is pointed out that the temperate flora has similarities with that of temperate regions of the Northern Hemisphere, e.g., Erica spp.

This southern vegetation is today moving northwards. Along the south coast, it is making use of both the mountains and the plains. In the inland parts, the fynbos is also ousting the tropical flora from the mountain tops. Today, members of this flora have penetrated the KwaZulu-Natal Mistbelt e.g., Athanasia acerosa (Curry’s post weed), Northern KwaZulu-Natal e.g., Passerina filiformis, and the Highveld e.g. Stoebe vulgaris (slangbos or bankrotbos).

Changes of Tropical and Temperate Flora

The balance between these floras has varied considerably. There is evidence that tropical flora has at least once progressed and invaded temperate areas. It is fairly certain too, that before grass was evolved, the fynbos covered the larger part of South Africa. Since the evolution of grasses, however, the fynbos has been pushed back.

Today, however, the fynbos and Karoo vegetation are moving eastwards and northwards at a phenomenal speed into the territory of the tropical vegetation.

Apart from ploughing and other mechanical disturbances, there is little doubt that sheep and cattle are still, and have been in the past, the major cause of deterioration of the tropical grass vegetation.

More specifically, continuous selective overgrazing can be singled out as the primary cause of retrogression. This is because the good grazing grasses become eaten out and replaced by less useful species in the wetter parts, but possibly not replaced at all in the drier parts so that the soil becomes exposed. Even in the wetter parts, the cover of the soil is reduced. This leads to increased loss of water by runoff and sheet erosion while wind erosion may occur too. This reduces the depth of the soil and makes the recovery of the vegetation slow and difficult. Finally, due to increased runoff, rivers are called upon to carry more water after rains. The first effect of this is to silt the rivers up, filling pools and smothering vlei vegetation. The next effect is that the water channels become scoured out and deepened so that the water draining into them falls over a bank and dongas start eating back.

The major and most widespread changes in vegetation that have taken place and are still taking place today, are the following:

• The forests and scrub forests have largely disappeared. This has been due to disturbance by fire and due to indiscriminate exploitation. Much of the area has been cleared and ploughed for sugar cane and other tropical crops.

 

Scrub forests or the Cape shrubland is a threatened ecosystem

 

• The most striking and alarming change is the spread of Karoo at the expense of sweet grassveld. The pioneers of this vegetation have penetrated the country to the east of the Drakensberg.
• The fynbos shows the biggest movement of all, having spread from the Western Province to Grahamstown; to the Mistbelt of KwaZulu-Natal; to Northern KwaZulu-Natal; and the Highveld.
• There is also an insidious movement of Vachellia karroo. This tree species is now spreading inland up the river valleys into higher altitudes in the Karoo and beyond into grassveld. It is thickening between Bloemfontein and Brandfort; it is found in the heart of the Free State plains; thickets are developing in the grassveld of some areas of the North West Province; it is moving from the Valley Bushveld into the grassveld of the Eastern Cape, the KwaZulu-Natal.

The extent of veld deterioration in the Karoo and adjoining regions