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ORIGIN OF KAROO FLORA

J P H Acocks 

 

The origin of the Karoo flora is of particular interest. Bews (1925) supposed it to be derived from the Bushveld via the Fish River Scrub. That seems likely enough for the Great Karoo and Little Karoo, where most of the large shrubs are actual Bushveld species and many of the succulents are either Bushveld species or closely related, but not all the Karoo flora can be so derived. The succulent habit is not peculiar to anyone vegetation type, but is rather a reaction to habitat, in particular to a permanent scarcity of moisture. Succulents are represented in all the veld types of South Africa. Those of the Karoo, therefore, can be derived from both the southern and the tropical floras. An interesting point is that our solitary representative of the Cactaceae (Rhipsalis) is a forest species. The other important constituent of the Karoo, the non-succulent shrublet or Karoobush, has become relatively scarce in much of the Little Karoo and Great Karoo, but is still dominant in the Upper Karoo and the False Karoo types; the origin of this plant-form is not to be sought in the tropical flora, where it scarcely occurs, but in the Fynbos. Common genera in the Karoo like Chrysocoma, Hermannia, Euryops, Pteronia, Eriocephalus Selago, Walafrida and Lightfootia are all well represented in the Fynbos; Pentzia and Phymaspermum, too, in some forms of Fynbos; and there is a very good transition from Fynbos and Arid Fynbos through Mountain Rhenosterveld and the Western Mountain Karoo to the ordinary short kind of Karoo, both in species-composition and growth-form. There is no such transition from Karoo to the arid types of Bushveld, the Karoo simply petering out in this direction. This is well seen in the southern part of South West Africa, where, on the other hand, the contribution of Acanthaceae, Euphorbiaceae and Amarantaceae to the Karoo flora by the tropicalflora is clearly seen, as is the contribution of shrubs and trees to the north-western forms of Karroid Broken Veld. To-day there appears to be a transition to the Bushveld in the south-east, because the Karoo has invaded the Noorsveld and Fish River Scrub, with Pentzia incana playing an important part; but there are indications that, in the natural state of affairs, there would be a grass savanna and bush clump veld separating the Noorsveld, etc., from the Karoo. In this event the undergrowth in the Noorsveld would have been grasses of tropical origin, e. g. Themeda, Setaria, Eanicum, Eragrostis, Enneapogon and Aristida, all of which are still to be found in protected places, and they form a dense grassveld, dotted with small trees and a few large Karoo bushes, in protected parts of the surrounding Karroid Broken Veld.

It would seem, therefore, that the Karoo has a strong Fynbos affinity, especially the Upper Karoo and the Western Mountain Karoo; and that when Man disturbs the tropical grassveld and scrub, and induces a Karoo invasion, it is these Fynbos-derived elements of the Karoo which play the leading part. That is to say, the changes in vegetation which are occurring to-day are an artificial reversal of the evolutionary replacement of the southern scrub vegetation by a sward of grass of tropical origin. That is in broad terms, of course; there are minor changes too, the most important of which is the replacement of Tropical Grassveld by Bushveld via Thornveld, with the Karoo ever moving forward. The view is taken that the ecology of South Africa is something dynamic.

(An extract from Memoirs of the Botanical Survey of S A No 40, 1975)

 

Published

Karoo Regional Newsletter Dec. 1977