THE CAPE FLORA AND THE KAROO
— A winter rainfall biome versus a fynbos biome!
by M. B. Bayer, Curator, Karoo Botanic Garden, Worcester
Veld & Flora March 1984
The humorist Groucho Marx when once propositioned said “for that you can include me out!” That is just how someone living in the Cape not within eyeshot distance of an erica, a protea or a restio, must feel — included out. Naturally one wants to be in on the show and Nordenstam’s1 statement that there is a “tendency to particularise the Cape flora” is a comforting one for those out in the cold.
Several recent papers on the Cape flora (Goldblatt2, Taylor3, 4 and Oliver, Linder and Rourke5) strengthen the exclusiveness of the club, and currently Cape flora and fynbos (sensu Taylor6) seem to be synonymous. Karroid species (non fynbos sensu stricto) are only called in when necessary to boost the numbers of species in, and stress the richness of, the Cape flora.
Geological pattern Peter Goldblatt points out that the correlation between the Cape floristic region and mediterranean climate is a loose one. But it is his concession that the Cape region follows a geological rather than a climatic pattern which is more interesting. This is not only because of the obvious relationship between Table Mountain sandstone and fynbos, but because geological considerations seem to have been lost in the recognition of the Karoo-
Namib region. This phytogeographic region extends from Angola in the north, to the south-eastern Cape and is the effective inland boundary of the Cape region. White’s Karoo-Namib2 covers the Kalahari sands, basement granites, bushveld granites and, predominantly, shales the Karoo system. and tillites of The paper by Oliver et al is a cruel shot in the eye, because in setting out to reappraise the boundaries of the Cape flora, only species typical of fynbos are used. A predetermined result.
Fynbos formation in new perspective There is no denying that fynbos is a “separate plant formation distinct floristically and physiognomically from the formations of the Republic”3 but it is time that its geological associations are brought into proper perspective. Oliver et al write of “tongues of vegetation” … rendering exact and simple definition of their (fynbos, Karoo and evergreen forest) boundaries very difficult. My contention is that such a definition is difficult because geology, and hence soil, overrides all the other ecological factors which operate to determine vegetation.
Winter rainfall should have been the natural choice in selecting an agent responsible for the uniqueness of fynbos, and one must ask why this is not done. Goldblatt skirts the issue and simply bases his Cape flora “solidly” on Acocks7, and includes non-fynbos vegetation types for practical reasons — in order to draw a rigid line about the Cape flora region. The area under winter rainfall is by-passed because it and the fynbos area do not coincide. Instead, there is a fairly abrupt end to the fynbos which coincides with the limits of the Table Mountain and Witteberg sandstones.
Currently the winter rainfall area is fairly well-defined by Poynton8 as extending in an arc from the mouth of the Orange River to Cape Agulhas. Poynton does not define “winter rainfall” but it is assumed to be where 60% or more of annual rainfall occurs in the winter. The zone of uniform annual rainfall parallels that of winter rainfall in the north, but widens southward and eventually ends between Port Elizabeth and East London. If that is the present situation, then what of the past?
Menace of an expanding Karoo Nature lovers and conservationists are concerned by shrinkage and loss of fynbos — and agriculturists by the north-westerly migration of the Karoo. Acocks10 paints a grim picture of the Karoo and semi-desert invading the rich grasslands of the Orange Free State and Eastern Cape. Although shrinkage of fynbos and expansion of the Karoo are not directly linked (the first is attributed to “mechanical” destruction, and the second to pastoral land use), there may be a connection. The future will become history and so then, what of the past?
The results of palynological research9 supported by other disciplines, shed a whole new light on the real Cape flora. It appears that as recently as 12-15 000 years ago, the world was experiencing a minor ice-age. During this time the system of mediterranean climate was situated northwards by as much as 6° latitude. This means that the northern limits of winter rainfall would have been in the north of South West Africa instead of at the mouth of the Orange River as it is now. Cyclonic winter rainfall would havebeen reaching across the southern Kalahari, across the Orange Free State and deep into Transvaal.
Shifting rainfall regimes Acocks gives a map which depicts the extent of Karoo elements, and these fit beautifully into the conceptof an extended winter rainfall region. Now it begins to make sense. The Cape flora is not just fynbos— it is a plant formation which coincides with the expansion and contraction of winter rainfall and all associated phenomena. As the winter rainfall zone has movedsouthwards, so has the Karoo shrunk accordingly. The flora of the Kalahari highveld, Afro-montane and Tongaland-Pondoland regions have advanced along the face of the retreating Karoo and advancing summer rainfall regime.
Acocks’ hypothesis of an expanding desert is true in the sense that grazing pressure particularly on summer rainfall grasses, has reversed the process. Non-palatable Karoo species have regained lost ground. Probably only because summer rainfall species competition has been reduced. Remotely perhaps because of a climatic swing back to a winter rainfall regime (heralded by the current devastating summer drought) or artificial aridification as a result of reduced ground cover.
With the possibility now established that Karoo is also Cape flora, a whole host of plant species can waltz into the limelight and take a bow. A winter rainfall biome can now be recognized within which other factors such as geology, altitude, latitude and longitude can operate. However, it is still important to explain adequately the vast difference in species endemism which exists between the north-west and the south-east areas of this winter rainfall biome.
Thoday’s 1925 map of drought severity in NW Cape This is remarkably easy because as long ago as 1925 D. Thoday10 in a study of Passerina presented a telling map illustrating the severity and incidence of drought. This shows that the north-western Cape (Namaqualand and Bushmanland) has to contend with extremely severe summer drought. Thoday’s map can be exterpolated to a glacial age with little imagination. Coupled with seasonality of rainfall, and history of climatic shift, we now have a scenario in which the Karoo can be properly related to fynbos. It can be suggested that high species endemism in the north west is due to geological and climatic extremes. Low endemism (?) in the Eastern Cape is because of relative equability of climate (rainfall) during glacial and post-glacial shifts.11
The fynbos derives its special place because of geology and sustained high winter rainfall, which impose further isolating mechanisms over location at the tip of a continent. The whole purpose of this little exercise is not only to gain access for a Karoo lover into a fynbos club. It is to find a foothold for a feeling that the Cape flora is a very ancient one — the concept of only fynbos representing a Cape flora reduces our capacity to observe and understand what we have and how we are to keep it. Fynbos is not an element which had advanced and retreated like an army. There has been a retreat with a rising sea level and constricting coastline as the glacial age passed, and there has been some change along the interface between summer rainfall and winter rainfall. But the fynbos has been reasonably secure on its geological base, and so has the core of species comprising the Karoo.
Stronger urge to conserve The endemnicity and distribution of many species such as in the genera Aloe, Commiphora, Pachypodium, Tylecodon, Adromischus, Crassula, Conopytum (and most of the mesem genera), Euphorbia, Asclepiadaceae, Geraniaceae and Oxalidaceae all support the wider concept of the Cape flora as a winter rainfall biome. This endemism suggests a permanence beyond minor climatic change (eg. Aloe can perhaps be argued back to the time of continental drift 140 000 000 years ago). Such permanence suggests that we need a still broader and stronger urge to conserve — or we will plough, bulldoze and trample away the florally greater equivalent of the great library at Alexandria.
REFERENCES
1. B. Nordenstam: Phytogeography of the genus Euryops (Compositae). A contribution to the phytogeography of Southern Africa. Opera Botanica 23; 1-79 (1969).
2. P. Goldblatt: An analysis of the flora of Southern Africa: its characteristics, relationships and origins. Ann. Missouri Bot. Garden 65; 369 436 (1978).
3. H. C. Taylor: Phytogeography of fynbos 13; 231-235 (1980).
4. – Is fynbos the natural climax vegetation of the south-western Cape Province of South Africa, in proceedings of the International Symposium on Vegetation Science, ed. A Schwabe-Braun, Cramer, Vaduz, p 385-391 (1981).
5. E. G. H. Oliver, W. P. Linder and J. P. Rourke: Geographical distribution of present-day Cape taxa and their phytogeographical significance. Bothalia (in press).
6. H. C. Taylor: Capensis in biogeography and ecology of Southern Africa, ed W. T. A. Werger, The Hague, p 171-229 (1978).
7. J. P. H. Acocks: Veld types of South Africa. Mem. Bot. Surv. S. Africa 28: 1-192 Govt. Printer Pretoria (1953).
8. R. J. Poynton: A silvicultural map of Southern Africa. South African Journal of Science 67: 58-60 (1971).
9. E. M. van Zinderen Bakker: The evolution of late-quartemary paleoclimates of Southern Africa. Palaeoecology of Africa 9, Balkema,Cap e Town, p 160-202 (1977).
10. D. Thoday: The geographical distribution and ecology of Passerina. Ann. Botany 39: 175-208 (1925).
11. R. M. Cowling: Patterns of plant endemism in the south eastern Cape. The Naturalist 27: 17-36 (1982).