African Wildlife & Environment Issue 82

FAUNA, FLORA & WILDLIFE

have several new species described (such as Vachellia kosiensis, V. natalitia, V. dyeri, V. theronii, V. robbertsii and V. inconflagrabilis ). A. karroo is not just a common, highly variable, and widespread shrub and small tree, but it also has a whole host of economic and traditional uses to many of the people of South Africa (and to an entire range of animals). But all of this is for another day. Before I elaborate further on why A. karroo has such devilish spines I need to go back in geological time to when plants first colonised the land.This was in the Cambrian ~500 Ma, and it was not until the early Cretaceous (~145-100 Ma) that angiosperms, (flowering plants) evolved, only becoming dominant in the Palaeocene (~66-56 Ma). The first known Acacia-like fossils seem to be from the late Oligocene (~25 Ma), and we now believe that Vachellia species colonised the early savannas ~8 Ma and the Senegalia species followed ~3 Ma years later (this is one recent

Clearly once woody this gave them a height advantage in seeking maximum sunlight in the primordial forests, a typical chicken-and-egg conundrum. Over the years I have read much about plant evolution but have never found a satisfactory explanation for the evolution of woodiness. From what I have been able to gather it is just reported that ‘in time plants became woody’. My explanation of this is that plants, because they are unable to excrete the excess products of photosynthesis (which, with almost unlimited CO2, sunlight and water ~500 Ma, the plants were unable to ‘switch-off ’ photosynthesis) they had to sequester excess CHO (=carbohydrates that start off as the 6-carbon-chain glucose molecule) somehow - otherwise it would 'clog-up' the living cells where photosynthesis was taking place. Back then, I argue, that the key nutrients for protein synthesis (NPK+ - these are N itrogen, P hosphorus, potassium [ K ] and other more minor elements such a calcium, etc.) were limited; having been leached out of the substrate over the earlier millennia. My explanation for woodiness is that the excess CHO was converted into long-chain carbohydrate molecules, such as cellulose and lignin (and many others), to remove the excess CHO from living cells. Early plants thus became woody thanks to CHO sequestration. This woodiness then gave them a competitive height advantage - so became genetically fixed. To me the logic of this seems compelling. Anyone else with other ideas? Considering A. karroo spines, they are simply the lignification of the stipules. Thus, Vachellia species developed spines because on many occasions there is insufficient NPK+, so the closest place to sequester CHO is at the base of the leaves, which are the solar panels that manufacture CHO. These spines then served a secondary function that was to moderate browser defoliation, giving spiny plants a competitive advantage that then became genetically fixed (another chicken-and-egg situation). In less favourable environments these spines became even more fearsome because of a shortage of NPK+ and not, as many contend, to further protect the plants from herbivory. This consideration is further supported by other observations and deductions that I shall expand on in the next episode. Watch this space…

argument I have read to further justify the separation of the two genera). I have only included the timelines above to give some indication of how many millions of years it has taken for our present trees to evolve, and to remind us that evolution takes many a turn. To understand why trees of today are structured and function as they do, we must understand what drove their evolution. Here, for those sufficiently interested, I suggest you read The Emerald Planet – how plants changed the earth’s history by David Beerling, Oxford University Press, 2007. In my journey to understand why A. karroo has such devilish spines my first question is: “What drove the early herbaceous plants to became woody?” Note the very heavily spine-covered shrub in the foreground while the bigger trees in the background is not as spinescent. Could this be because they have better access to nutrients?

Prof Eugene Moll Department of Biodiversity and Conservation Biology

University of the Western Cape eugenemoll74@gmail.com

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