African Wildlife and Environment Issue 66

FAUNA, FLORA & WILDLIFE

FAUNA, FLORA & WILDLIFE

- because there is every chance that there is more of the right habitat close by. A mechanism of short distance dispersal has, therefore, survival advantages. Since miombo covered enormous tracts of land, populations

Why are there so many species of BRACHYSTEGIA?

Semi-deciduous miombo woodlands once covered huge tracts of land on the central African plateau. Here the soils are generally granitic and rather poor in nutrients available to plants.

Eugene Moll

and charcoal. The same goes for vast tracts of miombo cleared for cultivation, resulting in people having to now rely on other fuels for cooking. Today, these once huge tracts of miombo cannot support the many pastoralists that eke out an existence there, and are now covered by coppicing shrubs and rank, ‘sour’ grass. In the historical past, when human population numbers were sparse, people practised ‘swidden’ (also known as ‘slash-and-burn’) agriculture, and moved sites every couple of years, allowing the woodlands to once again regenerate. This story for Africa is not unique; globally we have decimated the world’s woodlands and forests and there seems to be no stopping this carnage. In miombo the dominant tree species are mostly legumes, particularly of Brachystegia (= msasa) with many species, and its close allies Julbernardia and Burkea , all in the family Caesalpinioideae. Back in the 1990s Mike Bingham, a Zambian forester, gave a fascinating paper at an AETFAT Conference in Malawi. He suggested that humans may well have provided the opportunity for this speciation in Brachystegia. Letme,with thehelpofMikeBingham’s ideas, suggest how this speciation was perhaps made possible. Firstly, we know that hominids evolved in Africa and that they used fire over the evolutionary past. We also know that the first hearths in caves date back ~ one million years, and that it could well be that before then hominids used fire in the landscape (the great John Phillips suggested that perhaps this was for another million years). We also know from soil and off-shore cores that fires have been a feature in Africa for five million years and more. This means that African vegetation is fire-adapted and that some species rely on fire for their survival. If one looks at the number of African leguminous trees that flower and produce their pods in a neat layer above the canopy, there are a surprising number of species in genera like Afzelia, Brachystegia, Burkea, Guibourtia, Millettia and Peltophorum that do this. This begs the question why? Many of these genera have hard, woody pods that open explosively on hot, windy days, shooting the seeds up to 50 m away from the parent tree. Mike Bingham suggested that because trees only grow to reproductive maturity if they occupy a suitable habitat, that scattering their seedsnearby isanadvantage

Frank White’s seminal work, The Vegetation of Africa , is a UNESCO & AETFAT descriptive memoir published in 1983, with a companion wall-map. White describes woodlands as “Land with an open stand of trees the crowns of which form a canopy from 8 to 20 m or more in height… The crowns of adjacent trees are often in contact but not densely interlocking” and he estimated that woodlands once covered some 40% of Africa. Althoughmiombowoodlandsweremostly comprised of large areas of tall trees, there was not a great deal of homogeneity. In some areas one or two species were locally common and even formed almost monospecific groves. There were also areas where tall trees give way to patches of shorter trees, some of which were the same species that occurred as understory trees to the canopy giants; genera such as Diplorhynchus, Faurea, Monotes, Protea and Uapaca . There were also shrubs scattered through the region, many with huge woody underground organs (= geoxylic suffrutices); sometimes called “the underground forests of Africa”! In addition, there was an almost continuous layer of tussocked grasses that in places reach three m, and from autumn could carry raging hot fires. In spring a number of geophytes flowered: orchids, gladioli, flame lilies, and many more. Of course, all of this was way before modern humans had a massive impact on the vegetation of most of the continent that essentially started with European colonisation and mechanisation in the late 1800s. This fragmentation and clearing of miombo woodlands for agriculture (maize in places, and tobacco in the poorer sites characterised by Uapaca groves) is still happening to remaining islands. Living in Kabwe, Zambia, in the early 1950s, we had in our front garden a very large Brachystegia spiciformis tree that was almost a metre in diameter and at least 12 m tall. Across the road there was open ground with other big miombo trees, and huge termite mounds surrounded by “elephant” grass. Within a few kilometres bicycle ride I use to collect bags of the fruits of Uapaca and Parinari in summer, and made my own chewing gum from the latex of Diplorhynchus . Then we moved to Harare where I used to collect wild flowers for my mother in patches of miombo that ran through the city. Today there are no trees within many kilometres of the city; they have all been cleared for timber, firewood

The woody pods of Brachystegia spiciformis and spring foliage (Photograph: Tim Peatling)

of species using short distance dispersal mechanisms can, and do, become isolated. Over millennia new but closely related species evolved because of genetic isolation, Brachystegia being the prime example. Its explosive method of short distant seed dispersal allows the seeds to settle close to the parent plant, possibly in an abandoned field, once the swidden agriculturalists have moved on. So, the evolution of a dispersal mechanism that enabled the seeds to occupy suitable adjacent habitat had advantages. At the other end of the scale, there are some monotypic genera in Africa that, because they rely on animals moving over great distances for their dispersal, have continual genetic mixing, thus preventing speciation. Prime examples are Adansonia , Kigelia and Sclerocarya (baobabs, sausage trees and marulas) where elephants and/or humans are the chief agents of dispersal. As an interesting aside Afzelia , the pod-mahogany, is a legume species similar to Brachystegia in that the pods are held above the canopy, yet there is but one species in the genus in southern Africa. What could explain this anomaly? Possibly the reason is the large black seeds, with their bright red, waxy aril, which were and still are valued by people for their magical properties (and as adornments in the form of necklaces) were widely disseminated, maintaining genetic flow preventing speciation! Thus, the morale of this tale is that one size does not necessarily fit all, and speculation about the whys and wherefores in African tree ecology and evolution is a fun pursuit!

Miombo woodland tree dwarfing the elephant enjoying the shade.

Eugene Moll emoll@telkomsa.net

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53 | African Wildlife & Environment | 66 (2017)