African Wildlife & Environment Issue 83

FAUNA, FLORA & WILDLIFE

As it adapted to a range of different, seasonal, environmental and geographic types it slowly began to change – such that modern taxonomists now recognise several ‘new’ species. Not only was its radiation successful, but it also adapted new growth forms and strategies to become prominant in some Bushveld environments with a wider range of browsing animals. It also had to contend with bush-fires - so the ability to resprout from a rootstock, and even have epicormic buds hidden below the bark for post-fire resprouting, was honed (in the forest, disturbance was possibly limited to wind-fall, so resprouting may already have been a part of its armoury). It had already evolved protection from browsing by lepidopteran larvae by developing functional extra-floral-nectaries (ENFs) on the leaf-stalks. But in the Bushveld, there were bigger leaf-munchers as well as seasonal constraints to growth (so being deciduous was another advantage). The harsher Bushveld environments that were also more nutrient poor (because there was little or no decomposition, hence nutrient cycling) forced the evolution of secondary leaves from older twigs in drier years. Finally, to encourage mammalian browsers to not prune off shoots of ‘unlimited’ growth the species evolved secondary, sacrificial leaves that were more nutritious and had greater biomass on the older wood (and these leaves only had vestigial ENFs since they did not need protection from being eaten). The evolution of spines to sequester CHO assisted with regulating the rate of mammalian herbivory, so became genetically advantageous. Thus, over evolutionary time Vachellia karroo became a very successful tree. Below the lepidopteran larva that the ants have not yet discovered.

The spines being the way the tree sequesters CHO photosynthate that cannot be used for growth since that part of the tree, or the growth of a whole tree, is being limited because those nutrients to make protein (=NPK) are limiting optimal growth! Thus, excess CHO-photosynthate is being sequestered into long chain CHO molecules (i.e. cellulose and/or lignin for example). That then was the genesis of my ‘devilish thorns’ essay (see AW&E # 82). Having satisfied myself that spinescence was not primarily a herbivore defence mechanism , but was initiated as a way to sequester excess photosynthate, that then gave the plant an added defence mechanism against browsers, I then proceeded to consider the fate and role of PRIMARY and SECONDARY leaves. To do this I had to make many more field observations and laboratory measurements, and the results amazed me because I began to get an inkling of just how evolution may have played a role in acacia survival in the Bushveld. To briefly recapitulate for A. karroo , annual new stem growth (i.e. twigs) only produce PRIMARY leaves, while the SECONDARY leaves grow from axillary buds on twigs that may be up to four years old – those axillary buds are essentially shoots of limited growth. If one collects the leaves from the same number of nodes on new growth (=shoots of ‘unlimited’ growth) and from old growth (=shoots of ‘limited’ growth) it turns out that more than double the biomass is produced on the old-growth. If one then does a nutritional analysis on these two different kinds of leaves the results are astonishing. Primary leaves have more tannin, more fibre and less protein than secondary leaves! Clearly, as a mammalian browser, if you are to maximise your nutrition it is better to browse secondary leaves because they provide more biomass AND nutrition than primary leaves (thus leaving shoots of unlimited-growth relatively undamaged, and thus facilitating their extension to reach a point where they are beyond the reach of the mammals). How clever is that! In conclusion, let me summarize about Vachellia karoo , that left the primordial forest ~8 Ma to establish itself as a most successful Bushveld species – particularly in the southern and western regions of South Africa.

29 | African Wildlife & Environment | Issue 83 (2023)