African Wildlife & Environment Issue 82

FAUNA, FLORA & WILDLIFE

Recently, while holidaying in the Little Karoo, I was again impressed by large, paired spines of the Sweet Thorn. I was reminded of small trees I had seen in the Beaufort West area, where some trees look snow-covered because there were so many spines covering all the branches, and yet adjacent trees had very few spines. I asked the question WHY? Such a contrast between trees in the same area got me thinking, and I am still thinking... Why does Acacia karroo have such devilish spines? The condensed version of a complex topic. Episode 1

Eugene Moll

Photograph from the top of the Bokkeveldberge looking over the Vanrhynsdorp vlakte with the drainage-lines etched out with 'invasive' A. karroo as far as the eye can see

Range & Forage Science. 21(3) , 211-212). In this case the author suggests that this whiteness increases the tree’s visibility to browsing mammals, so these animals can be wary of where and how to feed on these plants. There are also many other papers on how and what spines do to aid many aspects of acacia biology and survival. Finally, there are some intriguing papers on ants that invade the much-expanded spines of some species and how these insects defend these acacias from browsers. In fact, all bushveld lovers cannot fail to be impressed by the spines and prickles on acacias (especially if you are walking carefully through dense stands of these spiny shrubs and trees in rhino country). Most of us have also been regaled by stories told by guides, and other bushveld experts, about how acacias and other trees and shrubs defend themselves from browsers.

I once saw an extremely, unusually long, white, spine on Acacia karroo (the Sweet Thorn, and now strictly speaking Vachellia karroo ). This was just over 300 mm long and no more than 10 mm diameter (just longer than the old measure of one foot, for us old-timers!). That was on a tree in the St Lucia secondary dune fields back in the 1960s - on a collection trip with Dr James (Jim) Henderson Ross who was doing his PhD on the ‘Acacias’ of KZN at the time. Jim later wrote up the sub-family for the Flora of South Africa, and then went on to compile a monograph on all the African species (Ross, J. H., 1979.A conspectus of the African Acacia species. Mem. Bot. Surv. S. Afr. 44 ). I am aware of some of the vast literature that has investigated various aspects of acacia taxonomy and biology, and there are many that only consider A. karroo . One paper that stands out for me focuses on white spines (Midgley, J. J., 2004. Why are spines of African Acacia species white? African Journal of

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