Senegalia nigrescens
(writing in progress)
W.r.t. raciality in acacias: genus Senegalia for the complex of entities including burkei, welwitschii and nigrescens
What are currently called burkei, welwitschii and nigrescens, all occurring commonly in Kruger National Park, and all their so-called subspecies, constitute a poorly-defined complex of acacias that I suspect to be racial rather than speciated. There is not enough geographical segregation for subspecies to be recognised; and one species tend to blur into another without geographical separation. What appears to be a well-known and distinctive species, such as knobthorn (A. nigrescens), is actually just part of a much more complex collection of entities. For all I know, even what we’ve called Acacia burkei, in our sampling site on Ecca shale along the Orpen road, was actually A. welwitschii.
Now, all these species have recently been transferred from Acacia to genus Senegalia. Genus Senegalia consist of many species not just in Africa but also in the Americas, Asia, and even New Guinea, and some of the few that are likely to be familiar to you are S. caffra (found as far southwest as the eastern Cederberg) and S. mellifera (which is such a problem in woody encroachment). An example of a species of Senegalia native to Australia is S. albizioides of north Queensland, which is bramble like (a climber rather than free-standing woody plant).
I suggest that we refer, in future, to this hypothetically racial complex including burkei, welwitschii and nigrescens as occurring in genus Senegalia. A handy way to conceive of Senegalia is as acacias with prickles (hooked spines). This genus has been domesticated in tropical Asia for its leafy greens (see photo below).
Coming back to KNP: everyone knows about Senegalia nigrescens, but what few realise is that ‘knobthorn’ is part of a complex of entities which seem to be more racial than speciated, and I have yet to learn to distinguish S. welwitschii from S. burkei in the field. Even within S. burkei, the shapeless form we noted on our recent visit to KNP does not necessarily apply everywhere in this Park, because S. burkei can also have an approximately flat top. Much more scrutiny is needed because conventional taxonomy does not seem to apply. The last thing we need is to lose precision even more by lumping all these entities within the genus Acacia, which was meaninglessly large until the recent breakup into Senegalia, Vachellia, etc.
In summary, what have been called nigrescens, welwitschii and burkei (and any so-called subspecies of these) may perhaps belong to a single species of Senegalia as an extreme example of raciality.
The extremely large bipinnate leaves of Senegalia pennata of southeast Asia, which is used as an edible leafy green:
Knobthorn (Mimosaceae: Senegalia nigrescens)
Botanical psyche: guardedly generous to megaherbivores.
Strategic benefit: receives nutrients in urine and dung from megaherbivores.
Why then is it prickly? Prickles do not deter savanna elephant, southern giraffe or hook-lipped rhino. They just reduce the rate of foraging to ensure that the plant survives to feed them another day.
Global perspective: The surprising generosity of knobthorn helps explain why acacia savanna in southern Africa is far more productive for large animals than are comparable savannas on other continents, such as the chaco of South America or the brigalow of Australia.
For readers with a keen eye for plants
- Preserving buds: knobthorn has curved prickles rather than impaling thorns, designed not to draw blood but to slow down the eating and to prevent stripping of the buds that can regenerate after defoliation.
- Managing the savanna elephant: more than other acacias, knobthorn has extremely tough wood – combining hardness and flexibility – which helps this tree to work with, rather than fight, the strength of the elephant’s proboscis.
- Supplying food at the right height: abrasive knobs on the bole cannot prevent the savanna elephant from toppling the tree, but are designed to control the force applied – allowing the bole to be bent to the ground but[awkward repetition of ‘but’ in same sentence] not snapped. This allows the knobthorn to send out new branches from the prostrate bole, keeping production of leaves and flowers within reach of elephant, giraffe and rhino.
- Feeding giraffes during drought: flowers are produced not in the rainy season but at the height of the dry season, when any produced lower than 5.5 metres above ground are likely to be eaten by the giraffe in its time of greatest need.
(writing in progress)
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