Possible raciality in Vachellia, a genus of acacias

(writing in progress)

There is a pattern of variation in acacias that does not fully conform to the concept of species or subspecies.

Perhaps the most obvious example is Acacia aneura (https://www.inaturalist.org/taxa/188884-Acacia-aneura and http://worldwidewattle.com/speciesgallery/aneura.php), a dominant tree or large shrub over much of semi-arid Australia (https://www.qld.gov.au/environment/plants-animals/habitats/regrowth/regrowth-guides/mulga/mulga-description and https://en.wikipedia.org/wiki/Mulga_(habitat) and https://www.qld.gov.au/environment/plants-animals/habitats/habitat/mulga).

In Africa, a possible example is Vachellia karroo (https://www.inaturalist.org/taxa/502780-Vachellia-karroo). This has recently been split into several closely-related species, but this reclassification has not fully solved the problem.

The entity now called Vachellia karroo (sensu lato) defies classification according to the textbook concept of species or subspecies. This is because it has various forms that do not seem to be separated objectively in terms of geographical or reproductive barriers.

‘Vachellia karroo’ is a bewildering range of entities, and it is easy to see why botanists have, for a century of more, put the lot in the ‘too-hard’ basket and just called them all ‘Acacia karroo’. 

In this Post, I point out a handy reference (Barnes et al. 1996) on this group of plants. I discuss several features of the biology of Vachellia, relative to the question of raciality as opposed to speciation.

(Dave Ward has published on the phylogenetic conundrum in V. karroo.)
 
in some ways V. karroo (sensu lato) behaves like a weed, senescing after only a few decades and providing generously to animals ranging from bees through baboons to megaherbivores. Its gum is palatable and its seeds poorly defended. Its shoots are infested by a psyllid bug, and it is so palatable to certain moth caterpillars that they can kill the plant. Yet, despite this ‘live-fast, die-young’ weediness, the plant has remarkably dense wood.
 
I would like to know more about why one form was formerly called ‘Acacia inconflagrabilis’, indicating that it refuses to burn, being some sort of fire-damping plant.
 
I have not read that Acacia robusta (including clavigera) or A. grandicornuta are particularly closely related to V. karroo. However, both of these plants strike me as little more than giant versions of V. karroo, adapted to places such as Kruger National Park, and distinguished phenologically more than morphologically.
 
The recent splitting off of several distinct ‘species’ may be welcome in practical terms. However, it has not really solved the phylogenetic conundrum (see abstract below from Taylor & Barker 2012). This is because (as also pointed out by Barnes et al. 1996) the various forms intergrade, rather than being morphologically or genetically distinct.

Furthermore, V. karroo has been recorded naturally hybridising with A. tenuispina and A. rehmanniana, the latter not even being considered part of this species-complex and being instead a potentially flat-topped form of acacia.

Vachellia karroo sensu lato seems to constitute a ‘superspecies’ with V. seyal, which replaces it over a vast area farther north in Africa.

And to add to all the confusion, Vachellia nilotica ssp. adstringens is vegetatively almost indistinguishable from V. karroo. This is despite the great difference in the form and function of the pods: V. nilotica has carob-type pods and dispersal by large mammals whereas V. karroo has deshiscent pods and, I suspect, dispersal mainly by whirlwinds.
 
It is interesting that V. karroo has a deep taproot. This emphasises the question of why it does not grow taller on e.g. the Highveld (https://en.wikipedia.org/wiki/Highveld).
 
http://www.sciencedirect.com/science/article/pii/S0254629912000932

http://onlinelibrary.wiley.com/doi/10.1111/j.1095-8312.2011.01757.x/full
 
http://www.bodley.ox.ac.uk/users/millsr/isbes/ODLF/TFP32.pdf

There is surprising intraspecific variation in acacias, which tends to throw doubt on conventional taxonomy.

This can be seen in the flat-topped shape of certain acacias in particular. This is because this is an easily visible feature that shows as much intraspecific variation as any other, and is only partly explained by regional subspeciation.
 
Here I offer an example, in the form of Vachellia sieberiana (https://www.inaturalist.org/taxa/559085-Vachellia-sieberiana).
 
Vachellia sieberiana is particularly conspicuous in parts of South Africa, where it has achieved the status of a ‘landscape symbol’ owing to the fact that it has not only a flat crown, but such an extremely flat crown that it almost looks like a caricature. Overall this is probably the prime example of a flat-topped acacia in South Africa, partly because it extends to altitudes, e.g. on the foothills of the Drakensberg, where there are few other trees.
 
Vachellia sieberiana can best be thought of as a ‘beautiful weed’ in the sense that it acts as a pioneer on old farmland. It may look like an icon of the primeval savanna, something Pleistocene and anachronistic in the vulgar, makeshift world of anthropogenic land-use, but actually it is promoted by disturbance and dresses up this rude vigour in the most elegant of costumes.
 
I have hypothesised in the past that this extreme growth-form is part of a mutualism with large herbivores, in which the plant provides a shady canopy in return for the nourishment provided by urine and faeces deposited under the tree. Vachellia sieberiana is certainly mutualistic with large herbivores in its fruit-form, because it has the ‘carob’-type pod which is indehiscent and retains starch in the ripe pod-wall, together with a tendency for the ripe pods to fall to the ground. This makes the fruits attractive to various species of ungulates which disperse and sown the seeds. The idea is that V. sieberiana, like e.g. V. tortilis, offers not only carob-like pods but also shade, in a complex mutualism with large herbivores.
 
Another interesting and puzzling fact about V. sieberiana is how soft its wood is (about 0.655 tonnes/cubic metre air-dry) compared to certain other flat-topped congeners, some of which have wood almost twice as dense. While I do not understand why some acacias have extremely dense wood and other, closely related, acacias have rather flimsy wood, the flimsy wood of V. sieberiana seems consistent with its ‘woody weediness’, rapid growth, and by implication short lifespan as a plant of disturbance.
 
Vachellia sieberiana illustrates a theme that crops up repeatedly in acacias: that the same species can be flat-topped in certain parts of its habitat and not flat-topped in others.
 
Vachellia sieberiana is the most common tree in the Ishasha savannas of Queen Elizabeth National Park, making it perhaps the most prominent acacia in the prime big-game country of Uganda. It forms the backdrop to photos of the extremely dense populations of topi (Damaliscus jimela) in that national park.

But, as the photos below show, no South African who knows the Tugela Valley or the Nelspruit area would recognise V. sieberiana in this prime national park located right on the equator. This is because, for some reason, the species does not have the same crown-form.

Vachellia sieberiana near Melmoth, KwaZulu-Natal, showing the familiar extreme flat-topped form of the crown, so familiar to South Africans and depicted to good effect in the paintings of Pierneef:
 
http://tropical.theferns.info/plantimages/2/3/23a1237f188990f7bcf3542ff50e424721a4a19a.jpg
 
The following is the same species on the equator in western Uganda, yet it lacks anything more than a slight flattening of the crown.
 
Vachellia sieberiana, Queen Elizabeth National Park, Uganda:
https://4.bp.blogspot.com/-4iH9BRZlMJw/Vr6lqLAfvOI/AAAAAAAAL_8/hrbyUiJMqk8/s1600/Acacia%2Bwoodland%2BQENP%2BUganda%2B0710.JPG
 
Vachellia sieberiana with Damaliscus jimela in foreground, Queen Elizabeth National Park, Uganda:
http://queenelizabethnationalparkuganda.com/wp-content/uploads/2015/01/Queen_Elizabeth_National_Park_2.jpg

Vachellia sieberiana with Loxodonta africana in foreground, Queen Elizabeth National Park, Uganda:
http://cctv-africa.com/wp-content/photo-gallery/2016/07/Queen_Elizabeth_National_Park_023.jpg
http://tropical.theferns.info/plantimages/2/3/23a1237f188990f7bcf3542ff50e424721a4a19a.jpg

(writing in progress)

Publicado el 28 de junio de 2022 por milewski