Query about a lack of spines in Vachellia davyi
@tonyrebelo @troos @botaneek @andrew_hankey @robertarcher397 @wynand_uys @rob_palmer @drmckenzie @katebraunsd @warrenmcc @wolfachim @simon_attwood @lindaloffler
I find Vachellia davyi (https://www.inaturalist.org/observations?verifiable=true&taxon_id=595943&locale=en-US) puzzling, for two reasons.
Firstly, this species seems to lack spines, despite belonging to a genus renowned for its spinescence.
Secondly, if it does have a capacity to produce spines, it is unclear to me whether this is
- restricted to saplings, and
- induced only by damage (such as that inflicted by ungulates).
There are currently 18 observations of V. davyi in iNaturalist, and few show obvious spines.
When I visited Ithala Game Reserve in 2000, I was told by an authority on the local ecology that V. davyi lacks spines, has long, strippable branches, and was being exterminated in the reserve by Giraffa giraffa.
However, at least one description of this species does mention stipular spines.
Can anyone please shed light on this puzzle?
If V. davyi really is underprotected by spines, what is it about this species, ecologically, that might have protected it from large herbivores?
And if V. davyi can produce effective spines, then under which circumstances does this occur?
Comments
Hello, Antoni
I think you'll find paired, more or less straight thorns on all examples of V. davyi that are bigger than a sapling.
The thorns are not well developed close to the growth tips where iNat observers took closeups of the flowers or young leaves.
(Edit: Of course, I meant stipular spines, not thorns)
I viewed images of the observations that you linked to, and I see thorns in some of them.
https://inaturalist-open-data.s3.amazonaws.com/photos/211104940/original.jpg
https://inaturalist-open-data.s3.amazonaws.com/photos/44683266/original.jpeg
https://inaturalist-open-data.s3.amazonaws.com/photos/57991961/large.jpg
https://inaturalist-open-data.s3.amazonaws.com/photos/30398294/original.jpg
https://inaturalist-open-data.s3.amazonaws.com/photos/30398302/original.jpg
Kind regards
According to J.D.Carr the spines of V. davyi vary in length from 2mm up to 25mm. That may be the reason why one sometimes can't see the very small spines on some photos.
Anyhow, Giraffe feed on many species of spiny Vachellia.
Greetings, Wolf
All of the V. davyi trees I have seen have spines, not nearly as obvious though as V. karroo or even V. sieberiana. The leaves cover them a bit as these thorns are quite short. It is interesting that giraffes would be a threat to them as the trees I am familiar with are not very tall and I would think are not at the preferred browsing height of a giraffe. They are often less than 2-3m in height and are coppiced shrubs, unlike many other species of Vachellia that grow much taller with a browseable crown. Next time I observe this species I will make sure to get a close-up image of its thorns.
@wynand_uys @wolfachim @simon_attwood Many thanks to all of you for your most helpful replies.
@richardgill @benjamin_walton
I have found the following entry in my field notes from a visit to Ithala Game Reserve in 2000:
"Vachellia davyi is fairly common in the vicinity of Kwa Sambane rondawel, but mainly as dead skeletons in the formerly cultivated areas, i.e. those farmed for herbaceous crops before 1973. Pooley states the maximum height of the plants as 3 m, but I think 5 m would be more correct. I am struck by two features of V. davyi: 1) corky bark, clearly indicating a fire-resistant species, capable of surviving even the most intense fire in thatch-grass; and 2) non-spinescence in the crowns, despite a) presence of spines at heights of about 1-1.5 m, and b) accessibility of all crowns to Giraffa giraffa. My interpretation: this is a 'fire-loving' African acacia, which has been killed in the formerly cultivated areas despite the perpetuation of fairly intense fire, and because of relatively intense pressure from herbivores - not just in terms of direct stripping, but more importantly because of a tipping in the balance in the cycling of nutrients between fire on one hand and herbivory on the other."
By Pooley, I refer to https://books.google.com.au/books/about/The_Complete_Field_Guide_to_Trees_of_Nat.html?id=_x1FAAAAYAAJ&redir_esc=y.
So, my current interpretation of V. davyi is as follows:
This species is nominally capable of producing the same sort of stipular spines that characterise the whole genus, Vachellia. However, it does not respond to damage by large herbivores in the same way as e.g. V. xanthophloea (https://www.inaturalist.org/observations/122965582), i.e. by growing large spines in defence of the recovering shoots. This, I suspect, is ultimately because the whole nutritional strategy of V. davyi is adapted to combustion and the deposition of ash, as opposed to herbivory and the deposition of dung and urine. The result is an apparent paradox: a species of Vachellia that allows itself to be defoliated to the point of mortality by ungulates, without seeming to defend itself.
Unlike coexisting and ecologically comparable Vachellia sieberiana (https://www.inaturalist.org/observations/17469948), V. davyi is incapable of overtopping giraffes.
Another way of putting this is that V. davyi is evolutionarily specialised for corky bark, not induced spinescence. So, when the newly-proclaimed Ithala Game Reserve was still understocked with herbivores, and managers relied on burning of grasses, such as Hyparrhenia, in the successional recovery on former crop-fields, a niche was available to V. davyi, for up to two decades. However, with the attainment of full or 'overfull' populations of Giraffa and Strepsiceros in the 90's, this niche waned.
Porter R N (1983) The woody plant communities of Itala Nature Reserve. Natal Parks, Game and Fish Preservation Board, Pietermaritzburg.
According to the above study, one of the vegetation types in Ithala Game Reserve is 'woodland' of Vachellia davyi and Dichrostachys cinerea, about 4 m high, on quartzite bedrock. Protea caffra occurs in the same community.
In my field notes from 2000 in Ithala Game Reserve, I find several mentions of what I take to be Vachellia davyi.
These were specimens of tree form, with flat-topped crowns <4 m high. This resembled the growth-form of sympatric Vachellia nilotica, but spines were restricted to branches at heights <2.5 m. The crowns were fully and freshly green in early September.
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