What Galpin wrote in 1926 about woody encroachment on the Springbok Flats

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The Springbok Flats (https://en.wikipedia.org/wiki/Springbok_Flats and https://www.britannica.com/place/Springbok-Flats) of Limpopo province, South Africa, is biogeographically significant plain, because it:

  • is unusually fertile owing to its volcanic substrates, and thus comparable to the Serengeti plain (in the restricted sense) in Tanzania,
  • may have been the largest patch of naturally treeless grassland in southern Africa, beyond the Highveld (https://en.wikipedia.org/wiki/Highveld),
  • was probably an important part of the original migratory circuit for large ungulates in the northern part of South Africa, and
  • ostensibly marked the limit of the distribution of Antidorcas marsupialis (https://www.inaturalist.org/taxa/42283-Antidorcas-marsupialis), despite the lack of historical verification beyond the name 'Springbok Flats'.

One of the reasons why so little is known of the original ecology of the Springbok Flats is that its fertility attracted the Boers (https://en.wikipedia.org/wiki/Boers) early in the settlement of the Transvaal (https://en.wikipedia.org/wiki/Transvaal_(province)), with rapid extermination of wild mammals as the plain was settled. Here, crops could be grown without fertilisers.

I have before me the 100-page report, 'Botanical Survey of the Springbok Flats, Transvaal (1926)', Botanical Survey of South Africa, Memoir no. 12. The author is E E Galpin (https://en.wikipedia.org/wiki/Ernest_Edward_Galpin).

Although published 26 years before I was born, this publication is still relevant today.

The point of particular interest is that, already at that time, encroachment by woody plants had become a problem on the Springbok Flats.
 
One of the points that comes out clearly in Galpin (1926) is that Vachellia karroo (https://www.inaturalist.org/taxa/502780-Vachellia-karroo)), and many other species of trees and shrubs, were encroaching on the Springbok Flats, depending partly on soil type.
 
The Springbok Flats includes not only cracking clays (vertisols, https://en.wikipedia.org/wiki/Vertisol) and deep, clay-rich, non-cracking loams (both over basalt, and perhaps dolerite too), but also sandy soils derived from sandstones. On the sandy soils there was already much woody encroachment as early as 1920. However, this was by taxa such as Combretaceae, rather than by acacias.
 
On the vertisols, Galpin mentions encroachment by

On the basaltic loam, he mentions encroachment by

On sandy soils (which occur in the central Springbok Flats), Galpin mentions encroachment by

He points out how odd it is that Burkea africana (https://www.inaturalist.org/taxa/340238-Burkea-africana) showed no such encroachment, if anything decreasing at that time.
 
At the transition from sandy to basalt-derived soils, Galpin mentions encroachment by Combretum apiculatum (https://www.inaturalist.org/taxa/340059-Combretum-apiculatum) and C. molle (https://www.inaturalist.org/taxa/342738-Combretum-molle).
 
I infer that:
Acacias such as Vachellia karroo have been prominent among the encroachers. However, there have been many spp. of non N-fixers in the system too, with the same tendency. Thus, ultimately, the encroachment process has not been about N-fixation, or even about any particular lineage of woody plants. It has been more general than that, and in some sense almost any woody plant has participated.

Galpin devotes 8 pages of his report to woody encroachment (pp. 80 ff).
 
Already in 1926 the Springbok Flats were being invaded from all sides by acacias and other woody plants. This invasion by acacias was most intense on the deep basaltic loams.

“Places that eight years ago were grassy fields, with a few Acacias scattered about, are now being thickly overgrown with young Acacias.” Galpin blames this on the European farmers not burning the grass as frequently as the indigenous pastoralists had done, but his argument seems questionable.
 
Galpin understood that, overall, the pastoral value of the pasture eventually declines with woody encroachment, notwithstanding the fact that "many of our sweetest and most fattening grasses, such as Panicum maximum and allied species included under the general name of Buffelgras, Urochloa spp., and Pennisetum cenchroides..., form dense thickets (sic) under their protection and afford the best grazing."
  
Galpin (1926) states on page 82: "On basaltic loam soils, Acacia karroo ... appears to be a comparatively recent addition to the flora, judging by the absence of old trees, but has not become one of the dominant trees. It is increasing the most rapidly of all, and over-running considerable stretches of veld, which only seven or eight years ago consisted of grasslands, dotted here and there with a few scattered trees."
 
From the above, it seems that:

  • the grassland of the Springbok Flats, at least on this particular soil type, was never completely treeless, and
  • the encroachment (by Vachellia karroo) started in earnest about 1917, just before the end of the First World War.

Galpin (1926) goes on to write:
"Dichrostachys ... thrives in light, sweet, sandy loams, and, in many places where the basaltic loams have been largely admixed with sand, is increasing enormously and forming dense jungles of thorny growth, particularly in places where old lands have gone out of cultivation."
 
Another noteworthy aspect is the status of Boscia (https://www.inaturalist.org/observations?place_id=9074&taxon_id=82545&view=species) on this soil type. "Mature trees, many of them of very great age, are locally frequent in basaltic loam soils and seed abundantly, yet I have never succeeded in finding a young plant there...there seems to be nothing improbable in the possibility of this remarkable tree being locally a survival from a past age when desert conditions prevailed in this area as referred to by Dr. P. A. Wagner [in] this memoir..."
 
The point of the latter observation is that, even if no other trees occurred on the basaltic loams of the Springbok Flats in pre-European times, then at least Boscia seems to have been present. So, again, we cannot really refer to the natural vegetation as fully treeless.
 
On the adjacent vertisols of the Springbok Flats, the woody encroachment by 1926 was less extensive. However, it was still a noteworthy phenomenon.

Galpin (1926) writes of this soil type:
 "This grassland association is being steadily invaded by trees, chiefly Acacias, on all sides from the adjoining Basaltic Loam...’. Acacia karroo once again features although equally important on these ‘black cotton soils’ were Acacia litakunensis [now called Vachellia tortilis], Acacia benthami [now Vachellia nilotica] and A. gerrardi. ‘Here and there, considerable groves of one or other species have already been formed, for the most part close to the outside boundaries of the association, whilst isolated scattered seedlings are to be found as pioneers sprinkled far and wide in the central grasslands. The curious stoloniferous dwarf Acacias, A. permixta, var. glabra, and A. natalitia, forma stolonifera, collectively known as ‘Fijndoorn’, found growing only in the black turf [= basaltic vertisols] and widely spread all over the grasslands, also appear to be steadily increasing in number."
 
My inference from the above facts:
 
Some may perhaps assume that the treelessness of cracking clays (e.g. in Mitchell grassland in Australia, https://en.wikipedia.org/wiki/Mitchell_Grass_Downs, as well as parts of Africa) is owing to the physical damage to roots inflicted by swelling and cracking.

However, it is clearly not the case that this damage excludes woody plants, even if it perhaps excludes ‘trees’. This is because some acacias are adapted to cope with such damage by becoming ‘stoloniferous’.

So, one cannot satisfactorily explain the treelessness of cracking clay soils in this mechanistic way.  And it is interesting that one of the indigenous acacias of the Springbok Flats, already by 1920 starting to encroach on to the cracking clays of the area, has been a vast encroacher in the Mitchell grassland after its introduction to Australia (probably from India). I refer to Vachellia nilotica (https://www.inaturalist.org/taxa/557826-Vachellia-nilotica and https://keyserver.lucidcentral.org/weeds/data/media/Html/vachellia_nilotica.htm#:~:text=Prickly%20acacia%20(Vachellia%20nilotica)%20is%20a%20Weed%20of%20National%20Significance,and%20economic%20and%20environmental%20impacts. and https://www.daf.qld.gov.au/__data/assets/pdf_file/0007/73753/prickly-acacia.pdf).
 
The bottom line:
Galpin (1926) is a reference for the Springbok Flats:

  • probably not being completely treeless in its original state, in the early nineteenth century, and
  • being subject to noticeable encroachment by Vachellia karroo, other acacias, and many other woody plants, as early as 1917.

Coming back to the present:
The following relevant study was done on the Springbok Flats: https://pubmed.ncbi.nlm.nih.gov/28662068/ and https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0179848.

Publicado el julio 30, 2022 03:45 MAÑANA por milewski milewski

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