Seasonal exhaustion of the water supply by acacia woodland on Ecca substrate in Kruger National Park

@jeremygilmore @ludwig_muller @tonyrebelo @troos @botaneek @joshua_tx @jrebman @cwbarrows @grnleaf @alastairpotts @adriaan_grobler

(Also see https://www.inaturalist.org/journal/milewski/68753-vegetation-on-basalt-in-kruger-national-park-does-not-exhaust-its-water-supply-in-drought-even-where-there-are-trees-6-m-high#.)

Vegetation varies greatly in height within a given area, in many parts of the world. Here is a patch of treeless grassland, and there nearby is a patch of woodland.

What determines this kind of variation in the height of the vegetation?

Many or most naturalists might answer: 'the amount of water available'.

However, this is an unsatisfactory answer.

There are various regions on Earth that remained treeless grassland despite copious rainfall - the prime example being the Pampas (https://en.wikipedia.org/wiki/Pampas) of Buenos Aires province.

By the same token, there are extensive tracts of tall vegetation under semi-arid climates - the prime example being the Great Western Woodlands (https://en.wikipedia.org/wiki/Great_Western_Woodlands) in Western Australia.

Tall, arborescent vegetation tends fully to use the water in the ground. By contrast, short vegetation in equally rainy climates tends to leave much groundwater unused. The result, in the latter case, tends to be seasonal marshes.

It is with this conceptual framework in mind that I participated, at the end of a drought in 2016 (https://www.youtube.com/watch?v=LxQDZExuhGs and https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0209678), in a study of the vegetation in Kruger National Park in South Africa.

Our main aim was to sample soils, to see if the balance of nutrients could help to explain why certain vegetation is low, i.e. fails to use all the water available to it.

However, this also brought an opportunity to record the seasonal behaviour (https://en.wikipedia.org/wiki/Phenology) of the plants, as evidence for or against water as a controlling factor.

The Park near Satara (http://wikimapia.org/834858/Satara-Camp-Kruger-NP) contains low vegetation on basaltic soils, and tall vegetation on soils derived from Ecca sediments (https://en.wikipedia.org/wiki/Ecca_Group). (This geological type occupies only small areas in Kruger National Park, not evident at the scale seen in https://en.wikipedia.org/wiki/Ecca_Group#/media/File:Geology_of_Karoo_Supergroup.png.)

And, as predicted, the latter vegetation - called 'Delagoa Thorn Thicket' - showed signs that it had reached the limits of its supply of water.

In this Post, I describe 'Delagoa Thorn Thicket', southwest of Satara. Furthermore, I show that the phenological status (https://en.wikipedia.org/wiki/Phenology) of this vegetation provides a useful contrast with that on basalt, particularly the relatively treeless vegetation north of Satara.
 
The woody vegetation on Ecca substrate, although called thicket, is better-described as woodland featuring Senegalia burkei (https://www.inaturalist.org/taxa/594417-Senegalia-burkei). It proved to be taller and denser than the vegetation on basalt, under the same climate. Also, the values for canopy cover on Ecca far exceeded those on basalt, despite the fact that the main tree in both cases was a species of Senegalia.

The difference in the height of the vegetation is partly because there is less suppression by the African bush elephant (Loxodonta africana) on Ecca than on basalt.

This woodland on Ecca substrate, dominated by Senegalia burkei, proved to be tall and dense, i.e. largely unsuppressed by megaherbivores. Not only was S. burkei here generally about 2 m taller than the dominant Senegalia nigrescens (https://www.inaturalist.org/taxa/594427-Senegalia-nigrescens) in the nearby sampling area on basalt, but those plant spp. shared between the two areas tend to be taller on Ecca than on basalt.

The following two observations are close to the study area: https://www.inaturalist.org/observations/10898460 and https://www.inaturalist.org/observations/84617218.

In keeping with its arborescent structure, this vegetation on Ecca turned out to have minimal anticipation of rainfall in the shooting of new foliage. Virtually all of the plants, including evergreens, were bare of leaves during my visit.
 
I visited the vegetation on Ecca on 18 Nov. 2016, about a week after the drought-breaking rains wet the soil in the nearby basalt area to a depth of about 5 cm. I assume that a similar amount of rain fell on Ecca, where the a clay-rich soil is derived largely from rocks ranging from shale to sandstone.

Unlike the basalt areas, the ground on Ecca had no special feel underfoot as we walked around. It merely had a firm, ostensibly well-drained surface, with a shallow layer of sand, over what was a clayey topsoil. The latter was without the darkness of the basaltic soil, and without the fluffiness – let alone the clogginess experienced in the relatively treeless area north of Satara - of the basaltic soil.
 
The main species of Grewia on Ecca was bare, and thus hard to identify. However, it was possibly Grewia bicolor (https://www.inaturalist.org/taxa/431074-Grewia-bicolor), identical to that in the nearby woody vegetation on basalt. This Grewia, like Euclea divinorum, is lignotuberous and probably clonal.

The only difference between this sampling area on the Ecca substrate and the nearby sampling area on the basalt, as regards Grewia, was that G. bicolor? was taller on Ecca. I had a similar impression in the case of Flueggea virosa (https://www.inaturalist.org/taxa/340143-Flueggea-virosa).
 
Senegalia burkei differs from its close relative, S. nigrescens, as follows:

  • it is generally taller,
  • it is not as frequently felled or broken by the African bush elephant; and
  • when it is broken, it tends to die rather than to survive in a leaning or horizontal position, as S. nigrescens does.

A great difference between this sampling area and the woody vegetation on basalt nearby was that there was abundant evidence (old faecal middens) of the impala (Aepyceros melampus, https://www.inaturalist.org/taxa/42278-Aepyceros-melampus) here on Ecca. However, most of this population of the impala had quit this area because of a lack of food.
 
The sampling area on Ecca was extreme in the bareness of the ground. There was little grass apparent, even in the form of dead matter. Furthermore, herbaceous germination was still negligible at the time of my visit, despite it having been a week since the rains started. The only herbaceous plants noted were the occasional individual of Cissus, one specimen of a toxic succulent, and one geophyte, noted in only one place.
 
As in the woody vegetation on basalt nearby, most of the individuals of Dichrostachys cinerea (https://www.inaturalist.org/taxa/129706-Dichrostachys-cinerea) here on Ecca were dead. It is also noteworthy that

Furthermore, I did not record a single living specimen of Combretum imberbe (https://www.inaturalist.org/taxa/340408-Combretum-imberbe) in this vegetation on Ecca.
 
In order to emphasise how extremely bare the plants were at the end of this drought, I repeat that virtually the only species I saw with green leaves, in any of the sample plots, were Commiphora spp. (these leaves usually being fresh), Capparis tomentosa, and euphorb shrub indet.

Note that although Euclea divinorum (https://www.inaturalist.org/taxa/343032-Euclea-divinorum) is technically an evergreen, it had in fact lost most of its leaves in this case, owing to the severe drought.

Even in the case of the three spp. of Commiphora found here (all of them restricted to small plants, i.e. suppressed) there were only a few leaves showing, i.e. the shooting of the tiny leaves was only in its early stages and it is possible that it started only a week before, i.e. immediately after the first rain (as opposed to anticipating the rains).

The only species I found to possess a fairly complete set of new leaves was an unidentified shrub which proved to be vanishingly rare. Capparis tomentosa (https://www.inaturalist.org/taxa/342724-Capparis-tomentosa), which is potentially evergreen, was so rare in this vegetation, and of such small size, that I could easily have overlooked it completely.
 
Plot 1: Canopy cover 55%, of which contributions are Senegalia burkei 70% (up to 7 m high, three individuals), Euclea divinorum 20% (up to 2.5 m high, four individuals), Dichrostachys cinerea 5% (up to 3 m high, five individuals), Grewia bicolor? 3% (1 m high, one individual), Gymnosporia maranguense 2% (up to 1 m high, two individuals, https://www.inaturalist.org/taxa/586682-Gymnosporia-maranguensis). Tallest plant is 7 m (S. burkei).
 
Plot 2: Canopy cover 25%, of which contributions are S. burkei 85% (ranging in height from saplings 0.2 m high to mature tree 7 m, altogether five individuals), Grewia bicolor? 10% (up to 1 m high, four individuals), D. cinerea 3% (0.2 m high, one individual), Commiphora africana 1% (0.5 m high, one individual, shooting new foliage, https://www.inaturalist.org/taxa/505821-Commiphora-africana), Flueggea virosa 1% (1 m, one individual). Tallest plant is 7 m (S. burkei).
 
Plot 3: Canopy cover 35%, of which contributions are S. burkei 89% (ranging in height from saplings 0.25 m high to mature trees 7 m high, altogether nine individuals), Grewia bicolor? 6% (up to 1 m high, four individuals), Dichrostachys cinerea 4% (up to 2.5 m high, two individuals), Euclea divinorum 1% (1 m high, one individual; even this technically evergreen species has tired foliage, partly shed in this drought). Tallest plant is 7 m (S. burkei).
 
Plot 4: Canopy cover 70%, of which contributions are S. burkei 95% (up to 9 m high, ten individuals), Flueggea virosa 3% (up to 2.5 m, six individuals), Grewia bicolor? 2% (0.5 m high, three individuals). Tallest plant is 9 m (S. burkei).
 
Plot 5: Canopy cover 30%, of which contributions are Euclea divinorum 40% (up to 2.5 m high, eight individuals), S. burkei 30% (up to 5 m high, two individuals), Spirostachys africana 20% (4.5 m high, one individual), Flueggea virosa 5% (1 m high, one individual), Grewia bicolor? 5% (up to 1.3 m high, two individuals). Within plot is fallen old dead tree of Combretum hereroense. Nearby are Vachellia grandicornuta (https://www.inaturalist.org/taxa/595949-Vachellia-grandicornuta), Capparis tomentosa (in leaf), euphorb shrub indet. (in leaf), and Carissa bispinosa? Tallest plant is 5 m high (S. burkei).
 
Plot 6: Canopy cover 22%, of which contributions are Spirostachys africana 55% (5.5 m, two individuals), Gardenia volkensii 26% (up to 2.8 m, three individuals), Zanthoxylum humile 15% (1.2 m high, two individuals, https://www.inaturalist.org/observations?taxon_id=596428), Grewia bicolor? 2% (up to 0.5 m, two individuals), Carissa bispinosa? 1% (0.5 m, one individual), Euclea divinorum 1% (o.75 m, two individuals). Tallest plant is 5.5 m (Spirostachys africana).
 
Plot 7: Canopy cover 30%, of which contributions are Euclea divinorum 55% (up to 4 m high, seven individuals), S. burkei 30% (0.2 m high in case of saplings and 7.5 m high in case of mature tree, altogether three individuals), Spirostachys africana 10% (up to 2.5 m, one clonal individual), ?Zanthoxylum humile and Commiphora glandulosa 5% (0.5-1.5 m high, three individuals, of which at least one is shooting new foliage).
 
Plot 8: Canopy cover 57%, of which contributions are S. burkei 45% (height range 0.2 m for sapling to 8 m for mature tree, altogether two individuals), Spirostachys africana 19% (6 m high, one individual/clone), Euclea divinorum 10% (2.5 m high, one individual), Grewia bicolor? 10% (up to 1 m, four individuals), Dichrostachys cinerea 5% (1.5 m high, one individual), Gardenia volkensii 5% (up to 1.8 m high, two individuals, https://www.inaturalist.org/taxa/431063-Gardenia-volkensii), Flueggea virosa 3% (0.3 m high, one individual), Maerua parvifolia 2% (0.4 m high, one individual, a few leaves still present, https://www.inaturalist.org/taxa/544900-Maerua-parvifolia), Commiphora africana/schimperi 1% (0.2 m high, one individual, one of the few species shooting fresh foliage). Tallest plant is 8 m (S. burkei). Nearby is big dead tree of Combretum hereroense, which would have been up to 12 m high when alive.
 
Plot 9: Canopy cover 25%, of which contributions are S. burkei 75% (ranging from 0.3 m high saplings to 6 m high tree, 10 individuals), Cassia abbreviata 10% (2.5 m high, one individual, suppressed), Grewia bicolor? 10% (up to 1.5 m high, seven individuals), Dichrostachys cinerea 5% (1.2 m high, one individual). Once again, it is noticeable that D. cinerea here is mainly in the form of dead individuals. Tallest plant is 6 m (S. burkei), but one dead specimen of S. burkei would have been 15 m high when alive. Just outside plot is old, dead tree of Combretum imberbe, which was perhaps 15 m high when alive.
 
Plot 10: Canopy cover 15%, of which contributions are Grewia bicolor? 40% (up to 0.5 m high, four individuals), Euclea divinorum 30% (2 m high, one individual), Zanthoxylum humile 20% (1.5 m high, one individual), Dichrostachys cinerea 5% (0.4 m high, one individual), Commiphora africana 3% (0.3 m high, one individual), Commiphora schimperi 2% (0.5 m high, one individual, https://www.inaturalist.org/taxa/582635-Commiphora-schimperi). Dichrostachys cinerea in plot is dead. Several individual trees of S. burkei in plot are also dead. Tallest plant is 2 m high (Euclea divinorum). Nearby is Ehretia rigida?, 2 m high, with a few tired-looking leaves still attached.
 
Plot 11: Canopy cover 25%, of which contributions are S. burkei 65% (range from 0.4 m sapling to mature tree 8 m high, altogether seven individuals), Grewia bicolor? 25% (up to 1 m high, four individuals), Dichrostachys cinerea 5% (up to 2.5 m high, three individuals), Lannea schweinfurthii 4% (2 m high, one individual, just starting to shoot new foliage, https://www.inaturalist.org/taxa/340118-Lannea-schweinfurthii), Commiphora africana 0.5% (0.5 m, one individual, shooting new foliage), Commiphora schimperi 0.5% (0.5 m high, one individual, shooting new foliage). Tallest plant is 8 m (S. burkei).
 
Plot 12: Canopy cover 30%, of which contributions are S. burkei 24% (up to 4 m high, two individuals), Grewia bicolor? 24% (up to 2 m high, six individuals), Spirostachys africana 19% (up to 5.5 m high, two individuals), Gardenia volkensii 15% (2.3 m high, one individual), Ehretia rigida? 7% (up to 2 m high, two individuals), Zanthoxylum humile 5% (2 m high, one individual), Dichrostachys cinerea 2% (2.5 m high, one individual), Gymnosporia maranguense 2% (2 m high, one individual), Euclea divinorum 1% (1 m high, one individual), Commiphora africana or schimperi 1% (up to 1.2 m, two individuals). Some of these plants are aggregated on a possible termite mound (old, low), the clump consisting of Grewia bicolor?, Ehretia rigida?, Commiphora africana/schimperi and Gardenia volkensii. Tallest plant is 5.5 m (Spirostachys africana). Near plot is big dead tree of Spirostachys africana, which would have been 14 m high when alive, making it exceptionally tall for its species.
 
Plot 13: Canopy cover 30%, of which contributions are S. burkei 60% (ranging from sapling 0.3 m high to mature tree 8 m high, altogether eight individuals), Spirostachys africana 18% (up to 6 m high, two individuals), Grewia bicolor? 10% (up to 2.5 m, four individuals), Flueggea virosa 5% (up to 1.5 m high, two individuals), Euclea divinorum 5% (up to 2.5 m high, two individuals), Commiphora 2% (up to 1. m high, two individuals). Tallest plant is 8 m (S. burkei).
 
Plot 14: Canopy cover 55%, of which contributions are S. burkei 60% (ranging from saplings 0.8 m high to mature trees 8 m high, altogether 6 individuals), Flueggea virosa 20% (up to 2.5 m high, nine individuals), Grewia bicolor? 10% (up to 0.8 m high, nine individuals), Dichrostachys cinerea 5% (0.5 m high, five individuals), Euclea divinorum 3% (1.5 m high, one individual), Zanthoxlum humile 1% (0.5 m high, one individual), Commiphora schimperi 1% (0.3 m high, one individual). Tallest plant is 8 m (S. burkei).
 
Plot 15: Canopy cover 28%, of which contributions are S. burkei 70% (up to 8 m high, three individuals), Flueggea virosa 5% (0.5 m high, one individual), Ehretia rigida? 5% (up to 0.5 m high, two individuals), Grewia bicolor? 5% (up to 1.5 m high, five individuals), Cassia abbreviata 5% (2 m high, one individual, https://www.inaturalist.org/taxa/147000-Cassia-abbreviata), Dichrostachys cinerea 5% (1.2 m high, one individual), Commiphora africana 2% (0.5 m high, one individual), Commiphora schimperi 2% (0.5 m high, one individual), Zanthoxylum humile 1% (0.2 m high, one individual). Tallest plant is 8 m (S. burkei).
 
PHENOLOGICAL NOTES:
 
All the mimosoid legumes were totally bare, as were Grewia, Flueggea, Gardenia, Gymnosporia, and Spirostachys.
 
Even Euclea divinorum, which is technically evergreen, had lost most of its leaves in this drought. Similar comments apply to Carissa, Maerua and Ehretia, which have evergreen tendencies.
 
Commiphora was more diverse and common here, on Ecca, than on the basalt. However, the shooting of foliage by Commiphora is unremarkable, considering

  • how small (suppressed) the individuals of this genus were in this area,
  • how small the leaves are,
  • how few leaves had yet appeared, and
  • that Commiphora may be able to store some water in its roots.

To summarise, there were virtually no species on Ecca that had anticipated the rains in shooting new foliage. There was also not a single functionally evergreen species. And, unlike the pattern on nearby basalt, there was negligible tendency for leaves to appear on the tallest plants.
 
My interpretation is that the nutritional regime in this substrate favours woody plants over herbaceous plants. The impala (Aepyceros melampus) is seasonally abundant, and this is certainly typical habitat for the hook-lipped rhino (Diceros bicornis). However, the vegetation has little attraction for the African bush elephant or square-lipped rhino. The woody plants reach something close to their full potential as set by the water supply.

There is thus no ‘surplus’ water in the soils, after a long and severe drought, for shooting of foliage in anticipation of the rains, or even for the maintenance of the tough leaves of evergreens. Even a week after the drought-breaking first rains, virtually the only green leaves to be found belonged to small, scattered plants with some sort of water-storage in the plant body.
 
The bottom line is that my predictions have been borne out: if one arranges the three sampling areas in the vicinity of Satara in the order Woodland on Ecca, savanna on basalt, and nearly treeless vegetation on basalt, the resulting cline in woody height and cover corresponds nicely to an inverse cline in incidence of green leaves during my visit, within a week of the first drought-breaking rains.

The implication is clear:
The low, open vegetation is low and open not because it lacks water, but because some other factor – such as competitive inferiority to grasses and extreme suppression by large herbivores – limits the demand for water by the woody plants, overall. This in turn ‘conserves’ some water in the ground, which can be drawn upon, even after severe drought, for the shooting of new flowers and foliage in anticipation of the drought-breaking rains.

Posted on August 03, 2022 05:48 AM by milewski milewski

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