Vegetation on basalt in Kruger National Park does not exhaust its water supply in drought, even where there are trees 6 m high

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The aim of this study, covered by a series of Posts, was to find out why vegetation on certain substrates in Kruger National Park tends to be treeless, and how this relates to the water supply.

Please see https://www.inaturalist.org/journal/milewski/68752-seasonal-exhaustion-of-the-water-supply-by-acacia-woodland-on-ecca-substrate-in-kruger-national-park#. Woodland on Ecca substrate in Kruger National Park exhausted its water supply in the drought of 2016.

By contrast, I show in the current Post that woody vegetation on nearby basalt, somewhat suppressed by megaherbivores, did not exhaust its water supply. The sampling area was located just southwest of Satara Rest Camp, and reached via Orpen Rd.

When one drives past this sampling area, what one sees essentially is a 6 m high stratum of Senegalia nigrescens over a 1.5 m high stratum of Flueggea virosa.

There are two basic ways in which a tendency towards treelessness arises, in the current context:

  • arborescent spp. to fail to establish, and/or
  • arborescent spp. establish, but to fail to reach their potential size as individual plants.

By suppression, what I mainly refer to is this failure of woody plants to reach their potential size.

In the wooded vegetation on basalt in Kruger National Park, the suppression seems to be enacted mainly by the African bush elephant.

The first rains fell in the Satara area on 11-12 November 2016. My visit, starting three days later, was on 15-17 November 2016. I observed that the soil had been wetted to a depth of only about 5 cm. I did the fieldwork so soon (six days or less) after these first rains that the herbaceous plants had hardly responded yet.

The following is a list of the tree and shrub species I recorded in the sampling area for wooded vegetation on basalt, just southwest of Satara, at this time, with my interpretations w.r.t. suppression.

SUPPRESSION OF ARBORESCENCE:
 
Senegalia nigrescens (https://www.inaturalist.org/taxa/594427-Senegalia-nigrescens):

This is the dominant woody plant on basalt near Satara. Its suppression is complex and subtle.

There are occasional full-size (>15 m high) specimens of S. nigrescens in the area sampled. However, most individuals are about 6 m high, suggesting that they are either suppressed or still growing.

My interpretation is that they are suppressed, and that the agency of suppression is a combination of

  • browsing by the southern giraffe (Giraffa giraffa, which can continue to reach nearly to the top of any specimen 6 m high), and
  • breakage/felling by the African bush elephant (which browses the species but also just breaks it, seemingly as a kind of horticultural policy).

Senegalia nigrescens is also known for the stripping of its bark by the proboscidean. However, this form of disfigurement was not noticed in this area.

Combretum apiculatum (https://www.inaturalist.org/taxa/340059-Combretum-apiculatum) is completely absent from the sampled area, because it is absent from basalt soils. However, S. nigrescens is somewhat like C. apiculatum, in that it

  • has ‘co-evolved’ with the African bush elephant,
  • has extremely dense wood, and
  • manages to live on after being broken, and even uprooted, by the proboscidean.

I doubt that the population of S. nigrescens in the sampled area is reduced in terms of population density by the forces of suppression. I.e. I doubt that the number of individuals would be any greater, were there freedom from damage by megaherbivores.

However, I suspect that the holding of the height of most specimens at 6 m or less is an example of suppression by a combination of physical disfigurement (mainly gross damage by the proboscidean) and intense browsing (including by the southern giraffe).

Wildfire occurs in this area, as evident from the abundance and flammability of Bothriochloa. Senegalia nigrescens is known to be sensitive to fire, taking a long time to recover vegetatively from scorching. (The same probably applies to Vachellia tortilis.)

I would emphasise the importance of megaherbivory here more than the importance of combustion. However, it is undoubtedly true that both megaherbivory and fire have acted together to suppress S. nigrescens on basalt in Kruger National Park.
 
Flueggea virosa (https://www.inaturalist.org/taxa/340143-Flueggea-virosa):

This shrub is so common in this vegetation that it forms a ‘lower stratum’, with a distinctive appearance even as one drives by on the nearby road. It is the only woody plant here that is both common and unsuppressed. Flueggea virosa seems to be naturally multi-stemmed, and it has not been visibly disfigured by the African bush elephant.
 
Sclerocarya birrea (https://www.inaturalist.org/taxa/340245-Sclerocarya-birrea):

Occasional mature trees occur in this vegetation. However, there are no growing individuals, only occasional saplings held down by the African bush elephant and other large herbivores.

So, this species is suppressed in the sense that its population is missing an entire range of sizes intermediate between ‘suppressed sapling’ and ‘mature tree’. Even the suppressed saplings are so scarce that it is easy to imagine that, if the density of population of the proboscidean increases, S. birrea will eventually be extirpated from this area.
 
Combretum imberbe (https://www.inaturalist.org/taxa/340408-Combretum-imberbe):

This species is potentially important for any study of the height or woodiness of vegetation. This is because it can grow so tall on basalt (up to nearly 20 m), and has such extremely dense wood (> 1 tonne per cubic metre air-dry).

Combretum imberbe, while present in this vegetation, is

  • not fully expressed, and
  • does not seem to be regenerating.

It occurs mainly as dead specimens. I saw no saplings or seedlings. Combretum imberbe is found here in suppressed form, as well as the occasional living, mature tree. It is one of the few spp. in this vegetation that is not multi-stemmed when kept suppressed at only 4 m high.

It thus presents a similar case to S. birrea. However, it is closer to extirpation, and this is not necessarily owing to physical damage by the African bush elephant.

Combretum hereroense (https://www.inaturalist.org/taxa/340062-Combretum-hereroense):

This species is too scarce in this area to matter. Any disfigurement by the African bush elephant is thus irrelevant.
 
Combretum collinum (https://www.inaturalist.org/taxa/426116-Combretum-collinum):

This species is scarce in this area. It is suppressed, in the sense that the only individuals seen were extremely small, possibly having been held at the sapling stage for many years by damage by the African bush elephant and other large herbivores.
 
Vachellia tortilis (https://www.inaturalist.org/taxa/489563-Vachellia-tortilis):

This is a significant species in this vegetation, despite its population being sparse relative to that of S. nigrescens. It is suppressed in the sense that the full height and form of the species is never attained in this area. All individuals of V. tortillis here retain the appearance of juveniles.

This species is not particularly disfigured by the African bush elephant. However, my impression was that it is so heavily browsed that it cannot surpass the height of even the greater kudu (Strepsiceros strepsiceros), let alone megaherbivores.
 
Vachellia exuvialis (https://www.inaturalist.org/taxa/595947-Vachellia-exuvialis):

This species is too scarce in this area to matter much. However, it is somewhat suppressed, in the sense that even those few individuals that do occur do not express their full size, owing to browsing pressure from the African bush elephant, the greater kudu, and the southern giraffe.
 
Vachellia grandicornuta (https://www.inaturalist.org/taxa/595949-Vachellia-grandicornuta):

As for V. exuvialis.
 
Vachellia robusta (https://www.inaturalist.org/taxa/559229-Vachellia-robusta):

As for V. exuvialis..
 
Dichrostachys cinerea (https://www.inaturalist.org/taxa/129706-Dichrostachys-cinerea):

As for V. exuvialis. It is noteworthy that this is the only acacia more numerous in the relatively treeless area on basalt, north of Satara (see my other Post), than in the current area on basalt, just southwest of Satara.
 
Dalbergia sp. indet. (https://www.inaturalist.org/observations?place_id=9074&taxon_id=68655&view=species):

As for V. exuvialis.
 
Philenoptera violacea (https://www.inaturalist.org/taxa/340211-Philenoptera-violacea):

As for V. exuvialis.
 
Ziziphus mucronata (https://www.inaturalist.org/taxa/340228-Ziziphus-mucronata):

As for V. exuvialis.
 
Strychnos spinosa (https://www.inaturalist.org/taxa/169414-Strychnos-spinosa):

This species is too scarce to matter much. However, it is noteworthy that the few plants present seem not to be disfigured. I suspect that fire is more important in the suppression of this species than megaherbivores are.
 
Diospyros mespiliformis (https://www.inaturalist.org/taxa/340214-Diospyros-mespiliformis):

This species is certainly present only as ‘saplings’, that are unlikely to produce fruit. These ‘saplings’ are certainly disfigured to some extent by the African bush elephant. However, it is possible that the main factor limiting this species in this area is insufficient water.
 
Grewia possibly bicolor, but unidentified because leaves were absent:

This species forms part of the same ‘stratum’ as Flueggea virosa. However, it presents a contrast to that species, in being extremely suppressed by the African bush elephant and other large herbivores. Unlike F. virosa, Grewia bicolor? is not only much-browsed, but repeatedly broken. This is revealed by stout old stems, substantial enough that, if this species were spared from damage by large herbivores, the plants would attain the size of trees.
 
Ximenia caffra (https://www.inaturalist.org/taxa/340166-Ximenia-caffra):

This species is too scarce to matter much. However, it is somewhat like F. virosa, in appearing free of disfigurement.
 
Euclea natalensis (https://www.inaturalist.org/taxa/430991-Euclea-natalensis):

This species is too scarce to matter much. It is rather nebulous in terms of disfigurement and suppression.
 
Lannea schweinfurthii (https://www.inaturalist.org/taxa/340118-Lannea-schweinfurthii):

This species is too scarce to matter much. However, it presents an interesting contrast with its close relative and look-alike, S. birrea. The only representation of L. schweinfurthii in this area is by old but low individuals, which have been

  • repeatedly broken by the African bush elephant, and
  • browsed by this species and other large herbivores (probably including the plains zebra, Equus quagga).

I suspect that the plants of L. schweinfurthii manage to produce fruits, despite this suppression. However, the main point is that, here again, we have a potentially substantial tree that is not allowed to express arborescent form in this area, and the mechanism of this limitation is certainly damage by large herbivores.
 
Cassia abbreviata (https://www.inaturalist.org/taxa/147000-Cassia-abbreviata):

This is a special species. When it reaches adulthood, it seems virtually immune to damage by large herbivores including the African bush elephant. However, as a sapling it is suppressed. The agency of suppression seems to be the proboscidean, which

  • disfigures the plant by gross breakage, and
  • eats the foliage possibly more as medicine than as food.

Cassia abbreviata nowhere grows in populations dense enough to matter much to the overall height or woodiness of the vegetation. However, it is noteworthy that even such a toxic species is suppressed in this area, and held at the sapling stage.
 
Ficus stuhlmannii? (https://www.inaturalist.org/taxa/340304-Ficus-stuhlmannii):

This species is too scarce to matter much. However, it is intriguing because it manages to grow as a strangler, despite the suppression/scarcity of its ‘host’ trees. There is some deep meaning to this which I have yet to cogitate fully.
 
In summarising so far:

Acacias are abundant on basalt in Kruger National Park. Not only is the dominant tree Senegalia nigrescens, but there are several species of Vachellia, plus Dichrostachys, as scattered individuals.

Despite this abundance, the acacias ALL remain suppressed in this area, not so much in terms of numbers of individuals per unit area, but more in terms of the size and form attained.

This vegetation on basalt is, in a sense, the antithesis of the vegetation onngranite, associated with large mounds east of Phalaborwa (see my other Posts). Whereas that vegetation virtually lacked acacias, this wooded vegetation on basalt is dominated by acacias.

The only individual of any acacia that I observed, in this sampling area on basalt, to have escaped suppression, was the single individual of S. nigrescens that I estimated to have a height of 18 m (possibly a slight overestimate).

None of the other acacias reach their full potential in this area. This includes even V. exuvialis, which has limited potential, because - even at full size - it is a small and spindly as woody plants go. Dichrostachys cinerea, which has well-known potential as a woody encroacher, is kept severely in check on basalt. The other legumes, namely Dalbergia and Philenoptera, show a pattern parallel to that of the acacias. They belong on this substrate, but cannot surmount the damage by large herbivores. Thus, they do not express their full size.

The non-legume Flueggea virosa acts, as it were, as an indicative counterpoint to the acacias.

This member of the Phyllanthaceae is not as palatable or spinescent as the acacias (lacking pinnate leaves and being defended more chemically than physically). It is spared from gross damage, to the point that it has become the only common woody plant in this area to form a non-suppressed stratum. I predict that if the ‘megaherbivory’ were experimentally relaxed to some extent, F. virosa would soon decline in abundance, its niche being usurped by the various acacias.

Both of the rather nebulous strata in this vegetation present evidence of suppression by large herbivores. The fact that the upper stratum is held within the reach of both proboscidean and giraffe is explained by physical suppression. The fact that it is a phyllantha, rather than Vachellia or Dichrostachys, that forms the lower stratum is explained by a sort of ‘compensation’ for the persecution of the acacias by a more chemically defended alternative shrub.

I do not know how Flueggea virosa responds to fire. However, I suspect that this multi-stemmed species is killed above ground, but regenerates rapidly by producing new stems after each fire. These, I suspect, grow within the first subsequent wet season back to the full height of about 1.5 m.

So, I interpret the multi-stemmed growth-form of F. virosa, here, as an adaptation to fire rather than megaherbivory.

As regards S. birrea, this species ‘belongs’ in this area just as much as the acacias do. However, in a sense it is the opposite of F. virosa: relatively defenceless against both megaherbivory and fire.

Were megaherbivory to be experimentally relaxed in this area, I predict that S. birrea would be released from suppression by large herbivores, but subject to suppression by fire. Its niche is to regenerate in cohorts in those occasional times when pressures from both megaherbivory and fire are alleviated (which is why it tends to mark periods when humans have intervened in the past).

DESCRIPTION OF SAMPLE PLOTS:
   
Plot 1: Canopy cover 22.5%, of which the contributions are Senegalia nigrescens 50%, Flueggea virosa 30% (about 1.5 m high), Grewia sp. indet. 15% (1 m high, multi-stemmed, apparently clonal, with old burnt stumps much older than the insubstantial above-ground parts alive today would indicate), Lannea schweinfurthii 5% (1 m high, multi-stemmed). Tallest plant is 6 m (S. nigrescens). In general vicinity are Cassia abbreviata (saplings hammered/suppressed by elephant), Ximenia caffra, Euclea natalensis, Vachellia tortilis, Vachellia exuvialis, Diospyros mespiliformis (the last-named restricted to 2 m high ‘saplings’, in reality old and weathered-looking because hammered by elephant; clumped with other woody plants rather than free-standing), and scattered large (emergent) mature individuals of Sclerocarya birrea. Flueggea virosa is abundant in this vegetation and is consistently about 1.5 m high.

Plot 2: Canopy cover 30%, of which the contributions are Senegalia nigrescens 95% (up to 6 m high, as in plot 1; even the individuals felled or broken by elephant were only 8 m when upright), Philenoptera violacea 2% (individual suppressed by elephant), Combretum collinum 2% (tiny individuals), Dichrostachys cinerea 1% (tiny individuals). Present in the plot is a previously 8 m high individual of S. nigrescens, broken by elephant but not felled. Just outside plot are sapling (suppressed) Cassia abbreviata and ‘sapling’ Diospyros mespiliformis, and a dead individual tree of Combretum imberbe. Nearby but not in plot are Flueggea virosa, Vachellia tortilis (juvenile) and the occasional juvenile of Vachellia grandicornuta. Tallest plant is 6 m (S. nigrescens).

Plot 3: Canopy cover 20%, of which the contributions are S. nigrescens 50% (up to 6 m high, as in the case of plots 1 and 2), Grewia sp. indet. 40% (0.5 m high, hammered, multi-stemmed), Flueggea virosa 10% (about 1.5 m high as usual). Tallest plant is 6m but if plot had been shifted by a mere 10 m it would have been dominated by a huge, emergent tree of Sclerocarya birrea. Also next to but outside plot is Dalbergia sp. indet. (1.5m high, with smooth photosynthetic stem, unlike corky bark of the form of Dalbergia found in the plots near Nyamunda Dam).

Plot 4: Canopy cover 20%, of which contributions are S. nigrescens 71% (up to 6 m high as in plots 1-3, and once again some individuals previously broken or felled by elephant, even the felled individuals having been only 6 m high when originally upright), Flueggea virosa 26% (2 m high), Dichrostachys cinerea 2% (1 m high, only one individual), Vachellia exuvialis 1% (suppressed seedling). Nearby is a strangler fig (Ficus stuhlmannii?). Also nearby are Vachellia robusta (1 m high, hammered by elephant) and Cassia abbreviata (suppressed sapling, i.e. broken by elephant). One uprooted individual of S. nigrescens has brought up many stones of basalt with its exposed roots, from a depth of <0.5 m. Tallest plant is 6 m.

Plot 5: Canopy cover 22.5%, of which contributions are Flueggea virosa 50% (extremely multi-stemmed), S. nigrescens 45% (up to 4.5 m high, not 6 m as in the previous plots), Dichrostachys cinerea 4%, Vachellia exuvialis 1%. Within the plot is a dead individual of Combretum imberbe with bole diameter >25 cm; its height when alive and intact must have been >8 m. Nearby are Combretum hereroense, Strychnos spinosa (one individual, not suppressed), Ziziphus mucronata, and a fully-grown individual of Senegalia nigrescens which I estimated to be 18 m high! In retrospect this may have been an exaggeration but it certainly represented the full height of this species of acacia, exceeding the height of the mature individuals of S. birrea in this area and being of a similar size to mature Combretum imberbe. Tallest plant is 4.5 m (S. nigrescens).

Plot 6: Canopy cover 48%, of which contributions are Senegalia nigrescens  98% (6 m high, several individuals in plot previously felled by the elephant), Dichrostachys cinerea 2% (sapling, only one individual in plot). Nearby is Peltophorum africanum (1 m high, suppressed). Tallest plant is 6 m (S. nigrescens).
 
Plot 7: Canopy cover 10%, of which contributions are Combretum imberbe 48% (overhanging foliage of a tall mature individual located just outside the plot, with bole diameter 75 cm), Dichrostachys cinerea 48% (0.2-1.5 m high), Senegalia nigrescens 4% (absent from this plot except for one felled individual, alive only at the base, with bole diam. 30 cm, and one suppressed sapling 0.5 m high, neither showing any foliage). Near plot is suppressed individual of V. tortilis, 2 m high. Tallest plant unmeasured but may be estimated from the photos (look for the branches of the adjacent tall tree of C. imberbe that overhang the plot).
 
Plot 8: Canopy cover 20%, of which contributions are Combretum hereroense 60% (up to 3.5 m high, 3 individuals), S. nigrescens 30% (5 m high, one individual), Dichrostachys cinerea 10% (suppressed, 0.1-0.5 m high, 4 individuals). Nearby is Flueggea virosa. Tallest plant is 5 m high (S. nigrescens).
 
Plot 9: Canopy cover 22%, of which contributions are Senegalia nigrescens 65% (up to 4.5 m high, two individuals), Combretum imberbe 30% (4.5 m high), Philenoptera violacea 3% (0.5 m high), Dichrostachys cinerea 1% (1.5 m high, suppressed), Vachellia exuvialis 1% (0.5 m high). Nearby are Flueggea virosa, old stumps of Sclerocarya birrea and old dead trees of Combretum imberbe. Tallest plants are 4.5 m high (both S. nigrescens and C. imberbe).
 
Plot 10: Canopy cover 30%, of which contributions are S. nigrescens 80% (up to 6 m high, four individuals of which one felled and another broken), Flueggea virosa 10% (up to 3 m high near plot and up to 2 m in plot, one individual), Dichrostachys cinerea 5% (0.5 m high, one individual), Cassia abbreviata 5% (1 m high, one individual). Near plot is Peltophorum africanum (2.5 m high, hammered and multi-stemmed) and Diospyros mespiliformis. Tallest plant is 6 m (S. nigrescens).
 
Plot 11: Canopy cover 18%, of which contributions are S. nigrescens 65% (up to 4.5 m high, three individuals of which two have been broken so that their foliage now only reaches to 2.5 m high), Vachellia tortilis 30% (4 m high, one suppressed individual), Dichrostachys cinerea 5% (only 3 individual still alive in plot, of which one is dead above 10 cm high). Near plot is Ozoroa engleri? and suppressed sapling of Cassia abbreviata. Within plot are several dead individuals of D. cinerea, felled bole of V. tortilis of diam. 15-20 cm, and old dead stump of C. imberbe. Tallest plant is 4.5 m (S. nigrescens).
 
Plot 12: Canopy cover 22%, of which contributions are Flueggea virosa 30% (1-2 m high, three individuals), Grewia sp. indet. (possibly bicolor) 30% (up to 1.5 m high, three individuals), Combretum imberbe 18% (two suppressed individuals, one of which is 4.5 m high and the other of which is 0.3 m high), Senegalia nigrescens 17% (two individuals, one of which is 4 m high and the other of which is 1 m high and barely alive), Dichrostachys cinerea 5% (1 m high, one individual). Tallest plant is 4.5 m high (C. imberbe).
 
Plot 13: Canopy cover 15%, of which contributions are Combretum hereroense 65% (up to 4 m high, four individuals), Ziziphus mucronata 25% (2.5 m high, one individual), Dichrostachys cinerea 8% (1 m high, four individuals), Senegalia nigrescens 2% (0.3 m, one individual sapling). Near plot are Ozoroa engleri? and Peltophorum africanum. Tallest plant is 4 m (C. hereroense).
 
Plot 14: Canopy cover 15%, of which contributions are S. nigrescens 44% (4.5 m high, one individual), C. imberbe 44% (4 m, one individual), C. hereroense 10% (2 m, one individual), D. cinerea 2% (0.5 m, only one individual, suppressed). There are several dead individuals of D. cinerea in this plot, once again showing that this species has suffered mortality in this area. Near plot is Peltophorum africanum 3 m high. Tallest plant is 4.5 m high (S. nigrescens).
 
Plot 15: Canopy cover 15%, of which contributions are Ximenia caffra 50% (2 m high, two individuals), Flueggea virosa 40% (1.5 m, two individuals), D. cinerea 10% (1 m high, only one individual). Inside plot are dead log of C. imberbe and dead individual of D. cinerea. Near plot are several individuals of V. tortilis, one of them felled but still alive, and big mature tree of Sclerocarya birrea, 15 m high. Tallest plant is 2 m high (Ximenia caffra).
 
PHENOLOGICAL NOTES:

Bothriochloa (see https://www.inaturalist.org/observations/112483387) had not yet started to produce new leaves, despite up to six days having elapsed since the first rain.
 
Senegalia nigrescens varied individually.

I did indeed find evidence that S. nigrescens retained access to water, even after severe drought. One full-size individual (which I estimated to be 18 m high) had anticipated the rains in its foliage growth. In this way it resembled the coexisting large tree, Sclerocarya birrea.

However, most individuals were still bare. Some were shooting foliage, but all the foliage was still early in development.

The ‘emergent’ large trees of both S. nigrescens and S. birrea had produced new leaves well before the first rains fell, which means that their deep roots had access to water despite the drought.

In the case of the main cover of S. nigrescens in the wooded area on basalt, which was about 6 m high, the situation was nuanced.

While some individuals showed early shooting of foliage during my visits on 15-17 Nov. 2016, others had not yet produced any shoots. Furthermore, even the most advanced of these 6 m high ‘reproductive juveniles’ of S. nigrescens was not as advanced, phenologically, as the mature ‘emergent’ of this species. Our game guard, Happy, assured me that these 6 m high juveniles do produce flowers in anticipation of the rains during normal years, even though they failed this year during the drought.

I do not know whether they produce any fruits in a normal year, because, after all, the 6 m high trees remain within reach of defoliation by both proboscidean and giraffe. The few saplings of S. nigrescens that I found in this vegetation were bare.

Peltophorum africanum is an ‘evergreen’ present in this vegetation; some individuals were shooting foliage.

At least one suppressed individual of Vachellia tortilis, only 2 m high, was fully and freshly green, despite the fact that the only herbaceous response to the rain by this time was a few inconspicuous, tiny seedlings of dicotyledonous herbs.

Combretum hereroense was shooting new foliage, probably having anticipated the rains somewhat.

Dichrostachys cinerea seems to have suffered considerable mortality in this area, during the drought.
 
Other deciduous spp. that had anticipated the rains in their foliage growth, in the woody vegetation on basalt, were:

  • Vachellia exuvialis,
  • Vachellia robusta,
  • Combretum hereroense,
  • Ximenia caffra,
  • Cassia abbreviata,
  • Philenoptera violacea, and
  • Strychnos spinosa.

These plants were relatively small: shrubs rather than trees, partly owing to suppression by herbivores. Therefore, it is noteworthy that - despite their limited rooting depth - they obviously managed to find some water at the end of a long drought, a considerable number of days before the first rain fell.

Ximenia caffra is a root hemiparasite. This could help to explain where it gets enough water to anticipate the rains in shooting foliage.
 
Deciduous woody spp. in this woody vegetation on basalt that had not anticipated the rains in their foliage growth, i.e. those which remained totally bare of foliage during my visits 3-5 days after the first rains, were

  • Grewia sp. indet. (which I suspect to be G. bicolor),
  • Dichrostachys cinerea,
  • Ziziphus mucronata,
  • Flueggea virosa,
  • Combretum collinum,
  • Vachellia grandicornuta,
  • Dalbergia sp. indet., and
  • Lannea schweinfurthii.

Certain spp. in the woody vegetation on basalt are ‘evergreen’. The following retained leaves even after this noteworthy drought:

  • Combretum imberbe,
  • Euclea natalensis,
  • Diospyros mespiliformis,
  • Peltophorum africanum, and
  • Vachellia tortilis.

Combretum imberbe in this habitat is evergreen in the qualified sense that the plants, whether juvenile or mature, may have rather sparse foliage during the dry season. Even in this severe drought, they did not become bare, as did all the other spp. of Combretum.

Vachellia tortilis is in a category of its own among acacias in its phenology. This is because it retains leaves even during the dry season – and proved true to this pattern even after this severe drought. This applied just as well to the relatively small individuals, many of them visibly damaged by the African bush elephant, that occurred scattered among the dominant S. nigrescens in the wooded vegetation on basalt.

There is a noteworthy distinction between the ‘evergreenness’ of V. tortilis and that of Ebenaceae. The former has short-lived leaves that are relatively frequently replaced even during the dry season, in contrast to the tougher, longer-lived leaves of Ebenaceae.

Peltophorum africanum may possibly have anticipated the rains in shooting new foliage, despite being evergreen. This would makes sense, because the leaves retained in drought – instead of being green as in V. tortilis – were faded and tired-looking.
 
Dichrostachys cinerea seems to have suffered considerable mortality in this vegetation, although I cannot say whether these individuals died in this drought or before it.
 
So, in summary:

It is true that the main trees and shrubs in this woody vegetation on basalt are deciduous, remaining bare of leaves for the dry months of winter. However, there are three categories of woody plants that show the availability of water in this substrate, even after severe drought.
 
Firstly, there are the three ‘emergent’ spp., particularly Senegalia nigrescens and Sclerocarya birrea. These, at least in the case of full-size specimens, >15 m high,

  • manage to find enough water to flower at the height of the dry season, and then (about a month later in the case of S. nigrescens)
  • also manage to find enough water to shoot foliage in anticipation of the first rains.

Please note that my observations on felled specimens showed that neither S. nigrescens nor S. birrea possesses a taproot. So, it is impressive that they manage to find enough water to defy the drought in their fresh shooting after their regular annual period of dormancy.

Combretum imberbe conforms to this pattern in the sense that it, too, bears leaves at the height of the drought. The difference is that it is never bare.
 
Secondly, there are sundry spp. that manage somewhat to anticipate the rains in their foliage growth, despite being small and presumably shallow-rooted, and, in some cases, present only in a form suppressed by the African bush elephant. An example of Combretum hereroense. 
 
Thirdly, there is Vachellia tortilis.

This acacia is known to have shallow, spreading roots rather than deep roots. It is typical of the basalt substrate, even though, in the wooded vegetation referred to here, the species

  • was neither dominant not emergent, and
  • was greatly eclipsed by S. nigrescens in both plant size and abundance.

The fact that V. tortilis managed to keep producing leaves, even in such a severe drought, is significant.
 
INTERIM CONCLUSION:

There was indeed considerable evidence of ‘surplus’ water in this woody vegetation on basalt, just southwest of Satara, even after the severe drought of 2016.

Furthermore, this is relative in a way correlated with the height of the vegetation.

On one hand, there was less evidence of ‘surplus’ water than I found in the relatively treeless vegetation on basalt north of Satara (see https://www.inaturalist.org/journal/milewski/68875-phenological-evidence-of-relationship-between-water-and-height-of-vegetation-on-basalt-in-kruger-national-park#).

On the other hand, there was far more evidence of ‘surplus’ water than I found on Ecca, a few km away – where Senegalia burkei showed no anticipation of the rains of the sort described above, and the other woody plants in the community followed a consistent pattern of remaining bare.

I therefore attribute this difference, between basalt and Ecca, to the far greater height and density of the vegetation on Ecca, which seems to have exhausted the water supply.

Please also see https://www.inaturalist.org/journal/milewski/68875-phenological-evidence-of-relationship-between-water-and-height-of-vegetation-on-basalt-in-kruger-national-park#.

Anotado por milewski milewski, 04 de agosto de 2022 a las 05:11 PM

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As a zoological footnote to this study:
This woody vegetation, although not dense in the scheme of things, was dense enough to attract two cover-dependent Carnivora: African civet (the faeces of which I saw) and side-striped jackal (spotted during our fieldwork, in broad daylight). Neither of these spp. would occur in the relatively treeless vegetation on basalt, north of Satara, where the black-backed jackal was instead common.

Anotado por milewski hace 2 meses (Advertencia)

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