Journal archives for August 2022

03 August, 2022

Information on wood density in the mimosoid legume shrub, Dichrostachys cinerea

@tonyrebelo @jeremygilmore @wynand_uys @troos @botaneek @graham_g @richardgill @jan-hendrik @ludwig_muller

An article by Fernandez et al. (2015) presents values for the wood density of Dichrostachys cinerea (

These were determined on an OVEN-DRY basis, instead of the usual air-dry basis.

Oven-dry values somewhat underestimate the air-dry values. However, the difference should not be much more than 10%, because air-dry wood contains only about 10% water.
Stems with diameter <2 cm have oven-dry wood density 820 kg per cubic metre, whereas stems with diameter >2 cm have oven-dry wood density 840 kg per cubic metre.
These values are more or less in line with the information I gave in a previous Post.

I have not calculated the likely air-dry values. However, they sound like about 900-940 kg per cubic metre, the former value probably lacking any heartwood, and the latter value including a narrow core of heartwood. (Fernandez et al. 2015 do not mention the question of heartwood at all.)
The paper compares wood density for D. cinerea (including bark) with those for various other taxa of trees (including bark). Again, all values are oven-dry.

The comparison is worthwhile, despite being somewhat complicated to relate to data elsewhere.

Values are: Dichrostachys 780, Populus 360, Salix 600, Eucalyptus 540, Allocasuarina 650, Leucaena 430, and Paulownia 270 (all values include bark).
My commentary is as follows.
Eucalyptus has a pattern, which I noticed many years ago when cutting down saplings in my garden, of having soft wood in the saplings despite the hard wood in the fully-formed mature stems.

This explains why the wood density is only 540 in Eucalyptus compared with 600 in Salix.

This pattern of ‘juvenile softness’ is not so marked in the case of Allocasuarina (

Populus and Paulownia ( are extremely light-wooded trees, growing extremely rapidly, and useful as plantations. This is because the wood, although not dense (wood density air-dry of only about 300-400 kg per cubic metre), is suitable for various objects, and can produced quickly.

On the basis that it is an angiosperm, not a gymnosperm, Paulownia has a reputation as ‘the fastest-growing hardwood’. This genus originates in China, and is grown for its light but durable wood. This is used for boat-building and making surfboards.

The wood density of Dichrostachys is 2.9-fold greater than that of Paulownia.

Salix, although related to Populus, has somewhat denser wood, which is actually denser than that of Sclerocarya and Lannea.

Leucaena, although a legume and thus related to Dichrostachys in that way, has wood far less dense than that of Dichrostachys. In terms of air-dry wood the respective values would be about 470 for Leucaena vs 860 for Dichrostachys (bear in mind that in both cases these values include bark).
The bottom line seems to be that Dichrostachys does indeed have dense wood for such a fast-growing, weedy plant, whether compared with poplars and willows, eucalypts and casuarinas, or other legumes.

Indeed Dichrostachys seems to achieve, in mere sticks with diameters of a few cm, the kinds of densities achieved as mature boles in plantation eucalypts.

Of course, many naturalists in South Africa value sticks of Dichrostachys as fuel for barbecues. However, this paper helps to quantify why these sticks make such good 'coals' for roasting meat.
Putting the bottom line into an even more meaningful generalisation:

Dichrostachys as a ‘woody weed’ (e.g. in Kruger National Park) has a SHRUBBY growth-form instead of a tree form. However, its great ‘woodiness’ makes up for the thinness of its stems, w.r.t. 'biomass' (which is actually partly necromass) and carbon-content.

Dichrostachys is

  • a rather spindly plant,
  • always multistemmed = shrubby (in an inverted cone shape, rather than having a bole), and
  • limited in height (usually only a few metres, despite its potential as a species of growing to about 7 m high),

However, the amount of wood contained in this plant is surprising.

So, any measurements of stem diameters are only part of quantifying this formidable woodiness, and would not do the plant justice without including a factor for wood density.

Posted on 03 August, 2022 04:17 by milewski milewski | 0 comments | Leave a comment

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

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 ( 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 ( 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 ( and, 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 ( of the plants, as evidence for or against water as a controlling factor.

The Park near Satara ( contains low vegetation on basaltic soils, and tall vegetation on soils derived from Ecca sediments ( (This geological type occupies only small areas in Kruger National Park, not evident at the scale seen in

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 ( 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 ( 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 ( 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: and

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 (, 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 (
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, 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 ( here on Ecca were dead. It is also noteworthy that

Furthermore, I did not record a single living specimen of 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 ( 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 (, 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, 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,, 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 (, 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,, 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,, Flueggea virosa 3% (0.3 m high, one individual), Maerua parvifolia 2% (0.4 m high, one individual, a few leaves still present,, 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, 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,, 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,, 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).
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 03 August, 2022 05:48 by milewski milewski | 0 comments | Leave a comment

04 August, 2022

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

@jeremygilmore @ludwig_muller @tonyrebelo @troos @botaneek @joshua_tx @jrebman @cwbarrows @grnleaf @richardgill @wynand_uys @charles_stirton @mr_fab @graham_g @marcoschmidtffm @sedgesrock @andrew_hankey @adriaan_grobler @careljongkind @alexdreyer @rob_palmer @reubenheydenrych @alastairpotts

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 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.

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 ( 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 (

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 (

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 (

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 (

This species is too scarce in this area to matter. Any disfigurement by the African bush elephant is thus irrelevant.
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 (

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 (

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 (

As for V. exuvialis.
Vachellia robusta (

As for V. exuvialis..
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. (

As for V. exuvialis.
Philenoptera violacea (

As for V. exuvialis.
Ziziphus mucronata (

As for V. exuvialis.
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 (

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 (

This species is too scarce to matter much. However, it is somewhat like F. virosa, in appearing free of disfigurement.
Euclea natalensis (

This species is too scarce to matter much. It is rather nebulous in terms of disfigurement and suppression.
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 (

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? (

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).

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).

Bothriochloa (see 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.

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

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

Posted on 04 August, 2022 17:11 by milewski milewski | 2 comments | Leave a comment

06 August, 2022

Woody plants surrounding treeless lawns on sodic substrates in southern Kruger National Park, part 2: adaptations to intense herbivory at the edges of the grazing lawns

@jeremygilmore @ludwig_muller @tonyrebelo @troos @botaneek @joshua_tx @jrebman @cwbarrows @grnleaf @richardgill @wynand_uys @charles_stirton @mr_fab @graham_g @marcoschmidtffm @sedgesrock @andrew_hankey @adriaan_grobler @careljongkind @alexdreyer @rob_palmer @reubenheydenrych @ricky_taylor @robertarcher397 @alastairpotts

...continued from


The following species of woody plants occurred in the plots in treeless vegetation, on the sodic patches along the N’waswitshaka River in Kruger National Park.

All woody plants in these plot were <10 cm high, except where otherwise specified.

I include an unidentified faboid legume, although this might not be regarded as a woody plant. My visit was during drought, preventing identification. @troos any ideas?

The species recorded would not seem to be a candidate for a woody plant, had I seen it in other vegetation systems in Kruger National Park.

I exclude a pliable, quasi-karoid perennial that was frequently encountered in these plots (as well as the treeless plots on Ecca sediments near the Orpen Road southwest of Satara). The latter is probably about as ligneous as the faboid sp., but was invariably extemely low (<10 cm).

I could not distinguish Vachellia grandicornuta from V. exuvialis in the extremely suppressed form of acacias found in these treeless plots. However, I assume V. grandicornuta, simply because it was this species that was prominent in the adjacent woody vegetation.

After each plot number I give the total woody cover, assessed as a percentage of the surface area of the plot.

Plot 9 (approx. 1 % as usual including Indigofera): Vachellia grandicornuta, faboid sp., Grewia bicolor (40 cm high), Grewia hexamita (40 cm high) (also see notes of Jessica & Zurelda) 
10 (see notes of Jessica & Zurelda) 
11 (< 1 %): faboid sp., Vachellia
12 (< 1 %): Gardenia volkensii, Dichrostachys cinerea
13 (< 1 %): Dichrostachys cinerea, faboid sp. 
14 (5 %): Dichrostachys cinerea (up to 20 cm high), Vachellia grandicornuta, faboid sp. 
15 (0 %): no woody plants whatsoever 
16 (0 %): no woody plants whatsoever 
17 (< 1 %): Vachellia grandicornuta, faboid sp. (up to 40 cm high)  
18 (0 %): no woody plants whatsoever 
19 (< 1 %): Dichrostachys cinerea, Vachellia grandicornuta 
20 (< 1 %): Vachellia grandicornuta, faboid sp. 
22 ( 1.5 %): Senegalia nigrescens, Vachellia grandicornuta, Dichrostachys cinerea (these three spp. totalling 27 individuals in plot), faboid sp. 
23 (2.5 %): Senegalia nigrescens, Vachellia grandicornuta, Dichrostachys cinerea (these three acacias totalling 14 individuals in plot), faboid sp. (particularly numerous in this plot) 
24 (1 % for acacias and 2 % for faboid sp., totalling 3 %): Senegalia nigrescens, Vachellia grandicornuta, Dichrostachys cinerea (these three acacias totalling 47 individuals), faboid sp. (hundreds of individuals) 
25 (1 %): Vachellia grandicornuta, Dichrostachys cinerea (10 individuals) 
26 (0 %): no woody plants whatsoever 
27 (< 1 %, or more precisely <0.1 %): Dichrostachys cinerea (only one individual, <10 cm high as usual) 
28 (< 1 %): Vachellia grandicornuta (6 individuals), Indigofera sp.
29 (< 1 %): Vachellia grandicornuta, Dichrostachys cinerea (approx. 10 individuals), faboid sp.

These results mean the following.

The tallest woody plant in these plots was only 40 cm high, and most individuals were only <10 cm high.

The greatest canopy cover of woody plants was 5 %, and several plots scored 0%.

My estimate of the average canopy cover for these 20 plots would be <1 %, and this consisted of plants generally 5-10 cm high.

This reflects the extreme suppression of the acacias, and the marginally woody (= arguably herbaceous) status of faboid sp. I do not know how tall this faboid species is capable of growing if protected from herbivory. To estimate canopy cover excluding the faboid sp: this would be about half of that including the faboid sp. However, the exact figure matters little, as it would remain as <1 %.

In summary so far:
It is noteworthy that there is no ‘woody flora’ – even a distinctive flora of small shrubs - associated with this treeless lawn vegetation on sodic patches along the N’waswitshaka River. The only woody species (apart from the questionably woody faboid sp.) are suppressed individuals of species common in the adjacent shrubland/savanna/woodland/forest.


One of the most intriguing findings of this study is the vegetative modifications seen in, and at the edges of, the treeless patches.

These reflect the suppression of woody plants by herbivores, particularly the impala.

In the case of V. grandicornuta, I am surprised how miniaturised the plant can remain in its suppressed form on the lawns.

The stipular spines of this species grow to several inches long in fully-formed specimens, browsed by the greater kudu (Strepsiceros strepsiceros) and the hook-lipped rhino (Diceros bicornis). By contrast, the miniatures have tiny spines and leaves, even when the specimen is plainly old enough to have a small woody rootstock. The plant is suppressed not only in size, but also in form. (I have noticed a similar phenomenon, elsewhere, also for Vachellia exuvialis.)

This miniaturisation applies also to S. nigrescens. However, this is less striking, because the prickles are not as miniaturised as are the stipular spines of V. grandicornis.

Dichrostachys cinerea warrants particular mention.

The species frequently occurs as tiny individuals in the treeless plots, as in the case of the true acacias. However, these tiny ‘saplings’ are spineless. The spines of D. cinerea are essentially extremely stiff twigs. It stands to reason that these cannot be miniaturised, as can the stipular spines of Vachellia.

However, the result is a considerable difference in form between these suppressed ‘saplings’ and grown-out individuals: seemingly structurally defenceless against the impala in the case of the former. It is only with practise that D. cinerea can even be recognised in the treeless plots.

In the case of S. africana, the growth-form is much more interesting than conveyed by any simplistic portrayal of the species as chemically defended.

The form of spinescence is similar to that in Dichrostachys. However, it is induced by browsing, disappearing as the plant surpasses the reach of the southern giraffe (Giraffa giraffa).

There is a certain scope for miniaturisation in this ‘quasi-spinescence’, which exceeds that in D. cinerea.

The 'spines' are sharp-pointed, leaf-bearing twigs at approximately right angles to the stems bearing them. These ‘spines’ are smaller at height <0.5 m than at height >1.5 m, showing remarkable adaptive plasticity.

Furthermore, the form of the whole plant (probably just a ramet,  is surprisingly modified/distorted/suppressed near the edges of the treeless vegetation. Here, S. africana ‘pioneers’ as virtual ‘hummocks’, in some cases only 0.5 m high, and far broader than tall, shaped and maintained by the impala.

I observed a similar pattern in Terminalia prunioides ( This species did not occur in our sample plots, but was noted in passing, in the vicinity. Please see and and

In the case of Euclea divinorum, E. natalensis and Diospyros mespiliformis, there is no noticeable capacity for miniaturisation. Although individuals can be suppressed in the size of the plants, this is not reflected in the growth-forms. These species are defended from herbivory chemically (e.g. by means of tannin), not structurally.

Posted on 06 August, 2022 00:38 by milewski milewski | 0 comments | Leave a comment

07 August, 2022

mopane vega

(writing in progress)
I hypothesise as follows.
In the original situation, with the migratory system intact, this vega would have been free of the sagebrush (Pechuel-loeschea leubnitziae) as well as Bothriochloa.
With the demise of the migratory system (particularly the great densities seasonally of eland as well as gnu and zebra) the vega has been partly encroached by the sagebrush and the grass stratum has been encroached by Bothriochloa. This is now taken as ‘natural’ because the original migration has bee forgotten so completely that it is not even missed, i.e. scientists don’t even suspect that it once played an important role here.
Philenoptera violacea and Vachellia tortilis have not encroached, remaining at more or less their original numbers. This is because the current population density of the elephant is great enough to keep these woody plants in check, given that they are favourite targets of the elephant. The relationship of these two species to the elephant differs: P. violacea tends to be disfigured more and eaten less, while V. tortilis tends to be disfigured less and eaten more. However, the effect is similar in this context and that is that neither of these trees amounted to much in the original situation and neither amounts to much today, in terms of having usurped the habitat of the original sweet grasses.
Flueggea virosa has not encroached either; this is interesting because it is only too easy to image this vega choked with F. virosa in the way it is partly choked by the sagebrush. Here, the reason I give is that F. virosa is readily eaten by the impala as well as kudu and probably hook-lipped rhino (although obviously avoided by the elephant) and it is not a plant that thrives under even moderately intense herbivory (relying more on toxicity than on spinescence). Flueggea virosa may have encroached a bit on the knobthorn savanna west of Satara Rest Camp, but there is no sign of such encroachment anywhere in the Letaba area.
It is interesting that I did not even see Dichrostachys cinerea in this vega, i.e. it seems to be completely absent here to this day. Whereas D. cinerea seems to have encroached on gabbro in the southern Kruger Park, and it certainly has also partly encroached on basalt e.g. near Lower Sabie Rest Camp, it has completely failed to encroach in this vega. The best explanation I can offer at the moment is that D. cinerea cannot tolerate even slight seasonal waterlogging.
I doubt that the lack of migratory ungulates has made much difference to the nutritional regime at the level of what we have been analysing in the soils; I see the difference more at the mechanistic level of biomass-reduction by trampling and eating. In other words, I doubt that the encroachment by the sagebrush and Bothriochloa can be ‘fixed’ by fertilisation; instead what would ‘fix’ it is amelioration of the intensity of grazing and trampling by certain herbivores.
Were the elephant to be reduced or removed from this system, I predict that both P. violacea and V. tortilis would encroach on this vega, converting it to a savanna.
In terms of the practicalities and tactics of completing a good study, in which various loose ends are tied up, I suggest organising for the following two bits of data to be gathered while we still have a chance.
Firstly, we really should establish which species of Bothriochloa it is that grows in this vega. There are at least two possible spp. but it’s really not good enough that at the moment we can only say ‘Bothriochloa sp.’. Perhaps we could ask Tercia or Michele to collect this grass from this location next time they drive past with a game guard? (Also someone should go back to the original description by U. de V. Pienaar to see if he actually names the species of Bothriochloa in his description).
Secondly, we really should find out how successful the square-lipped rhino was in this vega during the heyday of this species of rhino in Kruger Park about a decade ago. The square-lipped rhino never became as successful in the Letaba area as farther south in the Park, but it is possible that at one stage before poaching (and possibly even today) this vega was fully inhabited by this species of rhino. Various people in the Park will know the answer and I suggest we ask them before the memory fades. The first person I’d ask is the section ranger, Andrew Desmet, who has an impressively long record in the Park and will probably be able to answer our question right off the top of his head if we make sure we phrase it clearly with sufficient context. The context is not just ‘does the white rhino occur in that drainage line’ but more ‘how common is the white rhino in that drainage line in the scheme of things, i.e. relative to its general occurrence in the northern Kruger Park and relative to its general occurrence in the southern Kruger Park, in both cases before the current spate of poaching?’

I think we can eliminate the simple explanation I suggested.

We cannot explain the commonness of Bothriochloa in the mopane vega north of Letaba as a manifestation of any sort of phosphorus-poverty. The reality is that here we have a situation where a sour grass has come to be abundant in a sweet environment. This is analogous to a partly run-down farm, i.e. an intensely disturbed environment where, for one reason or another, the most is not being made of the soil by the regime of consumption by animals, and a relatively unproductive and ‘stagnant’ plant has been allowed partly to ‘take over’ along the lines of an indigenous ‘noxious weed’. I’m come full circle to the explanation I suggested already more than four months ago, when I first saw how choked-up with Bothriochloa the mopane lands on basalt are in Kruger Park. This explanation is that the northern part of Kruger Park is no longer grazed as Nature intended, by a migratory system, and that as a result the most nutrient-rich environments such as this vega have languished somewhat, their productivity being partly stifled by what amounts to ‘mismanagement’. I.e. Bothriochloa is here not as a ‘healer of overgrazing’ (although that is likely to be the case in places in the southern half of Kruger Park, where there is still something closer to the original migratory system). On the contrary, Bothriochloa is here as a symptom of not enough grazing; through our extirpation of a whole migratory system of herbivory we’ve managed to sour a sweetveld, and none of the burning regimes maintained in the mopane lands in the last half-century have compensated for the basic problem that the best lands in northern Kruger Park are no longer grazed intensively enough during the green season. I suggest to you that in the original situation, a thousand years ago and perhaps even several centuries ago, this vega north of Letaba would have had so many large grazers on it, in the form of not only gnu and zebra but also eland and possibly an extinct gazelle, that Bothriochloa would have been absent. For that matter, the sagebrush would have been virtually absent too, for its proliferation is in line with that of Bothriochloa even though its niche is far more restricted in Kruger Park. 
Subject: simpler explanation for: corrected version of: Kruger analysis: first set
On second thoughts perhaps the rationale could be as simple as this, in order to answer the question: ‘why was Bothriochloa, a sour grass, so common in an area P-rich enough to exclude trees?’
Does the following answer make sense? The P/B ratio is great enough to exclude trees, but the absolute availability of P is not enough to support ‘sweet’ grasses.
Is it the case that the major cause of treelessness in the Kruger Park, which seems to be P-richness, could not possibly apply to ‘sour’ grasslands in southern Africa? These grasslands, common on the Highveld, cannot conceivably be P-rich given that the grasses are so nutrient-poor. So I presume that a major cause of treelessness in sour grassland is a too great P/B ratio, although the absolute values for P and B will be much smaller than those in Kruger Park?

One take on the Pienaar grasslands is that P as a catabolic nutrient and B as an anabolic nutrient are controlling the major difference in veg height between the grassland and the tall mopane.

This is perhaps the most mundane/prosaic possible result, lacking any kind of conceptual elegance along the lines of trace elements, catalysis, etc. However, the picture is all the more meaningful for that. Phosphorus is in a sense the ‘master nutrient’ (as indicated by the name ‘superphosphate’ and its effects have played out in the difference between the relatively sterile mopane country and the Pienaar grassland (which I do not think of as a dambo because I don’t see it as seasonally waterlogged). Fertilise the mopane soil with enough P and the grasses ‘outcompete’ the mopane, attracting large mammals which keep recycling the phosphorus. The role of B is a wonderful eye-opener.

It is a surprise to find that basalt is relatively P-poor and relatively B-rich, because this is the opposite of what I would have predicted, based on a) my general concept that basalt is a P-rich rock and b) the fact that basalt can be 20-fold poorer than shale in B.

If we take these mopane vegas as essentially alluvial, somehow what seems to have happened is that the P in the surrounding mopane soils has been washed into and concentrated in the vega (which is essentially a drainage line) but the boron has not! Why not? It is not just the concentration of P in the vega, but also the corresponding ‘depletion’ of the B, that has allowed grasses to ‘outcompete’ woody plants in the vega, not so? But which could be the geomorphic/landform processes leading to this inversion of the P/B ratio between the general, surrounding mopane on basalt and the drainage line? The nutrient picture we’ve discovered is simple, but all the more remarkable for that because it seems that such a simple and gross characterisation of the soils has escaped all other ecologists in Kruger Park? What we’ve found here is that the difference between mopane and grassland is a matter of the P/B ratio. One could put it differently as follows: what is wrong with most of the mopane lands, which leads to them being choked up with mopane instead of having treeless grassland attractive to grazing mammals, is that they are too P-poor (and too B-rich).
There are at least three other aspects of these mopane vegas that may be worth considering. Firstly, this was one of the few situations where I spotted signs of the mole-rat, Cryptomys, in Kruger Park. Note the agricultural analogy, with this rodent ‘ploughing’ the soils in a way that it is presumably not doing in the surrounding mopane. Secondly, an important component of the vegetation in this ‘mopane vega’ was the daisy shrub Pechuel-loeschea leubnitziae, which is an obscure plant for even those who are otherwise knowledgeable about the flora of Kruger Park but which is quite significant biogeographically and ecologically. This species is effectively a ‘sage’ in the sense of ‘sagebrush’ in North America, i.e. an aromatic daisy shrub associated with nutrient-rich soils. Because daisies are generally associated with soil disturbance, it is significant that, in a land of grewias and acacias and flueggeas etc. etc. etc. it is instead a ‘sage’ bush that forms a ‘buffer’ between the nearly treeless grassland and the mopane. This indicates how distinctive this nutritional situation is, of having a switch in the P/B ratio from the mopane to the drainage line. As I’ve previously mentioned, part of the fascination of this sage bush for me is having observed the plains zebra eat it with gusto during a drought. It is  obvious that the elephant, the hook-lipped rhino and most of the bovids are not eating this shrub and there is little surprise there because the thing about daisies is that they tend to be aromatic and thus unpalatable. But the zebra has its own niche in all of this and the valid generalisation that equids are specialised grazers hides this interesting nuance, this interesting ability of the monogastric grazer to exploit ‘sage’ as if it were ‘the grass you have when you don’t have grass’. I now realise how much we’ve lost in not being able to observe how the extinct quagga grazed the Karoo lands, where it no doubt remained mainly a grazer but also ate certain asteraceous shrubs, perhaps Pentzias, as part of its diet at certain times. Thirdly, it seems highly significant that even in this extremely fertile situation it was still Bothriochloa that patchily dominated. I would assume that these mopane vegas were more heavily grazed, seasonally, before say 1800 than they have been for the last century; I suspect that a former migration of zebra, gnu, eland and perhaps an extinct species of gazelle vanished undocumented from the mopane lands of what is now Kruger Park, and that one of the results of this loss has been the increase in Bothriochloa as a sign of ‘stagnation’ in the ‘natural’ regime of consumption on these exceptionally fertile soils. I.e. I might be able to understand that the mopane soils, with much B but relatively little P, have become choked with Bothriochloa; but to find this same grass so common on the vega soils really tells me that something is wrong with the modern regime of herbivory in Kruger Park.

If we take Bothriochloa as a sign of ‘healing’ of the veld, what could it possibly be healing from? The alternatives are a) overgrazing and b) stagnation, which are essentially opposites. I don’t see the Letaba area as overgrazed (in the way I might see the Satara area), so stagnation seems the more likely problem. How can a soil be so fertile as to convert mopane to treeless grassland, yet retain an aspect of infertility that leads it to be ‘wasted’ on a grass as poor as Bothriochloa? Where is the dystrophy in this soil that leads it to produce a rubbish grass instead of just ‘sweet’ grasses and lawning grasses?]

(writing in progress)

Posted on 07 August, 2022 01:34 by milewski milewski | 0 comments | Leave a comment

Why did humans take so long to evolve?

@tonyrebelo @jeremygilmore @ludwig_muller @paradoxornithidae @beartracker @botswanabugs @alexanderr @magdastlucia @chewitt1 @dejong @jwidness @raymie @jacob_dirsuwei @ungulateunion @gingko_biloboa1 @cullin @kokhuitan @zarek @loarie

One of the enduring mysteries of the evolution of humans is why it took so long for us to evolve: more than 60 million years after the origin of primates.

Primates ( arose early among mammals. In most ways, their anatomy remains primitive. This includes Hominidae (

Humans have specialised hind feet ( However, in other respects we remain primitive and unspecialised relative to other mammals, in anatomical terms.

Therefore, it is puzzling that humans did not evolve already in the Miocene ( or Pliocene (

The unspecialised, primitive anatomy of humans can best be seen relative to cetaceans. The latter are obviously extremely modified relative to the ancestral mammals that arose in the time of dinosaurs (,the%20pen%2Dtailed%20tree%20shrew.).

Furthermore, even the anatomy of the brain is more modified in cetaceans than it is in humans ( and and,Homo%20%5B3%2C%204%5D. and

On land, various important lineages of mammals, such as chiropterans (, perissodactyls (, and proboscideans (, are incomparably more modified anatomically than are primates, including humans.

The human pes (, although unique among mammals, is not as modified as the manus ( of bats (

The feet of equids (, which are unguligrade ( plus reduced to one remaining claw per foot, are far more modified anatomically than any part of the human body.

The proboscis ( and and dentition of proboscideans are far more modified anatomically than any part of the human body.

And yet cetaceans (including forms with braininess similar to that in humans, and and and and, chiropterans, equids, and elephant-like forms evolved before hominids did.

The human hand, and the opposability of its digits, are functionally remarkable, as manifestations of the unrivalled technical ability of humans.

However, the manus itself is hardly different, anatomically, from those of the first amphibians to crawl out of the primeval slime. The advances have been neurological rather than anatomical, i.e. matters of software rather than hardware.

Furthermore, many kinds of amphibians have some dexterity ( Some show opposability ( comparable to that seen in hominids.

It is easy to assume that there was some special contingency in Pliocene Africa, without which hominid-like primates could not have begun to arise (

However, American monkeys ( seem in various ways (apart from their currently limited body size) to be even better-suited to 'human' ancestry than apes are.

Many of the American monkeys have heads reminiscent of miniature humans ( and and and and

Cebidae ( are brainier than most apes, with a capacity for tool-use.

Furthermore, the anatomical modifications for brachiation (, in certain American monkeys, would seem somewhat preadaptive for bipedality.

Humans are unrivalled, in the animal world, in our ability to invent and apply tools (including weapons) and artificial constructions.

There are at least three levels of adaptation involved in the evolutionary progression of animals, viz.

  • anatomical,
  • cognitive (depending on the size of the brain, rather than its anatomy), and
  • cultural (particularly facilitating social spreading of technical innovations).

Humans are anatomically unadvanced, except somewhat in the case of the foot. However, our cognitive capacity is advanced (albeit not necessarily more than that of certain cetaceans). And our cultural advancement is so far beyond that of other animals that we are in a category of our own.

The basic evolutionary innovation of humans is technical development, beginning with the manipulation of fire for cooking.

Opportunities to benefit from a cultural-technical strategy have presumably existed ever since any mammal evolved to a certain level of braininess.

The main advances of human evolution have been matters of neurological software and cultural versatility. Therefore, it would seem possible for these advances to have been made as soon as any primate lineage achieved a level of braininess seen in many clades of animals today, including

Given the evolutionary advantages conceivable in a wide range of biomes at various times in the last 50 million years, why could these cultural-technical advances not have been produced by natural selection already, say, 30 million years ago - instead of beginning only five million years ago with the appearance of Australopithecus (

Posted on 07 August, 2022 01:40 by milewski milewski | 6 comments | Leave a comment

Even during drought, savannas on basalt and granite in Kruger National Park underutilise their groundwater

@tonyrebelo @jeremygilmore @ludwig_muller @wynand_uys @troos @alastairpotts @adriaan_grobler @richardgill @graham_g @sedgesrock @paradoxornithidae @beartracker @doug263 @craigpeter @andrew_hankey @magdastlucia @williammcfarland

In several Posts, I have reported my observations on the responses of vegetation to the drought of 2016, in Kruger National Park in South Africa. The phenology of the woody plants on three substrates (Ecca, basalt, and granitic) indicates that the height and density of woody plants is not necessarily determined by the the supply of water to deep roots.

Other aspects of the environment seem to be more important than water. I refer in particular to the nutritional regime in the soil, and the associated regime of herbivory and suppression of woody plants by large mammals ( and and and

The nutritional regime on Ecca substrate in Kruger National Park strongly favours woody plants over herbaceous plants. Thus, the biomass there is maximally expressed relative to the rainfall and the dry-season regime: a woodland of trees about 8 m high, dominated by Senegalia burkei, which remained bare of flowers and leaves until the drought broke.

This vegetation, although providing food for the impala (Aepyceros melampus) by day during the rainy season, does not show much obvious damage by the African bush elephant.

Senegalia burkei tends to die if its bole is broken by the proboscidean. However, this breakage seldom happens. Instead, the height and spacing of trees and tall shrubs on Ecca substrate seem to represent what - in old textbooks - might be called the ‘climax vegetation’ for this climate, which is characterised by mean annual rainfall of about 575 mm, with a dry season usually lasting half of the year.

The trees on Ecca substrate grow to the maximum height sustainable under this rainfall on deep soil. The dominant woody plants are fully dormant during dry seasons, after virtually exhausting the groundwater available to their deepest roots.
This situation can serve as a ‘standard’ against which to compare two types of savannas in Kruger National Park: savanna on a basalt plain, and savanna on the ridgetops of undulating granitic terrain.

The former is dominated by Senegalia nigrescens, and the latter is dominated by Terminalia sericea and Combretum apiculatum.

In both situations, the African bush elephant routinely persecutes the dominant woody plants, by breaking the boles and, in the case of C. apiculatum, uprooting the whole plant.

The mean rainfall in savanna of S. nigrescens on the basalt plain is about 550 mm, while that on the granitic ridgetops east of Phalaborwa is about 475 mm. Both are similar enough to that on Ecca substrate that we would not expect major differences in the height and spacing of woody plants, given that the average length of the dry season is about six months throughout ( 
What my fieldwork in Sept.-Nov. 2016 showed was that

  • the woody plants in these two savannas are far lower and more spaced-out than those on Ecca substrate, and
  • this is associated with underutilisation of groundwater by the relatively low and open vegetation, even during drought.

In the case of the basalt plain, S. nigrescens retains a single bole, but is often broken by the African bush elephant (tending to survive this disfigurement, unlike its close relative S. burkei). Even the unbroken individuals seem suppressed by herbivory. This is because they seldom exceed 6 m high, despite the species being capable of reaching 15 m under this rainfall.

In the case of the granitic ridgetops we sampled east of Phalaborwa, T. sericea never retains its normal bole, being converted to a multi-stemmed shrub by the African bush elephant. Coexisting C. apiculatum, which has a natural tendency to be multi-stemmed, is so often uprooted by the proboscidean that a considerable proportion of the population lives in a prone position. The branches lie on the ground, but continue to shoot foliage and to produce seeds.
Senegalia burkei, and most of the associated woody species on Ecca substrate, remained bare throughout the dry season, until the drought-breaking rains of mid-November 2016. By contrast, on the basaltic plain, there was some anticipation of the rains in the refoliation of S. nigrescens and the associated woody species.

In the case of S. nigrescens, some individuals started to shoot new leaves before the first rains fell. This anticipation occurred even earlier in the case of e.g. Sclerocarya birrea and Combretum hereroense.

In the case of C. apiculatum on granitic substrate, most individuals came into prominent bud just before the first rains, so that within days they rapidly produced both new leaves and the first flowers – in contrast to T. sericea which was relatively slow to respond.

Legumes tended to anticipate the rains on both basaltic and granitic substrates. However, there was an interesting difference between these substrates. This was that there was no member of the mimosoid legumes present in our plots on granitic substrate, other than Dichrostachys cinerea, which anticipated the rains weakly, if at all.

On both basaltic and granitic substrates, Cassia abbreviata ( and Ozoroa engleri? ( occurred in short forms, suppressed by continual breakage by the African bush elephant. These species strongly anticipated the rains, producing new leaves several weeks before the end of the dry season.
What this seems to add up to:
On both basaltic and granitic substrates, the nutritional regime in the soils has effectively constrained the establishment and growth of woody plants, relative to herbaceous plants. At the same time, the nutritional regime, as expressed by all plants, has invited intense activity by megaherbivores, leading to suppression of even those woody plants that have been able to establish in competition with grasses.

Because woody plants require more water per land area than grasses do, the constrained trees and large shrubs on basaltic and granitic substrates did not exhaust the water supply in the soil even in the drought of 2016.

This inadvertent sparing of water may help to explain my findings, viz. that the lower, sparser vegetation types (on basalt and granite) showed some refoliation during the drought, whereas no such refoliation occurred on Ecca substrate, except by a few scarce and small plants such as Commiphora, which seems to store water in its roots, and appeared to have been suppressed by herbivory even in the woodland of S. burkei.

PHENOLOGICAL OBSERVATIONS from Kruger National Park, Sept.-Nov. 2016, with particular reference to anticipation of the rains by plants:
This year (2016) there was no widespread anticipation of the rains by flowering acacias. Even Senegalia nigrescens largely failed to flower this year, owing to the drought.
Vachellia robusta ( anticipated the rains extremely conspicuously in Sept. 2016, in its restricted habitat of riverbanks. This involved mainly vigorous, bright new foliage, grown at a time when coexisting plants, such as Diospyros mespiliformis, were actually shedding foliage.

Vachellia robusta is, in Kruger National Park, certainly anomalous phenologically.
Vachellia tortilis is, just as I have known for many years now, effectively ‘evergreen’, and a real anomaly.
Full-size, mature, tall Senegalia nigrescens on basalt anticipated the rains with foliage growth.
The rest of the acacias, including Dichrostachys cinerea, hardly anticipated the rains, in their shooting of foliage.

A possible partial exception was 6 m (not full-size) Senegalia nigrescens on basalt near Satara.

On Ecca substrate near Satara, Senegalia burkei remained bare a week after the first rains, except in the runoff zone next to a tarred road. Vachellia nilotica ( and Vachellia grandicornuta seemed similar in this tardiness.
Combretum imberbe seems to be more or less the combretaceous equivalent of Vachellia tortilis: effectively evergreen at all ages and sizes.
Combretum hereroense and C. apiculatum seem, in some areas, to anticipate the rains in shooting foliage. However, this is variable, and, in some places, failed, at least in this drought year.

By the end of this visit to Kruger National Park, I became certain that Combretum hereroense differs from C. apiculatum phenologically. The former anticipates the rains, whereas the latter does not.

By 20 Nov. 2016, the foliage of C. hereroense was so full and mature that this species looked like an evergreen, whereas in the same areas C. apiculatum was still in bright and small-leafed shoot.

However, puzzlingly enough, C. hereroense on gabbro substrate near Ship Mountain ( was tardy in its shooting of foliage, and individually variable; I would not describe that population as anticipating the rains. I wonder if the form on gabbro might instead be C. zeyheri (, a species with which I am unfamiliar (our game guard did call it hereroense, but perhaps he was wrong?).
Terminalia prunioides anticipated the rains with foliage growth. So may have Terminalia sericea in some areas, but the latter species in particular was surprisingly variable (unpredictable). I certainly saw stands of T. sericea that did not anticipate the rains, and seemed, if anything, to lag.
Colophospermum mopane generally anticipated the rains, being about on a par with S. birrea in the time before the rains when its new leaves started to appear. However, as in the case of T. sericea, I was surprised by the degree of local variation among populations.
Cassia abbreviata is unusual in its exemption from herbivory by large mammals once adult. Even its fallen flowers seem to be shunned. This species certainly anticipated the rains in both flowering and shooting of foliage, and in Sept. 2016 was the prime example of this phenomenon, away from the riverbanks.

Sclerocarya birrea is also a clear example of anticipation of the rains, although not as extreme as Cassia abbreviata.

Unlike S. birrea, Lannea schweinfurthii ( did not anticipate the rains.
Trichilia emetica ( seems to be evergreen in its natural habitat, as are Capparis tomentosa ( and Euclea spp.

However, I learned in Nov. 2016, on Ecca substrate near Satara, that even Euclea divinorum can lose its leaves in drought, and it certainly did not anticipate the rains.
Some of the deciduous species are tardily and briefly deciduous, e.g. Diospyros mespiliformis and perhaps Schotia brachypetala ( The former fruited at the driest time, and the latter, if memory serves, flowered at the driest time. The leaves turned yellow before falling in the former species, but fell in a green state in the latter species. More data are needed.
Zanthoxylum humile ( was fully bare by Nov. 2016, and did not anticipate the rains.
Ehretia rigida? ( approaches an evergreen in its behaviour, being at least as ‘evergreen’ as coexisting Euclea divinorum on Ecca substrate near Satara. It has accordingly semi-scelrophyllous leaves, dull and ragged by mid-Nov. 2016. Ehretia obtusifolia? ( near Skukuza seems different, including in phenology.
Phragmites mauritianus ( is evergreen, although it looked ragged in the drought. In this way it is quite different from Phragmites communis (, which I have not seen in Kruger National Park.

Posted on 07 August, 2022 05:33 by milewski milewski | 0 comments | Leave a comment

Phenology of woody vegetation on basalt in Kruger National Park, showing water surplus where woody growth constrained by nutritional regime favouring grasses

(writing in progress)

The question is: what evidence is there that woody vegetation on basalt near Satara in Kruger National Park, which is relatively low and open, is limited by competition with herbaceous plants (and by the damage inflicted by large herbivores supported partly or mainly by the herbaceous plants) rather than by sheer lack of water? I.e. what evidence is there that it is the nutritional regime, rather than the climate, that constrains the height and density of the woody cover?
Herbaceous plants, other things being equal, need less water than woody plants do; this is largely a function of body sizes and absolute surface areas, together with the exponential increase in wind speed with height above ground.
Therefore, if the nutritional regime (mediated partly by the actions of herbivores) favours herbaceous plants (particularly grasses) over woody plants (particularly trees) then one would predict that the water supply in the soil would not be exhausted even in drought, and that deciduousness of relatively deep-rooted plants in the community would be a phenological tactic rather than a dormancy enforced by an inability to afford the water costs of transpiration.
On this basis, I would predict the following if the height and density of woody plants is constrained mainly by the nutritional regime on basalt in Kruger Park.
1) Some woody plants should be evergreen even if all herbaceous plants are deciduous.
2) Some woody plants should anticipate the rains in shooting either flowers, or new foliage, or both, rather than waiting for the first rains to fall.
3) The woody plants showing either evergreen tendencies or anticipation of the rains should include small or relatively shallow-rooted species, or shrubby species, or individuals suppressed by physical damage by large herbivores.
4) Some woody plants should be able to produce flowers and/or foliage even in times so dry that all the grasses and other herbaceous plants are fully dormant.
Now let’s examine the reality of what I found on my two visits to this woody vegetation, on 15 and 17 Nov. 2016, 3-6 days after the first drought-breaking rains that wet the clay-rich soil to a depth of about 5 cm.
The vegetation to which I refer consists of an upper stratum dominated by Senegalia nigrescens over a lower woody stratum dominated by Flueggea virosa, over a grass stratum dominated by Bothriochloa. There are various other species in the mix in all three strata. Most individuals of S. nigrescens are only 6 m high or lower, despite the potential of this species to exceed 15 m high; and the stand of S. nigrescens is open, with the individuals on average >?20 m apart (it may be possible to estimate this from Google Earth?).
Prediction 1):
It is indeed true that several woody plants in this vegetation are evergreen, and this evergreenness is of several different types. Euclea natalensis and Diospyros mespiliformis are evergreen in the most prosaic way, having semi-sclerophyllous, long-lived, simple leaves; however these species are scarce here, fail to reach their full size, and may not be reproductively mature. Peltophorum africanum is evergreen despite having bipinnately compound leaves which look tired by the end of the dry season. Vachellia tortilis continues to grow bipinnately compound leaves through the dry season and did not become bare even by the end of this severe drought. Since all these species are < 3 m high here and the root system of the last-mentioned species in particular is shallow and spreading, this is evidence that water remains available throughout the year and the drought cycle. Lastly, Combretum imberbe (including suppressed saplings) retains leaves continually, even though it belongs to an otherwise deciduous genus and differs from the Ebenaceae and Peltophorum in tending to be quasi-spinescent as a sapling.
Prediction 2):
Various woody species did indeed anticipate the rains in producing flowers and or new foliage. The clearest examples were Sclerocarya birrea (both tall, mature individuals and suppressed saplings) and tall, mature individuals of S. nigrescens. However, even the somewhat suppressed individuals of S. nigrescens, about 6 m high, tend to produce flowers at the height of the dry season, when most deciduous trees in Kruger Park are still bare. Other spp. anticipating the rains in the shooting of foliage were Ximenia caffra, Combretum hereroense, Vachellia exuvialis, Vachellia robusta, Philenoptera violacea, Cassia abbreviata, Strychnos spinosa, and possibly Peltophorum africanum. It is noteworthy how many of the species which produce fresh foliage during drought are legumes (or, in the case of Ximenia caffra, a root hemiparasite which ostensibly parasitises legumes); however, it is also noteworthy that one species of legume, namely Dichrostachys cinerea, remained completely bare and moreover had suffered noticeable mortality in this area.
Prediction 3):
It was indeed true that those woody spp. showing green foliage during dry conditions included suppressed individuals with body sizes far inferior to their potential. This applied particularly to V. tortilis, Cassia abbreviata (of which no mature, full-size individual was seen in this sampling area), P. violacea and S. birrea.
Prediction 4):
It was indeed true that the shooting of new foliage by various woody spp. during drought was unaccompanied by any shooting of new foliage by any grass or other herbaceous plant. The main grass apparent in the sampling area was Bothriochloa, which remained dry and dormant even six days after the first drought-breaking rains. During my second visit on 17 Nov. 2016, several deciduous woody species (e.g. Ximenia caffra, Sclerocarya birrea, and V. tortilis) had grown much of their foliage but the only green apparent in the herbaceous stratum was tiny, scattered, freshly-germinating seedlings of unidentified dicotyledonous herbs, probably annuals. Furthermore, the most conspicuous members of the lower woody stratum, namely Flueggea virosa and Grewia sp. indet. (bicolor?), remained completely bare during both my visits. Thus it can be said that there was a great disparity between the tallest plants and the shortest plants in this vegetation phenologically, with the former clearly anticipating the rains but the latter tending not to anticipate the rains and, in the case of the herbaceous stratum, showing no anticipation of the rains whatsoever. It is understandable that there is some incentive for the woody plants to conform to this prediction because the dry season is a time when the woody plants can avoid the superior competitiveness of the (relatively shallow-rooted but metabolically more powerful on a per cell basis) herbaceous plants for nutrients. However, the failure of D. cinerea to capitalise in this way – and indeed to a large extent even to survive in this area - was one of the noteworthy findings of this study. What emerged was an interesting contrast between D. cinerea and V. tortilis in phenology and persistence: both spp. were suppressed in this area and had sparse populations, but the former failed to produce any foliage or flowers during this drought, and showed evidence of much mortality, whereas the latter proved not only resilient but also functionally evergreen, something unusual among ‘acacia’ spp. and not attributable to a deep rooting system.
(writing in progress) 

Posted on 07 August, 2022 06:44 by milewski milewski | 1 comment | Leave a comment

Phenological evidence of relationship between water and height of vegetation on basalt in Kruger National Park

Please see

I turn now to the relatively treeless area on basalt, north of Satara in Kruger National Park. This continues my documentation and discussion of phenological patterns, as revealing ‘surplus’ water in the soil, even during the drought of 2016.

As far as I know, the first rain fell at the same time in both of our sampling areas on basalt in the vicinity of Satara, on 11-12 Nov. 2016.

I attempted to verify this, by checking how deeply the rain had penetrated the soil. I found that a scratch in the soil showed penetration to be about 5 cm, in both sampling areas.

Despite this commonality in the timing of the drought-breaking rainfall, the phenological state of the woody plants was relatively advanced in the relatively treeless area.

This can perhaps be seen as consistent with the even greater ‘surplus’ of water in the soil here than in the woody vegetation on basalt (detailed in a recent Post).
The specific observations are as follows.
The following species were already in some leaf during my visit on 15 November 2016:

The species of Grewia common in the woody vegetation on basalt – which I suspect to be G. bicolor - was uncommon in this relatively treeless vegetation on basalt. However, here it had already started to shoot leaves, and flowers had appeared. By contrast, on the same day (15 Nov. 2016) I observed it as completely bare in the woody vegetation on basalt.

Dichrostachys cinerea had already started to shoot leaves (from the base of the stem). This means that this species seemed more advanced here than in the woody vegetation on basalt; I even saw a few inflorescences here, already.

Typical evergreenness was not exemplified by any plant in this relatively treeless area area. However, Combretum imberbe was somewhat evergreen. Even a suppressed ‘sapling’, only 1 m High, of C. imberbe (observed in plot 2), was in leaf, not bare.

In the case of Vachellia tortilis, the usual pattern was apparent.

This species had not become fully leafless regardless of the drought, being ‘evergreen’ in a way more dynamic (repeatedly producing and shedding small leaves) than the usual pattern of evergreenness.

We observed the hook-lipped rhino (Diceros bicornis) here, right out in the open, foraging on a suppressed individual of V. tortilis.

This defiance of the drought by V. tortilis is important, because V. tortilis is one of the most important woody plants in this relatively treeless area on basalt.

It is hard to generalise about Senegalia nigrescens in this relatively treeless area. This is because it was so scarce here. However, it seems significant that the two individuals I found in or near the plots, both of them tiny (= ‘suppressed saplings’ <10 cm high), were already in leaf, i.e. apparently advanced relative to their conspecifics in the woody vegetation on basalt.

Flueggea virosa and Terminalia prunioides were still largely bare. However, in contrast to the same species in the woody vegetation on basalt, F. virosa retained a few old leaves, and had already shot some new leaves by the time of my visit.

So, I can say the following with certainty:
Flueggia virosa remained completely bare in the woody vegetation on basalt, where it was so common as to constitute a stratum of the savanna. By contrast, the same species, growing more scattered in the relatively treeless vegetation on basalt, had not become completely bare - and was already starting to produce new leaves.
The anticipation of the rains by woody plants in this relatively treeless area on basalt was not a major phenomenon. What I saw during my visit was, After all, just a small proportion of the full foliage, and there was individual variation within each species.

However, the fact that fresh leaves were apparent at all, on most of the woody plants, shows

  • a difference from comparable plants in the woody vegetation on basalt, and
  • some anticipation, even if only by a week or so, of the rains, rather than merely a response to the actual fall of rain.

The amount of new tissue produced by each species may seem minor. However, the sheer consistency of the difference across the floristic spectrum makes for a clear result, overall.

Not a single species of woody plant here failed to show green leaves on at least some individuals, at a time when this shooting could not have been merely responsive to the first rains.
It is important to note that I found virtually no green grass in any of these plots in the relatively treeless area on basalt. The only exception was a few fresh leaves on a few dry, worn tussocks of Bothriochloa. This was a species uncommon in the relatively treeless vegetation on basalt, despite its dominance in the woody vegetation on basalt.

(Even though Bothriochloa was uncommon here in the relatively treeless vegetation, it was if anything ahead of its conspecifics in the woody vegetation on basalt, in responding to the rain that fell in both areas, three days previous to my visit.) 

It is significant that the woody plants were ahead of the grasses phenologically. This implies some degree of anticipation of the rains, and in turn some degree of underutilisation in the water supply in the soil.
To summarise:
Yes, the woody plants in the relatively treeless vegetation on basalt did indeed anticipate the rains to some extent in their shooting of foliage, and yes, this pattern was more marked than in the woody vegetation on basalt.

All of this certainly supports the idea that there was still water available in this ‘black cotton soil’ at the end of a severe drought, in which the lawn grasses virtually disappeared from view even as dry remnants.

And, likewise, it supports the interpretation that the ‘bareness’ and ‘openness’ and ‘arid appearance’ of this relatively treeless plain north of Satara was not a consequence of drought in either the sense of a dry season, or the sense of an unusual drought. Instead, it resulted from a combination of

  • the ‘outcompeting’ of woody plants by lawn-forming grasses, and
  • the suppression of woody plants by large herbivores including the African bush elephant, both species of rhinos, and the southern giraffe (Giraffa giraffa).

The bottom line seems to be this:

If anyone assumes that the basalt plain north of Satara has such ‘open’ vegetation because this is a dry part (either climatically or edaphically) of Kruger National Park, they are probably mistaken.

This relative treelessness has little to do with the water supply, and more to do with nutritional regimes, mediated by herbivory.

The green leaves apparent on woody plants during my visit, right at the end of a severe drought, although small in absolute quantity, are revealing because they prove that water is not ‘limiting’ in this ecosystem, as far as woody plants are concerned. As a corollary, the deciduousness of woody plants, such as Cordia and Dichrostachys, is a ‘tactic’ rather than something imposed absolutely by the hydrological regime.

Posted on 07 August, 2022 07:09 by milewski milewski | 0 comments | Leave a comment

Ecological notes on Strychnos in Kruger National Park

(writing in progress)
During fieldwork in Kruger National Park at the end of the drought in 2016, I recorded two spp. of Strychnos, namely

  • S. spinosa (in the woody vegetation near large termite mounds in the vicinity of Nyamunda Dam), and
  • S. madagascariensis.

Both species were seen only in extremely suppressed form, not as mature or adult individuals.

Strychnos spinosa looked clonal to me. Possibly so is S. madagascariensis although the signs of this were not apparent.

Both spp. were bare at the time of sampling, although in the case of S. madagascariensis there were a few small leaves still green right at the base of the stems, at ground level.
The aim of this Post is firstly to document the wood density, and secondly to discuss the agencies of suppression of Strychnos in Kruger National Park.
Van Wyk (1974) gives the air-dry densities of the wood of the two spp. as S. spinosa 730 kg per cubic metre and S. madagascariensis 850 kg per cubic metre.

Unlike e.g. Dichrostachys cinerea, neither species of Strychnos possesses heartwood.
An interesting ecological fact is that neither S. spinosa nor S. madagascariensis occurs on basalt in Kruger National Park, despite their widespread occurrence on granite. Both spp. are most prominent (i.e. some individuals have escaped suppression) in the Pretorius Kop area, where rainfall is relatively copious.
I see gross disfigurement by the African bush elephant as a major suppressor of various woody plants in Kruger Park. However, I did not get the impression that this was the agency responsible for the low, apparently clonal stands of S. spinosa in the woody plots near Nyamunda Dam.

The effects of the proboscidean were apparent in those plots, chiefly in the form of individuals of Combretum apiculatum uprooted years ago.

However, the suppression of S. spinosa, and the mallee growth-form of Combretum collinum, and possibly C. zeyheri, in those plots looked like the results of fire rather than the results of breakage by the African bush elephant. There was plenty of evidence of past fires in these plots in the form of blackening of boles at ground level.
The role of fire raises an interesting pattern, because Strychnos produces fleshy fruits. Endozoochory is generally not associated with fire-prone vegetation.
In general, woody plants with fleshy fruits tend, in Kruger National Park as elsewhere, to dominate mainly in fire-free patches of vegetation.

Strychos madagascariensis, and especially S. spinosa, have not only fleshy fruits but spectacularly large fleshy fruits, far larger than those of most of their congeners in Africa and around the world.

So, to find that S. spinosa persisted in the Combretum bushveld of the woody plots near Nyamunda Dam, despite what seems to be a fire regime capable of converting single-boled small trees to multi-stemmed shrubs, is anomalous.

Given this role of fire here, I would have expected the floristics of the communities to be such that woody taxa other than Strychnos would have come to dominate.

Yet, the reality was that, in several of the woody plots near Nyamunda Dam, S. spinosa was the commonest woody species in the sense of having the greatest number of woody stems in the plot.
My explanation of this anomaly is only partial, but goes as follows.

The ‘populations’ of S. spinosa in the woody plots near Nyamunda Dam are held at such limited height that they are reproductively immature (unlike e.g. Lannea schweinfurthii on basalt, which I suspect to be capable of producing fruit even in suppressed form).

Hence S. spinosa, in terms of this incidence, is effectively not a fleshy fruit-producing plant.

Van Wyk (1974) points out that this species is puzzling in Kruger National Park in occurring in two distinct forms that are so different that they would hardly seem to belong to the same species.

I suggest that one of these forms effectively abandons sexual reproduction, and relies instead on vegetative reproduction to persist in vegetation that burns frequently and intensely enough to convert taxa, with the potential to grow into trees, into shrubs.

Another odd feature of some of these plots was the presence of populations of a bizarre monocotyledonous herbaceous plant, Xerophyta retinervis. This connection may be significant in ways I have not yet thought out.

If memory serves, at least some spp. of Xerophyta are resurrection plants, and perhaps this species found in our plots in Kruger National Park is one of these?

Anyway, Xerophyta is obviously resistant to fire.

Neither Strychnos spinosa nor Xerophyta retinervis occurred in the woody plots on granite east of Phalaborwa, despite the similar ecological situations, and this seems consistent with the lesser role of fire, and greater role of the elephant, in the latter area.
(writing in progress)

Posted on 07 August, 2022 07:33 by milewski milewski | 0 comments | Leave a comment