New interpretations around the marula, Sclerocarya birrea

@troos @botaneek @jeremygilmore @ludwig_muller @tonyrebelo @wynand_uys @phil183 @ricky_taylor @andrew_hankey @francoisdurandt @robert_taylor @pieterwinter @richardgill @bartwursten @andrewdeacon @warrenmcc @brendenpienaar @mrpopp @magdastlucia @jan-hendrik @graham_g @grantegen @arista_botha @alastairpotts

First I examine the symbiosis between the marula (Sclerocarya birrea, https://www.inaturalist.org/taxa/340245-Sclerocarya-birrea) and large mammals.
 
Both the African bush elephant (Loxodonta africana) and the human species treat the marula as a favourite plant for consumption.

Indeed, the fleshy fruits of the marula are extreme in that they are uneaten by birds.

The relationship of the fruit to its large mammalian consumers is so strong that it seems possible that the very evolution of the marula reflects the selective pressures of this symbiosis.

The seed-casings are too hard to be damaged by even the massive jaws of the proboscidean, and the seeds are passed intact and viable by virtue of superficial digestion.

But the questions arise:

• which species – elephant or human - has most profoundly shaped the nature of the whole plant? and
• is it possible that the incidence of the marula in, for example, Kruger National Park is more anthropogenic than ‘natural’?

This is how I see it.
 
It is impossible for the human species to have shaped the marula much by selective pressure, simply because until a few thousand years ago our species lacked the means to affect this tree much. Although the human species did indeed evolve in the presence of the marula, until recently our tool-use was too primitive for us to exert much pressure by chopping down other, competing trees, or by protecting and fertilising the marula as a quasi-domestic plant.
 
However, a sound argument can be made for the African bush elephant having shaped marula, by being its main disperser and sower. The gross damage inflicted by the megaherbivore on this soft-wooded and generally defenceless plant would be balanced by propagation.

The African bush elephant does not spare the marula. However, it defecates so many intact seeds, in conditions so conducive to the growth of seedlings, that a case can be made for a loose mutualism between plant and elephant. The main structural adaptation of the marula to withstand abuse seems to be its almost miraculous ability to regenerate bark, even after what seems like complete ring-barking (van Wyk, 1974, https://www.abebooks.com/first-edition/Trees-Kruger-National-Park-Volume-Set/30239678186/bd and https://antiquarianauctions.com/lots/trees-of-the-kruger-national-park-2-volume-set).
 
And yet I would argue that the incidence of the marula in Kruger National Park is as much anthropogenic as natural. How are the two arguments compatible, in terms of a timeline?
 
The marula was, from the start, shaped evolutionarily by proboscideans (https://en.wikipedia.org/wiki/Proboscidea). But then, in the last few thousand years, the DISTRIBUTION of this species was shaped mainly by the human species. This is because the combination of rapid growth and the fruit-qualities and –fermentation that suited proboscideans happens also to suit humans. In parts of Africa, anthropogenic promotion of the marula may have been via deliberate plantings and continual protection.

In Kruger National Park, such intensive management is unlikely because the disease-prone low-lying savannas were not settled continually or densely. However, what is likely is that, for hundreds of years before the establishment of the park, the population of the African bush elephant in the area was kept modest by killing for the ivory trade (which was with Arabs before Europeans arrived).

I suggest that the current population of the elephant in Kruger National Park is the densest seen in centuries, and that any recent decline in the incidence of the marula reflects a reversion towards a ‘natural’ regime, rather than a loss of something ‘natural’.
 
So, we may not have changed the nature of the plant, but we may have spread it beyond the scope of its natural (i.e. Pleistocene and before) dispersers, the proboscideans. This anthropogenic spreading has been achieved despite the fact that humans avidly eat and thus destroy the seeds, and because we

• spare the tree when we clear land for horticulture,
• transport the seeds to new areas to plant, and
• protect the plant from the elephant, both directly and indirectly.

It is probably unknowable what the original, ‘natural’ distribution of the marula was, before the advent of farming. However, as van Wyk (1974) notes, the species is vulnerable to low temperatures. So, I suspect that the marula had a relatively restricted distribution, somewhere in tropical Africa, contingent on a balance in its relationship with the African bush elephant - which both plants the species and destroys the plants by bark-stripping and breakage.

I suggest that the currently wide range of the marula in Africa is a result of the spreading of this plant by the human species – all the way to Senegal in far West Africa and the Sudan in northeast Africa, and southwards to the Magaliesberg (near Pretoria, https://en.wikipedia.org/wiki/Magaliesberg) and the southern border of KwaZulu-Natal (https://en.wikipedia.org/wiki/KwaZulu-Natal).

The presence in Madagascar is attributable to deliberate transportation of the seeds by humans within the last thousand years. The natural dispersal of the marula to this island is implausible, either mechanistically or chronologically.
 
My argument is based on a certain version of ‘anthropogenic’.

I do not envisage human pressures as having shaped the marula much (although the current selective breeding w.r.t. the semi-domesticated populations in South Africa is another thing). The main agent of its shaping has been proboscideans. However, I do see the human species as being more effective in taking the fully evolved species and multiplying

• its range across Africa plus Madagascar, and
• its coverage of landforms and soils within Africa.

The result is that the marula, of all the trees in Kruger National Park, is the most ubiquitous in this park (van Wyk, 1974). Or at least it was before the African bush elephant was released from culling, and reached its current densities of population with a certain reduction in the incidence of the plant.

Such reduction in the marula is particularly evident in

• the vicinity of large mounds, built by termites, east of Phalaborwa (https://en.wikipedia.org/wiki/Phalaborwa), and
• the vicinity of the sodic patches along the N’waswitshaka River southwest of Skukuza (https://en.wikipedia.org/wiki/Skukuza).

Perhaps I can elaborate my interpretation by means of a comparison between the marula and the leadwood (Combretum imberbe, https://www.inaturalist.org/taxa/340408-Combretum-imberbe).

Both the marula and the leadwood

• are particularly common on the basalt plains of Kruger National Park,
• form ‘emergent’ trees up to about 18 m high, which are remarkably tall and massive in what is generally the least-vegetated of the widespread landforms of the park, and
• can remain suppressed for years, eventually outstripping the megaherbivores in height and growing above the reach of the African bush elephant and the southern giraffe (Giraffa giraffa giraffa).

Both species are ‘iconic’ of the low-lying savannas, because of their size and grandeur, and their extreme adaptations. However, the two species contrast in life-strategy.

The marula has the least dense wood of any of the common large trees of Kruger National Park. By contrast, the leadwood has the densest wood of these trees. When a large individual of the marula dies, its bole decomposes rapidly. By contrast, when a large individual of the leadwood dies, its bole may continue to stand for centuries.

The marula is palatable in most of its parts (foliage, fruits, seeds, cambium, and even roots) to the African bush elephant. The leadwood is relatively unattractive as food, because

• its leaves are relatively tough (semi-evergreen),
• it adopts a quasi-spinescent juvenile form when wholly within reach of the megaherbivores,
• it lacks any fruit-pulp, and
• its seeds are – if in line with its congeners – toxic.

I suggest that the ‘natural’ (i.e. Pleistocene) regime was for the leadwood to be the main ‘emergent’ (i.e. taller than Senegalia nigrescens, https://www.inaturalist.org/taxa/594427-Senegalia-nigrescens) tree on these basalt plains, to the virtual exclusion of the marula. However, with reduction of the population of the megaherbivore by the human species, and some horticulture by Bantu (https://en.wikipedia.org/wiki/Bantu_peoples) in parts of what is now Kruger National Park, the marula was promoted.

After the park was established, and before the African bush elephant reached its current numbers, the fortunes of the marula increased to the point that van Wyk, in 1974, wrote “In large areas...the knobthorn is gradually disappearing with the marula gaining dominance.”

I gather that this increase has since been reversed with the ban on culling of the elephant. I also infer that current concerns about an excessive population of the megaherbivore are partly based on a false standard: the preservation of large trees of the marula as a ‘natural’ part of the vegetation.
 
To summarise so far:
I see proboscideans as being responsible for shaping the nature of the marula, with the human species responsible for taking this species and extending its range geographically (e.g. to Natal or to Senegambia, and certainly to Madagascar) and in terms of habitat (e.g. to the widest possible variety of landforms and soils within Kruger National Park).

Turning to a different topic:
 
One of the puzzles of Kruger National Park is why two tree species, closely related and similar in appearance, coexist here. I refer to the false-marula (Lannea schweinfurthii, https://www.inaturalist.org/taxa/340118-Lannea-schweinfurthii), which like the marula is widespread in the park, and covers soils derived from various parent materials.

Given that this seems to be a case of biological redundancy, how do these two types of trees relate to each other?
 
Although there is obvious evolutionary convergence between S. birrea and L. schweinfurthii, the two species are functionally different in significant ways.
 
One difference between Sclerocarya and Lannea is that the latter genus seems to have a greater capacity to survive in suppressed form under intense herbivory. At least one species of Lannea, occurring only marginally in the Kruger National Park, forms a geoxylic suffrutex (‘underground tree’, https://en.wikipedia.org/wiki/Geoxyle). Although L. schweinfurthii is not specialised in this way, it nevertheless can survive in grossly disfigured form, as a kind of stunted, felled plant, growing on its side and repeatedly regenerating foliage from a bole without increasing its height for decades – a capacity unknown in the marula.
 
I cannot recall seeing the marula in Kruger National Park as stout individuals only 1-7 m high; this species seems to occur only as suppressed saplings (< 1 m high) or adults (> 7 m high). I infer that, in the marula,

• regeneration is in cohorts rather than continually, and
• the latest cohort of mature individuals, which reach up to 18 m high, grew beyond reach of the tallest herbivores during a period when the populations of these herbivores were at an ebb, about a century ago.

Lannea schweinfurthii is different; most of the individuals I saw in Kruger National Park are substantial but suppressed. I.e. they have obviously been broken when already well-grown juveniles, but have survived this felling in a way that seems beyond the marula.
 
Another difference is as follows.

I have seen the marula as suppressed saplings, surrounded by elephant-dug trenches, in Colophospermum mopane -Combretum apiculatum bushveld east of Phalaborwa, in the vicinity of Masorini and Shikumbu koppies. I have seen no such thing in the case of L. schweinfurthii.
 
Many types of trees in Kruger National Park exist in a long-term suppressed condition. Among these, however, L. schweinfurthii is unusual in having exceptionally insubstantial wood. Sclerocarya birrea is unusual among the common large trees of Kruger National Park in that its wood is not dense. Its relative, L. schweinfurthii, is similar in this respect – and may have even less substantial wood.

Van Wyk (1974) gives similar values of 560-570 kg per cubic metre for these two species, but Dale & Greenway (1961) state that wood density in L. schweinfurthii is only 400 kg per cubic metre. This means a wood as light, when air-dry, as that of a cricket bat made in England from the plantation willow Salix alba (https://www.inaturalist.org/taxa/54841-Salix-alba).
 
Whereas the fleshy fruit and dispersal biology of the marula are specialised for large mammals, that of L. schweinfurthii is ‘normal’ in that the fruit and its 'stone' are unremarkable for an endozoochorous plant. The seeds of L. schweinfurthii seem to be dispersed mainly by baboons (Papio spp.) and/or birds - although so edible to the human species that several congeners are called ‘grape’.
 
I do not know if the suppressed individuals of L. schweinfurthii are capable of producing fruit while held in the zone of intense herbivory by repeated breakage by the African bush elephant. I suspect that they are, because the ‘underground tree’ Lannea edulis (https://www.inaturalist.org/taxa/490675-Lannea-edulis), in higher-lying parts of Mpumalanga (https://en.wikipedia.org/wiki/Mpumalanga), is known for its bright red fruits despite being so 'dwarfed' that it is effectively a part of the herbaceous stratum.  
 
An odd fact about L. schweinfurthii is that, according to my own observations, its fresh foliage is eaten by the plains zebra (Equus quagga chapmani, https://www.inaturalist.org/taxa/43335-Equus-quagga) in Kruger National Park.

My observation was on the basalt plain just north of Lower Sabie Rest Camp (https://en.wikipedia.org/wiki/Lower_Sabie), at a time just after the break of a serious drought (late Nov. 2016), when the green grass was still so short that grazing was particularly time-consuming.

Because all living equids are effectively specialised grazers, it is always of interest when any species of zebra is observed eating any woody plant. For example, for the plains zebra in Kruger National Park,

• the only record by Anon. (1960) of ‘browsing’ on an indigenous woody plant was for Schotia capitata (a ‘dwarf’ species of Caesalpiniaceae, https://www.inaturalist.org/taxa/560140-Schotia-capitata), and
• the only record of fleshy fruits being eaten was for the marula - further testimony to the special appeal to ungulates of the fruits of Sclerocarya birrea.

I infer the following:
With the increase in the population density of the African bush elephant in Kruger National Park over the last few decades, there has been widespread suppression of L. schweinfurthii. One of the incidental effects of this is a slight facilitation for the plains zebra. This is because the foliage of this relative of the marula emerges within reach of the equid - at a season when the plains zebra needs the bulk, but cannot accept the foliage of most woody plants.

The involvement of Lannea makes for a relationship between elephant and zebra that I would not have anticipated.

Posted on 07 de julho de 2022, 09:00 AM by milewski