Some perspectives on genus Combretum in Kruger National Park

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Here are some thoughts, quotes, and quantifications following on visits I made to Kruger National Park in 2016, w.r.t. genus Combretum.

The African bush elephant (Loxodonta africana) has such an extreme relationship with its food-plant, Combretum apiculatum (https://www.inaturalist.org/taxa/340059-Combretum-apiculatum), in the area east of Phalaborwa (https://en.wikipedia.org/wiki/Phalaborwa), that the ‘normal’ state of this plant is to be uprooted.

This phenomenon is hardly shown in photos on the Web, the closest being https://www.inaturalist.org/observations/93233816. However, I observed this time and again in our sampling plots.

It is also true to say that the ‘normal’ state of Sclerocarya birrea (https://en.wikipedia.org/wiki/Sclerocarya_birrea) in this area is to be suppressed to sapling size, with a proboscidean-dug circular trench around the ‘sapling’.

However, the difference is that C. apiculatum is a dominant woody plant here, maintaining a reproductive population. By contrast, S. birrea does not seem to get even close to reaching reproductive maturity in this area.
 
Observing this apparent distortion/victimisation of plants by an animal prompted me to read more on the genus, Combretum, as a whole.
 
The following may perhaps shed some light on the specific interaction that I found so striking in Kruger National Park.
 
Combretum, although absent from Australia, occurs as far away from Africa as tropical and temperate South America.

This is an important genus, for example, in the southwestern Amazonian forests (https://www.inaturalist.org/observations?place_id=6783&taxon_id=81496&view=species). Combretum also extends as far south as Buenos Aires, reaching the same latitude as in temperate South Africa (https://www.inaturalist.org/observations?place_id=6878&taxon_id=81496&view=species and https://www.inaturalist.org/taxa/284830-Combretum-fruticosum).
 
Combretum is not only speciose and near-cosmopolitan, but it has greater plasticity in growth-form than do many genera of woody plants.

Even within Kruger National Park, the diversity of growth-forms is impressive, as follows:.

• Combretum microphyllum is a liane (https://www.inaturalist.org/taxa/340115-Combretum-microphyllum),
• C. mossambicense is a lax scrambler (https://www.inaturalist.org/taxa/211071-Combretum-mossambicense),
• C. erythrophyllum is a willow-like, shapeless, self-standing plant with wood of comparable lightness to the willows it resembles,
• Combretum apiculatum seems to be effectively multi-stemmed (https://www.inaturalist.org/observations/85011474 and https://www.inaturalist.org/observations/47282820), despite having extremely dense wood,
• Combretum hereroense differs from co-existing C. apiculatum in anticipating the rains in its shoot-growth (at least on granite), and
• Combretum imberbe is unusual in several ways: extremely calcium-rich wood, evergreenness, and the possession of a quasi-spinescent juvenile stage when within reach of the southern giraffe (Giraffa giraffa).

These differences in growth-form are so great that one might be forgiven for assuming that different families, let alone genera, are involved.

And yet the form of the fruit seems consistent, with anemochory (https://en.wikipedia.org/wiki/Seed_dispersal#Wind) and protection of the seeds from granivores (such as the local species of parrot, https://www.inaturalist.org/taxa/19050-Poicephalus-cryptoxanthus) by means of toxicity (https://li01.tci-thaijo.org/index.php/anres/article/view/242359 and https://www.researchgate.net/publication/324039541_Effect_of_Combretum_molle_Combretaceae_seed_extract_on_hematological_and_biochemical_parameters).
  
Towards a comparative view of C. apiculatum vs C. imberbe (https://www.inaturalist.org/taxa/340408-Combretum-imberbe), let me first examine C. erythrophyllum.

It is noteworthy that Combretum erythrophyllum (https://www.inaturalist.org/taxa/135389-Combretum-erythrophyllum) occurs in the Sabie River bed (https://en.wikipedia.org/wiki/Sabie_River). The species shares this environment with Salix mucronata (https://www.inaturalist.org/taxa/593611-Salix-mucronata) and Morella serrata (https://www.inaturalist.org/taxa/492352-Morella-serrata).

Not only has a typical genus of the Northern Hemisphere penetrated Kruger National Park in willow-like form, but even one of the local combretums has adopted a similar, willow-like form.

Combretum erythrophyllum is a peculiar in that

• the only place where it is common in Kruger National Park is along the Sabie River, and
• it ranges as far southwards as the banks of the Orange River (https://en.wikipedia.org/wiki/Orange_River) in the Northern Cape, the western Free State, and East London in the Eastern Cape (https://en.wikipedia.org/wiki/Eastern_Cape).

Combretum erythrophyllum is not a small plant (it reaches up to 12 m high with up to 60 cm stem diameter). However, it tends to grow

• as a large shrub rather than a tree,
• in dense stands rather than the spaced-out populations associated with most of its congeners, and
• extremely rapidly (about 5 m in three years).

Another remarkable aspect of C. erythrophyllum is that it seems immune to the African bush elephant.

As van Wyk (1974, https://www.abebooks.com/Trees-Kruger-National-Park-Wyk-P/22486254279/bd) states on page 424:

“In times of food scarcity, as in the early spring, it has been noticed that giraffe eat large quantities of the young leaves, twigs and flowers. Elephants make little or no use of the species.”

This suggests that C. erythrophyllum has a different strategy from C. apiculatum, even though both coexist with the proboscidean, and both are effectively large shrubs (https://www.inaturalist.org/observations/77812637) rather than typical trees.

Combretum erythrophyllum has about half the wood density of C. apiculatum, correlating with an apparent difference in growth-rates. I infer, although van Wyk does not mention this, that C. erythrophyllum is

• adapted more to damage from flooding than to damage from the African bush elephant, and
• chemically defended against the proboscidean in some obscure way.

However, my point is that C. apiculatum is far from simply being a typical combretum. If anything, what is typical for Combretum it is to be a lax woody plant, similar to C. mossambicense.

This makes both C. apiculatum and C. imberbe ‘peculiarly African’ in their

• extreme wood density, and
• extreme relationships with large herbivores.

Combretum imberbe is odd for its genus in several ways.

Firstly, it is quasi-spinescent. Is there any other species, in this genus of hundreds of spp., that has any kind of spinescence at all? Or is C. imberbe truly unique in this way?

When still within the foraging height of the southern giraffe, C. imberbe in Kruger National Park looks more like a gymnosporia (https://www.inaturalist.org/observations?place_id=any&taxon_id=185532&view=species) than like a combretum, a case of evolutionary convergence in a context of heteroblasty (https://en.wikipedia.org/wiki/Heteroblasty_(botany)).

Secondly, C. imberbe has an evergreen tendency. This does not seem to be stated in the literature. However, I have noted this phenological phenomenon repeatedly in my various diaries/journals, from several visits to Kruger National Park and elsewhere.

Van Wyk (1974), disappointingly, just calls C. imberbe ‘deciduous’, without further discussion. This hardly does justice to this species. However, one of the adjectives he uses to describe the leaves, i.e. ‘hard’, seems consistent with evergreenness.

Thirdly, C. imberbe seems strangely reluctant to adopt a multi-stemmed form. It is, as it were, bent on becoming a tree from the start.

Fourthly, C. imberbe grows peculiarly tall for a plant adapted to seasonal drought. It reaches 20 m in Kruger National Park, on basalt plains where (e.g. north of Satara) the vegetation is otherwise low and open, to the point of being treeless grassland.

And fifthly, C. imberbe shows a combination of slow growth and extremely dense wood.
 
Van Wyk’s (1974) account is so engaging – particularly w.r.t. the relationship to fire - that I will quote extensively, from pp. 430-431:
 
“Unlike the other species ‘'[of Combretum], the small fruits are shed within a very short time. Large trees are frequently hollow and also often have a large hole on one side close to the ground. Such trees are easily set alight during veldfires and hollowed out even more. The species is famous for its excellent firewood because it burns slowly and the large coals last a long time. Many reports by local rangers mention trees that began burning during a veldfire and kept on burning for days and even weeks afterwards. The best eyewitness account of all, however, is given by a former ranger in the Shingwedzi section of the Kruger National Park who, while on a foot patrol about two months after a veldfire, came upon a leadwood tree which was still smouldering. During that time a number of showers had fallen on it and the grass around it was bright green and in full flower! Frequently dead trees which are ignited by lighting cause veldfires, sometimes many days after the thunderstorm. The species contains so much lime that its ashes can be used as whitewash for walls...Trees broken apart by elephants go on growing. Another typical phenomenon is that those which have been pushed over completely seldom if ever die but give rise to a number of upright trees which grow out of the recumbent stems.”
 
We have seen, above, that the typical growth-form of the genus Combretum, if there is such a thing, is a scandent/scrambling, multi-stemmed plant rather than an upright tree. This applies to southern Africa, as elsewhere.
 
I now see that the following is somewhat ironic.

The African bush elephant habitually forages on C. apiculatum, one of the minority of this genus that has evolved wood so dense that it sinks in water even when dry. The proboscidean violently returns the plant back towards the horizontal, into a kind of ‘forced recumbency’. This is despite the herculean performance of C. apiculatum in reinforcing its stem, to a density exceeding even that of the renowned C. imberbe.

Combretum imberbe is

• by far the tallest of its genus in southern Africa,
• reputed to live for more than a thousand years, and
• so physically durable that the boles remain standing for decades after death, leading to the name 'leadwood'.

Here are data on air-dry wood densities.

The values for C. apiculatum (which is effectively a large shrub in Kruger National Park, rather than a true tree) and C. imberbe are respectively 1230 kg/m3 and 1200 kg/m3.

Compare these with (in decreasing order):

• 960 kg/m3 for Combretum collinum,
• 910 kg/m3 for Combretum hereroense,
• 860 kg/m3 for Combretum molle,
• 770 kg/m3 for Combretum woodii,
• 750 kg/m3 for Combretum zeyheri, and
• only 670 kg/m3 for Combretum erythrophyllum.

All of the above spp. occur in Kruger National Park.
 
What this means:
Combretum apiculatum, most individuals of which in Kruger National Park are effectively large shrubs (https://www.inaturalist.org/observations/58033115), and which is commonly subjected to non-lethal uprooting by the African bush elephant, actually has denser wood than the renowned leadwood. Its wood is nearly twice as dense as that of a particularly willow-like species (C. erythrophyllum), which grows taller (up to 12 m with up to 60 cm stem diameter) than C. apiculatum (up to 9 m with up to 40 cm stem diameter).

Publicado el 08 de agosto de 2022 a las 05:24 AM por milewski