Growth-form in Combretum, part 1

(writing in progress)

Combretum is an important genus of woody plants in Africa.

Various spp. in this genus Combretum have various growth-forms. The typical combretum may be more like a scandent shrub than like a leadwood.
 
See, for example, the combretum flora of an example country, Kenya. My source is Dale & Greenway (1961), ‘Kenya Trees and Shrubs’.
 
There are 22 spp. of Combretum listed from Kenya, excluding those spp. which are specialised as lianes.
 
Of these the following spp. are ‘trees’: binderianum, deserti, exalatum, fischeri, ghasalense, mechowianum, microlepidotum, molle, collinum, zeyheri, totalling only 10/22 spp.
 
The following are ‘scandent’, ‘lianes’, or ‘scrambling shrubs’ (typically in the undergrowth of forests): aculeatum, butyrosus, constrictum, denhardtiorum, padoides, tanaense, totalling 6/22 spp. That’s right, more than a quarter of the combretums in Kenya are more like vines than like trees.
 
The following are ‘shrubs’ ‘bushes’ or ‘small shrubs’: apiculatum, contractum (which may be quasi-spinescent, see herbarium specimen below), ukambense, volkensii, totalling 4/22 spp.
 
This means that 10/22 of the spp. are scandent or shrubby, and Dale & Greenway do not even list those spp. of Combretum that are downright lianes, such as C. microphyllum, which I know from Kruger Park but which is also widespread in tropical Africa.
 
This leaves only two spp.: elgonense (for which no information on growth-form is given) and schumannii, which is an unusually tall tree (to about 20m high) in forest, with wood of density >100 kg/m3, i.e. comparable with leadwood although different in habitat. Combretum imberbe itself does not reach Kenya. 
 
To summarise: only one species of Combretum in Kenya is a tree exceeding about 10 m high. Most of the spp. in this genus in Kenya are scandent or shrubby.

In particular, please note that Dale & Greenway describe C. apiculatum, which is so important near large mounds (built by termites) east of Phalaborwa in Kruger Park, as a shrub (despite the extreme density of its wood).
 
Combretum contractum in Kenya, showing hint of quasi-spinescence comparable with that of juvenile C. imberbe in Kruger Park:

One of the lasting impressions of my visits to Kruger Park is that the African bush elephant has such an extreme relationship with its food-plant, Combretum apiculatum, in the area east of Phalaborwa that the ‘normal’ state of this plant is to be uprooted!

I saw this time and again. (It is also true to say that the ‘normal’ state of Sclerocarya birrea in this area is to be suppressed to sapling size with an elephant-dug circular trench around the ‘sapling’; however, the difference is that C. apiculatum is a dominant woody plant here which maintains a reproductive population, whereas S. birrea does not seem to get even close to reaching reproductive maturity in this area.)
 
Given this bizarre relationship between plant and animal, I have done some reading on the topic of the whole genus, Combretum, to shed more light on the specific interaction we saw.
 
The following are some of my findings.
 
Combretum, although absent from Australia, occurs as far away from Africa as South America. It is an important genus, for example, in the southwestern Amazonian forests.
 
Combretum is not only speciose and near-cosmopolitan, but it has greater plasticity in growth-form than many genera of woody plants. Even within Kruger Park, the diversity of growth-forms is impressive.

Combretum microphyllum is a liane, C. mossambicense is a lax scrambler, C. erythrophyllum is a willow-like shapeless self-standing plant with wood of comparable density to the willows it resembles, Combretum apiculatum seems to be effectively multi-stemmed despite having extremely dense wood, and Combretum hereroense differs from co-existing C. apiculatum in anticipating the rains in its shoot-growth (at least on granite), and C. imberbe is unusual in several ways: a) extremely calcium-rich wood, evergreenness, and the possession of a quasi-spinescent juvenile stage when within reach of the giraffe. 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 and protection of the seeds from granivores (such as the local species of parrot) by toxicity.
 
It is interesting that Combretum erythrophyllum occurs in the Sabie River bed, in the same environment as Salix mucronata and Morella serrata. As if it is not enough that a typical Northern Hemisphere taxon penetrates the Kruger Park in willow-like form, even one of the local combretums has adopted a similar willow-like form.

Towards a comparative view of C. apiculatum vs C. imberbe, let me first mention C. erythrophyllum, which is a peculiar species in that the only place where it is common in Kruger Park is along the Sabie River (it is also peculiar in ranging as far as the banks of the Orange River in the Northern Cape, the western Free State, and East London in the Eastern Cape).

Combretum erythrophyllum is not a small plant (it reaches up to 12 m high with up to 60 cm stem diameter), but it tends to be a huge shrub rather than a tree, to grow in dense stands rather than the spaced-out populations associated with most of its congeners, and to grow extremely rapidly (about 5 m in three years).

The interesting thing about it is that it seems immune to the elephant. As van Wyk (1974) 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.” (bold emphasis mine). So what’s interesting is that this species, erythrophyllum, has a different strategy from apiculatum even though both coexist with the elephant and both are effectively large shrubs 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 elephant, and that it is chemically defended against the elephant.

But my point is that C. apiculatum is far from simply being a typical combretum. If anything is typical in Combretum it is to be a lax woody plant similar to C. mossambicense, making both C. apiculatum and C. imberbe ‘peculiarly African’ in their extreme wood density and their extreme relationships with large herbivores.
 
Combretum imberbe is peculiar for its genus in several ways.

• It is quasi-spinescent, whereas I cannot offhand think of a single other species in this genus of hundreds of spp. which has any kind of spinescence at all. When still within the foraging height of the giraffe, C. imberbe in Kruger Park looks more like a Gymnosporia than like a combretum, a case of evolutionary convergence.
• It has an evergreen tendency, something poorly stated in the literature but which I have detailed notes on in my various diaries/journals from several visits to Kruger Park and elsewhere. Van Wyk (1974) disappointingly just calls it ‘deciduous’ without further discussion, which does not do this species justice in my opinion although one of the adjectives he uses to describe the leaves, i.e. ‘hard’, seems consistent with evergreenness.
• It seems strangely reluctant to adopt a multi-stemmed form, being bent on becoming a tree from the start, as it were.
• It gets peculiarly tall for a plant adapted to seasonal drought (it reaches 20 m in Kruger Park, on basalt plains where treeless grassland seems more the order of the day).
• It combines slow growth with extremely dense wood.

Van Wyk’s (1974) account is so interesting – particularly w.r.t. the relationship to fire - that I’ll 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.”
 
Do you see how much more there is to combretums in Kruger Park than meets the eye, and how far the literature is from satisfactorily characterising and comparing the strategies of the various spp.?
 
Having read a bit more about Combretum, I can point out the following.
 
Firstly, the typical growth-form of the genus, if there is such a thing, is a scandent/scrambling, multi-stemmed plant rather than an upright tree. This applies to southern Africa too. So C. mossambicense, which I hopped up and down about in Kruger Park when I met it for the first time a few weeks ago, only excited me so much because of my relative ignorance. In fact, this is far more typical of the genus than is C. imberbe.
 
I now see that it is a bit ironic that the African bush elephant habitually takes C. apiculatum, one of the minority of this genus that has evolved wood so dense that it sinks in water even when dry, and puts the plant back on its side again, into a kind of ‘forced recumbency’ despite the herculean performance of C. apiculatum in reinforcing its stem to a density exceeding even that of the famous C. imberbe, which is by far the tallest combretum in southern Africa, is reputed to live for more than a thousand years, and has boles so durable that they remain standing for decades after death.
 
Here are the data. The wood densities of C. apiculatum (which is effectively a large shrub in Kruger Park, rather than a tree) and C. imberbe are respectively 1230 kg/m3 and 1200 kg/m3, compared with 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 these spp. occur in Kruger Park.
 
What this means is that C. apiculatum, most individuals of which in Kruger Park are effectively shrubs and a species frequently uprooted by the elephant, actually has denser wood than the famous leadwood, and its wood is nearly twice the density 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).
  
One of the lasting impressions of my visits to Kruger Park this year is that the African bush elephant has such an extreme relationship with its food-plant, Combretum apiculatum, in the area east of Phalaborwa that the ‘normal’ state of this plant is to be uprooted! We saw this time and again in our plots, did we not? (It’s also true to say that the ‘normal’ state of Sclerocarya birrea in this area is to be suppressed to sapling size with an elephant-dug circular trench around the ‘sapling’; however, the difference is that C. apiculatum is a dominant woody plant here which maintains a reproductive population, whereas S. birrea does not seem to get even close to reaching reproductive maturity in this area.)
 
Given this bizarre relationship between plant and animal, I’ve done some reading on the topic of the whole genus, Combretum, to shed more light on the specific interaction we saw.
 
The following are some of my findings.
 
Combretum, although absent from Australia, occurs as far away from Africa as South America. It is an important genus, for example, in the southwestern Amazonian forests.
 
Combretum is not only speciose and near-cosmopolitan, but it has greater plasticity in growth-form than many genera of woody plants. Even within Kruger Park, the diversity of growth-forms is impressive. Combretum microphyllum is a liane, C. mossambicense is a lax scrambler, C. erythrophyllum is a willow-like shapeless self-standing plant with wood of comparable density to the willows it resembles, Combretum apiculatum seems to be effectively multi-stemmed despite having extremely dense wood, and Combretum hereroense differs from co-existing C. apiculatum in anticipating the rains in its shoot-growth (at least on granite), and C. imberbe is unusual in several ways: a) extremely calcium-rich wood, evergreenness, and the possession of a quasi-spinescent juvenile stage when within reach of the giraffe. 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 and protection of the seeds from granivores (such as the local species of parrot) by toxicity.
 
It’s interesting that Combretum erythrophyllum occurs in the Sabie River bed, in the same environment as Salix mucronata and Morella serrata. As if it is not enough that a typical Northern Hemisphere taxon penentrates the Kruger Park in willow-like form, even one of the local combretums has adopted a similar willow-like form.

to be continued in...

(writing in progress)

Publicado el 21 de junio de 2022 por milewski