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 August 07, 2022 07:33 AM by milewski milewski


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