Dwarf ebonies, part 3: adaptation of Diospyros to winter-rainfall and adjacent climates is unique to South Africa

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The most obvious 'dwarfing' of ebonies in southern Africa belong to the genus Euclea (see my last Post).

This is because Euclea:

  • is, unlike its cosmopolitan relative Diospyros, restricted to Africa and Arabia,
  • tends to be shrubby even in its most arborescent forms,
  • is nowhere species-richer than in southern Africa, and
  • penetrates the winter-rainfall climates of South Africa in the cases of at least nine species, four of which are completely restricted to these climates.

However, confamilial Diospyros - an exceptionally widespread and speciose genus globally (https://en.wikipedia.org/wiki/Diospyros) - also participates in the syndrome of dwarf ebonies to a surprising degree. This is partly because Diospyros is incomparably more widespread and ecologically differentiated in southern Africa than on climatically analogous landmasses.

Three forms of Diospyros in South Africa qualify as dwarf ebonies:

  • Diospyros glabra in mesic fynbos throughout the southwestern Cape,
  • Diospyros austro-africana in various vegetation types from the southwestern Cape through the Great Karoo to the Highveld, and
  • Diospyros scabrida var. cordata in certain types of scrubby vegetation (other than fynbos and succulent thicket) in the Eastern Cape.

In all three cases the plants tend to be low shrubs associated with rocky substrates (see https://www.inaturalist.org/observations/67144217 for an extreme example).

None of these species is genotypically dwarfed to the degree seen in e.g. Euclea tomentosa, E. acutifolia and E. polyandra, because all can grow several meters high where protected from wildfire.

Nor can any species of Diospyros be called sclerophyllous. Euclea becomes sclerophyllous rather than small-leafed where the whole plant is diminutive. Although the foliage is also adapted in the case of Diospyros, the leaves of dwarf ebonies of this genus are reduced in size rather than being lignified.

Diospyros glabra (https://www.inaturalist.org/taxa/569146-Diospyros-glabra) is restricted to the acidic soils of the heartland of the Cape Floristic Region (https://en.wikipedia.org/wiki/Cape_Floristic_Region), part of which is in a winter-rainfall climate. It has penetrated fynbos (e.g. https://www.inaturalist.org/observations/67105857 and https://www.inaturalist.org/observations/91888971) more deeply than has any species of Euclea, and it seems to be more adapted to a regime of frequent wildfires than is any other species of dwarf ebony (e.g. see https://www.inaturalist.org/observations/20952415).

What this means is that Diospyros glabra is the prime example of an ebony exclusive to the Cape Flora. There is no similar plant in any analogous flora of nutrient-poor, fire-prone environments elsewhere on Earth (e.g. https://en.wikipedia.org/wiki/Kwongan).

The only feature of D. glabra that remains incongruous with fynbos is its fleshy fruit, attractive to seed-dispersing birds (https://www.inaturalist.org/observations/10084476 and https://www.inaturalist.org/observations/70307544 and https://www.inaturalist.org/observations/11278386 and https://www.inaturalist.org/observations/68855126 and https://www.inaturalist.org/observations/77290881 and https://www.inaturalist.org/observations/75933818 and https://www.inaturalist.org/observations/71651195 and https://www.inaturalist.org/observations/71303126 and https://www.inaturalist.org/observations/71057924 and https://www.inaturalist.org/observations/70489280 and https://scholar.ufs.ac.za/handle/11660/4322).

More illustrations of Diospyros glabra:

https://www.inaturalist.org/observations/89676893
https://www.inaturalist.org/observations/91614370
https://www.inaturalist.org/observations/63725719

It occurred to me that Diospyros glabra might be regarded as the biogeographical and ecological equivalent of Vaccinioideae, a tribe of Ericaceae notably absent from the Cape Flora and most of Africa. However, this idea did not stand up to scrutiny.

The following illustrate Diospyros austro-africana (https://www.inaturalist.org/taxa/583880-Diospyros-austro-africana), which has possibly the smallest leaves of any member of this genus worldwide:

https://www.inaturalist.org/observations/99336091
https://www.inaturalist.org/observations/79033492
https://www.inaturalist.org/observations/74022426
https://www.inaturalist.org/observations/11353292

The following illustrate Diospyros scabrida var. cordata (https://www.inaturalist.org/taxa/598746-Diospyros-scabrida-cordata):

https://www.inaturalist.org/observations/10851680
https://www.inaturalist.org/observations/70094165
https://www.inaturalist.org/observations/69870118
https://www.inaturalist.org/observations/20551835
https://www.inaturalist.org/observations/41098873
https://www.inaturalist.org/observations/22290772

It is easy for South African naturalists to take for granted the floristic integration of Diospyros into various types of low vegetation. However, the oddness of this emerges from comparison with other landmasses having similar latitudes and climates.

South America is extremely different from Africa in the incidence of Diospyros. Not only is there no counterpart for dwarf ebonies, but most of South America at the latitudes of South Africa lacks Diospyros completely.

North America has a few species (e.g. see https://en.wikipedia.org/wiki/Diospyros_virginiana) of Diospyros at the latitudes of South Africa, but none is comparable with dwarf ebonies.

In Australia there are 12 indigenous species of Diospyros, the most southerly of which (https://www.inaturalist.org/taxa/538020-Diospyros-australis) reaches as far south as the southern tip of South Africa. However, this species is restricted to the east coast, and all but the eastern and northern 5% of Australia is naturally devoid of this genus (https://www.jstor.org/stable/43869000).

Despite the many floristic links among southern continents, no species of Ebenaceae occurs in any Australian (or South American) biome comparable with those described above in South Africa.

In partial summary, all continents and subcontinents ecologically comparable with the southwestern parts of South Africa have a negligible indigenous incidence of Diospyros.

Therefore the exceptional nature of southern Africa with reference to dwarf ebonies applies to Diospyros as much as to Euclea. The intercontinental difference is categorical in the sense that dwarf ebonies are exclusively southern African.

to be continued...

Publicado por milewski milewski, 09 de enero de 2022

Comentarios

Diospyros galpinii (https://www.inaturalist.org/taxa/583884-Diospyros-galpinii) is a geoxylic suffrutex rather than a dwarf ebony.

Publicado por milewski hace 4 meses (Marca)

Diospyros ramulosa (https://www.inaturalist.org/taxa/583889-Diospyros-ramulosa) is also restricted to the winter-rainfall climate, mainly under semi-arid conditions. However, it does not qualify as a dwarf ebony because it remains one of the tallest shrubs in the vegetation on rocky or bouldery slopes. Diospyros acocksii (https://www.inaturalist.org/taxa/583879-Diospyros-acocksii), like ecologically similar D. ramulosa, also tends not to be stunted enough to qualify as a dwarf ebony.

Publicado por milewski hace 4 meses (Marca)

Samuel et al. (2019) treats the southern African genus Royena as distinct from Diospyros and closer related to Euclea. This should certainly be of interest in the light of your discussion, @milewski. Most of your dwarfs seem to be in Royena and not Diospyros.

Publicado por pieterwinter hace 4 meses (Marca)

@pieterwinter Hi Pieter, Many thanks for this useful information. I recall that when I was an undergraduate at University of Cape Town half a century ago the name Royena was used. It looks like the taxonomy is coming full-circle.

Publicado por milewski hace 4 meses (Marca)

Very interesting, thanks.

Publicado por graham_g hace 4 meses (Marca)

@graham_g Hi Graham, Many thanks to you, with regards from Antoni.

Publicado por milewski hace 4 meses (Marca)

I love these journals, thank you!

Publicado por jeremygilmore hace 4 meses (Marca)

Pre-ripe display in Diospyros lycioides https://www.inaturalist.org/observations/23046264.

Publicado por milewski hace 4 meses (Marca)

@jeremygilmore You're most welcome and it is individuals like you who are the future.

Publicado por milewski hace 4 meses (Marca)
Publicado por milewski hace 4 meses (Marca)

Rhamnus ilicifolia is another example of a shrub combining xeromorphic leaves, adaptation to wildfires, vegetative regeneration, fleshy fruits, and seed-dispersal by birds (https://www.inaturalist.org/observations/102118515 and https://www.inaturalist.org/observations/100906513 and https://www.inaturalist.org/observations/97555498 and https://www.inaturalist.org/observations/96999527 and https://www.inaturalist.org/observations/96658317 and https://www.inaturalist.org/observations/96319846). Although not as abundant as Arctostaphylos, it is typical of chaparral vegetation under a mediterranean-type climate in California. Although convergent to some extent with dwarf ebonies, it differs from them in the following ways: a) the leaves are spinescent, and b) the genus is mainly temperate, not tropical.

Publicado por milewski hace 4 meses (Marca)

@milewski I am coming back to these ;posts about Diospyros dichrophylla (very interesting about the Erica link!). I am just about to post some images of D dichrophylla from a cave site I visited a few years ago which had very large Diospyros dichrophylla formign the "Green Wall" across the opening. The flowers were almost pitch black! I had never seen that before but I thibnk I did then read that sometimes they can be. I just wondered if you have ever seen this or know anythign about it, it seems an odd mutation when normally the flowers are pale lemion?

Publicado por yvettevanwijk1941 hace 4 meses (Marca)

@yvettevanwijk1941 Hi Yvette, That is interesting but I have no explanation for the blackness of the flowers...

Publicado por milewski hace 4 meses (Marca)

I will tag you when I upload the observations.

Publicado por yvettevanwijk1941 hace 4 meses (Marca)

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