Dwarf ebonies, part 4: tough-wooded versions of ericas, resilient from megaherbivory?

...continued from https://www.inaturalist.org/journal/milewski/61065-dwarf-ebonies-part-3-adaptation-of-diospyros-to-winter-rainfall-and-adjacent-climates-is-unique-to-south-africa#

We have seen that various species show the syndrome of dwarf ebonies to various degrees.

The following list is in decreasing order, based on degree of diminution of the whole plant and the leaves, degree of sclerophylly, degree of resilience from wildfires, degree of tolerance to nutrient-poverty, and degree of restriction to winter-rainfall climates:

Diospyros glabra (the most extreme example of a dwarf ebony)

Euclea tomentosa

Euclea acutifolia

Diospyros austro-africana

Euclea racemosa racemosa

Euclea polyandra

Euclea coriacea

Euclea scabrida var. cordata

Euclea crispa var. ovata

Sideroxylon inerme

Diospyros lycioides (if qualifying as a dwarf ebony then only at the southwestern extreme of its distribution)

Diospyros dichrophylla

We have also seen that southern Africa is globally odd in the extension of ebonies (Ericales: Ebenaceae-Sapotaceae), in evergreen and non-spinescent form, to temperate latitudes and winter-rainfall climates.

In order to understand this pattern, one should first realise that southern Africa is equally odd in its lack of most other clades of Ericales.

Any claim that southern Africa lacks Ericales may at first sound ignorant of the unrivalled number of species of Erica (https://en.wikipedia.org/wiki/Erica_(plant)) in the Cape Flora. This number, about 700, happens to be similar to the number of species of Diospyros worldwide (https://en.wikipedia.org/wiki/List_of_Diospyros_species).

However, Ericales contains many families besides Ericaceae, and there are six subfamilies of Ericaceae besides Ericoideae. All of these are absent from southern Africa except for one isolated species. I refer to Vaccinium exul (https://www.inaturalist.org/taxa/595940-Vaccinium-exul), which itself occurs in the subtropics of South Africa, nowhere near the southwestern Cape.

More particularly, what is missing from the otherwise diverse floras of fynbos, karoo, and grassland in southern Africa is any member of Ericales combining lignified or diminutive leaves with fleshy fruits attractive to seed-dispersing birds.

This allows us to reframe the dwarf ebonies as uniquely African 'erica-substitutes'.

From an adaptive viewpoint, why would such floristic substitutions have occurred in Africa?

One possible reason is that Africa has been unusual in the intensity of its regime of damage to woody plants by large mammals.

Within the African context, the Cape Floristic Region (containing mainly fynbos vegetation) is regarded as being unsuitable for large ungulates with their mainly tropical associations. However, relative to comparable ecosystems on other continents the southwestern Cape of South Africa was surprisingly rich in large mammals (https://cdn.24.co.za/files/Cms/General/d/2276/69b122dacb504e22826fa5c7da943188.pdf).

The fauna of fynbos included the largest-bodied extant land mammal (Loxodonta africana) and the largest-bodied antelope (Taurotragus oryx oryx).

This raises the possibility that exceptionally tough wood helped to make Ebenaceae-Sapotaceae better-adapted than ericas - particularly Vaccinioideae, Arbutoideae and Epacridoideae - to a regime of physical breakage.

The available information is insufficient to test this idea, partly because the mechanical properties of wood are a complicated, technical subject. However, it is intriguing that Coates Palgrave mentions the following for Diospyros lycioides: "the tough black roots rapidly blunt ploughs and other farming implements".

Publicado el enero 12, 2022 06:18 MAÑANA por milewski milewski


The densities of the wood of various species of Diospyros are difficult to compare because density and specific gravity have been measured in inconsistent ways.




Publicado por milewski hace más de 2 años

Diospyros glabra cannot be seen as convergent with most vacciniums (i.e. forms of Vaccinioideae) because a) most vacciniums are deciduous (not evergreen) and exceed D. glabra in the size of the whole plant, and b) there are no vacciniums in fire-prone shrubby vegetation (chaparral or matorral) in parts of California or the Mediterranean Basin with a mediterranean-type climate.

Publicado por milewski hace más de 2 años
Publicado por milewski hace alrededor de 2 años

Euclea racemosa, Euclea schimperi, Euclea divinorum, and Euclea natalensis form a spp.-complex. Each species is variable, and all seem capable of interbreeding. They seem to vary their chemical anti-herbivory defences as a matter of life-history strategy, rather than as a simple reflection of nutrient-poverty in the local soils.

During a visit to Ithala Game Reserve in 2000, I found it significant that Euclea natalensis was 'heavily browsed' there.

According to Pooley, E. natalensis is capable of growing up to 18 m high, in patches of fire-free forest. It occurs from coastal dunes (where it can be 'very stunted in salt spray zone') to the Midlands of Natal, and extends to tropical Africa. Its wood is so hard and tough that it is avoided as firewood in Zululand.

Publicado por milewski hace más de 1 año

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