Searsia (Anacardiaceae) in perspective, part 1

The genus Searsia of the Anacardiaceae is extremely important in the vegetation of the part of South Africa corresponding climatically to the southwest of Western Australia.

This genus

  • straddles the divide between vegetation dominated by pyrophiles and vegetation dominated by pyrofuges,
  • tends not to dominate any particular stand of vegetation (although tending to be common where it occurs),
  • has complex patterns w.r.t. fruit type and spinescence,
  • has no counterparts in eastern or northern Australia, and
  • has previously been confused taxonomically, in Asia and even in northern Australia, with Rhus, which is actually quite different.

So, the absence of Searsia from southwestern Australia is a minor question relative to the major question of the absence of this genus from Australasia generally. But the latter question is salient simply because Searsia is overall such an important genus in South Africa, across such a wide spectrum of vegetation types.
It is odd that Searsia disappears so abruptly once one leaves southern Africa. However, the genus persists into Mediterranean North Africa and the Middle East in the form of at least one species, namely S. pentaphylla, see . And there is at least one species in India, namely S. mysorensis.
Searsia in the southwestern Cape of South Africa is always recognisable by its trifoliate leaves, but otherwise varies greatly in life-form. It ranges from small shrubs to trees large enough to form forests, from non-spinescent to grossly stem-spinescent, from fleshy-fruited (most spp.) to non fleshy-fruited (e.g. S. tomentosa, S. incisa and S. angustifolia, common on the Cape Peninsula in scrub intermediate between pyrophilic and pyrofugic), and from fully evergreen (never really sclerophyllous) to semi-deciduous. One species (S. pyroides) even has toxin-laden spines.
In pyrofugic scrub/thicket near the littoral in the vicinity of Cape Town, the main spp. are S. glauca, S. crenata and S. laevigata. Two of these spp. have rigid branchwork representing a kind of semi-spinescence.
In fynbos, including the borders of pyrofugic forest patches, a main species is S. lucida. This is fully evergreen and completely non-spinescent, and has fruits that never quite seem to ripen to anything clearly describable as zoochorous.

However, there are also various minor spp. such as S. rosmarinifolia, S. dentata and S. longispina, again tending to hug the less fire-prone sites in fynbos such as boulderfields. Searsia longispina is more stem-spinescent than any common shrub in southwestern Australia.
In the semi-arid Karoo, Searsia undulata is a conspicuous shrub, because it tends to be larger than the predominant cover of small shrubs of Asteraceae. It is semi-spinescent in the sense of having rigid branchwork, and is evergreen but non-sclerophyllous. It is typically bird-dispersed for the genus, with small rather dull fruits that certainly have fruit-pulp and certainly ripen, but tend to dry out and become hollow as they ripen.
In the pyrofugic forest in the Knysna area, an important species is S. chirindensis, which can have long, stout stem-spines at least where broken by megaherbivores.
In drainage lines in the Karoo, immediately adjacent to the area of mediterranean climate, S. lancea is a prominent large shrub or small tree, and is evergreen.
The relationship between Searsia and pyrofugic vegetation is rather nebulous. However, what remains true is that in ‘pure’ fynbos, i.e. the most pyrophilic vegetation in southwestern South Africa, Searsia tends to be either absent or present only as occasional plants of non-spinescent, evergreen, non-zoochorous spp.

The genus Searsia does not really lend itself to features such as serotiny, bradyspory, or sclerophylly. It is never leaf-spinescent; and, even though it is ‘resinous’, I know of no species that is particularly flammable – the function of the ‘resin’ being defence against folivores.
Any biologist thoroughly familiar with the vegetation of the southwestern Cape of South Africa, if told that there is analogous vegetation in southwestern Australia, would surely assume that there must be some lookalike for Searsia in the Australian area, whether phylogenetically related or evolutionarily convergent. But they would be wrong.

This failure of southwestern Australia – and indeed the rest of Australia – to produce anything that I could call a counterpart to the various searsias (which are in many ways so diverse that they make for a remarkably plastic genus) is for me one of the most surprising aspects of the intercontinental comparison. It was among the first anomalies I noticed when I came to Australia in 1977.
In summary:
Searsia is a nebulous genus in various ways, tending to defy generalisation in its ecological role. It seems to play the part of a ‘filler-in’ in the vegetation, in many ways. Furthermore, one can never really call it a dominant in any vegetation type – although it approaches this in the case of

  • S. glauca near the littoral, where it tolerates wind-shearing well, or
  • S. undulata or S. lancea in moister microsites in Karoo, which are the tallest plants in the community.

Searsia is certainly shaped by large ungulates. However, it does not becomes downright spinescent without its branches being actually broken.

Searsia can best be thought of as a genus of ‘transitional scrub’. It is easy to imagine this or a lookalike shrub being common in places in southwestern Australia, including the relatively nutrient-rich microsites, protected from intense wildfires, in the Geraldton area, or some of the protected swales along the coast and on coastal islands.
At a biogeographical level, it is true to say that Searsia is a kind of ‘balancer’. On one hand it is true that there is a remarkable overlap between Australia and southern Africa in the sense that both landmasses contain Canthium, Carissa, Diospyros, Grewia, Flueggia, Solanum, Dioscorea, etc. etc. However, on the other hand the two landmasses are worlds apart when it comes to Searsia.

Searsia does occur in ‘rainforest’ (e.g. in the form of S. chirindensis) and it also occurs in pyrofugic vegetation of drier sorts in the form of the glauca-undulata group, plus the phreatophytic S. lancea.

No species of Searsia has fruit that remains succulent until separation from the plant by fruit-eaters, as seen in confamilials such as Sclerocarya caffra. In the Rhus/Malosma/Searsia complex, the fruits vary, according to genus and species, in the adherence of endo- and mesocarp, and resinousness vs waxiness,

Generally speaking, the spherical drupes of certain spp., e.g. Searsia dentata, Searsia crenata, Searsia chirindensis, and Searsia pyroides, are eaten by birds and monkeys while still fleshy. By contrast, the non-spherical, discoid fruits of e.g. Searsia undulata, Searsia burchellii, and Searsia lancea, are eaten by these animals also when 'post-ripe, i.e. when the exocarp of has dried without wrinkling (Rodney Moffett, who has taxonomically revised Rhus sensu lato, and is an author of, in email).

In Searsia incisa, the exocarp tends to crack. There is no suture as such, and some drupes simply crack spontaneously (owing to weathering or physical pressure) due to the fruit being relatively large, the stone small, and the exocarp thin. In his species one also finds some infructescences with the drupes so tightly packed that, when the individual fruits separate, it may give the impression of dehiscence.

Also see

to be continued in

Publicado el julio 1, 2022 02:39 MAÑANA por milewski milewski


The seed, in the diaspore of Searsia, is in a 'stone', technically called a pyrena.

Publicado por milewski hace más de 1 año

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