Spinescence in the cycad Encephalartos, with reference to Macrozamia
The cycad family Zamiaceae (https://en.wikipedia.org/wiki/Zamiaceae) occurs on all of the southern continents (Australia, southern Africa, and South America).
A familiar genus in southern Africa is Encephalartos (https://www.inaturalist.org/observations?place_id=any&taxon_id=129815&view=species). This is closely related to Macrozamia (https://www.inaturalist.org/observations?taxon_id=129824) of southern Australia. Both genera penetrate temperate latitudes.
Foliar spinescence is far less widespread in southern African than in Australian plants (https://www.researchgate.net/publication/6213166_Ecology_of_Australia_The_effects_of_nutrient-poor_soils_and_intense_fires).
However, this does not hold food for cycads.
We can begin with Encephalartos transvenosus (https://www.inaturalist.org/taxa/135963-Encephalartos-transvenosus) of southern Africa. This species somewhat resembles the Australian Macrozamia communis (https://www.inaturalist.org/taxa/135902-Macrozamia-communis) in degree of spinescence and local commonness.
Here is some background reading: https://en.wikipedia.org/wiki/Encephalartos_transvenosus and http://www.junglemusic.net/Encephalartos_Species/Encephalartos_transvenosus.html.
Among the many southern African spp. of Encephalartos, E. transvenosus is both the tallest (most arborescent) and the locally commonest. It is perhaps common enough locally to qualify as ‘dominant in the vegetation’. And, unlike the case of M. communis, it seems to form the tallest statum of the vegetation, and is not merely a component of the understorey.
Here are some details to consider.
Firstly:
Encephalartos transvenosus may be as foliar-spinescent as M. communis. However, comparison is complicated by a different arrangement of the spines.
Secondly:
It is widely accepted that the local commonness of E. transvenosus is anthropogenic, representing a kind of ‘indigenous horticulture’ for non-utilitarian reasons.
Based on the descriptions in Coates-Palgrave (https://books.google.com.au/books/about/Trees_of_Southern_Africa.html?id=jtvZAAAAMAAJ&redir_esc=y and https://books.google.com.au/books/about/Trees_of_Southern_Africa.html?id=TR8gAQAAIAAJ&redir_esc=y), I note the following points about foliar spinescence in 18 spp. of southern African Encephalartos.
Firstly:
It is odd that spines on the tips of the main pinnae are seldom mentioned, even though drawings show clearly that the leaf-tips are acute.
Secondly:
The proximate (small) pinnae are described as spine-like in some of the spp., but in most this is stated not to be the case.
Thirdly:
Some spp. of Encephalartos (including E. transvenosus) have toothed (spinescent) margins to the main pinnae (a feature lacking in M. communis). However, this feature tends to be rather inconsistent in the genus, and absent in certain spp. of Encephalartos.
As a result, I infer that at least some of the spp. of Encephalartos lack leaf-spines completely. Can the same be said for any spp. of Macrozamia?
Encephalartos transvenosus is moderate within its genus, w.r.t. the incidence/degree of development of leaf-spines. It shares with M. communis the incidence of leaf-spines formed from the proximate pinnae, and it has marginal spines on the pinnae which M. communis lacks. However, I have the impression that E. transvaalensis may possibly lack spines on the tips of the main pinnae, which M. communis may possibly possess.
Encephalartos is known to have provided indigenous people with stem-starch. Thus, this genus qualifies as a human food-plant beyond any consumption of its seeds or ‘fruit-pulp’. Has any species of Macrozamia ever been recorded as a food plant for aboriginals w.r.t. stem-starch?
Anyway, the preservation, and possible promotion, of a dense stand of E. transvenosus in one part of South Africa (where a National Monument was declared in 1936) seems to reflect an attachment by the local Bantu horticulturalists for cultural reasons beyond any utilitarianism of foraging. (Unlike Australian aboriginals, the people in question certain cultivated annual crops and probably kept livestock too.)
Hence it seems reasonable to conclude that, even if E. transvenosus is as foliar-spinescent as M. communis, and even if it is dominant in small patches of vegetation, the difference is that the vegetation in which M. communis is abundant is the ‘more natural’. Even if the local stratum-dominance by M. communis is in some sense anthropogenic, the local dominance of E. transvenosus seems more clearly anthropogenic.
The following shows leaves of Encephalartos latifrons (https://en.wikipedia.org/wiki/Encephalartos_latifrons) of probably fire-protected environments in the Eastern Cape, South Africa. Note the rigidity (sclerophylly) and spinescence of the pinnae, probably including the tips of the pinnae. This species is extreme in a genus of which most spp. have rather weak foliar spinescence.
The following shows the leaves of Encephalartos transvenosus, which is unusually tall (arborescent) and grows in unusually dense stands in a few areas in Limpopo Province of South Africa. This species is certainly foliar-spinescent but, unlike Macrozamia communis, the main leaf-spines are on the margins of the pinnae.
My source of the above information on foliar spinescence in Encephalartos, namely Keith Coates Palgrave, seems ambivalent about whether the tips of the leaflets (as opposed to the spines on the margins of the leaflets) are ‘pungent’.
A far clearer and more detailed source of information is D. L. Jones (2002, https://books.google.com.au/books/about/Cycads_of_the_World.html?id=lFEeAAAACAAJ&redir_esc=y).
From Jones (2002) it is clear that in many spp. of Encephalartos (but not all), the tips of the pinnae are indeed spinescent (‘pungent’). In this sense, there is strong similarity to Macrozamia.
However, the situation is surprisingly complex in Encephalartos. This genus shows much variation in the degree of foliar spinescence and in the placement of the spines.
At least one species lacks spinescence entirely.
Others, with specific epithets such as ‘ferox’ and ‘horridus’, are extremely foliar-spinescent.
Some have the proximal pinnae spinescent in a ‘reduced’ way, i.e. the leafy function reduced but the spinescent function emphasised; in other congeners these proximal pinnae are not pungent.
In some spp., there are marginal spines on the pinnae; in others not.
Furthermore, in some spp. the spines derived from the proximal pinnae can be bifurcate or even trifurcate.
So the full range of variation in Encephalartos, w.r.t. foliar spinescence, is that at least one species is not at all leaf-spinescent, whereas certain species possess all of the following:
- leaf-spines on the tips of the pinnae,
- several leaf-spines on each margin of each pinna, and
- leaf-spines on the relatively small proximal pinnae, where the structure is better described as a spine than as a pinna.
Yes, what this means is that in some spp. of Encephalartos the proximal pinnae are not spinescent at all, whereas in others these proximal pinnae are so spinescent that the whole pinna has been converted into not just a spine but a trifurcate spine.
I conclude, overall, that Encephalartos (Africa) is more foliar-spinescent than Macrozamia (Australia). In both genera, there is virtually no spinescence in certain spp. However, even the most foliar-spinescent of Macrozamia spp. lacks marginal spines and bi- or trifurcate proximal spines, features that are far from rare among the spp. of Encephalartos.
There is also at least one species of Encephalartos, and a particularly leaf-spinescent one at that, that is described as forming virtually impenetrable thickets. The size of these thickets is not stated, and they are probably less than 100 square metres. However, there is a hint of dominance of the vegetation at a small scale.
And there is another surprising aspect to foliar spinescence in Encephalartos, as follows, which none of my readings on Macrozamia prepared me for. This is the question of deciduousness.
Many plants on Earth are deciduous, but in general deciduous leaves tend not to be sclerophyllous (https://en.wikipedia.org/wiki/Sclerophyll), let alone tip-lignified enough to be called ‘pungent’. Sclerophyllous leaves by their nature are somewhat ‘green-woody’, and tend to live relatively long before falling. They are ‘structurally invested’ rather than being energetically ‘cheap and disposable’.
In general in the floras of the world, if one finds a sclerophyllous, or even semi-sclerophyllous, plant then one can predict with confidence that it is ‘evergreen’. And even more so if the leaf in question also happens to be spinescent, not so?
Although this is an exaggeration, spinescent leaves ‘dream of dying by combustion’ rather than ‘dreaming of ‘'dying from seasonal senescence’. Evergreen leaves (epitomised by aromatic eucalypts) may be programmed to die, but more by fire rather than by means of either herbivory or abscission. And although it may be true that most spinescent leaves are not ‘aromatic’ enough to provide ‘oil’ as a fuel for ‘crown fires’ in the way seen in hummock grasses (Triodia, https://en.wikipedia.org/wiki/Triodia_(plant)), they often grow in the midst of flammable vegetation.
Based on the above pattern, which is a fairly reliable pattern in plant ecology in my experience, one would not expect any species of Encephalartos to be deciduous.
And yet several spp., found in several widely separate parts of Africa, are indeed deciduous. The leaves tend to fall each year, so that no leaves remain photosynthesising during the cool, dry season. One of these spp., which lives in tropical West Africa, is also particularly cold-sensitive.
What this means:
I now realise something of which I hardly even conceived in the past: that there is such a thing as a large-leafed plant that is both foliar-spinescent and deciduous.
I do not know if this anomaly has been pointed out as such in the literature.
However, it is of particular interest in terms of ‘ecological strategies’.
My impulse is to explain this strategy in the context of fire-prone environments (such as the savanna habitat of the West African species). However, it occurs also in at least one cliff-hugging species of Encephalartos in South Africa, where it cannot be explained by fire.
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I have discovered another case relevant to the question: which vegetation types on Earth are dominated by leaf-spinescent plants?/This case is about the cycad Macrozamia communis./Here is some background reading:http://www.pacsoa.org.au/wiki/Macrozamia_communis and https://davesgarden.com/guides/articles/view/1619 and https://anthropologyfromtheshed.com/project/the-ancient-practice-of-macrozamia-pit-processing-in-southwestern-australia/#/Macrozamia communis is superficially similar to a miniature date palm (Phoenix), with a similar configuration of leaf-spinescence./However, Macrozamia communis is unlike date palms in being sclerophyllous and fire-prone; and its zoochory is peculiar relative to that of date palms./The spines of M. communis are located at the tips of the proximate pinnae of the large pinnate leaves. The cones of this species are also spinescent, presumably defending the immature cones from predation of the forming seeds by (extinct?) animals. When the female cones are mature, they naturally ‘dehisce’ (fall apart) to scatter the reddish diaspores on to the nearby ground, attracting seed-dispersers in return for the food-reward offered by the edible coating (sarcotesta) on the seeds./Macrozamia communis is not the most leaf-spinescent of the 40 species in its genus (all of them restricted to Australia), but it is unusual in growing in particularly dense communities as an understorey to eucalypt forests, particularly on coastal sands. The best-known examples of dominance in the understorey by M. communis are near Batemans Bay, New South Wales./I cannot claim that there is any vegetation type dominated by M. communis, despite the dominance of the understorey by this species in certain vegetation types. As far as I know, the dominant plants in all cases are the eucalypts which form the canopy, such as Corymbia maculata and Angophora costata. However, dominance by a leaf-spinescent plant even in the understorey is noteworthy, because I know of no vegetation on other continents in which leaf-spinescent cycads dominate the vegetation even in the lower stratum, and I know of few forests or woodlands of any kind, worldwide, in which he understorey is dominated by leaf-spinescent plants./There are places, particularly near Batemans Bay, where the stand of M. communis is so dense, and the pinnae so spinescent, that it is challenging for a person to walk through the vegetation safely (see the first link above)./I do not know whether crown fires, which are expected in most types of eucalypt forest in southern and southeastern Australia, occurred in the types of forest in which M. communis is most common. I suspect that these particular forests were somewhat exempt from crown fires, one line of evidence for this being that the local eucalypts may not be as aromatic (= oily, = fuel-rich) as expected for southern eucalypts in Australia. If I am right, the natural regime of fire in these forests was more or less restricted to the understorey, fires recurring every decade or so. The cycad itself is probably ‘pyrophilic’ in the sense that it promotes fire by means of the flammability of its living and dead parts, and in turn it is promoted by fire, its regeneration and reproduction being facilitated by the deposition of its own ash, which contains nutrients scarce in the soils inhabited by M. communis. /It is possible that the density of M. communis, in the vegetation types in question, is partly aboriginal-anthropogenic. Macrozamia was one of the dietary staples for aboriginals in parts of Australia, the part eaten being the toxic seed or, in the case of southwestern Australian Macrozamia riedlei, the ‘fruit-pulp’ (technically the sarcotesta). Although we can assume that Macrozamia was naturally dispersed, regardless of human interference, by emus (Dromaius) in prehistoric times, the relationship between Macrozamia and emus is odd relative to ‘typical’ syndromes of endozoochory by birds or mammals. It is well-known that emus swallow large objects and digest them only superficially, defecating them more-or-less intact. So it is likely that emus would have swallowed the bright-hued diaspores of Macrozamia, superficially digesting the oily sarcotesta and defecating the intact seeds, covered in the remains of the sarcotesta. Two problems arise. Firstly, it is hard to see how the emu would digest enough of the sarcotesta for a specialised mutualism to arise between the plant and the bird. Secondly, it is difficult to explain concentrated populations of Macrozamia based on dispersal and sowing by non-human animals. Because aboriginals relied on Macrozamia for food (the food being the seeds in the case of M. communis), it makes sense that these people might have ‘quasi-farmed’ M. communis in places, by planting a proportion of the seeds rather than eating them, and by deliberately setting fire to the stands at a certain frequency intended to promote the cycad relative to competing plants of the understorey. Because
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