Carpobrotus, a plant paradoxically combining succulence with fire-proneness

@tonyrebelo @jeremygilmore

(writing in progress)

Also see https://www.inaturalist.org/journal/milewski/73681-how-should-we-describe-and-interpret-the-fleshy-fruit-of-carpobrotus-edulis-aizoaceae#
 
Recently, a fellow naturalist - and well-published vegetation scientist - watched as a pure stand of Carpobrotus (presumably C. edulis, https://www.inaturalist.org/taxa/49322-Carpobrotus-edulis) burned with flames 0.4 m high, in late spring at Simon's Town (https://en.wikipedia.org/wiki/Simon%27s_Town) in the southwestern Cape of South Africa.

Although the weather was warm at the time, it was not yet summer, let alone late summer.

So, I assume that, if a pure stand of this succulent mat-plant could burn in late spring, then this indicates that it would burn even more readily in the combination of heat and drought of late summer.
 
The surprising observation – that Carpobrotus is not only scorched by fire inflicted by nearby plants, but actually burns in its own right - seems like the tip of some conceptual iceberg.

Everything I had known about C. edulis indicated a fire-shunning niche, in which any fire experienced by this species has deleterious effects rather than promoting the plant.

It seems obvious that C. edulis, as a mat-forming succulent, should stop a fire in its tracks. However, my colleague's observation suggests that this plant instead ‘invites fire in’ to some degree, possibly by having a structure and chemical composition of its stems and dry leaves that actually fuels the fire at ground level.
 
Could it be that Carpobrotus may turn out to be that counterintuitive thing, a fire-adapted succulent?
 
There are various reasons why Carpobrotus should not burn easily, or benefit ecologically from fire. The niche of C. edulis is ostensibly to avoid fire, suppress fire, and occupy the fire-free parts of the landscape. It seems natural to assume that Carpobrotus is the antithesis of e.g. fynbos (https://en.wikipedia.org/wiki/Fynbos), because neither its growth-form nor its leaf texture seem, on the face of it, to encourage combustion at all.
 
Here are the factors suggesting that dominance by Carpobrotus edulis would be mutually exclusive with fire:

• It is a pioneer plant in successional terms, later replaced by other plant communities that may eventually carry fire. For such a pioneer of bare ground (of the sort not caused by fire but caused instead by wind, erosion, deposition of fresh sand, etc.) to invite or encourage fire seems self-defeating.
• It is fully succulent, the water in the leaves surely dampening any fire. It is also fully evergreen, remaining succulent even at the hottest, driest time of year.
• It is mat-forming, so that its biomass does not seem spatially arranged in a way conducive to fire.
• Its normal habitats are buffered from fires partly by being littoral. With the beach on one side, there is reduced likelihood that fire in the vicinity will reach the stands of C. edulis.
• It produces fleshy fruits and seeds dispersed by animals. Endozoochory is not generally associated with fire-prone plants, which instead tend to disperse their seeds inanimately. Can readers think of a single plant species, indigenous to the Fynbos Biome, which combines leaves edible to humans (even in small amounts) with encouragement of fire?
• It seems to benefit from intense grazing, e.g. at De Hoop (https://en.wikipedia.org/wiki/De_Hoop_Nature_Reserve), where the mats of Carpobrotus seem to have grown with the increase in the densities of indigenous grazing ungulates such as Damaliscus pygargus pygargus (https://www.inaturalist.org/taxa/42275-Damaliscus-pygargus), Equus zebra (https://www.inaturalist.org/taxa/43330-Equus-zebra), and Taurotragus oryx oryx (https://www.inaturalist.org/taxa/75192-Tragelaphus-oryx). Since these mats are surrounded by lawn at De Hoop, it seems as if the plant is thriving in a situation naturally protected from any fires in the surrounding limestone fynbos.
• It tends to grow on relatively nutrient-rich, albeit sandy, soils. The nutrient-richness is derived from the cation-rich status of fresh sea sand, plus marine aerosols. Succession on such soils is likely to produce strandveld (https://en.wikipedia.org/wiki/Cape_Flats_Dune_Strandveld) rather than fynbos. Strandveld may sometimes burn but in general it is a fire-suppressing vegetation type in which the dominants have fleshy fruits.
• It has neither mycorrhizae (https://en.wikipedia.org/wiki/Mycorrhiza) nor cluster roots (https://en.wikipedia.org/wiki/Cluster_root). Virtually all perennial plants adapted to fire have either mycorrhizae or cluster roots.

Carpobrotus forms, in a sense, a ‘succulent lawn’. Lawns tend not to be fire-prone or promoted by fire. Any plant that is both fully succulent and ‘lawn-forming’ would seems to have little to gain from combustion.
 
Although some species of Aloe (https://www.inaturalist.org/observations?place_id=any&taxon_id=71956&view=species) are, in a sense, fire-adapted succulents, the whole growth-form of these aloes seems designed to protect the plant, particularly the apex of the rosette, from destruction by burning (https://www.jstor.org/stable/4217000 and https://asknature.org/strategy/dead-leaves-insulate-against-fire/ and https://opuntiads.com/oblog/aloes-dead-leaves-and-fire/). The form of these aloes seems to indicate that they do well in fire-prone grasslands despite, not because of, fire.
(@tonyrebelo Does any species of Aloe actually qualify as fire-dependent or fire-promoted, as opposed to merely fire-tolerant?).
 
Ash is seldom favourable to perennial succulents. There are of course various ephemeral/annual/pauciennial succulents that grow briefly on ashbeds in various parts of the world. However, Carpobrotus is different, because it is

• perennial,
• capable of vegetative regeneration, and
• generally a pioneer of surfaces laid bare by agencies other than fire.

I would sum up the succulent plants on Earth as follows: some may tolerate fire, but almost all survive despite, rather than because of, fire. I.e. I can hardly think of a species of succulent, anywhere, which I know to benefit from fire in the sense that it would not do better if fire was completely excluded from the landscape in which it naturally grows.

The only exceptions that springs to mind are some species of ?Lampranthus (https://en.wikipedia.org/wiki/Lampranthus) in dry, sparse fynbos (e.g. near Vanrhynsdorp, https://en.wikipedia.org/wiki/Vanrhynsdorp). These persist into mature stands, and seem to regenerate along with heathy plants and restios in a regime partly nourished by ash. (@tonyrebelo ?)
 
However, if we think more broadly about the genus Carpobrotus, there are some hints of a relationship to fire.
 
One species of Carpobrotus is strongly associated with the fecal middens of Oreotragus oreotagus (https://www.inaturalist.org/taxa/42384-Oreotragus-oreotragus) in fire-prone mountain fynbos. Since these middens are small (< 2 m diam.), it seems likely that they are consumed by the burning of the surrounding vegetation. Is this species adapted somehow to fire?
 
Several species of Carpobrotus are indigenous to Australia. Since fire is a more pervasive and ubiquitous feature of southwestern Australian than southwestern Cape landscapes, it seems odd that a genus of leaf-succulents would diversify here unless somehow favoured by fire. None of the species indigenous to Australia, in my experience, grow in extensive mats, as does C. edulis. Therefore, none can much buffer itself from surrounding fires. The common species near Perth grows near the shrub Olearia axillaris (https://www.inaturalist.org/taxa/428475-Olearia-axillaris), which seems more flammable than any analogous dune-pioneer daisy in the Fynbos Biome.
 
Carpobrotus edulis, originating in South Africa, has been introduced to California - where it now covers large areas, particularly on coastal sandy soils. The paper below documented, as long ago as 1980, that this succulent benefited from fire in much the same way as expected in fire-prone plants with either sclerophyllous foliage or flammable substances in their leaves. Carpobrotus edulis, where invasive in California, is known to regenerate better after fire than in the absence of fire. Although succulent plants may survive/tolerate fires in some situations, it is highly unexpected for a succulent perennial plant to grow better on ashbeds than off them, in competition with indigenous plants.

It seems safe to assume that Carpobrotus edulis is incapable of burning intensely (in the way that e.g. a fynbos of similar biomass would burn). I take it for granted that any fire in a stand of C. edulis will always be relatively mild. However, the question resolves to: given that C. edulis is capable of carrying (and encouraging) a mild fire, does the plant benefit from this in any way? What is it, exactly, about its structure and composition that allow fire to take hold despite the water content of the leaves?
 
I suggest the following explanation for how Carpobrotus manages to burn despite being fully succulent.
 
The noteworthy fact about Carpobrotus seems to be that the leaves are persistent when dead, much as they are in plants specially adapted to fire, such as the Australian grass-tree Xanthorrhoea (https://www.inaturalist.org/observations?place_id=any&taxon_id=83581&view=species). (I realise that in almost every other way there is no resemblance between Carpobrotus and Xanthorrhoea.)
 
The leaves could be eaten by some animal once dry; they could decompose rapidly; they could at least fall off. Instead they seems to remain attached, nicely arranged with the air spaces among them needed to make a good kindling. What this amounts to is that the whole ‘canopy’ (if one can speak of a mat being three-dimensional enough to have a canopy) of Carpobrotus sits above a bed of kindling. The fire seems to be able to burn the aerated three-dimensional framework of stems and dry leaves, unimpeded by the water in the green leaves because the latter lie above the flames, and hot air rises.
 
So the central question seems to be: why do the leaves not fall off once dead and dry?
 
It might be instructive to collect some of the dead, naturally dry leaves (from an unburnt stand of Carpobrotus), and test them for flammability. I predict that they will turn out to be fair kindling, as opposed to being fire-dampeners. It might also be interesting to smell/chew them, for any noticeable odour/taste of terpenes/phenols. It would also be interesting to know just how much water remains in the air-dry dead leaf. However, I suspect that the main reason why the dead, dry leaves of Carpobrotus can catch fire relatively quickly is that the wax, which is initially used to seal the succulent green leaf against desiccation, remains on the dead leaf – somehow combined with enough porosity to allow shrivelling.
 
Invasive plants may seem to take advantage of various disturbances to gain ground. This may be why South African ecologists have not paid much attention to reports of an affinity with fire on the part of C. edulis in California. However, is it possible that, even in its natural habitat in the southwestern Cape, the regeneration of C. edulis and its congeners are promoted rather than retarded by fire?
 
http://www.jstor.org/stable/41424698?seq=1#page_scan_tab_contents
 
Californian coastal sage differs from coastal fynbos (where C. edulis originates) in that the dominant shrubs tend to be drought-deciduous. I think this difference is related to the greater nutrient-richness of the coastal sage (https://en.wikipedia.org/wiki/Coastal_sage_scrub) than the coastal fynbos. Despite its nutrient-richness, coastal sage does burn and is in some ways adapted to fire.
 
One particular species of daisy, Ericameria ericoides (https://www.inaturalist.org/taxa/60955-Ericameria-ericoides), which is common in places in Californian coastal sage scrub, is particularly fynbos-like. Indeed, it is called ‘mock heather’ by Californians, who have no fynbos-like members of the Ericaceae in their indigenous flora, although the broad-leafed erica Arctostaphylos (https://www.inaturalist.org/observations?place_id=any&taxon_id=47179&view=species) is an extremely common, fire-prone shrub in chaparral, and occurs also in coastal sage scrub.
 
Please see the illustrations below of Ericameria ericoides, which despite its name is a daisy and thus, I suppose, most comparable with Metalasia muricata (likewise a daisy, https://www.inaturalist.org/taxa/527592-Metalasia-muricata) in the Simon's Town area. Ericameria ericoides tends to replace Artemisia on sand dunes.
 
The name 'turpentine bush' suggests that Ericameria ericoides is extremely flammable. I assume it to be evergreen (unlike the drought-deciduous dominant Artemisia that tends to dominate Californian coastal sage scrub), and more resinous than most ericoid daisies in the Fynbos Biome. I suspect that a stand of E. ericoides on the Californian coast would burn intensely in the dry season. However, see https://www.jstor.org/stable/41425852.
 
Carpobrotus edulis, instead of shunning stands of E. ericoides, invades them. In this situation at least, invasive C. edulis is probably prone to fire in California.
 
I have yet to find information on the fire relationships of E. ericoides. However, a congener turns out to be poorly adapted to fire, and recovers mainly in periods between fires (please see below).
 
Asteraceae: Ericameria ericoides:
https://farm8.staticflickr.com/7376/10383818436_e391d42978_z.jpg
 
http://www.laspilitas.com/images/grid24_24/12719/images/plants/332/ericameria-ericoides-mock-heather.jpg

https://farm9.staticflickr.com/8738/16684848954_e53621508d_z.jpg

(writing in progress)

הועלה ב-יולי 4, 2022 02:37 לפנה"צ על ידי milewski