Evolutionary plasticity of ericas in Australia vs their conformism in southern Africa - the extreme case of Richea pandanifolia

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Dear Naturalist familiar with ericas in southern Africa, can you spot the erica in the following photo (https://www.inaturalist.org/observations/130707229), taken in Tasmania?

(By 'erica', I mean any member of Ericaceae.)

If you are aware of Erica arborea (https://www.inaturalist.org/taxa/82688-Erica-arborea) of Europe and East African mountains as far south as Malawi - which has the typical small, dull-evergreen leaves of its genus, but grows as tall as 7 m - you may suspect that I am referring to the most conspicuous trees in this scene.

And that would be remarkable enough, as an example of phylogenetic plasticity (see comments in https://www.inaturalist.org/journal/milewski/81613-metrosideros-angustifolia-myrtaceae-the-pacific-face-of-fynbos#).

However, this is not the case. The trees in the photo, resembling 'tree-heaths', actually belong to a different family, Myrtaceae.

The 'mystery erica' takes centre-stage in the following (https://www.inaturalist.org/observations/105090939 and https://www.inaturalist.org/observations/2749930).

The species in question grows as tall as 12 m. The following (https://www.inaturalist.org/observations/62969379 and https://www.inaturalist.org/observations/130603304), with human figures for scale, show specimens about 4 m and about 5 m high respectively.

The following (https://www.inaturalist.org/observations/130910367) shows branching of the bole, which is unusual for tall specimens. It also shows the persistence of the dead leaves (https://www.inaturalist.org/observations/55115512).

The leaves are up to 1.5 m long.

The following (https://www.inaturalist.org/observations/525192) shows the inflorescence.

The following are additional illustrations of the erica in question, namely RICHEA PANDANIFOLIA (https://www.inaturalist.org/taxa/143879-Richea-pandanifolia ):

https://www.inaturalist.org/observations/130687661

https://www.inaturalist.org/observations/105359385

https://www.inaturalist.org/observations/18509777

DISCUSSION

In southern Africa, the foliage-form of ericas is so consistent that they have been used to define a category, namely 'ericoid' (https://en.wikipedia.org/wiki/Ericoid), that includes several other families and many other genera in fynbos.

This is an example of homoplasy (https://en.wikipedia.org/wiki/Homoplasy).

In Europe, ericas are not as predictable in growth-form as in southern Africa, because

However, even in Europe, the most familiar ericas are 'heaths' or 'heathers', similar to those in fynbos. I refer to Erica (https://www.inaturalist.org/observations?place_id=97391&taxon_id=55776&view=species), Calluna (https://www.inaturalist.org/taxa/119450-Calluna-vulgaris), Daboecia (https://www.inaturalist.org/taxa/371651-Daboecia-cantabrica), Phyllodoce (https://www.inaturalist.org/taxa/166760-Phyllodoce-caerulea), and Andromeda (https://www.inaturalist.org/taxa/119892-Andromeda-polifolia).

In Australasia, ericas are common and diverse. However, relatively few species (e.g. https://www.inaturalist.org/observations?place_id=any&taxon_id=525466&view=species) resemble Erica in fynbos.

Many genera and spp. of ericas in Australia can be described as shrubby, with small, evergreen leaves, and are adapted to fire regimes. However, they tend to deviate from Erica (and similar genera in Europe) in lacking rolled/narrow leaves, and in featuring foliar spinescence and fleshy fruits.

More relevant to this Post:
It is in the relatively cold climates of southeastern Australia, particularly Tasmania (https://en.wikipedia.org/wiki/Tasmania), that ericas deviate morphologically and ecologically to the degree of being initially unrecognisable even as dicotyledonous plants - let alone ericas.

In its growth-form, Richea pandanifolia shows homoplasy with arborescent monocotyledonous plants, e.g.

Furthermore, R. pandanifolia has deviated from the relationship with fire, typical of ericas in southern Africa and Australia. It occurs in fire-free vegetation, whether this is 'heathland' (e.g. that characterised by stunted, winter-deciduous Nothofagus, https://en.wikipedia.org/wiki/Nothofagus_gunnii) or 'rainforest' (https://www.alamy.com/rainforest-with-pandanus-trees-richea-pandanifolia-in-pine-valley-image6393269.html).

Overall, the plant most resembling R. pandanifolia, worldwide, seems to be Yucca gigantea (https://www.inaturalist.org/observations/154505105 and https://www.inaturalist.org/observations/152082012 and https://www.inaturalist.org/observations/124675181 and https://www.inaturalist.org/observations/106218202 and https://www.inaturalist.org/taxa/201452-Yucca-gigantea).

This species, which belongs to the same family as cultivated asparagus (https://en.wikipedia.org/wiki/Asparagaceae), is indigenous to central America.

However, the ecological similarity is limited, because Y. gigantea is tropical, whereas R. pandanifolia occurs farther south than the southernmost tip of Africa.

Posted on July 16, 2023 08:20 PM by milewski milewski

Comments

Is Richea mycorhizal?

Posted by tonyrebelo 10 months ago

@tonyrebelo
The mycorrhizal status of Richea pandanifolia is a good question. I have yet to find a reference on this.

Posted by milewski 10 months ago

Ericoids are not plants associated with North America. However, there is at least one example: https://www.inaturalist.org/taxa/131247-Ceratiola-ericoides

Posted by milewski 10 months ago

I beg your pardon? ("By 'erica', I mean any member of Ericaceae." - why the sudden change to 'ericoids'?)
https://www.inaturalist.org/observations?captive=false&place_id=97389,97394&taxon_id=133387&view=species

Posted by tonyrebelo 10 months ago

@tonyrebelo

Thanks for asking.

'Erica' is a taxonomic/phylogenetic category. 'Ericoid' is a category of growth-forms.

Most ericoids on Earth are not ericas, because many genera of Asteraceae (particularly in South Africa), Myrtaceae (particularly in Australia), Bruniaceae, Dilleniaceae, etc. have the ericoid growth-form.

A similar distinction can be drawn between proteas and proteoids. However, this is less useful because the growth-form category 'proteoid' seems never to have been used on any landmass other than southern Africa. This is possibly because a) broad-leafed proteas in Australia are generally so much more sclerophyllous and foliar-spinescent than those in southern Africa, and b) even in South Africa, 'proteoid' seems never to have been applied to any family other than Proteaceae.

'Restio' and 'restioid' are like 'protea' and 'proteoid' in that the growth-form category hardly transcends the family.

'Ericoid' is well-recognised enough to feature in Wikipedia (https://en.wikipedia.org/wiki/Ericoid). By contrast, Wikipedia does not similarly feature 'proteoid' or 'restioid'.

Your further thoughts?

Posted by milewski 10 months ago

I fully understand the distinction. We routinely differentiate between ericoid and ericaceous.
You have just changed your tune. Richea can hardly be called Ericoid. In fact, Ericoid is a rather specialized strategy.
Also, you are talking "growth forms" - that is not conventional. Ericoid usually refers to a leaf form (small, sclerophyllous, rolled leaves with a narrow ventral caniculus, typically - but not exclusive to the concept - arising in whorls along the stem): although it is typical in Erica, it also occurs in many other Fynbos genera (often exclusively so - Phylica, Stilbe, etc.). I dont know what an Ericoid growth form is - there is nothing that comes to mind that is typically ericoid in the growth form line, other than perhaps a predilection to stunted, wispy, small, thin branched shrubs (with many exceptions - in Erica (e.g. Eriica arborea, caffra, tristis): hardly a "conformist" concept).

Posted by tonyrebelo 10 months ago

@tonyrebelo

If I have stated or implied that Richea pandanifolia is ericoid, I am unaware of doing so, because the whole point of the Post is how far from ericoid this plant is.

Turning to the question of what, exactly, 'ericoid' means:

I have invoked 'ericoid' as a growth-form category, whereas your view is that the term refers only to the form of the foliage (which would mean, for example, that E. arborea remains ericoid despite growing to the size and shape of a tree).

Which of us is right?

I note that the Wikipedia site recognises some ambiguity, but calls ericoid a 'habit'. The following (https://www.anbg.gov.au/cpbr/cd-keys/peakey/key/The%20Pea%20Key/Media/Html/21_plant_habit.html), for what it is worth, explains this term.

I take it that 'habit' is approximately synonymous with 'growth-form'. I prefer the latter term, partly because 'habit' already has a common meaning in the behaviour of animals. In other words, 'growth-form' is less ambiguous than 'habit'.

Furthermore, in the literature on fynbos, it seems common to refer to three strata, namely proteoid (tallest), ericoid (intermediate), and restioid/graminoid (lowest). This stratification implies a modest range of plant sizes, centering on about 1 m high. It is certainly true that the ericoid stratum could be as low as 0.5 m, or as tall as 1.5 m, but there does seem to be an association with height within that range, viz. 0.5-1.5 m.

Is this approach no longer in vogue, among ecologists in fynbos?

Posted by milewski 10 months ago

No: they are not strata - restioids for instance can be in any strata, and proteoids include ground proteas.
The terms are most typically used as leaf classes: large leaves, fine leaves and leafless graminoids. In terms of cover and visual affect and taxonomy it is Fynbos is dominated by Proteas, Ericas and Restios (with Protea meaning the family (or at least the big 3: Protea, Leucadendron, Leucospermum), and Restios including sedges, but Erica typically only referring to the genus (the other near-endemic ericoid families are often listed separately: Bruniaceae, Stilbaceae).
But it is not used in the sense you mention.
We also have Fynbos types: Protoid, Ericaceous, Asteraceous, Restioid, Grassy and scrub (which Laco insisted on calling Thicket Fynbos, which is wrong; note too that the -oid and -aceous is very deliberate) - see https://opuntiads.com/oblog/wp-content/uploads/2021/11/Fynbos-Biome.pdf

Posted by tonyrebelo 10 months ago

@tonyrebelo
Okay, many thanks, I have corrected the Post accordingly.

Posted by milewski 10 months ago

Fascinating write-up on these beautiful plants! As someone who has admired the delicate beauty of fynbos ericas, it is very interesting for me to see the diverse forms of other related plants from the same family.

Posted by dinofelis 10 months ago

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