Leaf-spinescence in Orites, an intercontinental genus of proteas

@tonyrebelo @gregtasney @nyoni-pete @fionagumboots @nigelforshaw @nicfit @kenharris @ninakerr01 @chrisclarke25 @nicklambert @arthur_chapman

Leaf-spinescence - which is perceived as 'prickly' (i.e. somewhat painful) on human skin - occurs on all vegetated continents.

However, it is most noticeable in Australia. This is partly because sclerophylly (lignification of the foliage, https://en.wikipedia.org/wiki/Sclerophyll) is extremely well-developed on this continent.

In Australia (see page 408 in https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1469-185X.2007.00017.x and https://onlinelibrary.wiley.com/doi/10.1111/j.1469-185X.2007.00017.x ):

  • an exceptional number of species are leaf-spinescent,
  • extensive vegetation types are dominated by leaf-spinescent plants (particularly Triodia, https://en.wikipedia.org/wiki/Triodia_(plant)), and
  • most of the leaf-spinescent species belong to genera restricted to this continent.

This is exemplified by Proteaceae.

There are so many leaf-spinescent species of Proteaceae in Australia that it would be hard to count them all. By contrast there are none in either Africa or South and central America, despite the indigenous occurrence of 14 genera in the former and at least five genera in the latter.

The proteaceous genus Orites is worth focussing on because:

  • it has a disjunct 'Gondwanan' distribution, shared between Australia (6 species) and South America (2 species), and
  • its Australian species vary extremely in habitat, and the size and shape of their leaves.

It is true that Orites:

However, what seems to emerge is that the leaf-spinescent species of Orites occur only in Australia, varying in the position of the sharp features on the leaves.

The eight species of Orites can be categorised as follows:

  • two species in South America (fiebrigii of Bolivia and myrtoideus of Chile), neither of which seems to be leaf-spinescent,
  • two species in Australia (lancifolius and revolutus) which are not leaf-spinescent despite being somewhat sclerophyllous,
  • two species in Australia (milliganii and acicularis) which are sclerophyllous and leaf-spinescent (in different ways from each other), and
  • two species in Australia (diversifolius and excelsus), both of which vary from shrubs to trees and have complex patterns of variation in leaf-form.

In the case of the two last-named species, there is leaf-spinescence (albeit weak) in at least some of these forms. This applies particularly to 'juvenile' foliage in the case of the rainforest tree Orites excelsus.

Within Orites in Australia, there is:

  • a ‘kwongan-like’ (https://en.wikipedia.org/wiki/Kwongan) pattern in the shrub genera of alpine Australia, with an emphasis on sclerophylly in wildfire-prone vegetation, which in some species extends to leaf-spinescence, and
  • a ‘rainforest-like’ pattern aligned with e.g. Macadamia (https://en.wikipedia.org/wiki/Macadamia), in which the vegetation is free of wildfire but any leaf-spinescence is more associated with 'juvenile' foliage.

Neither O. milliganii nor O. acicularis dominates vegetation. However, they collectively make for a considerable element of leaf-spinescence in the alpine vegetation of Tasmania. This contrasts with the situation in mainland alpine Australia where the local species of Orites, namely O. lancifolius, is not leaf-spinescent and instead closely resembles O. myrtoideus of Chile.

A third species common in heathland in alpine Tasmania, namely O. revolutus, is not leaf-spinescent. However, as a classic example of evolutionary convergence, it has similar foliage to the daisy Olearia ledifolia (https://en.wikipedia.org/wiki/Olearia_ledifolia). Furthermore, another species of Olearia, also growing in similar habitats, does qualify as leaf-spinescent (https://en.wikipedia.org/wiki/Olearia_pinifolia).

Is there any other continent on which proteas and daisies, growing side-by-side, have such similar foliage that they can be confused by naturalists (https://www.utas.edu.au/dicotkey/dicotkey/AST/ast/sOlearia_ledifolia.htm)?

The following illustrate the species in alphabetical order (note that Orites megacarpus has been transferred to a genus of its own, https://en.wikipedia.org/wiki/Nothorites).

Orites acicularis, of heathlands in Tasmania, has needle-like leaves qualifying as leaf-spinescent in a simple design.

Orites diversifolius of Tasmania occurs across a range of vegetation types, and varies from shrub to tree. Its leaves are variable but in at least certain situations it seems to exhibit (weak) leaf-spinescence. I do not know how much of this variation is between 'juvenile' and 'adult' foliage, as opposed to being habitat-based.

Orites excelsus is the ‘forest tree’ in the genus, growing up to 30 meters high. It shows complex heteroblasty, with the seedling leaves, 'juvenile' leaves and 'adult' leaves all different (https://en.wikipedia.org/wiki/Orites_excelsus). In the 'juvenile' foliage the leaf tends to be pinnasect and toothed. I infer, from the common name ‘prickly ash’, that the teeth sometimes qualify as leaf-spinescent (probably always weakly so, much as in genus Macadamia).

Orites fiebrigii (Bolivia; no close-up photos of the leaves are available)

Orites lancifolius

Orites milliganii is a shrub of the high-altitude heathlands in Tasmania. It is certainly sclerophyllous, and although the spines on the leaves do not look particularly sharp I think this species qualifies as leaf-spinescent.

Orites myrtoideus (Chile)

Orites revolutus

Posted on February 08, 2022 02:14 AM by milewski milewski


@fionagumboots Hi Fiona, In your experience, are the leaves of Orites diversifolius ever actually prickly, as opposed to just having nominal toothy edges? with thanks from Antoni

Posted by milewski over 2 years ago

The problem with Proteaceae in southern Africa is that the genera (although in Leucadendron this applies to sections, which Salisbury recognized as genera, under the premise that seed morphology determined genera - as in all other genera on the subcontinent), have rather uniform-shaped leaves. This is in total contrast to Australia, where most genera have an amazing array of leaf shapes.
Any consideration of prickles (and breadknives and other amazing shapes), must surely take this into account.
Whatever in Australia made prickly leaves, is probably a consequence of the wide variety of leaf shapes, rather than just one leaf shape. What pressures shaped leaves is Australia, is/was clearly absent in America and Africa. But what on earth were the selection pressures?

Posted by tonyrebelo over 2 years ago

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