Banksia contains many foliar-spinescent spp. but hardly achieves leaf-spinescent dominance of any vegetation types
(writing in progress)
It is one thing for a flora to contain foliar-spinescent spp. It is another thing for a vegetation type to be dominated by foliar-spinescent spp.
Where foliar-spinescent spp. become the major component of the vegetation, this indicates a particular success of a certain ecological strategy in plants.
There are relatively few vegetation types, on Earth, that are dominated by foliar-spinescent plants. I am currently searching for them.
One category that has come up is ‘banksia woodland’, a type of vegetation restricted to Australia.
My main point, in this Post, is to point out that all forms of Banksia woodland seem just to miss the mark, in qualifying as vegetation dominated by leaf-spinescent plants. My interpretation is that it takes rather extreme ecological conditions to favour foliar-spinescent plants to the degree that they actually dominate vegetation, and although various environments in Australia approach such conditions, few manage to qualify.
In the genus Banksia, most spp. have sclerophyllous leaves, and most have leaves with some form of ‘teeth’ on the margins. This combination provides the prerequisites for foliar spinescence of the leaf-margins, and indeed there are many foliar-spinescent spp. in the genus.
However, the spp. that tend to dominate in (low) woodlands tend not to fall just short of being actually spinescent, because the teeth on the leaf-margins tend to be neither sharp (‘pungent’) nor stiff (lignified) enough to produce spines capable of harming the human skin.
On the Swan coastal plain in the Perth region of Western Australia, there occurred - before suburban clearing - the most extensive low woodlands of Banksia anywhere in Australia.
The two most important spp. were, and in places still are, Banksia attenuata and B. menziesii. Both have toothed leaves, but neither is foliar-spinescent, because the teeth are not ‘sharp and stiff’ enough. A minor component of the same vegetation, namely Banksia ilicifolia (https://www.inaturalist.org/taxa/518359-Banksia-ilicifolia), is truly foliar-spinescent, but it nowhere dominates the stand.
There are small (smaller than 0.5 hectare) patches of vegetation in both eastern Australia and northern (tropical) Australia that have Banksia dominant.
However, once again the Banksia spp. that achieve prominence here to the relative exclusion of other trees turn out not to be foliar-spinescent. A typical example in New South Wales is Banksia integrifolia (https://www.inaturalist.org/taxa/244884-Banksia-integrifolia), while the sole species of Banksia in far-north tropical Australia is Banksia dentata (https://www.inaturalist.org/taxa/131164-Banksia-dentata). (The very name of B. dentata is suggestive of a ‘toothed’ leaf which fails to be ‘pungent’, as the specific epithets ‘pungens’ or ‘spinosa’ might indicate.)
What does it take for Banksia to dominate vegetation in the form of foliar spinescence?
In my experience it is only where the plants of Banksia are reduced from arborescent spp. to shrubby spp. that this line is crossed. Examples include Banksia baxteri scrub-heath on the south coast of Western Australia (e.g. Fitzgerald River National Park, https://en.wikipedia.org/wiki/Fitzgerald_River_National_Park).
Banksia baxteri (https://www.inaturalist.org/taxa/487473-Banksia-baxteri) does qualify as foliar-spinescent, but it forms large shrubs (killed by fire rather than regenerating vegetatively as do the low-tree spp. of Banksia which dominate Banksia woodlands elsewhere). And even in the case of Banksia baxteri, the patches of vegetation (which is a form of kwongan, https://en.wikipedia.org/wiki/Kwongan) where the upper stratum actually dominates (the lower stratum consisting of floristically-mixed low shrubs, many of them not leaf-spinescent) are limited in area to fractions of a hectare at a time.
So the following picture emerges:
Banksia contains many foliar-spinescent spp. (with the spinescence marginal to the leaf, as opposed to apical).
Banksia even contains several spp. with the ‘holly-leaf’ form of foliar spinescence (as indicated by the specific epithet ‘ilicifolia’).
Banksia attains dominance of the vegetation type in a few limited areas, although it is more typically only one component among many (some taller than it, like eucalypts, and others shorter than it, like the many low shrubs of kwongan and wallum, https://en.wikipedia.org/wiki/Wallum).
However, those spp. of Banksia that tend to be most dominant tend not to qualify as foliar-spinescent, while those spp. qualifying as foliar-spinescent tend to be either low (relatively small plants unlikely to dominate vegetation) or sparse as components of the vegetation in which they occur.
The following shows the global distribution of genus Banksia.
The following several photos show the leaves of Banksia dentata, which is the only fully ‘tropical’ species of Banksia but co-dominates a few small patches of vegetation, e.g. on Melville Island and on Cape York Peninsula. This shape would be sufficient for the leaves to qualify as unambiguously leaf-spinescent were it only the case that the ‘teeth’ were more ‘pungent’ than they really are. In reality, the leaves may qualify (at least on some individual plants) as leaf-spinescent, but overall the species is a ‘borderline case’ in terms of qualifying as leaf-spinescent.
The following picture of B. dentata is a painting, not a photo, but I include it because it shows particularly clearly that the ‘teeth’ on the margins of the leaves of this species are sharp enough to provide leaf-spinescence. Their shortcoming is not their sharpness but their rigidity, so that overall this species does not, according to the literature, qualify as ‘prickly’. As in all examples of Banksia woodland, this seems to be a case of ‘so near and yet so far’ w.r.t. achieving the status of vegetation dominated by leaf-spinescent plants.
(writing in progress)
תגובות
הוספת תגובה