Banksia Subg. Isostylis

''Banksia'' subg. ''Isostylis'' is a subgenus of ''Banksia''. It contains three closely related species, all of which occur only in Southwest Western Australia. Members of subgenus ''Isostylis'' have dome-shaped flower heads that are superficially similar to those of ''B.'' ser. ''Dryandra'', but structurally more like reduced versions of the "flower spikes" characteristic of most other ''Banksia'' taxa.

There are three species of ''Banksia'' subg. ''Isostylis'', ''B. ilicifolia'' (holly-leaved banksia), ''B. cuneata'' (matchstick banksia) and ''B. oligantha'' (Wagin banksia). ''B. ilicifolia'' is widely distributed and relatively common, but the other two species are rare and threatened.

Description

''Banksia'' subg. ''Isostylis'' shares with ''B.'' ser. ''Dryandra'' the property of having compact, dome-shaped flower heads. Structurally, however, ''Isostylis'' flower heads are quite different from those of ''B.'' ser. ''Dryandra'', having more in common with the erect flower spikes of other ''Banksia'' taxa. Specifically, ''Isostylis'' flower heads have an ovoid axis, suggestive of a greatly reduced flower spike, whereas ''Dryandra'' flower heads emerge from a flat receptable. Furthermore, ''Isostylis'' has thick follicles with a woolly coating, whereas ''Dryandra'' follicles are thin and hairless; and the involucral, common and floral bracts of ''Isostylis'' are unlike those of ''Dryandra''.

The ''Isostylis'' species are all upright shrubs or trees, with a single trunk. They generally have serrate leaves, although in rare cases ''B. ilicifolia'' may have entire leaves.

Taxonomy

Taxonomic history

''Banksia'' subg. ''Isostylis'' was first published by Robert Brown in his 1810 ''Prodromus Florae Novae Hollandiae et Insulae Van Diemen''; thus its full name, with author citation, is ''Banksia'' subg. ''Isostylis'' R.Br. Brown's arrangement was the first infrageneric arrangement of ''Banksia'', making ''B.'' subg. ''Isostylis'' ''Banksia''s first infrageneric taxon. Brown erected ''B.'' subg. ''Isostylis'' to contain ''B. ilicifolia'', which was then the only known ''Banksia'' with a dome-shaped inflorescence. He did not explicitly name a type species for the subgenus, but ''B. ilicifolia'' is treated as the type because it was the only member when the subgenus was published.

Twenty years later, Brown issued a supplement to his ''Prodromus'' entitled '' Supplementum Primum Prodromi Florae Novae Hollandiae''; another nine ''Banksia'' species were published, but there was no change to the 1810 arrangement, and no new ''Isostylis'' species.

In 1846, Édouard Spach promoted ''B.'' subg. ''Isostylis'' to genus rank in his '' Histoire Naturelle des Vegetaux: Phanerogames''. This was not accepted, and ''Isostylis'' (R.Br.) Spach is now considered a nomenclatural synonym of ''B. ''subg. ''Isostylis''.

When Carl Meissner published his arrangement of ''Banksia'' in 1856, he demoted both of Brown's subgenera to sectional rank, maintaining ''B.'' sect. ''Isostylis'' (R.Br.) Meisn. as a monospecific taxon containing only ''B. ilicifolia''. Meissner's rank and circumscription of ''Isostylis'' was retained by George Bentham in his 1870 arrangement for ''Flora Australiensis'', but Bentham also published a putative variety of ''B. ilicifolia'', ''B. i.'' var. ''integrifolia'', based on specimens collected by Ludwig Preiss near the Swan River in Western Australia. This was later overturned.

In 1905, James Britten challenged the genus name ''Banksia'', on the grounds that ''Banksia'' J.R.Forst & G.Forst had precedent over ''Banksia'' L.f. Britten adopted the name ''Isostylis'' for the entire genus, republishing the names ''Isostylis dentata (L.f.) Britten'' for '' B. dentata'' (Tropical Banksia), ''Isostylis ericifolia (L.f.) Britten'' for '' B. ericifolia'' (Heath-leaved Banksia), ''Isostylis integrifolia (L.f.) Britten'' for '' B. integrifolia'' (Coast Banksia) and ''Isostylis serrata (L.f.) Britten'' for '' B. serrata'' (Saw Banksia). This challenge failed, ''Banksia'' L.f. was eventually conserved, and his four names are now considered taxonomic synonyms of their respective names under ''Banksia'' L.f. None of them are considered members of ''Isostylis''.

The next change to ''Isostylis'' came in 1981, when Alex George promoted it back to subgenus rank, and published a second species, '' B. cuneata''. In discussing the subgenus, George commented that there had been calls to transfer ''Isostylis'' into ''Dryandra'', which was then a distinct genus. He argued, however, that the similarities between ''Isostylis'' and ''Dryandra'' were largely superficial, whereas the similarities with ''Banksia'' were much more important taxonomically. His conclusion was that the taxon should remain in ''Banksia'', although he did not rule out promoting it into a separate genus. A third ''B.'' subg. ''Isostylis'' species, '' B. oligantha'', was published by George in 1988.

In 1996, Kevin Thiele and Pauline Ladiges published a revised arrangement based on a cladistic analysis of morphological characters of ''Banksia''. They took up the question of an affinity of ''Isostylis'' with ''Dryandra'', finding George's arguments unconvincing but failing to find any further evidence for or against ''Isostylis''s placement within ''Banksia''. They eventually accepted both of George's subgenera, using each as an outgroup in the analysis of the other. Thus their analysis yielded no information about the circumscription and placement of ''Isostylis'', and their arrangement maintained ''Isostylis'' as a subgenus.

Thiele and Ladiges' arrangement was not accepted by George, and was largely discarded by him in his 1999 arrangement. The placement and circumscription of ''B.'' subg. ''Isostylis'' was unaffected and can be summarised as follows:

:''Banksia''
:: ''B.'' subg. ''Banksia'' (3 sections, 11 series, 73 species, 11 subspecies, 14 varieties)
::''B.'' subg. ''Isostylis''
:::'' B. ilicifolia''
:::'' B. oligantha''
:::'' B. cuneata''

Since 1998, Austin Mast has been publishing results of ongoing cladistic analyses of DNA sequence data for the subtribe Banksiinae. His analyses has provided compelling evidence for the paraphyly of ''Banksia'' with respect to ''Dryandra'', and suggest an overall phylogeny that is very greatly different from George's arrangement. Mast's results clearly recognise ''Isostylis'' as a distinct clade, but place it in a fairly recent position, within a clade that also contains '' B. elegans'' (Elegant Banksia) and '' B. attenuata'' (Candlestick Banksia). It falls a substantial distance from ''Dryandra'', suggesting that similarities between those two groups are indeed superficial.

Early in 2007, Mast and Thiele initiated a rearrangement of ''Banksia'' by merging ''Dryandra'' into it, and publishing ''B.'' subg. ''Spathulatae'' for the species having spoon-shaped cotyledons. They foreshadowed publishing a full arrangement once DNA sampling of ''Dryandra'' was complete; in the meantime, ''B.'' subg. ''Isostylis'' has been set aside. If maintained at all in Mast and Thiele's forthcoming arrangement, it will be at a lesser rank than subgenus.

Relationships within ''B.'' subg. ''Isostylis'' still remain unclear. Though Mast's studies found ''B. cuneata'' to be the most basal of the three species, a 2004 study of genetic divergence within the subgenus yielded both other possibilities: some analyses suggested ''B. ilicifolia'' as basal, while others suggested ''B. oligantha''. Further complicating the situation is the existence of a population of ''B. cuneata'' having both genetic and phenetic affinities with ''B. oligantha''. The origin of this population is unknown. It might have arisen through hybridisation, or it may be a transitional or even ancestral form. Finally, biogeographical factors suggest that ''B. ilicifolia'' would be the most basal of the three species: it occurs in the High Rainfall Zone where relictual species are most common, whereas the others are restricted to the Transitional Rainfall Zone, where more recently evolved species are most common.

Distribution

Species of ''B.'' subg. ''Isostylis'' occur only in Western Australia's South West Botanical Province. ''B. ilicifolia'' is widespread within 70 kilometres of the coast from Mount Lesueur in the north, south to Cape Leeuwin and east to Albany. The other two species occur further inland, and have quite limited distributions. ''B. cuneata'' occurs Brookton and Bruce Rock in the Avon Wheatbelt biogeographic region; while ''B. oligantha'' occurs slightly further south, in the vicinity of Wagin.

Ecology

Ecologically, ''B.'' subg. ''Isostylis'' is similar to other ''Banksia''s. As with other ''Banksia'' taxa, all three species have proteoid roots, roots with dense clusters of short lateral rootlets that form a mat in the soil just below the leaf litter. These roots are particularly efficient at absorbing nutrients from nutrient-poor soils, such as the phosphorus-deficient native soils of Australia. They lack a lignotuber, so shrubs are killed by bushfire; mature trees of ''B. ilicifolia'' have a limited ability to resprout from epicormic buds on the trunk. But like all ''Banksia''s they release their aerial seed bank following a bushfire. This adaptation, known as serotiny, ensures the rapid regeneration of populations killed by fire.

''B. cuneata'' and ''B. oligantha'' have been declared rare under both Western Australia's Wildlife Conservation Act 1950, and the federal Environment Protection and Biodiversity Conservation Act 1999. Threats include loss of habitat, ''Phytophthora cinnamomi'' dieback, and grazing on seedlings by feral rabbits.

Cultivation

None of the ''B.'' subg. ''Isostylis'' species are popular in cultivation. The two rare species are virtually unknown in cultivation. ''B. ilicifolia'' is better known, but its usefulness as an amenity plant is limited by the fact that it has very prickly leaves.

References

External links

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{{DEFAULTSORT:Banksia Subg. Isostylis
subg. Isostylis
Eudicots of Western Australia
Plant subgenera
Taxa named by Robert Brown (botanist, born 1773)