Taxonomy of Banksia

Classification of the plant genus Banksia

As with other flowering plants, the taxonomy of Banksia has traditionally been based on anatomical and morphological properties of the Banksia flower, fruiting structure and seed, along with secondary characteristics such as leaf structure and growth habit. Increasingly, molecular evidence from DNA is providing important new insights into relationships within the genus and between this and other genera in the Proteaceae. The genus is placed in family Proteaceae, subfamily Grevilleoideae, tribe Banksieae and subtribe Banksiinae. The most recent complete revision is that published in Alex George's 1999 monograph for the Flora of Australia book series; this recognises two subgenera, three sections, 13 series, 77 species, 6 subspecies and 18 varieties, and treats the traditionally related genus Dryandra as separate but closely related. Recent molecular cladistic analyses have provided an alternative view in which Dryandra is shown to be nested inside, and hence should be included within, Banksia. This interpretation has been controversial in some circles, and is not universally accepted.

Banksia (in the traditional sense, not including Dryandra), is a genus of around 80 species in the plant family Proteaceae. An iconic Australian wildflower and popular garden plant, they are easily recognised by their characteristic flower spikes and fruiting "cones". They grow in forms varying from prostrate woody shrubs to trees up to 35 metres tall, and occur in all but the most arid areas of Australia. As heavy producers of nectar, they are important sources of food for nectariferous animals such as honeyeaters and honey possums, and they are of economic importance to the nursery and cut flower industries. However they are threatened by a number of processes, including land clearing, frequent burning, and disease; and a number of species are rare and endangered.

In 1981, Alex George published his classic 1981 monograph The Genus Banksia L. f. (Proteaceae). George's arrangement was based on a variety of properties including leaf, style, pollen presenter, follicle and seed characters, with the criterion that a taxon was considered a distinct species only if it exhibited a "significant and consistent difference in the morphology of flowers and/or fruit". which remains the standard text on the genus, and the treatment of Banksia in the Flora of Australia series. As their cladogram differed substantially from the current taxonomic arrangement, they published a revised arrangement that accorded better with their results. Four varieties were promoted to species rank: B. conferta var. penicillata to B. penicillata (now B. conferta subsp. penicillata); B. gardneri var. brevidentata to B. brevidentata; B. gardneri var. hiemalis to B. hiemalis; and B. sphaerocarpa var. dolichostyla to B. dolichostyla. Two new series and eleven subseries were introduced; Banksia sect. Oncostylis and Banksia ser. Crocinae were discarded; and Banksia ser. Cyrtostylis was largely redefined. Six species were left incertae sedis. However, a number of Australian herbaria have continued to follow Thiele and Ladiges on some points, for example by recognising the four species that they promoted.

Following on from an earlier molecular study, Austin Mast and co authors published cladistic analyses of genetic data from DNA samples of almost all species of Banksia, along with five Dryandra species, in 2002 and 2005. Their results indicated the presence of two large clades of Banksia, which they named "/Cryptostomata" ("hidden stomates") and "/Phanerostomata" ("visible stomates"). The /Phanerostomata were defined as those taxa in which the leaf stomata occur superficially or in shallow pits. These taxa are typically tall shrubs and trees that occur in moist areas; they have unbeaked follicles and soft, short lived leaves that are in many cases needle like. The clade includes all eastern taxa of the series Salicinae and Spicigerae (that is, all taxa except B. serrata, B. aemula and B. ornata) and also the western Spicigerae, Quercinae, Grandes, Abietinae and Dryandroideae. The /Cryptostomata were defined as those taxa in which the leaf stomata occur in crypts with constricted entrances. These are usually small shrubs that occur on dry, infertile sandplains. They have beaked follicles and thick, tough, long lived serrated leaves. It includes all other western taxa, plus the eastern species B. serrata, B. aemula and B. ornata, and also appears to include Dryandra. It is worth noting that Dryandra does not appear especially closely related to the Isostylis group, which is instead most closely related to B. elegans and then B. attenuata. Thus, the results presented by Mast et al. strongly suggest that Banksia is paraphyletic with respect to Dryandra. Although they did not propose a new taxonomic arrangement, they did consider various options for adjusting the current accepted arrangement to remove the paraphyly. They conclude that the simplest and least disruptive solution would be to merge Dryandra into Banksia.