Banksia sphaerocarpa

Shrub or tree in the family Proteaceae widely distributed across the southwest of Western Australia

Banksia sphaerocarpa, commonly known as the fox banksia or round fruit banksia, is a species of shrub or tree in the plant genus Banksia (family Proteaceae). It is generally encountered as a high shrub, and is usually smaller in the north of its range. This species has narrow green leaves, and brownish, orange or yellow round flower spikes which may be seen from January to July. It is widely distributed across the southwest of Western Australia, growing exclusively in sandy soils. It is usually the dominant plant in scrubland or low woodland. It is pollinated by, and is a food source for, birds, mammals, and insects. First described in 1810 by botanist Robert Brown, B. sphaerocarpa has a complicated taxonomic history, and several taxa once classified as part of a broadly defined B. sphaerocarpa have since been named as species in their own right. At present, most authorities recognise five varieties; the largest variety, B. sphaerocarpa var. dolichostyla (ironcap banksia), is sometimes given species rank as B. dolichostyla. B. sphaerocarpa is classified as Not Threatened under the Wildlife Conservation Act of Western Australia, although two varieties have been placed on the Declared Rare and Priority Flora List—var. latifolia has been designated a Priority Two – Poorly Known taxon, and var. dolichostyla falls under Declared Rare Flora. None of the varieties are commonly seen in cultivation.

Banksia sphaerocarpa is a variable species that differs in plant size, flower size and leaf size across its range. The variability is marked enough that five varieties are recognised. This species is generally a shrub tall. Plants are smaller in northern parts of the range, and grow larger in the southeast, with var. dolichostyla and rarely var. caesia reaching in height. All varieties bear a lignotuber, a swollen starchy root crown from which the plant resprouts after bushfire. The new stems are finely hairy but become smooth with maturity. The leaves are stiff, narrow and linear, and measure in length, on a petiole long. Leaves of most varieties are in width, and have a pointed tip, The foliage is green, or a more pale blue grey in the case of caesia and dolichostyla. The inflorescences are generally globular and measure in diameter, although larger forms, such as var. dolichostyla, are more oval in shape. Flowering from January to July, the blooms are yellow, orange or brownish in colour. The old flowers fade to brownish and grey hues and remain curled around the flower spike. Up to 60 follicles develop on the globular spikes. The follicles are finely furred at first before becoming smooth and golden brown in colour, measuring long, high, and wide. The follicles are flat with pronounced 'shoulders', giving a rectangular appearance in cross section. and is credited to English botanist Robert Brown, though it is not possible to be certain on this point, as Brown incorporated the collections of junior expedition members into his herbarium without attribution. Neither Brown nor expedition horticulturist Peter Good mentions the collection in his diary. Brown published a formal description and name for the species in his 1810 On the Proteaceae of Jussieu. He did not designate a type specimen (a specimen to be representative of the species) for the species, but the one specimen in his collection has since been formally declared the lectotype for the species. He also did not state the etymology of the specific epithet, but it is accepted that the name derives from the Ancient Greek sphaera ("round"), and carpos ("fruit"), in reference to the shape of its infructescences. In Brown's arrangement of the genus, B. sphaerocarpa was placed between B. pulchella and B. nutans in taxonomic sequence; that is, an order that places related taxa next to each other. No subdivision of the genus was given, other than to separate a single distinctive species into a subgenus of its own. Meissner also published a variety, B. sphaerocarpa var. glabrescens, based on specimens collected by James Drummond; this is now considered a synonym of B. incana. For many years following Bentham's arrangement, the circumscription of B. sphaerocarpa was widely recognised as unacceptably broad. William Blackall informally published two varieties, var. pinifolia and var. violacea (properly Banksia violacea) in his 1954 How to Know Western Australian Wildflowers; and in 1966 the nurseryman Fred Lullfitz predicted that there were as many as eight taxa within the species. Several of these were recognised in George's revision of the genus for 1981 "The genus Banksia L. f. (Proteaceae)": B. micrantha, B. grossa, B. lanata, B. scabrella, B. telmatiaea, B. leptophylla and B. incana. George placed B. sphaerocarpa in subgenus Banksia because of its flower spike, section Oncostylis because its styles are hooked, and the resurrected series Abietinae, which he constrained to contain only round fruited species. He reduced variety latifolia to synonymy with variety sphaerocarpa, but conceded the species needed further review. George's arrangement remained current until 1996, when Kevin Thiele and Pauline Ladiges published an arrangement informed by a cladistic analysis of morphological characteristics. They retained George's subgenera and many of his series, but discarded his sections. Banksia ser. Abietinae was found to be very nearly monophyletic, and so retained. It further resolved into four subclades, so Thiele and Ladiges split it into four subseries. Banksia sphaerocarpa appeared in the second of these, initially called the ' grossa clade' for its most basal member. As with George's classification, B. grossa and B. micrantha emerged as close relatives of B. sphaerocarpa. This clade became the basis for new subseries Sphaerocarpae, which Thiele defined as containing those species with lignotubers, styles loosely curling around the infructescence (although this trait was reversed in micrantha), and "transversely aligned cells of the seed wing inner face". Other than the most basal B. grossa, these species also have shouldered follicles. Having found B. micrantha to be more closely related than B. sphaerocarpa var. dolichostyla to the other varieties of B. sphaerocarpa, they promoted var. dolichostyla to species rank as Banksia dolichostyla. Morphological support for this was given by the fact that the old styles of var. dolichostyla are quite different from those of other varieties, being stouter, and tending not to curl around the infructescence as the others do. Questioning the emphasis on cladistics in Thiele and Ladiges' arrangement, George published a slightly modified version of his 1981 arrangement in his 1999 treatment of Banksia for the Flora of Australia series of monographs. He added that he did not feel the characters of dolichostyla alone justified specific rank, being essentially merely larger in all parts than, but otherwise essentially similar to, var. caesia. Therefore, he retained it as a variety of B. sphaerocarpa. To date, George's 1999 arrangement remains the most recent comprehensive classification. The placement of B. sphaerocarpa in George's 1999 arrangement may be summarised as follows:

Early in 2007, Mast and Thiele initiated a rearrangement of Banksia by merging Dryandra into it, and publishing B. subg. Spathulatae for the taxa having spoon shaped cotyledons. They foreshadowed publishing a full arrangement once DNA sampling of Dryandra was complete; in the meantime, if Mast and Thiele's nomenclatural changes are taken as an interim arrangement, then B. sphaerocarpa is placed in B. subg. Spathulatae. In 2008, George reinstated B. sphaerocarpa var. latifolia, and published the new variety B. sphaerocarpa var. pumilio.

As a species, B. sphaerocarpa is not considered to be under threat, but two of the five varieties have been placed on the Declared Rare and Priority Flora List. B sphaerocarpa var. dolichostyla has been gazetted as "Declared Rare Flora – Extant" under the Wildlife Conservation Act 1950. Banksia sphaerocarpa var. latifolia, though not afforded legislative protection, has been identified as a "Priority Two – Poorly Known" taxon, because so few populations are known.

Various animals, including mammals, birds, and insects such as bees, wasps, ants and beetles, have been recorded visiting Banksia sphaerocarpa inflorescences, including the colletid bee species Hylaeus sanguinipictus. Botanist Stephen Hopper found pollen of B. sphaerocarpa on New Holland honeyeaters (Phylidonyris novaehollandiae) and honey possums (Tarsipes rostratus) at Cheyne Beach in a field study published in 1980. Knowledge of the breeding system of B. sphaerocarpa comes from a 2009 study of genetic structure within and across fragmented plant populations, which made a case study of B. sphaerocarpa var. caesia populations in the vicinity of Dongolocking, where the landscape has been severely fragmented by land clearing. Very little self pollination was observed. Most mating was between plants in the same population, but inter population mating accounted for 15–33% of seed, a "very significant contribution to overall reproductive dynamics". This figure was lowest in the smallest populations, which also exhibited lower rates of germination, smaller plants, and less genetic diversity than larger populations. One possible interpretation of this is that interpopulation mating confers a fitness advantage. There was also evidence of fine scale genetic structure, with plants tending to be more closely related to nearby plants than to more distant plants. The authors suggest that gene flow was probably always limited in these populations, even before they were fragmented. Banksia sphaerocarpa is one of five closely related Banksia species that have highly unusual flower nectar. Whereas other Banksia species produce nectar that is clear and watery, the nectar of these species is pale yellow initially, but gradually becomes darker and thicker, changing to a thick, olive green mucilage within one to two days of secretion, and eventually becoming "an almost black, gelatinous lump adhering to the base of the flowers". It was first noted by Byron Lamont in 1980; he attributed it to cyanobacteria that feed off the nectar sugars. Noting that many of these cyanobacteria had heterocysts, he speculated that they aid the plant by fixing atmospheric nitrogen, which is then washed off the flower heads by rain, and absorbed by the proteoid root mat. This purported symbiosis was investigated by Barrett and Lamont in 1985, but no evidence of nitrogen fixing was found. Further investigation by Markey and Lamont in 1996 suggested that the discolouration is not caused by cyanobacteria or other microorganisms in the nectar, but is rather "a chemical phenomenon of plant origin". Their analyses indicated that the nectar had unusually high levels of sugar and free amino acids,

Like most Western Australian Banksia species, B. sphaerocarpa is susceptible to dieback from the soil borne water mould Phytophthora cinnamomi. In one experiment, 43% of plants were dead within three months of inoculation, and the species was therefore rated as "moderately susceptible". In another experiment, 37% of plants were dead within three months, and 85% within a year; yet this study rated the species' susceptibility as "high". In the latter study, it took 35 to 40 days for the death rate to reach its maximum, and thereafter the death rate continued at that level throughout the year, only dipping slightly in the dry summer months.

None of the varieties of Banksia sphaerocarpa are commonly seen in cultivation. They are difficult to grow in the wetter conditions of Australia's east coast. Otherwise, they adapt well to gardens with good drainage, sandy soils and sunny aspects in drier and Mediterranean (winter moisture) climates, and are also frost tolerant.