Banksia petiolaris

Appearance

"Banksia petiolaris" is a prostrate shrub that can spread to a diameter of 2 metres. Its thick stems grow horizontally on the ground and are covered in fine hair. The new growth is more densely covered with velvety orange brown hair. The large, leathery, upright leaves arise vertically on petioles up to 15 cm high. The upper surface faces north and is inclined at around 15 degrees off vertical. The leaf blades can reach 60 cm in length and 4 cm wide. They are dull green with serrated margins and a white undersurface. Dead leaves remain on the plant. Flowering occurs in late spring. The cylindrical inflorescences are yellow in overall colour and range from 9 to 16 cm high. As the flower spikes age, they fade to a greyish colour, the old flowers persisting. Up to 20 woody seed pods, known as follicles, may appear on each spike. Covered with a fine grey fur, they are elliptical in shape and measure 2.8–3.8 cm in length, and 1.5–2 cm in width.

The obovate seed is 2.5–2.8 cm long and fairly flattened. It is composed of the triangular seed body, measuring 1.0–1.5 cm long by 1.4–2.0 cm wide, and a papery wing. One side, designated the "outer surface", is deeply pitted and the other is brown and smooth. The seeds are separated by a sturdy dark brown seed separator that is roughly the same shape as the seeds with a depression where the seed body sits adjacent to it in the follicle. The first pair of leaves produced by seedlings, known as cotyledons, are cuneate and measure 1.2–1.4 cm long by 1.8–2.0 cm wide. They are dull-green with a faint net-like pattern. The auricle at the base of the cotyledon leaf is pointed and measures 0.2 cm long. The first pair of leaves to appear after the cotyledons are 3.5 cm long and oval in shape, with 2–3 lobes or teeth on each side. The next set are 5 cm long with 7–10 teeth.

"B. petiolaris" is fairly uniform across its range, though plants may vary in leaf size. Its yellow flower spikes and white leaf undersurface distinguish it from other prostrate banksias.

Distribution

Endemic to Western Australia, "Banksia petiolaris" is found near the state's south coast from the vicinity of Munglinup east to Israelite Bay, concentrated in two disjunct ranges—an eastern one around Cape Arid National Park, and western one east of Scaddan. It is found in white sandy soils in kwongan or mallee heathland, but sometimes occurs with the taller "B. speciosa". It is often locally common, with many populations over 100 plants in size.

Habitat

Like many plants in Australia's Southwest, "Banksia petiolaris" is adapted to an environment in which bushfire events are relatively frequent. Most "Banksia" species can be placed in one of two broad groups according to their response to fire: "reseeders" are killed by fire, but fire also triggers the release of their canopy seed bank, thus promoting recruitment of the next generation; "resprouters" survive fire, resprouting from a lignotuber or, more rarely, epicormic buds protected by thick bark. "B. petiolaris" and the related "B. blechnifolia" are in the former category—rapidly growing plants killed by bushfire and regenerating by seed—while the other prostrate species are slow growing resprouters.

Like other banksias, "B. petiolaris" plays host to a variety of pollinators—insects such as bees, wasps and ants were all recorded in the 1988 "The Banksia Atlas" survey.

An assessment of the potential impact of climate change on this species found that its range is likely to contract by between 30% and 80% by 2080, depending on the severity of the change. Many of its western populations are found on road verges, rendering them vulnerable to resurfacing or widening of roadways. It does, however, show little susceptibility to the soil-borne water mould "Phytophthora cinnamomi", unlike many Western Australian banksias.

"Banksia petiolaris" has the longest-lived leaves of any flowering plant so far recorded—a study published in 1992 recorded a lifespan of up to 13 years for a single leaf, while leaves of 10 years of age ably maintained their ability to photosynthesize compared with young leaves. The authors concluded that leaf longevity is advantageous in plants in nutrient-poor soils, as the loss of valuable nutrients in leaf loss is minimised.