BPotD Work-Learn student Taisha is again the author of today’s entry. She writes:
Today’s photo is of Banksia robur, or the swamp banksia. This photo was taken in May of 2009 at the Royal Botanical Gardens, Sydney in Australia by sftrajan@Flickr (aka Eric). Thanks for the photo, sftrajan!
Swamp banksia is native to coastal eastern Australia, where it prefers areas with damp soil and full sun. A small shrub, Banksia robur reaches 2 to 3 meters in height. The egg shaped, or obovate, leathery leaves are toothed on the margins and have an obvious midvein visible from both sides. The underside of the leaves is fuzzy. The characteristic inflorescence of this genus is made up of hundreds of small florets attached to a rachis or central column. This species’ flowers are greenish with greenish-cream, almost metallic-looking claws, that later turn brown after anthesis. A small number of these flowers will develop into the fruits, called follicles. The follicles will only open after being burnt by fire or severe and extended drought.
He, Lamont, & Downes from the University of Western Australia looked at Banksia species and their favourable traits for survival in a fire-prone environment. They were hoping to uncover when these traits first appeared, at what rate they proliferated, and what the co-evolution of fire-dependent and fire-enhancing traits say about fire as a selective force on the evolution of Banksia species. They found that numerous members of the genus are both serotinous (dependent on environmental triggers, in this case fire, for seed release) and woody fruit-retaining.
This co-evolution of a fire-dependent trait and fire-enhancing trait were shown to be present within the genus since early fossil records from 60 million years ago in the Paleocene, implying that fire was then an evolutionary selective force. The retention of dead leaves, a trait first appearing 35 mya, in conjunction with retaining the dead floral parts ensures serotinous species burn hot enough to melt the resin that seals the valves of the capsules containing the seeds. Lastly, clonality–the ability to resprout via underground stems, rhizomes, and root suckers–allows for regrowth after a fire. Clonality is ensured by the meristematic tissues being insulated by soil and by being a poorer heat conductor than bark. This adaptation has been present within the genus since the Miocene, around 16-20 mya, which was noted to also be the era when climatic seasonality commenced and fires became more (or perhaps less) prevalent in the area.
He et al. suggest that fire has greatly influenced the evolution of clonality within Banksia, but that other pressures also contributed and thus resprouting should not be used as a single trait in trying to understand its evolution. Overall, the role of fire as an evolutionary selective force is controversial, but their results provide support for the hypothesis that there is an association between land plants and fire before the onset of seasonal aridity in Australia. Their results also suggest some land plant groups have evolved a set of traits in response to fire regimes and that these traits can be used to test evolutionary theory (see: He, T., Lamont, B. B., Downes, K. S. 2011. Banksia born to burn. New Phytologist. 191:184-196).