Smoke bush species show fire awareness

Signs of serotiny, an ecological adaptation in which seed release occurs in response to an environmental trigger rather than spontaneously at seed maturation, has been discovered in in two species of Conospermum.

More commonly known as smoke bush the Conospermum genus, named for its long branches of white flowers giving them a 'smoky' resemblance when seen from afar, is endemic to Australia and contains 53 species.

The majority of smoke bushes release their seeds at maturity, falling to the soil to form a seed bank and awaiting an environmental trigger to germinate, however C. capitatum and C. petiolare display a different reproductive strategy.

The two species are serotinous, storing their seeds in a protective succulent-like 'cone' on the plant for significant periods of time, with seed release only occurring as a response to an environmental trigger such as fire, playing a similar role to the woody structures found in many Banksia species.

Dr Xiaoying Zhao and Dr Philip Ladd from Murdoch University studied the species as part of a recent study into fire-prone vegetation in south-west WA.

"The particularly interesting thing about the two species is that they are two that are in a group (genus) where serotiny is not normal—seeds of most smoke bushes are stored in the soil until a fire comes through and cues the seeds to germinate," Dr Ladd says.

"In addition the two species do not look at all like smoke bushes—they do not have the abundant white flowers of what we would normally think of as smoke bushes."

Infructescences, or collections of fruits, from each species were collected and subjected to a range of heat and smoke treatments to study their germination requirements.

Dr Ladd says that smoke acting as a stimulus to germinate the seeds of C. capitatum is extremely unusual for a serotinous species.

"Normally, as serotinous species shed their seeds after a fire, the seeds only need water to begin their germination," he says.

In contrast, C. petiolare, like most serotinous species, has no requirement for smoke or heat to cue germination.

The reason serotiny arose in a genus where it is the exception is unknown, but having more control over the fate of the seeds may provide a reproductive advantage.

The infructescences' high moisture content and inconspicuous placement within the branches of the plant protects the seeds from wildfire temperatures up to 200C and predator detection.

Once the plant is burnt the infructescence's branches spread out and the seeds are revealed, their long hairs enabling wind dispersal into favourable microhabitats for germination.