Heavy road traffic promotes flowering of the grasstree, Xanthorrhoea preissii – a preliminary investigation

Abstract Grasstrees are renowned for their ability to flower after fire. They are a popular species for ornamental landscaping and anecdotal observations indicate much higher levels of flowering of Xanthorrhoea preissii than occur among unburnt plants in nature. Also, unburnt grasstrees in nature reserves beside major roadways in Perth, Australia, appear to flower much more frequently than those some distance from the roadway. We chose a reserve site 6 km south of Perth with prolific flowering of grasstrees beside a busy intersection. We showed that plants within 10 m of the roadway were 2.5 times more likely to flower than those in the following 50 m. This is on a par with burnt plants in a nearby reserve and those in the median strip of a nearby road. Of those that flowered, roadside plants, adjusted for size (height), had 60% more spikes Branching is induced by flowering, and roadside plants also had 60% more branches that those distant from the road without any difference in plant size. This means that the difference cannot be attributed to better growing conditions beside the road. Grasstrees have been shown to respond to the volatile hydrocarbon, ethylene, by flowering, and a major component of vehicle exhaust fumes is ethylene. We therefore hypothesise that prolific flowering by cultivated grasstrees in public places, and those beside busy roads, is a response to locally raised ethylene levels in the air. Much more remains to be known about this intriguing anthropogenic phenomenon.