How do some plants become flammable? Insights from Dracophyllum

When I was a kid one of my favourite activities on a sunny day (much to my parents’ despair) was to go into the garden with a magnifying glass and try to set fire to things – egg boxes, newspaper, bits of wood, dried leaves etc. Thankfully I grew out of that, but reading about the ins and outs of fire and flammability still stirs up a childlike curiosity in me. Fire is a challenge that plants in some parts of the world have to face frequently, and in such areas it is believed that frequent fire has led to the emergence of certain adaptive traits. These include rapid re-sprouting, underground tubers, fire-stimulated seed dispersal and high degrees of flammability in plants of these areas. What is not well understood is whether other environmental pressures may also produce flammable traits in plants. To redress this imbalance, Xinglei Cui and colleagues in a recent paper in New Phytologist examine the flammability of different Dracophyllum species in New Zealand, an area that seldom experiences fire.

Using a series of flammability measurements, Cui and colleagues find that flammability varies widely across the genus Dracophyllum, ranging from species that cannot be ignited to those that can easily be almost entirely destroyed by fire. The fact that New Zealand seldom experiences fire suggests that the high flammability of some Dracophyllum species is not driven by fire itself as an evolutionary pressure. So why does such high variation in flammability exist in Dracophyllum species if they don’t experience fire? Well, there are some clues. Members of a particular subgenus of Dracophyllum are measured by Cui and colleagues as being generally more flammable than other species. Cui and colleagues speculate based on previous studies that historic glaciation in New Zealand, and the associated cold, dry climates, may have driven the emergence of certain adaptive prevalent traits in  Dracophyllum such as small leaves (which help protect against drought and frost). Incidentally, the authors find that Dracophyllum species with small leaves are more likely to have high flammability. Historic climate events may therefore have promoted certain adaptive features in some Dracophyllum species that happened to also be flammability-promoting traits.

Incidental arising of flammability variation due to other environmental conditions is also supported by current climate variation between growth areas of Dracophyllum species in New Zealand. For example, the two closely related species Dracophyllum menzisii and Dracophyllum fiordense have rather different shoot flammability, with Dracophyllum menzisi being mildly flammable but Dracophyllum fiordense being fire-resistant. The likely reason for this is that  D. fiordense grows in wetter areas than D. menzisi, and consequently has larger leaves and higher moisture content. Whilst past and present climatic conditions have therefore apparently influenced that incidental appearance in flammability in Dracophyllum species, phylogenetics also plays a role. Although Cui and colleagues note that closely related species can vary markedly in flammability in some cases, they also observed the overall trend that closely related species were more likely to be similar in flammability compared to distantly related species.

Therefore, while flammability is a trait known to be driven by frequent fire exposure in some areas, it can also arise in areas with low fire exposure as an adaptation to other factors. This, as Cui and colleagues conclude, highlights that researchers need to consider the prevalence and history of fire in a particular area when studying flammability, and how it arose, in plants. There’s more to flammable plants than just keeping a bored kid entertained then!