Have plants evolved to self-immolate?

Abstract

By definition fire prone ecosystems have highly combustible plants, leading to the hypothesis, first formally stated by Mutch in 1970, that community flammability is the product of natural selection of flammable traits. However, proving the "Mutch hypothesis" has presented an enormous challenge for fire ecologists given the difficulty in establishing cause and effect between landscape fire and flammable plant traits. Individual plant traits (such as leaf moisture content, retention of dead branches and foliage, oil rich foliage) are known to affect the flammability of plants but there is no evidence these characters evolved specifically to self-immolate, although some of these traits may have been secondarily modified to increase the propensity to burn. Demonstrating individual benefits from self-immolation is extraordinarily difficult, given the intersection of the physical environmental factors that control landscape fire (fuel production, dryness and ignitions) with community flammability properties that emerge from numerous traits of multiple species (canopy cover and litter bed bulk density). It is more parsimonious to conclude plants have evolved mechanisms to tolerate, but not promote, landscape fire.

Keywords: biomass burning; evolution; fire regime; landscape fire; niche construction; plant regeneration; plant traits.

Figure 1

Diverse plant traits that affect vegetation flammability. (A) Surface fire in Amazonian rainforest leaf litter and ground cover vegetation during a severe drought, when leaf moisture context of living and dead foliage was very low (Photo: Mark Cochrane); (B) Koala (Phascolarctos cinereus), an iconic specialist mammalian herbivore involved in a co-evolutionary relationship with eucalypt leaf secondary chemical defenses. These defenses also make foliage exceptionally flammable (Photo Kath Handasyde); (C) New Caledonian maquis vegetation, which is dominated by sclerophyll species with phylogenetic links to Australian flammable heathland, yet has a poor capacity to recover from fire (Photo David Bowman); (D) leaf retention of Richea pandanifolius, a fire sensitive Gondwana rainforest giant heath, demonstrates that this trait is not universally associated with increasing flammability (Photo David Bowman); (E) low bulk density annual grass layer in eucalypt savanna is exceptionally flammable (Photo Don Franklin); (F) post-flowering die-off of the giant bamboo Bambusa arnhemica in frequently burnt eucalypt savanna. The dead bamboo is much less flammable than the grass layer in surrounding savanna (photo Don Franklin); (G) decorticating bark on a SE Asian tropical rainforest tree Cratoxylum cochinchinense demonstrates that this trait is not necessarily related to spreading fires via fire brands (Photo David Tng); (H) abrupt rain forest boundary in north Queensland which limits the spread of savanna fires, as evidenced by the shrubs burnt in the preceding dry season (Photo David Bowman).