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.

A quantitative assessment of shoot flammability for 60 tree and shrub species supports rankings based on expert opinion

“…Interestingly, the most fire-adapted indigenous species burned here, Mānuka, was not the most flammable of the indigenous species. Although our study was not designed to test their hypothesis, these findings support Bowman et al (2014) as we demonstrate high levels of flammability in species from communities that are rarely burnt. High shoot-level flammability was also recently reported for species that occur in the fire-free forest biome in South Africa (Calitz et al 2015).…”

“…This is the case for some tree fern species and also Cabbage Tree (Cordyline australis). The retention of dead leaf material increases plant flammability (Bowman et al 2014), and preliminary tests found dead tree fern and Cabbage Tree fronds were more flammable than the corresponding live material. For such species the flammability of the dead material will likely be a more important driver of plant flammability than that of a live shoot, yet the present method only measures this where dead material is part of a terminal shoot section.…”

“…The Mutch (1970) hypothesis suggests that plants from fire-dependent communities have evolved characteristics that increase their propensity to burn, though this has since been much debated in the literature (e.g. Snyder 1984;Bond and Midgley 1995;Schwilk 2003;Bowman et al 2014). Bowman et al (2014) showed that fireenhancing plant traits can exist in plant communities rarely exposed to fire, and proposed that such traits have therefore evolved independently of landscape fire as the result of other selective pressures, although some may be exaptations.…”

“…Snyder 1984;Bond and Midgley 1995;Schwilk 2003;Bowman et al 2014). Bowman et al (2014) showed that fireenhancing plant traits can exist in plant communities rarely exposed to fire, and proposed that such traits have therefore evolved independently of landscape fire as the result of other selective pressures, although some may be exaptations. Despite fire not being a selective pressure in the evolutionary history of the indigenous NZ species (with the possible exception of some .…”

Advances in understanding the influence of fire on the ecology and evolution of plants: a tribute to Peter J. Clarke

“…Unlike warm temperate rainforests, monsoon rainforest and tropical savanna are thus likely alternative stable states driven by fire . There is ongoing debate about the adaptive significance of plant traits linked to flammability (Bradshaw et al 2011;Keeley et al 2011;Bowman et al 2014). Mason et al (2016;this issue) explore this proposition in common native woody and herbaceous species of New Zealand, where fire has not been a strong selection pressure.…”