Toward a macroevolutionary understanding of live-leaf flammability in plant species of fire-prone forests

Abstract

Premise: The flammability of live leaves in canopies varies considerably among plant species. Identifying macroevolutionary processes that shape variation in leaf flammability contributes to an understanding of the phylogenetic underpinnings of wildfire dynamics.

Methods: We used a phylogenetic comparative approach to examine the macroevolution of live-leaf flammability in 75 plant species of fire-prone dry sclerophyll forests in eastern Australia. We estimated phylogenetic signal in leaf flammability, fitted a series of evolutionary models to test macroevolutionary hypotheses about leaf flammability, and assessed evolutionary correlations between leaf flammability and leaf water content (LWC), leaf mass per area (LMA), leaf area (LA), plant growth form, and fire response strategy.

Results: We detected weak phylogenetic signal, indicating that leaf flammability exhibited greater variation among closely related species than would be expected under phylogenetic conservatism. The evolution of leaf flammability was equally well described by an Ornstein-Uhlenbeck model and a Pagel's δ model, implying weak stabilizing selection and an acceleration in leaf flammability evolution over time. We found significant evolutionary correlations such that high leaf flammability was related to low LWC, low LMA, and large LA.

Conclusions: Our results show that live-leaf flammability is an evolutionarily labile trait in plant species of fire-prone forests. We suggest that the evolution of the three leaf traits in response to prevailing environmental conditions (such as LWC to water availability, LA to light capture, and LMA to herbivore defence) provide antagonistic selective forces that produce a macroevolutionary pattern of weak stabilising selection on leaf flammability.

Keywords: evolution; fire; ignitability; leaf traits; phylogeny; stabilizing selection; wildfire.