Tropical grasses fuel the majority of fires on Earth. In fire-prone landscapes, enhanced flammability may be adaptive for grasses via the maintenance of an open canopy and an increase in spatiotemporal opportunities for recruitment and regeneration. In addition, by burning intensely but briefly, high flammability may protect below ground buds from lethal temperatures. Despite these potential benefits of high flammability to fire-prone grasses, we do not know how flammability varies among grass species, or how trait differences underpin this variation. By burning leaves and plant parts, we aimed to experimentally determine how five plant functional traits combined to drive the three components of flammability (ignitability, sustainability and combustibility) in grass species of fire-prone South African grasslands at a time of peak fire occurrence. We also aimed to assess the influence of evolutionary history on flammability by building a phylogeny for the study species and testing for phylogenetic signals.
We demonstrate that grass flammability is predicted from easily measurable plant functional traits and is influenced by evolutionary history with some components showing strong phylogenetic signals. The five measured plant traits effectively predicted all components of flammability. Leaf ignitability and plant combustibility were particularly well predicted, with 70% and 58% of variation in these components respectively being explained by a combination of the traits. Total above-ground biomass was a key driver of combustibility and sustainability, whilst biomass moisture content was the main influence on ignitability.
Grasses are not homogenous fuels to fire. Rather, species differ in functional traits that in turn lead to significant variation in all components of flammability. This diversity is consistent with the idea that flammability may be an adaptive trait for grasses of fire-prone ecosystems. Fire-prone grasslands are expansive, yet are typically dominated by relatively few species. The finding here of significant variation in grass flammability therefore suggests that community composition could have a major effect on fire characteristics, with regions dominated by different species experiencing very diverse fire regimes.