Conceptualizing ecological flammability: an experimental test of three frameworks using various types and loads of surface fuels

Vegetation flammability remains poorly defined and involves many intercorrelated components and metrics. Schwilk (2015) proposed a flammability framework with only two axes: total heat release and rate of spread. Pausas et al. (2017) modified this framework by standardizing the heat release axis by fuel load, and adding a third axis of fuel ignitability. We tested these frameworks using data from a field experiment that quantified flammability metrics and survival of Callitris intratropica saplings in relation to fuel type (grass, litter, and mixed grass and litter, all air-dried) and fuel load. Principal components analysis showed PC1 was closely aligned with rate of combustion, flame height and temperature, and PC2 was aligned with duration of combustion. The Schwilk framework separated the fuel types according to rate of spread, and fuel loads according to total heat release. The Pausas framework was less useful in describing community-scale flammability because it removed the effects of fuel load, and there was no support for adding the ignitability axis. Both frameworks successfully predicted sapling mortality, an indicator of fire severity. In addition, the three flammability strategies proposed by Pausas et al. were not well-supported because they assumed unrealistically low heat release by ‘fast-flammable’ fuels. We conclude that the Schwilk framework is useful for conceptualizing community-scale flammability and facilitates modelling for fire management purposes, and exploration of evolutionary relationships.