Increasing radiant heat flux affects leaf flammability patterns in plant species of eastern Australian fire-prone woodlands.

Abstract

Leaf flammability is a functional trait that can vary widely among plant species. At present, however, the effects that increasing radiant heat flux have on variation in leaf flammability among species are not well understood. Yet, such effects could have important implications for wildfire models that take into account species' differences in flammability. We examined how five leaf flammability attributes spanning ignitibility (times to incandescence and flaming), sustainability (incandescence and flame durations) and combustibility (proportion of leaves entering flaming combustion) responded to increasing radiant heat fluxes (29.6 to 96.6 kWm-2 ) in 10 species of fire-prone woodlands. As radiant heat flux increased, times to incandescence and flaming became significantly faster and proportions of leaves entering flaming combustion became significantly higher. In contrast, incandescence duration became significantly shorter at high radiant heat flux. Differences among species in these flammability attributes decreased with increasing radiant heat flux, with species becoming significantly more similar to each other. Differences among species in flame duration, however, were not significantly affected by increasing radiant heat flux, with leaf flaming durations in each species remaining relatively fixed across the radiant heat flux gradient. Our findings show that leaf flammability is significantly affected by increasing radiant heat flux. We suggest that of the flammability attributes assessed in our study, flame duration is the most informative to include in wildfire models which explicitly consider species' flammability, given that differences among species in flame duration are maintained across a radiant heat flux gradient.