Background

Potential heat release (PHR) is the theoretical maximum amount of heat releasable by wildland fuels during fire and is a key determinant of fireline intensity. Understanding its variability and dynamics is important for predicting fire behavior and effects on ecosystems. In this study, we sought to understand PHR and its drivers at the forest-stand scale. We quantified PHR for stands of wet sclerophyll forest in south-eastern Australia from field measurements of fine fuel loads, fuel moisture content, and calorific values for each fuel type. We determined the effects of species composition and live-to-dead ratio on stand-level PHR by integrating forest composition data from another study in the same forest type.

Results

PHR varied from 19.8 to 77.6 MJ m⁻² between the sites surveyed. Large differences in PHR between forest stands were primarily due to substantial differences in fuel loads and the calorific values of eucalypt versus non-eucalypt litter. Litter and canopy fuels were the primary contributors to stand-level PHR (~ 40% and ~ 30% respectively) at sites dominated by Mountain Ash (Eucalyptus regnans F.Muell.). In contrast, understory fuels and litter were the main contributors to PHR at the scrub site, which had no Mountain Ash. Variations in species composition between forest stands had a negligible effect on stand-level PHR compared to variations in fuel load and live-to-dead ratio.

Conclusions

The significant variability in PHR could have implications for predicting fireline intensity across the landscape. Although canopy fuels are a major contributor to stand-level PHR in Mountain Ash forest, their contribution to actual heat release during a fire will depend on fire reaching the canopy via ladder fuels. Thus, forest structure and species composition may play a key role in heat release through aspects of fire behavior affecting the completeness of combustion. Further quantification of fuel consumption and live-and-dead fuel loads across the landscape is warranted for accurate determination of actual heat release during wildfire.