A soil heating index for interpreting ecological impacts of jarrah forest fires

Soil heating during forest fires gives rise to numerous ecological responses, many of which are poorly understood. To interpret, predict and manage these responses, such as fire-induced legume regeneration, it is necessary to identify the key fire, fuel and soil factors that affect heat transfer to the soil, and to determine functional relationships incorporating these factors. Heat transfer to the soil was investigated in the laboratory using jarrah forest litter as the heat source to develop a soil heating index for jarrah forest fires. Maximum soil temperature and heat load (the area beneath the temperature history trace) depended on the quantity of surface litter fuel consumed and soil and fuel moisture content, but was independent of fire rate of spread. A linear relationship incorporating these factors was developed to predict the maximum soil temperature at a depth of 10 mm, thus deriving the soil heating index. This relationship, together with knowledge of the temperatures necessary to achieve optimum germination of soil stored seed, can be used to interpret regeneration responses and to prescribe fires to optimise the regeneration of desired species.