Determination of Fire-Residence Time, and Its Role in the Survival of Eucalypts After a Bushfire

Abstract

Bushfires occur frequently in drought-prone eucalypt forests of Australia, and their intensity can vary within very wide limits. Permanent damage is caused when a tree's vascular cambium layer is heated above 60°C, but this may be prevented in eucalypts by a thick layer of bark. It can take several minutes of flaming combustion during a bushfire for sufficient heat to penetrate the bark and kill the cambium. By heating some small sections of wood or bark in an oven to temperatures between 300°C and 600°C, then grinding and polishing a transverse section, reflectance measurements have been made in vertical incident light through a microscope fitted with a photomultiplier. These readings show a linear correlation between the logarithm of reflectance and temperature. Using this correlation, some maximum temperature profiles were obtained across transverse bark sections cut from trees subjected to a low-intensity bushfire. These have been compared with some graphs which relate the maximum temperatures reached within bark to the distance from the outer surface after progressively longer periods. By using reflectance measurements made on sections of charred bark or, more simply, by measuring the thickness of the black outer layer of charred bark an estimate of the fire residence time of a bushfire can be obtained.