Fuel weight and understorey hazard dynamics in mature karri ( Eucalyptus diversicolor ) forests in southwest Western Australia

Fuel properties influence the behaviour of forest fires, so understanding how these change with time since fire is important for appraising the bushfire threat and planning and implementing bushfire mitigation operations. A space-for-time study in mature karri forests with fuel ages ranging from 1 to 92 years demonstrated that total fine-fuel weight (TFFW) increased with time since fire for about 30 years then plateaued at a mean value of about 50 t ha−1. For fuels older than four years, on average, 74% of TFFW was in the surface fuel layer (the litterbed) and 17% was in the near-surface layer (up to 1 m above the surface layer). Live understorey vegetation contributed only about 6% to TFFW. Predicting TFFW from time since fire was improved by including karri tree basal area. Mean understorey height (Uht) increased with time since fire, peaking at 6 m after about 30 years, then declining to about 4 m after 92 years. Mean understorey hazard (Uhaz), derived from Uht plus the proportion (%) of dead fuel in each fuel layer, followed a similar trend, peaking at 20–30 years post-fire, then declining. Although Uhaz had declined by 36% from the maximum value by 60+ years post-fire, it was 27% higher than the Uhaz value for young fuels (1−<5 years old). For a mean prescribed-burn interval of eight years, 50% of the forest fuel will be ≤four years old and so will be carrying about ≤19 t ha−1 of fine fuel (≤38% of the maximum value), with a Uhaz value of about <3.56 (<50% of the maximum value). Fuel weight and Uhaz directly influence fire intensity, flame size, spotting potential and rate of spread. Therefore, prescribed burning, done strategically and at the appropriate temporal and spatial scales, will make bushfires less damaging and easier and safer to suppress.