Predicting ignitability from firebrands in mature wet eucalypt forests

Forest ignitability varies spatially and temporally across landscapes as a function of fuel moisture. Wetter forests are often more difficult to ignite and therefore act as a barrier to the spread of wildfire. However, when these forests dry out and transition to an ignitable-state, the landscape is primed for large wildfires. Fire managers require information about forest ignitability to predict the timing of this switch. Yet, an understanding of the drivers and thresholds for ignitability in wetter forests remains incomplete. Our study sought to address this knowledge gap by evaluating a range of in-forest and landscape variables as predictors of ignitability in wet eucalypt forests. We conducted ignition experiments over two fire seasons at six sites in mature wet and damp eucalypt forest in south-eastern Australia. Each experiment involved 30 ignition attempts within each site on a single day using two types of flaming firebrands with different heats of combustion. The success and sustainability of each ignition attempt was recorded and this process repeated 8-9 times per site culminating in 1590 ignition attempts across a range of fuel moisture conditions. Ignition results were evaluated against 21 different in-forest and landscape variables. In-forest variables were generally stronger predictors of ignition and sustained ignition than landscape variables. The more energetic firebrand yielded more successful ignitions and sustained ignitions, but rankings for the predictor variables were similar for both types of firebrand. The strongest predictor of ignition and sustained ignition was in-forest vapour pressure deficit (VPD) (R-2 = 0.65 for ignition and R-2 = 0.55 sustained ignition), which is a measure of atmospheric dryness that is strongly associated with dead fuel moisture content. Past studies have identified strong links between VPD, burned area and fire severity at broader spatial and temporal scales. Our results show that VPD is also important at the time of ignition and could be used to predict wildfire risk in wet eucalypt forests.

Automated summary

Forest ignitability varies spatially and temporally depending on fuel moisture. Wetter forests act as barriers to the spread of wildfires, but become prime locations for large wildfires when they dry out. Our study found that in-forest vapor pressure deficit (VPD) is the strongest predictor of ignition and sustained ignition in wet eucalypt forests, and could be used to predict wildfire risk.