Laboratory determination of factors influencing successful point ignition in the litter layer of shrubland vegetation

Factors affecting ignition thresholds of the litter layer of shrubland vegetation were investigated using reconstructed litter beds in a laboratory. The factors investigated were fuel moisture content (FMC), litter type (primarily species), pilot ignition source, and wind. Litter beds made from 11 different litter types were ignited with point ignition sources. Litter from Allocasuarina nana (Sieber ex Spreng.) L.A.S. Johnson was used as the standard type across all experiments. Successful ignition was defined as fire spreading a fixed distance from the ignition point. Ignition success was modelled as a logistic function of FMC. Litter type had a major effect on ignitibility. The bulk density of the litter bed and the surface area of litter per volume of litter bed provided reasonably good predictors of the effect of litter type on ignition success. Low-density litter beds ignited at higher FMCs than dense litter beds. The two densest litter beds failed to ignite with the procedures used here. The ignition sources tested had significantly different effects on ignition success. Larger ignition sources were able to ignite wetter fuels than smaller sources. The presence of wind was found to have a different effect on ignition success depending on the location of the ignition source with respect to the litter bed. Wind decreased ignition success when the ignition source was located on top of the litter bed, but aided ignition when the ignition source was located within the litter bed.