SOIL TEMPERATURES DURING BUSHFIRES IN SEMIARID, MALLEE SHRUBLANDS

Soil temperatures were measured during 11 experimental fires in semi-a rid mallee shrublands in central NSW. Sensors were placed at depths fr om 1-10 cm beneath the soil surface in three fuel types; litter beneat h Eucalyptus shrubs, live hummocks of the grass Triodia irritans and l itter beneath shrubs of Acacia species. Weights of these fuels per uni t area were determined. Maximum soil temperature and its duration were related to fuel type and depth. Mean weights of Eucalyptus and Triodi a fuels were similar (0.35 kg m-2), while there was less Acacia fuel ( 0.1 kg m-2). Highest maximum temperatures were registered under Eucaly ptus litter (e.g. 140-degrees-C at 2 cm). Maximum temperatures under T riodia and Acacia litter were similar (e.g. 60-70-degrees-C at 2 cm). Durations were examined in two temperature classes (60-120-degrees-C a nd > 120-degrees-C) chosen to represent threshold for stimulation of g ermination and mortality, respectively, of soil-stored seeds. Temperat ures between 60 and 120-degrees-C were recorded only between 0-2 cm so il depth for Acacia and Triodia (one exception at 4 cm), No temperatur es > 120-degrees-C were recorded for these fuel types. Temperatures be tween 60 and 120-degrees-C were recorded to 5 cm depth under Eucalyptu s fuels while putative lethal temperatures for seeds occurred occasion ally at 0-2 cm depth. The results indicated greatest potential for sti mulation of germination and death of buried seeds under Eucalyptus fue ls, although the level of variability of temperature was highest under Eucalyptus fuels. Despite similar fuel loads, differences between tem peratures under Eucalyptus and Triodia fuels reflected the influence o f the depth of the fuel bed, with Triodia hummocks constituting a deep fuel bed and Eucalyptus litter a shallow fuel bed.