SOIL TEMPERATURES DURING BUSHFIRES IN SEMIARID, MALLEE SHRUBLANDS

Soil temperatures were measured during 11 experimental fires in semi-arid mallee shrublands in central NSW. Sensors were placed at depths from 1-10 cm beneath the soil surface in three fuel types; litter beneath Eucalyptus shrubs, live hummocks of the grass Triodia irritans and litter beneath shrubs of Acacia species. Weights of these fuels per unit area were determined. Maximum soil temperature and its duration were related to fuel type and depth. Mean weights of Eucalyptus and Triodia 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 Eucalyptus litter (e.g. 140-degrees-C at 2 cm). Maximum temperatures under Triodia 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 and > 120-degrees-C) chosen to represent threshold for stimulation of germination and mortality, respectively, of soil-stored seeds. Temperatures between 60 and 120-degrees-C were recorded only between 0-2 cm soil depth for Acacia and Triodia (one exception at 4 cm), No temperatures > 120-degrees-C were recorded for these fuel types. Temperatures between 60 and 120-degrees-C were recorded to 5 cm depth under Eucalyptus fuels while putative lethal temperatures for seeds occurred occasionally at 0-2 cm depth. The results indicated greatest potential for stimulation of germination and death of buried seeds under Eucalyptus fuels, although the level of variability of temperature was highest under Eucalyptus fuels. Despite similar fuel loads, differences between temperatures under Eucalyptus and Triodia fuels reflected the influence of the depth of the fuel bed, with Triodia hummocks constituting a deep fuel bed and Eucalyptus litter a shallow fuel bed.