Soil Temperatures During Experimental Bushfires in Relation to Fire Intensity: Consequences for Legume Germination and Fire Management in South-Eastern Australia

Abstract

1. Experimental fires were used to determine the influence of fire intensity and subcomponents of intensity (fuel consumption and rate of spread) on soil temperatures (at 0-10 cm depth) in shrub/woodland communities in the Sydney region of south-eastern Australia. 2. Temperatures > 60°C were achieved at depths of 0-3 cm. Such temperatures are sufficient to break seed dormancy in local legume species. 3. Maximum change in soil temperature was not significantly affected by Byram fire intensity. However, at 0-4 cm depth, maximum temperature change was significantly affected by the amount of fine fuel (particles < 6 mm thick) consumed on the ground (mainly litter, twigs and herbs). Fine fuel above the ground (shrub canopies), coarse fuel (6-25 mm thick) on the ground and rate of spread did not significantly affect maximum change in temperature in the top 4 cm of the soil. 4. The results indicate that 0.6-2.0 kg m −2 of fine fuel burnt on the ground is likely to stimulate germination of buried seeds of local legumes (seed dormancy broken at ≥ 60° C). 5. At the upper end of this fuel consumption range, germination is predicted to occur down to 3-cm depth, while consumption at the lower end of the range will stimulate germination to a depth of 1 cm. Post-fire germination in Acacia suaveolens was in accordance with these predictions. 6. These results are discussed in relation to the use of prescribed fire for hazard reduction. Prescribed fires with maximum potential for germination (maximum fuel consumption in the target range) will be close to, or in excess of, the limit for control unless the rate of spread is very low (< 0.10 M S −1 ), thereby imposing constraints on the use of prescribed fire to achieve both protection and conservation objectives