Fuel characteristics, vegetation structure and fire behaviour of 11–15 year-old rehabilitated bauxite mines in Western Australia

Summary

Bauxite mines rehabilitated in the early 1980s in Western Australia have built up substantial fuel loads that may be reduced through prescription burning. Fuel characteristics, vegetation structure and fire behaviour of 11–15 year-old rehabilitated bauxite mines were examined. Fuel loads were high in all pre-burn rehabilitation areas (23.4–35.1 t ha⁻¹) although large ranges in fuel load estimates of sample plots (6.3–49.3 t ha⁻¹) indicated the heterogenous nature of the fuel distribution that led to heterogeneity in fire behaviour. The vegetation structure of the rehabilitated areas differed from that of the native jarrah forest due to a lower proportion of live to dead plant material and the presence of a prominent mid-storey layer composed of dead Acacia plants. This mid-storey layer contributed 45% of the total fuel load and led to increased flame heights and subsequent higher levels of crown scorch regardless of the soil and litter moisture conditions. Seven prescription burns were carried out between 1994 and 1996 encompassing a range of fire intensities and seasons of burning. Autumn burns were generally of higher intensity than spring burns due to the drier nature of the fuel following summer drought. Re-accumulation of fuel in the first year following burning was rapid (6.4–9.5 t ha⁻¹) but had levelled off by the second year (6.1–10.3 t ha⁻¹). Vegetation structure of rehabilitated areas one and two years after burning indicated that the prescription burns had removed the mid-storey layer of senescent Acacia material and stimulated an increase in the proportion of live plant material, particularly following autumn burning. Maximum soil temperatures recorded by heat-sensitive crayons exceeded 200°C in the autumn burns and were generally well below 100°C in the spring burns. Hard-seeded species would have been stimulated to germinate in the top 2–5 cm following autumn burning and only in the top I cm following spring burning. Some seeds in the top 1 cm of soil may have been killed in the autumn burns. The fire management of rehabilitated areas will have to be different to that of the native jarrah forest due to differences in fuel characteristics, vegetation structure and fire behaviour of these two sites.