Fire management implications of fuel loads and vegetation structure in jarrah forest restoration on bauxite mines in Western Australia

Bauxite mines being restored to native jarrah (Eucalyptus marginata)-dominated forest in Western Australia have accumulated substantial fuel loads. Assessment of fire management aspects is necessary for the effective integration of restored areas into forest-wide management. Fuel characteristics, vegetation structure and fire behaviour of young (5- and 8-year-old) restored bauxite mines in Western Australia were examined. Pre-burn fuel loads were moderate in 5-year-old restoration (15.0tha−1) and high in 8-year-old restoration (29.8tha−1). Large ranges in available fuel load estimates of sample sites (2.2–60.8tha−1) indicated the heterogeneous nature of fuel distribution leading to variable fire behaviour. The vegetation structure of restored areas differed from that of the unmined jarrah forest due to the presence of a prominent mid-storey layer composed of number of acacias (Acacia pulchella, A. celastrifolia, A. extensa, A. drummondii and A. lateriticola). This mid-storey layer contributed 49% of the total fuel load in 5-year-old restoration, although the high proportion of live material (73.6%) inhibited fire development. In 8-year-old restoration the mid-storey layer contributed 46% of the total fuel load. The lower proportion of live material in these sites (27.9%), due to the senescence and death of the relatively short-lived acacias, led to increased fire intensities, flame heights and higher levels of crown scorch and defoliation. Prescribed burns were conducted in early summer 1997. Burns in 5-year-old restoration were of low intensity (<250kWm−1) while burns in 8-year-old restoration were of very high intensity (>7000kWm−1). Fuel re-accumulation was rapid in the first 2 weeks post-burn, with litter-fall rates 2–3.5 times that of unburnt control sites. Thereafter, litter-fall and fuel accumulation in burnt restored and unmined sites was comparable to that of unburnt control sites. Analysis of 1 and 2 years post-burn 5-year-old restoration indicated that the prescribed burns had failed to remove the mid-storey acacia layer and actually increased the proportion of dead standing material, whereas in the 8-year-old restoration, the prescribed burns removed the mid-storey layer of acacia shrubs and stimulated an increase in the proportion of live plant material, particularly near ground level. Maximum soil temperatures recorded by heat sensitive crayons exceeded 300°C in 8-year-old restoration burns but were less than 100°C in 5-year-old restoration burns. Hard-seeded species were stimulated to germinate in the top 2–3cm of the soil following burns in 8-year-old restoration and only in the top 0.5cm of the soil following burns in 5-year-old restoration. Some seeds in the top 1cm of the soil may also have been killed by the high temperatures generated in burns in the 8-year-old sites. Fire management of jarrah forest restoration will have to be different to that employed in unmined jarrah forest due to differences in fuel characteristics, vegetation structure and fire behaviour.