Fire regimes, fire-sensitive vegetation and fire management of the sandstone Arnhem Plateau, monsoonal northern Australia.
A fire history (1980 to the present) developed for Kakadu National Park, derived mostly from manual interpretation of LANDSAT MultiSpectral Scanner (MSS) imagery, was used as the temporal basis for undertaking rapid assessment of the effects of fire regimes on floristically diverse vegetation containing many regionally endemic species, occupying sandstone formations of the Arnhem Plateau, monsoonal northern Australia. Three broad vegetation types were identified through TWINSPAN classification of floristic composition at 108 sample plots: Allosyncarpia-dominated closed forest; open forest/woodland savanna with an annual grass understorey; and open shrubby heath interspersed with perennial hummock grasses. All sampled vegetation occurred on freely draining, oligotrophic substrates derived from sandstone parent materials; grazing and browsing of sampled vegetation were not significant factors. The three fire frequency parameters derived from the assembled fire history ('years unburnt', 'fire frequency', 'shortest interfire interval') were autocorrelated. Correlative analyses between vegetation structure and soil nutrient variables, and 'years unburnt', demonstrated generally increasing shrub densities, fuel loads and soil fertilities with increasing time since last burnt. Identified limitations in assembled data included: substantial between-plot structural variation for respective vegetation types, reflecting diverse ecological and fire histories of individual plots; and inability to describe accurately the important fire regime variable 'fire intensity' from the imagery used. Assessment of the reproductive maturity of obligate seeder shrub species indicated that, whereas vegetation at some sites could tolerate burning after 3-year intervals without loss of floristic diversity, many species required fire-free intervals of more than 5 years, particularly those occupying harsher, rocky sites. In contrast, fine grass and litter fuels were sufficient to support intense fires in all representative situations under late dry season climatic conditions within 1-3 years of having been burnt previously. Published fire history data for the period 1980-94 show that 40% of vegetation occupying sandstone-derived landforms in Kakadu National Park have been burnt at frequencies of at least 1 in 3 years; such fire frequencies evidently cannot be sustained without substantial loss of obligate seeder species, comprising 54% of the sampled shrubby heath flora. The data support other observations concerning the catastrophic impact of contemporary fire regimes on fire-sensitive vegetation types in other sandstone regions of northern and north-western Australia. Based on these insights, an established programme for strategic fire management of the Arnhem Plateau is outlined.