Fire regimes and the conservation of sandstone heath in monsoonal northern Australia: frequency, interval, patchiness

Heathlands are scattered across fire-prone monsoonal northern Australia mostly in dissected sandstone terrain. Such communities, although floristically depauperate by comparison with heathlands in southern Australia and southern Africa especially, share in common a relatively high proportion of fire-sensitive, obligate-seeder shrub species. The paper explores the implications of frequent fires, and associated short inter-fire intervals, on populations of obligate-seeder shrub species occurring in extensive heathlands occupying the western rim of the Arnhem Plateau, in the Northern Territory. Two studies are presented. With reference to published data concerning the maturation times of regional obligate-seeder shrubs, the first study reports on minimum and maximum intervals between fires determined from a 16-year fire history, 1980-1995, for the Plateau landform unit in Kakadu National Park, interpreted from LANDSAT MSS imagery. While species with maturation times of 5 or more years are common in the regional heath flora, minimum fire interval data for each 1 ha pixel indicate that 69% of heath habitats had been burnt at least once by fires recurring within 3 years, and 64% had a maximum fire interval of 5 years; 11% burnt only once or remained unburnt. The second study reports on the effects of an unreplicated experimental fire, involving observations on ca. 4000 individual shrubs, on ensuing heath floristic composition and abundance, undertaken 3 years after a wildfire had burnt the same site. Despite the experimental fire being highly patchy, substantial declines in the occurrence and density of many obligate-seeder shrub species were attended by increases in many herbs, including flammable grasses. Three years after the experimental fire the number of obligate-seeder shrubs was still less than half that pre-fire despite significant recruitment of some species in latter years. Collectively, these and other published data indicate that minimum fire return intervals of at least 4-5 years are required for conserving rapidly maturing tropical sandstone heath obligate-seeder shrubs, and longer still on sites comprising species with longer maturation times. For conservation management purposes individual fires should be small (especially in relation to the extent of any one tract of heath), patchy, and recurring intervals between fires should be varied as far as practicable.