Big fires and their ecological impacts in Australian savannas: size and frequency matters

Savannas are the most fire-prone of the earth's major biomes. The availability of various broad-scale satellite-derived fire mapping and regional datasets provides a framework with which to examine the seasonality, extent and implications of large fires with particular reference to biodiversity values in the tropical savannas of northern Australia. We document the significance of savanna fires in the fire-prone 'Top End' region of the Northern Territory, Australia, using 9 years (1997-2005) of National Oceanic and Atmospheric Administration (NOAA)-Advanced Very High Resolution Radiometer (AVHRR)-, Landsat Thematic Mapper (TM)- and Enhanced Thematic Mapper (ETM+)-derived fire mapping. Fire (patch) sizes from both AVHRR- and Landsat-scale mapping increased through the calendar year associated with progressive curing of grass and litter fuels. Fire frequency data at both satellite sensor scales indicate that regional fire regimes in higher rainfall regions are dominated by large (> 1000 km(2)) fires occurring typically at short (similar to 2-3 years) fire return intervals. In discussion, we consider the ecological implications of these patch size distributions on regional fire-sensitive biota. Collectively, assembled data illustrate that many northern Australian savanna flora, fauna and habitats embedded within the savanna matrix are vulnerable to extensive and frequent fires, especially longer-lived obligate seeder plant taxa and relatively immobile vertebrate fauna with small home ranges.