* Corresponding author. E-mail: ian.radford@tag.csiro.au Journal of Arid Environments (2001) 49: 795–807 doi:10.1006/jare.2001.0846, available online at http://www.idealibrary.com on Impact of prescribed burning on Acacia nilotica seed banks and seedlings in the Astrebla grasslands of northern Australia Ian J. Radford*, D. Michael Nicholas & Joel R. Brown CSIRO, Tropical Agriculture, PMB PO Aitkenvale, QLD 4814, Australia (Received 5 June 2000; accepted 24 April 2001) Acacia nilotica is a tree of international significance both as a beneficial plant and as a species prone to thicket formation and negative impacts on savannas throughout much of its range. While fire has been identified as a useful tool for controlling negative impacts of some Acacia species in Africa, A. nilotica adult trees are apparently fire tolerant. The effects of fire on seed bank mortality, viability and germination, and on juvenile plants of this species are unknown. We undertook a study to investigate the impacts of fire on seeds and first and second year seedlings in Australian Astrebla savannas. The primary objective was to determine the effects of control burns applied during the early stages of invasion and thicket formation. Because of high rates of seedling dessication post fire in 1997, supplementary watering was undertaken until fire mortality could be measured the following year. High seed mortality occurred for seeds located either in cattle dung (100%) or on the soil surface (c. 80%). Germination of those seeds that survived was enhanced following exposure to fire. Very high fire mortality (99%) was observed among first year seedlings and moderate mortality (60%) for second year seedlings even after a low intensity burn. Negative impacts on major pasture species were minimal. The implica- tions of these findings are discussed in relation to A. nilotica fire ecology and to the efficacy of fire as a tool for managing this species.  2001 Academic Press Keywords: fire ecology; invasive; woody weed; thicket formation; savannas Introduction Acacia nilotica (L.) Del. is an internationally important tree both as a dominant savan- na/woodland species in indigenous regions and as a plantation tree and invasive woody weed in exotic locations. It is a widespread indigenous species in Africa, Israel, Yemen, Pakistan, India and Burma and has been introduced to Iran, Vietnam, Indonesia and Australia (Ross, 1979; Brenan, 1983). Acacia nilotica provides timber, fire suppression, shade for livestock and fodder in native and exotic regions (Fagg, 1992; El Atta, 1993; Alikodra, 1987; Carter et al., 1991; Lowry et al., 1993). Although it is seen to have many benefits, A. nilotica is also seen as having negative impacts associated with increasing density in many regions (Pratt & Knight, 1971; Trollope, 1980; Carter, 1994; Mackey, 1997). Agricultural impacts in rangelands include reduced pasture growth, reduced 0140}1963/01/120795#13 $35.00/0  2001 Academic Press