Modelling the trade-off between fire and grazing in a tropical savanna landscape, northern Australia

As savannas are widespread across northern Australia and provide northern r angelands, the sustainable use of this landscape is crucial. Both fire and grazing are known to influence the tree-grass character of tropical savanna s. Frequent fires open up the tree layer and change the ground layer from p erennials to that dominated by annuals. Annual species in turn produce copi ous quantities of highly flammable fuel that perpetuates frequent, hot fire s. Grazing reduces fuel loads because livestock consumes fuel-forage. This trade-off between fire and grazing was modelled using a spatially explicit, process-orientated model (SAVANNA) and field data from fire experiments pe rformed in the Victoria River District of northern Australia. Results of si mulating fire (over 40 years) with minimal or no grazing pressure revealed a reduction in the shrub and woody plants, a reduction in grasses, and no i nfluence on the tree structure given mild fires. WI-tile mature trees were resistant to fire, immature trees, which are more likely associated with th e shrub layer, were removed by fire. The overall tree density may be reduce d with continual burning over longer time periods because of increasing sus ceptibility of old trees to fire and the lack of recruitment. Increases in stocking rates created additional forage demands until the majority of the fuel load was consumed, thus effectively suppressing fire and reverting to the grazing and suppressed fire scenario where trees and shrubs established . (C) 2001 Elsevier Science Ltd. All rights reserved.