The impacts of Andropogon gayanus (gamba grass) invasion on the fire danger index and fire management at a landscape scale

Effective fire management relies on the ability to assess the potential risk that a fire event poses to the community so that fire managers can ensure sufficient resources are available to respond to unplanned fires. Fuel loads are a major contributor to fire risk and any significant changes to fuel load should be incorporated into fire risk models. An example is the invasion of Australia’s tropical savannas by the high biomass African grass Andropogon gayanus Kunth (gamba grass). Fine fuel load (grass and twigs <6 mm) in a heavily invaded landscape has increased from 6 to 10 t ha−1. Consequently, in 2010, the Bureau of Meteorology created two 20 km2 radius ‘primary response zones’, which are defined areas of dense invasion in the greater Darwin region in whichfire risk is assessed using an estimate of increased fuel load. In this paper, we quantify the effect of the increased (invaded) fuel load on the assessment offire risk. We do this by modelling the daily McArthur Mark 4 Grassland Fire Danger Index (GFDI) for the fire seasons in 2012 and 2013 using both native (6 tha−1) and invaded fuel loads. We show that the number of days with GFDI >50 (the threshold for fire weather warnings and fire bans) has increased and the length of the severe-risk fire season has extended, resulting in substantial increases in fire management costs. This has safety and resource implications for fire management agencies as the area of invasion increases across northern Australia.