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Case study 2: Weeds compromise fire management: Gamba grass in northern Australia

Introduced invasive plants pose a major threat to biodiversity and ecological function throughout the world, and their ecological impacts can include significant changes to fire regimes. The management of such invasive plants is extremely challenging even when there is widespread support for their control. However, management is especially problematic when negative impacts in some situations need to be traded off against benefits in others. This is the challenge presented by the African Gamba Grass (Andropogon gayanus) in northern Australia.

Gamba Grass was introduced into northern Australia in the 1930s as a pasture grass, and it is highly valued by the northern pastoral industry. This introduced grass is now well-established outside pastoral systems, and its rate of expansion seems to be accelerating. Its success as an invader is due to exceptionally high seed production combined with an ability to colonise a wide range of habitats, regardless of canopy cover or ground disturbance.

Gamba Grass is an extremely tall (up to five metres) perennial grass that produces fuel loads that are on average four times higher than the native species it replaces (mostly annual species of sorghum). It cures later than annual sorghum, and remains erect for longer into the dry season. The higher fuel loads combined with changed fuel architecture results in fires almost an order of magnitude higher in intensity than those fuelled by native grasses. Furthermore, Gamba Grass rapidly re-sprouts following fire and can attain sufficient biomass to support another fire within the same dry season. It therefore has the potential to cause a dramatic alteration of regional fire regimes.

There is widespread concern that the high fire intensities fuelled by Gamba Grass are causing extensive tree death, precipitating what savanna ecologists refer to as a grass–fire cycle. Under this scenario, a decline in tree cover facilitates further grass invasion, leading to more severe fire and further tree decline. Such a self-perpetuating cycle has the potential to transform open forests and woodlands to treeless grasslands, as has indeed occurred following grass invasion elsewhere in the world. There is evidence that this is already happening in the Darwin region following Gamba Grass invasion.

The high fire intensities generated by Gamba Grass pose a threat not only to biodiversity and ecological function, but also to human life and property. Historically, the need to protect lives and property has not been a major driver of fire management in northern Australia because fires are of relatively low intensity and occur in landscapes sparsely populated by people. Invasion by Gamba Grass has now brought high intensity fire to where people live, introducing an unprecedented fire risk.

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