Turning up the heat: The impact of invasion of Australia’s tropical savannas by an alien grass

Background/Question/Methods

Tropical savannas are the dominant vegetation across northern Australia, covering approximately ¼ of the continent. The region is characterised by frequent fire, with approximately 50% burnt each year. During the 20^th Century, hundreds of grass cultivars were introduced to northern Australia for the purpose of improving agricultural potential. Many of these species are now major weeds with some species of particular concern due to their effects on fire behaviour. The most serious is Andropogon gayanus Kunth. (gamba grass), which was introduced from Africa for pastoral production but has now invaded significant areas of the Australian savanna region, including land managed for conservation, rural/residential purposes, agriculture, defence and mining. The objective of this research was to quantify the changes in fire regimes following broadscale invasion, and document environmental, cultural and economic consequences.

Results/Conclusions

Compared with native grasses, gamba grass forms substantially taller and denser stands that cure later in the dry season.  The large fuel loads resulting from gamba grass invasion support fires 5 to 10 times more intense than those fuelled by native grasses. The change in fire fuel properties means that flame height is significantly increased and we have recorded the first passive crown fires in the region in the gamba invaded areas. The change in fire regime caused a major decline in woody vegetation, which results in a major loss of above-ground carbon store. Fire risk has  increased dramatically and this presentation describes the transformation in changes in fire management practices required to mitigate this threat, resulting in a 10 times increase in the cost of fire-management resources.