Testing the grass-fire cycle: alien grass invasion in the tropical savannas of northern Australia

Invasive alien grasses can increase fuel loads, leading to changes in fire regimes of invaded ecosystems by increasing the frequency, intensity and spatial extent of fires. Andropogon gayanus Kunth. (Gamba grass), a tall perennial grass from Africa, is invading ecosystems in the Top End of northern Australia. To determine whether A. gayanus alters savanna fire regimes, we compared fuel loads and fire intensities at invaded sites with those from native grass savannas. Savanna invaded by A. gayanus had fuel loads up to seven times higher than those dominated by native grasses. This higher fuel load supported a fire that was on average eight times more intense than those recorded in native grass savannas at the same time of year (means 15700 ± 6200 and 2100 ± 290 kW$\m^{-1}$, respectively), and was the highest early dry season fire intensities ever recorded in the Northern Territory. These results suggest that A. gayanus is a serious threat to northern Australia's savannas, with the potential to alter vegetation structure and initiate a grass-fire cycle.