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Posted May 2009

The increasing fire risk for southeast Australia under climate change has, not surprisingly, become a hot topic. But most of the popular discussion of the Victorian fires and climate change has ignored the issue of how weeds can greatly influence fire risk.

Recent research in Alaska by Philip Higuera, from the University of Washington, and colleagues has demonstrated that vegetation can substantially alter the direct effects of climate change on fire regimes. For example, 10,500 years ago, when the Alaskan climate went from cool and dry to warm and dry, vegetation changed from flammable shrubs to more fire-resistant deciduous trees, and fire frequency declined sharply despite the warming.

Weed invasions can change the composition of ecosystems and greatly increase flammability. This is well-recognised for pasture grass invasions in northern Australia – with African gamba grass (Andropogon gayanus) fuelling fires eight times as hot as those in natural grasslands (see Rossiter et al 2003).

Pasture grass invasion in southern Australia can also increase fire hazards, but there has been very little research.  An assessment by John Stoner, from Deakin University, and colleagues found that Phalaris aquatica, a widely planted and highly invasive grass, increases the fuel load about three-fold over that of native kangaroo grass grasslands.

Invasion by exotic pasture grasses can also transform ecosystems not usually subject to fire into fire-prone systems, destroying fire-sensitive vegetation. This is occurring in Victoria, for example, with the invasion of invasive pasture grasses into chenopod (Halosarcia)-dominated shrublands.

Escaped garden plants can also increase fire hazards, particularly some Australian natives that invade outside their natural range. Geoff Carr of Ecology Australia has highlighted this risk for Victoria’s Surf Coast and hinterland, where scores of invasive natives have escaped, including over 60 species of wattles (Acacia), paperbarks/honey-myrtles (Melaleuca), hakea, eucalypts, coast tea-tree, Cape wattle (Albizzia), and kunzea.

Characteristics (in many species) that enable them to fuel very intense fires include abundant, highly flammable volatile leaf oils (as in eucalypts and honey-myrtles), very fine fuels (abundant small leaves and branches) and the formation of very dense thickets.  Invasive coast tea-tree and coast wattle often produce fuel loads greatly exceeding (perhaps by several orders of magnitude) the natural fuel loads in the invaded dunes, cliffs, heathlands, heathy woodlands or forests of the Surf Coast.

The severe fires of Ash Wednesday 1983 were a turning point, stimulating massive  recruitment of these Australian plants.  Now, 25 years later, there are vast accumulated fuel loads. In the event of a wildfire the intensity and rate of spread of the fire are likely to be greatly exacerbated.

References
Higuera PE, Brubaker LB, Anderson PM, Hu FS, Brown T. 2009. Vegetation mediated the impacts of postglacial climate change on fire regimes in the south-central Brooks Range, Alaska. Ecological Monographs 79: 210-219.

Rossiter NA, Setterfield SA, Douglas MM, Hutley LB. 2003. Testing the grass-fire cycle: exotic grass invasion in the tropical savannas of northern Australia. Diversity and Distribution 9: 169-176.

Stoner JR, Adams R, Simmons D. 2004. Management implications of increased fuel loads following exotic grass invasion. Ecological Management and Restoration 5: 68-69

Carr, G. (2009) Bush fires spark warning of weedy enemies within. Feral Herald 21:6-8. http://www.invasives.org.au/news.html