Bushfires are part of the Australian landscape and the psyche of its human inhabitants. This is particularly true as months of hot, dry weather approach.
Recent warnings have predicted a dire summer ahead with the potential for major fires across the continent.
So will this year’s fire season be particularly bad? What are the different types of bushfire activity found across our continent? And how can these bushfire types help us predict how climate change will affect future fire seasons?
More rain = more fire
Last year, floods and high rainfall affected many regions of the continent, including large tracts of the dry interior.
A wet year, following extended dry spells, has resulted in rapid growth of grasses and herbs. This leaves a relatively dense and continuous layer of ground cover blanketing what is often bare ground.
Such conditions are the precursor to major fires, particularly as herbaceous ground cover begins to dry or “cure”. Once cured it is then available as fuel to burn.
In other years we’ve heard warnings but for different reasons. During the major droughts of recent years, warnings of a “horror” fire season were targeted at the forests that fringe our coastline, particularly in the temperate south where most Australians live.
These are our wettest environments and where plant growth or “productivity” is high. The relatively dense cover of trees, shrubs and grasses in forests provides an ever-present fuel layer of ground litter: it’s mainly the fallen leaves of trees and shrubs.
For much of the year, these litter fuels are unavailable to burn (they are too wet). But periodic, prolonged and widespread droughts dry this fuel enough that fires can readily ignite and spread.
It’s not what starts fires, but what usually stops them
Most years, fires don’t happen because there is something stopping them. We call these things “key limitations”. When limitations break down, fire takes hold.
Where fire follows rain, as in dry ecosystems, the mass of fuel is normally too low and patchy to enable fires to spread. In wet years this limitation is overcome, as often-dormant plants start to grow.
By contrast, in our relatively moist, productive ecosystems, fuel mass and continuity is normally sufficient for fires to spread but is otherwise too wet for fires to take hold. Prolonged, extensive dry spells are needed to overcome this limitation.
In the dry country, big wet years typically occur a decade or more apart, with exceptional, extensive years – such as 2011 – occurring less frequently.
In wetter forests, major drought may occur as often as twice a decade. In cooler regions, droughts may be a decade or more apart.