Scale-dependent thresholds in the dominant controls of wildfire size in semi-arid southwest Australia

We aimed to characterize the size distribution of naturally occurring wildfires and to determine how fuel characteristics influence wildfire size in a vegetation mosaic of shrublands and woodlands in semi-arid southwest Australia. The shape of frequency-size distributions of fires can be used to elucidate shifts in the dominant drivers or constraints of fire size. We modeled the cumulative frequency-size distribution of wildfires in southwest Australia using a segmented linear model with two break points. Three fire size categories were defined by the break points of the segmented model: “small” (<2,286 ha), “medium” (2,286–41,020 ha), and “large” fires (>41,020 ha). Break points were interpreted as thresholds in fire size that may represent shifts in the dominant controls of fire spread. We tested whether the influence of fuel characteristics on fire spread varied among fire size categories by comparing the composition of vegetation types and fuel ages among burnt and adjacent unburnt areas using selection ratios (ωx). Small and medium fires burnt shrub-dominated vegetation types in proportion to their availability (ωx ≈ 1), but tended to burn smaller proportions of sparsely-vegetated woodlands (ωx < 0.62). In contrast, large fires tended to burn woodlands in proportion to their availability (ωx ≈ 1). Young fuels (<20 years) were under-represented in burnt areas regardless of fire size (ωx < 0.62), while older fuels (>30 years) burnt in proportion to their availability (ωx ≈ 1). Fire size in semi-arid southwest Australia is generally limited by fuel mass in recently burnt areas and by fuel continuity in sparsely vegetated woodlands. However, following unusually wet years and subsequent growth of ephemeral plants, fuels in woodland vegetation likely become spatially continuous and capable of supporting the spread of large fires.