Countervailing effects of urbanization and vegetation extent on fire frequency on the Wildland Urban Interface: Disentangling fuel and ignition effects

Abstract

Fire activity has been found to follow a humped relationship with population density, but the countervailing drivers and scale effects in this relationship have not previously been teased apart. This is important because it helps us to understand which aspects of fire risk are amenable to management. The likelihood of a fire occurring at the Wildland Urban Interface (WUI) can be broken into two components: that of ignitions occurring and that of the fire spreading from the ignition to the interface. We hypothesize that urbanization is a double-edged sword because it both increases the likelihood of ignition but also protects areas from fire spread. We investigated this hypothesis for Sydney Australia using 38 years of historical fire mapping by examining statistical relationships between wildfire count at 1250 points in the WUI and measures of vegetation clearing and urbanization at multiple scales (1km and 10km radii around sample points). The number of fires at a point was influenced negatively by the amount of un-vegetated land at both 1km and 10km radii and positively by urban land within 10km radii. There was also an interaction between un-vegetated land and urbanization such that fire activity is particularly high where some urban development has occurred but a considerable amount of vegetation remains. As predicted, urban centres provide both sources of ignition and a degree of protection from fire spread. Fire risk could best be reduced either by reducing fuel near the WUI or by reducing ignitions from city dwellers.