Vegetation patches

Should a patch of trees be treated the same as a forest when planning for bushfire protection?

I recently completed bushfire hazard assessments for two developments, the first bound by a forest reserve and the second bound by a parkland reserve. Both developments were constrained by similar types of hazardous vegetation; however, the patchiness of hazardous vegetation in the parkland reserve and the maintenance of vegetation around these patches resulted in the recommendation of fewer bushfire protection measures when compared to the development located adjacent to the forest reserve. The purpose of this article is to explain why the area of hazardous vegetation and the separation of these areas needs to be carefully considered when planning for bushfire protection.

Bushfire impact mechanisms can affect people and property through flame attack, radiant heat exposure, ember attack, wind attack, smoke hazard and convective heat exposure. Of these mechanisms, flame attack, radiant heat exposure and ember attack are most relevant to land use planning and building decisions that seek to reduce the risks to life and property in new developments.

At a landscape scale, the preferred metric for indicating the potential severity of these impact mechanisms is fire-line intensity. Fire-line intensity is a standardised measure of the rate that an advancing head fire would consume fuel energy per unit time per unit length of fire front. There is a mathematical equation to calculate potential fire-line intensity, but for the purposes of this article the equation can be simplified as a product of potential fuel load (tonnes/hectare), maximum landscape slope (degrees) and fire weather severity (McArthur’s Forest Fire Danger Index).

The fire line intensity equation described above will overestimate the potential fire-line intensity in areas with patches of hazardous vegetation (generally less than one hectare) which are surrounded by non-continuous vegetation. Reduced fire-line intensity is attributable to variations in fuel load and fuel distribution across these areas. Areas of non-continuous vegetation will prevent or limit head fire intensity and will interrupt the movement of a fire front into the patches of hazardous vegetation. In comparison, continuous areas of vegetation with a continuous fuel load will generally have a uniform fuel distribution that supports continuous flame spread.

The effect of vegetation patches and corridors of vegetation on potential fire-line intensity is considered in the Queensland and New South Wales guidelines for preparing bushfire prone area mapping and in the bushfire attack level assessment procedure in the Australian Standard for the Construction of Buildings in Bushfire Prone Areas (AS 3959-2009).