A fire which trickles through the undergrowth at knee height, slowly consuming dead leaves and twigs that have accumulated on the forest floor is called a ‘cool burn’. Such a fire can only be set after the opening rains in autumn or at the end of the rains in spring. The choice on which time is better for planned burning is a very controversial issue with no definite ‘right’ or ‘wrong’ answer.

A cool burn trickles through at knee height.
Photo - Lachlan McCaw, Autumn 1993 Landscope

The most important difference between spring and autumn fires is the amount of fuel consumed by the fire. This largely depends upon the fuel moisture content. In spring, following winter rains, the vegetation and soil are damp. In summer the entire forest is much drier and therefore the fire burns more intensely and spreads more rapidly. The flame height estimates the fire intensity, which is a measure of the heat energy output of a fire. In a spring burn, flames are usually half to one metre in height. In autumn the flames are approximately two to three times this in height. Therefore the autumn fires are generally more intense.

There is a great visual difference between a spring and autumn burn. A spring burn leaves the understorey a mottled green/brown. The brown leaves are those killed by the heat of the fire. Approximately 20% of forest ignited under a spring burn remains unburnt, whereas with an autumn burn nearly all of it becomes burnt. An autumn burn leaves the understorey mostly brown/black, indicating the vegetation to be mostly killed.

Some people claim that Fuel Reduction Burning in spring adversely affects wildflowers and breeding animals, therefore late summer or early autumn is a better time to burn. During spring the wildflowers are at a peak. Approximately 70% of plant species flower in late winter/spring. A spring burn will therefore destroy a higher proportion of flowers and foliage of low understorey plants, which will detract from the beauty of the forest. It will also reduce the food base for insects and other animals which feed on flowers and seeds.

Some species require a high intensity fire to regenerate. Plants such as the prickly moses (Acacia pulchella) and heartleaf poison (Gastrolobium bilobum) form dense thickets after summer and early autumn burns, but regenerate poorly after low intensity spring burns. Melaleuca viminea thickets require infrequent (25-30 years) and intense fires in summer or early autumn to regenerate. This affects animals such as the tammar wallaby which lives in the thickets.

A spring burn often has unburnt patches throughout the burnt area, which are extremely important as animal habitat and refuge areas, from which animals can recolonise burnt areas when vegetation regenerates. An autumn burn defoliates the understorey and, in a very intense fire, can scorch the overstorey - which causes a great disturbance to the habitat and birds and animals. Although most birds are nesting in spring, they are usually finished breeding by the time prescribed burning commences. Birds are highly mobile, and can survive in the unburnt patches of a spring burn before recolonising burnt areas when regeneration occurs.

Not all areas burn in a fire; some are left unburnt.
Photo - Rick Sneeuwjagt, Autumn 1994 Landscope

The choice between a spring or autumn burn is determined by what is suited to the particular environment, and what is practical. Most Fuel Reduction Burning is carried out in spring or early summer as there are more days when fuels are moist and weather is mild and predictable. This results in fires of low intensity, which can be easily and cheaply controlled. There are fewer suitable days in autumn which burning can be safely carried out. Dry fuels and unstable weather conditions in autumn increase risks of fire escaping.

In most forests, the ideal fire regime, one which results in minimal impact and damage caused by wildfires and one which caters for the natural environment, is a combination of spring and autumn burning. For example, a forest would be given two spring burns, then an autumn burn, at five to seven year intervals. Then a ten to fourteen year period would pass before the cycle was recommenced. The extended period between burns would allow the seed stored in the soil to replenish, and provide a range of vegetation ages throughout the forest. However, this is not practical in all areas due to the threat to human life and property. Forest areas around townships cannot have extended burning rotations applied to them as the risk is too great. Therefore the degree and frequency of a planned burn depends upon the individual area and the amount of fuel and species it contains, as well as the risk it poses to humans.