Research Letter #5

Does the time of fire occurrence matter?

Timing of prescribed fires is particularly important. Getting it wrong creates problems. If fires get away they may do all sorts of damage. Time of day affects whether or not fuels will burn and if weather conditions are going to improve, or get worse, as far as rates of fire spread are concerned. Some years have many more suitable days for prescribed burning than others (e.g. Gill et al. 1987).

As well as the conduct of the prescribed burning operation, there are effects of prescribed and unplanned ('wild') fires that are becoming increasingly important in the public mind. Smoke over Australia's cities is of concern in relation to health and visibility (Rye 1995; Sneeuwjagt and Smith 1995). At this time of the year in the ACT smoke from household fires tends to hang in the valleys where the suburbs lie, a seasonal effect.

Season of burn is one of the components of the fire regime and smoke problems are an example of its importance. There are others too. The classic Australian plant example is that of the annual native grasses of the genus Sorghum - the same genus as the crop plant - which grow particularly well during the northern Australian wet season and become the main fuel for fires in the dry season. If understoreys of the woodlands where this plant dominates do not burn in one, or especially two, dry seasons, there may be opportunity to burn the plant community in the wet season after a new crop of seedlings has just germinated. The fire in the accumulated dead grass kills the seedlings and, because all the seeds in the soil have germinated, the species is largely eliminated from the burned area (Stocker and Sturtz 1966). This practice is particularly useful for the creation of fuel breaks but there is a risk of unwanted fire while awaiting sufficient fuel to accumulate.

Many Australian shrubs depend for their regeneration after fire on seeds stored in the plant canopy (as in the cones of Banksia for example). If these plants are burnt in spring before a long seasonal drought, for example, it has been argued that the chances of them remaining viable in autumn (or whenever suitable moisture conditions exist), are less than those if a fire occurs in autumn just before seasonal rains (after McMahon 1984).

Concerns are often expressed about the effects of prescribed burning on nesting birds. If nests are destroyed by fire there may be no recruitment of juveniles to the population. There is a surprising lack of data on this point but Woinarski and Recher (1997) state that: "Probably the most important consideration is not the loss of individual nests, eggs or young, but the frequency of opportunity species will have to nest between successive fire events."

There are other seasonal effects among our plants and animals that have not been mentioned here. One effect is on the resprouting capability of trees and shrubs; and, there may be seasonal effects on the post-fire stimulation of flowering among some species. There is also anecdotal evidence suggesting that some orchid populations are devastated by repeated spring burning. There is much to be done and the observations of naturalists and other keen observers could be very valuable.

Literature cited:
  1. Gill, A.M., Christian, K.R., Moore, P.H.R. and Forrester, R.I. (1987). Bushfire incidence, fire hazard and fuel reduction burning. Aust. J. Ecol. 12, 299-306.
  2. McMahon, A. (1984). The effects of fire regime components on heathlands in the Little Desert, N.W. Victoria, Australia. MEDECOS IV Proceedings pp. 101-102.
  3. Rye, P.J. (1995). Contribution by bushfire smoke to photochemical smog. CALMScience Supplement 4, 129- 133.
  4. Sneeuwjagt, R.J. and Smith, R. (1995). Smoke management in Western Australia's south-west forests. CALMScience Supplement 4, 135-143.
  5. Stocker, G.C. and Sturtz, J.D. (1966). Use of fire to establish Townsville lucerne in the Northern Territory. Aust. J. Agric. & Anim. Husb. 6, 277-279.
  6. Woinarski, J.C.Z. and Recher, H.F. (1997). Impact and response: a review of the effects of fire on the Australian avifauna. Pacific Conservation Biology 3, 183-205.

Malcolm Gill
Division of Plant Industry, CSIRO
16 July 1998