Issue 9, March - April 1999


Linking fire management to rainfall patterns

Most Australian plants are not only very well adapted to fire, in some cases they depend upon it for survival. Eucalypt species are very fire resistant, and some even use the heat from the fire as their cue to drop seed or germinate.

Our grasses are also well adapted to fire, and regrow rapidly if burnt at the right time. Species like black speargrass particularly so. Their seed burrows into cracks in the soil, protecting it from fire, while the seeds of competing grasses are burnt.

From Dr Peter O'Reagain Principal Scientist QDPI

I SPENT 10 years researching South African native pastures before immigrating to Australia in 1995. The research taught me fire is one of the cheapest and most effective management tools available for pasture management.

However, the big challenge in our dry and variable environment is to link burning with rainfall. Forecasting tools like the Southern Oscillation Index (SOI) allow us to select years with a good chance of a reasonably good season. But even then, it is important never to burn too much in any one year. Also you need to spell a paddock for as long to ensure regrowth. A burn followed by a drought could mean a whole year with no pasture!

Out of Favour

Fire has unfortunately dropped out of favour. It is not applied nearly enough, and where it is, applied incorrectly. Sometimes people think burning is a waste of feed. In other cases, there is very little fuel to burn, because of heavy stocking rates.

As a result, woody weeds such as rubbervine, currant bush and chinee apple flourish. Many people have also noticed a thickening of our native trees, reducing grass production. Pasture condition has declined, and fire-tolerant species, such as black speargrass are dying out.

But before burning, we should ask: "Why am I burning?" Is it to control trees and woody weeds, or is it to remove dead grass and 'freshen up' the pasture? Hot Fires

This critical question determines how, and when, the fire will be applied. If the aim is to control woody weeds, then the fire must be very hot to ensure sufficient heat to kill the trees or saplings.

To achieve a 'hot' fire, there should be a lot of fuel present-at least 2000 kg per hectare. Also, the fire needs to be set when conditions are at their driest, preferably near the end of the dry season when things are very dry.

The time of the day the fire is applied is also critical to get a hot fire. Burning should be between 10am and 3pm when the relative humidity is low, and the fuel is at its driest.

It is also important for the fire burn with the wind, to get a 'head fire' so the heat is carried up into the trees to cause the most damage. Follow-up burns will almost always be required for woody weeds. If, however, the intention is to remove old grass, then the fire should be cool.

Cool Fires

Cool fires literally just singe the dead leaves off the plant, and should be lit early in the morning or late in the evening when the relative humidity is high. These fires should also be "head fires" and burn with the wind. This ensures the flame front passes over the plants quickly, causing minimum damage to the tussocks.

To avoid damaging grass, the grass must be dormant, usually in the dry season. But this could even be in the middle of the wet during a long dry spell, or just after the first rains before the grass has started to grow fully. Also, burnt areas should not be grazed until they have regrown sufficiently. If grazed too soon after regrowing from a fire, grass can be seriously weakened and die, leading to a decline in pasture condition and productivity.

So the basic rules are:

  • Decide why you want to burn.
  • Apply the appropriate fire under the right conditions.
  • Give the pasture a chance to recover before grazing.

Contacts

Dr Peter O'Reagain
Principal Scientist
Department Primary Industries & Fisheries
Tel: 07 4787 2155

Fax: 07 4787 4998

PO Box 976
CHARTERS TOWERS, QLD