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Northern Territory Agnote - Control of Woody Weeds with Fire

Woody weeds, or the nuisance increase in trees and shrubs, have taken over parts of the Alice Springs district since the 1970s. The reasons for this invasion and its effect are described in Agnote No 653, F85 'Woody Weeds in Central Australiaí and Agnote No 654, F89 ëWhy Do Woody Weed Problems Occur?í. These Agnotes mention the use of fire for control of woody weeds.

A separate Agnote (No 343, F20) deals with various mechanical methods of control.

Fires as a control agent

Managed fires provide the most economical long-term solution for woody weed control. These fires kill a large proportion of fire-sensitive species (particularly mulga) and reduce the vigour of others (e.g. witchetty bush and broombush) allowing grasses to regenerate.

Woody weed encroachment occurs in wet years, but the extra pasture growth present in such wet years provides additional fuel for managed fires. This results in good opportunities for control since trees and shrubs are most susceptible to fire as seedlings. However, it is often difficult to recognise the severity of emerging shrub densities until they are well above the pasture, by which time the opportunity for good burns is diminished.

Country with dense stands of well-established scrub is difficult, and often impossible to burn. Destocking in wetter years will assist pasture fuel to accumulate. Burning on hot windy days may carry the fire into adjacent areas of dense scrub that would not otherwise burn. In this way, fire can be used as a battering ram or bulldozer to wear down larger areas of scrub.

Requirements for successful control of wood weeds with fire

  1. Burn early. Seedlings are most easily killed by fire. Established trees and shrubs (apart from mulga) are harder to kill.

  2. There must be sufficient pasture accumulation as fuel. Destocking country following rain will assist pasture to accumulate as fuel.

  3. Dry grasses and spinifex provide the best fuel. The Aristida species (kerosene grass, mulga grass, wire grass) burn particularly well. Herbage species generally have low flammability - even when dry.
  4. Use a fire of the required intensity. Cool to moderately intense fires will thin young mulga without killing all adult trees. Hotter fires are needed to control witchetty bush and broombush.

    A fire of increasing intensity is required as the fuel supply becomes limiting or patchily dispersed. Hot fires offer the only change of crashing into adjacent dense scrub that has inadequate fuel accumulation.

  5. Use favourable weather conditions to maximise the result from available fuel levels. Temperature, wind speed and direction, and humidity must be right for the fire to carry at the required velocity and intensity.

  6. If a fire will not carry on areas that appear to have adequate fuel, try again on a hotter, drier or more windy day. Do not persist with a low-intensity fire that consumes the available pasture but kills very few shrubs.

  7. Consult with the Bushfires Council regarding fuel and weather conditions prior to burning. Their advice must be followed - particularly at times of high fire danger.

Post-fire pasture management

Low stocking rates, to enable the re-establishment of a vigorous pasture, will assist in woody weed control. Dense vigorous perennial grasses compete strongly against seedlings of woody weeds.

Follow-up fires may be required at 5 to 10-year intervals in some areas for control of woody weeds. The rate at which shrubs re-establish will be largely determined by subsequent rainfall.

While all care has been taken to ensure that information contained in this Agnote is true and correct at the time of publication, the Northern Territory of Australia gives no warranty or assurance, and makes no representation as to the accuracy of any information or advice contained in this publication, or that it is suitable for your intended use. No serious, business or investment decisions should be made in reliance on this information without obtaining independent/or professional advice in relation to your particular situation.

Andrew White
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