Use of fire in grazed woodlands – Queensland

Bill Schulke
Formerly Queensland Government

Australia’s woodlands are by their very nature unstable. In the absence of disturbance, a woodland will tend to thicken. The trees will increase in size until either competition between trees limits their growth or some calamity such as drought kills some of the trees. Old age is also a factor in tree death with the older trees often the first to die in a drought. Episodic wild fires along with climate change around 100,000 years ago and subsequent burning by Aboriginal people have also been implicated in the rise and dominance of fire-tolerant species in Australia, particularly the eucalypts.

Prior to European settlement, frequent and systematic burning by Aboriginal people was responsible for maintaining large areas as semi-open woodlands with a good layer of fire-tolerant grass growing between the trees. Periodic burning achieved this vegetative balance by keeping eucalypt and other native woody seedlings and suckers small enough to have minimal effect on pasture production.

Following white settlement fire was still used to help keep grazed eucalypt country open and productive but the intensity, timing, frequency and scale of fires differed from the Aboriginal regime.

In recent years burning has not been possible in many areas because the combined effects of poor summer rain and the introduction of Bos indicus cattle and the use of dry season supplements resulting have resulted in low fuel loads. Without regular burns the under-storey woody plants have been thickening up with a consequent loss of pasture production and ultimately also of carrying capacity.

Re-introduction of fire as a management strategy is essential for the long-term stability and productivity of grazed woodlands. Unfortunately, in many areas trees have grown above the height where fire will effectively control them, even when there is enough fuel to carry a fire.

Eucalypts

The ‘TRAPS’ woodland measurement program, part of the MLA sponsored project ‘Understanding the dynamics of Queensland’s grazed woodlands’, showed that fire killed very few small, suppressed eucalypts but regularly checked their growth. Suppressed lignotuberous (a large underground ‘root’ that stores food for the plant) suckers can survive like this for a very long time (monitoring shows more than 25 years). Other work in northern Australia’s savannahs has shown that most eucalypts taller than two metres are resistant to fire. Fire can be used to prevent woodland thickening in eucalypt woodlands and forests.

Wattles

The MLA-funded project on the effect of fire on wattles at Wigton near Gayndah in the Burnett highlighted the differences in the response to fire between wattle species. A spring burn killed almost all the mature wattle plants at the site. One species, early flowering black wattle (Acacia leiocalyx) responded to being burnt by producing a profusion of root and butt suckers as well as some new seedlings. Another black wattle species, A. grandifolia, only regenerated from seed and apparently did not have the ability to root sucker. Subsequent fires over the next five years reduced the population of both species to about the original population but kept those plants so small they had little effect on pasture productivity.

Without these fires the wattle regrowth would have completely dominated the pasture. The reaction of these two wattle species to fire is indicative of the effect on most wattles and, in fact, most acacias. Fire will keep wattles in check but as a general rule will not eliminate them.

Currant bush, Burrum bush

In central and north Queensland there has been an increase in the density of currant bush (Carissa ovata) and Burrum bush (C. lanceolata) over vast areas of cleared and uncleared semi-arid eucalypt woodlands. This represents a severe constraint on the availability of herbage to the grazing animal and reduces the already low carrying capacity of these beef cattle pastures.

In a study in the Mount Coolon area of central Queensland, plots heavily infested with currant bush were fenced to manage grazing and allow fuel to accumulate. These plots were burnt either once, twice or three times with burns either one or two years apart. After burning many of the larger currant bush plants fragmented into several smaller plants due to this shrub’s habit of rooting along its sprawling branches. Few plants died as a result of burning but the fire greatly reduced the overall canopy area of the currant bush and allowed pasture grasses to re-colonise the areas formerly covered. No new currant bush seedlings were found.

Burning currant bush in spring is a viable option for controlling its canopy expansion. Usually conditions at this time of year are hot and dry, causing some stress to the current bush, but there is a good chance of follow-up rain. This provides the greatest success by removing the above-ground portion of the plant at a time when pasture species will likely be able to respond to the reduction in canopy cover. A fire is required at least once every five years. For a good result at least one year’s growth of grass needs to be available as fuel for the fire.

Fire as a management tool

If fire is to be used to maintain pastures in grazed woodlands and prevent woodland thickening, it is necessary to have a practical fire management strategy for the whole property.

In the semi-arid grazed woodlands of central and north Queensland it is necessary to burn at least once every five to six years. In south-east Queensland the fire interval may need to be less with a fire every three to five years. Missing one of these fires means tree seedlings and shrubs will be able to grow above the height at which fire will knock them back down to ground level.

In order to guarantee a successful fire, sufficient fuel is required. The easiest way to achieve this is to take advantage of the better grass growth experienced in better seasons. Ensuring sufficient fuel in drier years is more difficult and requires a reduction in stocking rate.

Whilst many landholders are reluctant to reduce stocking rates in poor seasons, it is probably the best option in the long run as it allows annual rotational burning of all paddocks, helping guarantee a stable, productive and sustainable pasture. The cost of alternatives, such as chemical treatment or mechanical clearing, has increased significantly in recent years. In many situations the use of these alternatives has been restricted by legislation.

Conclusions

  • Australia’s woodlands are not static and change over time with climate and fire.
  • Fire is a preventative tool that checks the growth of eucalypts, acacias and other woody weeds to maintain a grassy understorey.
  • In the absence of fire, grazed woodlands will tend to thicken at the expense of pasture productivity and ultimately carrying capacity.
  • The use of fire to manage woodland thickening needs to be compared with chemical or mechanical alternatives, which are increasingly limited by economic or legislative controls.
  • As a management tool, the use of fire needs to be deliberate and planned. This often includes adjusting grazing management to ensure sufficient fuel for an effective burn.

Adapted from an article by Paul Back, a woodland ecologist and woody weeds specialist formerly Queensland Government.

Further information