Fire and weeds: what works, what doesn't

From Drs Shane Campbell, Qld. Dept. Natural Resources & Mines and Tony Grice, CSIRO

Using fire to manage rubber vine is proving extremely effective since the release of the biological control rust Photo: Mike Whiting, CSIRO

Surveys undertaken with land managers in northern Australia consistently identify control of exotic weeds as one of their key management issues. Presently there is a suite of weeds invading large tracts of land.

They range from grasses such as giant rat’s tail and grader, shrubs such as bellyache bush to larger woody plants like parkinsonia, mesquite, rubber vine, chinee apple and prickly acacia. Control techniques include chemicals, machinery, fire and biological control. Fire is relatively cheap compared to other options and it can be applied over large areas.

However, Dr Shane Campbell from the Dept. of Natural Resources & Mines Tropical Weeds Research Centre (TWRC) warns that burning is not effective against all weeds.

“It is critical that we understand how fires affect the different life stages of each weed we are trying to control,” he said. “This information then allows us to incorporate or exclude fire when developing integrated control strategies.” Some woody weeds invading our rangelands demonstrate the different responses that can occur following burning and how this affects management decisions.

Algaroba

Algaroba, the most widespread of the mesquite species growing in Australia, is very susceptible to fire. Even very large plants (greater than 10 metres in height) can be killed and some of the seed bank is destroyed thereby reducing the number of seedlings that emerge.

Rubber vine

Rubber vine is another highly susceptible species but this was not always the case. Before 1995 best kills by fire were only around 50 per cent. Since then a biological control agent—a rust fungus—has been released. The rust acts as a defoliant, reducing the vigour of the plant, making it more susceptible to control. Where once grass was excluded because of the vine’s dense foliage it has now returned, allowing a fuel load to accumulate and fire to spread within infestations. The rust also reduces seed production which in turn reduces the size of the seed bank. Consequently few seedlings emerge once the original plants have been controlled. For both algaroba and rubber vine, controlled burning can be implemented so they have minimal impact on the productivity of an enterprise or the ecological integrity of the area.

Bellyache bush

Bellyache bush plants appear to be highly susceptible to fire, but seedling recruitment after burning can be substantial. If follow-up controls are not implemented, the problem can be exacerbated. A major limitation of using fire to control bellyache bush is that once infestations thicken, grass is excluded and burning becomes impossible. In such situations an alternative primary technique may be needed, with fire used as a secondary treatment. TWRC’s Dr Faiz Bebawi is currently researching possible options, such as machinery and chemicals.

Prickly acacia

Prickly acacia is a plant that is only susceptible at the seedling stage so fire is definitely not a primary control option. However, after killing adults with machinery or chemicals, fire may be used to treat seedling regrowth—particularly after wet years when large numbers may appear.

Chinee apple

Chinee apple is a plant that appears to have no stages that are susceptible to fire. Even small plants less than 30 cm high can re-shoot following burning. It is a difficult plant to remove from an area once infestations become extensive or dense. However, it appears its seed bank does not live much more than a year or two, so there will probably not be much regrowth after the original infestations are killed. Management options are either chemicals or machinery, and both are expensive for treating thick infestations; it needs to be controlled while infestations are small.

Fire and grazing management

Wherever fire is used its success will largely depend on pre- and post-fire grazing management. It is important that enough fuel is available on which to undertake burning and it is equally important that after burning, pastures are able to recover so that they can compete with any weed seedlings. It is important to remember that while most native species are fairly tolerant of fire there may be some present within the weed infestations that are susceptible. In these situations other control options may be more appropriate. There is also a need to identify grazing and fire regimes that can be implemented to prevent the build up of weeds. The removal of fire from grazing systems is often highlighted as the primary explanation for why we have such weed problems in our rangelands.

More research needed

We still don’t know enough about the impact of fire on many weeds in northern Australia. These include giant rat’s tail grass, grader grass and parkinsonia. TWRC, in collaboration with Dr Tony Grice from CSIRO, is about to begin research on parkinsonia, particularly its susceptibility to fire at different seasons. This research is part of a broader cooperative project on the ecology and management of parkinsonia under the auspices of the CRC Australian Weed Management and involves CSIRO Sustainable Ecosystems and Entomology and NRM (Qld).