Effect of woody vegetation increase in the VRD

by Rodd Dyer, Department of Business, Industry and Resource Development, (Primary Industries) From. Managing for healthy country in the VRD, Tropical Savannas CRC 2000.

Shrub encroachment

For some time, it has been evident that native trees and shrubs are encroaching into open grasslands and woodlands in the VRD that are considered to be valuable grazing country. One of the most likely causes of this is the alteration of long established fire regimes, however, work is being undertaken by Rod Fensham of Queensland's Environmental Protection Agency to investigate this further.

Photographic evidence

Photographs have shown that the most significant increase in woody vegetation in the more productive pastoral lands of the district has been occurring since the 1950s and 60s. (To view some of these photographs, go to the Savanna Links story, "Tropical Savannas: Not what they used to be", web link below.) This change is comparable to that which has occurred in savannas in other parts of Australia, and in other parts of the world such as South Africa and the United States. Experience in these areas supports the theory that this increase is most likely due to the reducing frequency, extent and intensity of fires. Detailed work is being undertaken to explore this relationship in the VRD which can be viewed by clicking on the 'Grazing and fire' link above.

Changing management practice

In the past, pastoral leases were immense and levels of management were low. There were few roads and fences and the small but increasing cattle herds congregated their grazing around natural watering points. Most of the district would have been covered with a continuous layer of grassy fuel. Fires commonly lit by lightning would have regularly burnt with little or no means of control. Early pastoralists also used fire to lure cattle to "green pick" for ease of mustering. In addition, aboriginal people still moved relatively easily through the area carrying out traditional burning practices. An altered fire regime has therefore probably been caused by a reduction in the amount and continuity of grassy fuels and the active extinguishment and prevention of fires with the intensification of grazing on pastoral lands. The reduction in traditional aboriginal burning practices has also been a factor.

In contrast there is also evidence that fire regimes on rugged country often surrounding valuable grazing lands have been altered, with an increase in fire frequency, extent and intensity. These severe fire regimes are damaging the sensitive vegetation that grows in these areas.