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Much of Australia's grazing land is comprised of woodland where trees and native pastures coexist. In these ecosystems, trees and pastures compete for water, nutrients and sunlight , but there is also a mutually beneficial relationship between trees and pastures provided the balance is right.
The interaction between trees and shrubs and native pastures within grazing systems is known as the tree-grass balance.
When a favourable tree-grass balance exists, trees provide shade and shelter for livestock and support biodiversity. They also carry out key ecosystem functions, such as water and soil nutrient cycling, and contribute to healthy land condition by preventing erosion and salinity, storing carbon and enhancing soil condition.
There is increasing recognition of the role woody vegetation plays in grazing land ecosystems which has led to demand for woodland management that is both sustainable and compatible with broader environmental issues.
A changing landscape
Woodland density and structure is constantly changing due to climatic variation and other disturbances such as fire. In many regions there is woody thickening during higher rainfall periods and thinning during drought. Where fire has been a regular feature within the landscape, the removal of fire will often lead to woodland thickening.
What constitutes a natural balance is impossible to determine, what is important is establishing a balance that meets production and environmental objectives for a particular property and catchment.
In extensive systems where woody thickening has occurred, mustering costs can be increased by up to 30% and production suffers as pastures compete for scarce resources. Grazed woodland ecosystems evolved with fire, which suppresses woody thickening.
Without a disturbance such as fire, many land types will have a higher tree density.
Management options for woody thickening
The most economic and environmentally sustainable way to control woody thickening is with fire, provided conditions such as fuel load are satisfactory. Although not without risk, the following fire managemen guidelines can help maintain tree-grass balance:
- Burn every 5-6 years.
- Have at least 2000 kg DM/ha of fuel.
- Have fuel cover of at least 60%.
- Burn late in the dry season when pasture are dormant or at the start of growing season.