Why did reduced fire frequency allow woody shrubs to invade eastern Australian grasslands?

In eastern Australian grasslands, the encroachment of woody shrubs and fire-sensitive trees is primarily a consequence of reduced fire frequency. Frequent wildfires act as a critical ecological mechanism that maintains open landscapes by consuming grasses and suppressing species that lack fire-resistant adaptations.1

When fire is suppressed, fire-sensitive plants—such as the cypress pine (Callitris)—are no longer restricted to protected refuges like rocky outcrops or gorges. Without the regular disturbance of fire to clear the landscape, these woody species can expand into areas that were previously dominated by grasses. This pattern is consistent with broader ecological observations where, in the absence of burning, savanna and grassland ecosystems tend to transition toward denser vegetation or forest cover. Essentially, fire serves as a natural barrier that prevents fire-intolerant woody vegetation from outcompeting the grasses that define the open character of these regions.1