Australia: The Land Where Time Began

A biography of the Australian continent 

This belt is a bioclimatic zone where the predominant rainfall is in the summer, between November and April, 60 - 90 % of the mean annual rainfall occurring during this period. It extends south from a bit south of Charters Towers to the Queensland-New South Wales border. The rainfall pattern is bimodal, rainfall not being evenly distributed throughout the year. At either side of the summer rainfall peak are troughs in spring and autumn. The evaporation is high during the usually long hot summers, when there is heavy rain, much of which runs off. In the mild winters rainfall is less, as is evaporation, recharge of the soil water occurring in both summer and winter, though in summer there is also a large volume of surplus water, while in spring and autumn, there tends to be soil water deficits. When cyclones change to low pressure systems crossing the coast they bring flooding rain, recharging aquifers and maintain base flow in the main river systems. As well as recharging soil water, winter rain lessens the effects of the drying that occurs in spring, the driest part of the year. Because the weather of the Brigalow Belt is under the influence of the ENSO climate system, the rainfall is variable, leading to variable flows in the associated river systems that are among the most variable in the country.

Lying some distance from the east coast, this discontinuous belt is comparatively narrow, stretching parallel to the coast south from the Central Highlands of Queensland. The belt is cut into northern and southern sections by the dissected sandstone plateau of the Central Highlands, Queensland's most important inland watershed, covering an area of more than 15,000 km². It is the where the headwaters of 7 major rivers are situated. The sandstones of the highlands were deposited in the Great Artesian Basin during the Mesozoic, being subsequently buried by lava flows during the period of the Minerva Hills volcanics in the Springsure-Emerald district, undergoing deep weathering, erosion and dissection that resulted in the broken, canyon-type landscape of the present. There are now basalt remnants found on isolated ridge crests as well as outcrops in places. This has led to a diverse topography with variable soils that have a range of moisture regimes. The animals and plants are diverse in the landscape of ranges and tablelands that can reach 1,000 m in height. The average annual rainfall is about 760 mm.

In the central hub there are the most highly developed Brigalow and Brigalow softwood communities. These communities become more scattered to the north and south of the hub. Different associated species are characteristic of 3 different sections, the north, central and south. Dry sandstone plant communities of mixed eucalypt forest and grassy woodland, with Acacia-dominated understorey, occupy the higher, uniform sandy regions.

On the heavier clay soils that have a better moisture ratios, found on valley floors and scree slopes, there is a much wider diversity of plants. In these areas are brigalow scrubs with understoreys of mixed softwood, with complex pure softwood patches.

West of the Great Escarpment, at least 6 million hectares of Queensland were covered by Acacia harpophylla-dominated brigalow. In the core of the area at least 10 million hectares were covered with Acacia harpophylla as a co-dominant species.

As it is a legume, brigalow has symbiotic nitrogen-fixing bacteria, Rhizobium, in its roots. The soils of the Brigalow Belt are of better agricultural quality than the red earths of the mulga zone or the gravelly soils of the foothills of the Great Divide. By Australian standards, the soils of the brigalow country are good dual-purpose agricultural soils.

Brigalow is well adapted to its habitat, being drought resistant to cope with the erratic nature of the climate, and producing seeds that germinate and grow rapidly when they get sufficient spring rain, as occurs in La Nina episodes, but only a few times in a century. Both seeds and seedlings are very salt tolerant. Brigalow also resprouts to form dense masses of suckers, whipstick stands. The brigalow has been removed so thoroughly over its range that it is now considered a threatened ecosystem. The softwood scrubs that once covered large areas in the brigalow zone are in an even worse predicament, being as threatened as the brigalow, but awareness of them is much less than that of the Brigalow.

By 1985 the original 6 million hectares had been reduced to 30,000 hectares, that includes 1980 Ha or softwood scrub, that is now in reserves. Some small areas of scrub have since been added to the original conserved area.

The unique climatic zone, with complex hydrological characteristics, was adapted to by the brigalow communities evolving very specific adaptations. The ecosystems that replaced the brigalow communities don't have these adaptations that suit them to the hydrology of the region, making salinisation of large areas inevitable (White, 2003).

By 1861 the entire area had been occupied. The clearing of the area for agriculture has led to a number of serious problems, for future agriculture as well as for the environment. As well as biodiversity loss, other problems are erosion, soil degradation, river silting, degradation of rivers downstream of dams. As a result of the heavily grazed pastures and annual cropping the nitrogen content of the soil that had been fixed by the brigalow has been continually reducing, and not being replaced since the elimination of the brigalow.

At least 4 scientific papers are warning of serious impending problems with salinity following the clearing, but their warning also applies to the rest of Queensland. The spectre of the sort or salinity problems being experienced in the Murray-Darling Basin lies in wait for agriculture in Queensland, but it appears to remain unrecognised by those with the most to lose if the agricultural land becomes a saline desert (White, 2003).

Sources & Further reading

1. Mary E. White, Earth Alive, From Microbes to a Living Planet, Rosenberg Publishing Pty. Ltd., 2003