Australia: The Land Where Time Began
Last updated 08/04/2010
In the highlands, the softwood scrubs, comprised of shrubs and low trees with a closed canopy, about 5-8 m high, often with vines of a number of species such as Hoya australils, Capparis lasiantha, Jasminum spp., Cissus opaca, that together form an ecosystem that is complex and diverse.
At the time of the study by Jim Gasteen, many of the scrubs were expanding, with new ones being established, and some were shrinking, the transition between them and the grassy open eucalypt forest being the result of burning of the mid-height grass communities by graziers that occurred seasonally. It is believed that the area covered by the scrubs was previously much greater than at present, the remaining scrub probably being only in areas that were not reached by the fires.
Acid soils were found in the upper layers of the soil profiles under the scrubs that were investigated, changing to a slight trend towards alkalinity below 1 m. Throughout the highlands, where the dominant vegetation type is sandstone eucalypt communities on elevated acid sandy soils, the scrubs don't occur. The Triassic Moolaymber Formation (that contains plant fossils) is exposed where erosion has removed the sandstone capping platform. On the shales of this formation the soil that has formed is composed of friable loamy or silty clays. The softwoods grow in small dense thickets, up to 4-6 m on these soils, extending upslope on the sandstone scree slopes to the point where the sandstone is still intact. Sandstone slabs of varying sizes are found on and throughout the upper levels of the profile of these clay soils because of the proximity of the eroding sandstone. There is a sharp boundary where the sandstone is still intact.
In the scrubs the upper layer of soil is a thin layer that is silty or sandy, below which is a tangle of roots and with sandstone fragments and small floaters, all being covered by leaf litter. Apart from areas in which the many scrub turkeys have cleared areas around their nests. The Middle Tertiary basalts of the Minerva Hills are found in the centre of the Brigalow Belt, which determine the soil type and nutrient status. In the layered woodland and forests dominated by brigalow, blackbutt, bauhinia and wilga, the understorey is dominated by tree-pear (Opuntia tomentosa) and softwood scrub species. Brachychiton, Macropteranthes, and Cadelia emergent trees replaced the blackbutt and brigalow locally in areas where soils and moisture were favourable.
The soils the softwood scrubs grew on in the Expedition National Park (between Taroom and Arcadia Valley) differed from the adjoining grassy open eucalypt forest in the different proportions of sand and fragments found in the upper layers of the soil profile. Inside the scrubs the surface humus layer was comparatively rich. Well away from the weathering sandstone and basalt, on the lowland plains, the influence of these eroding rocks was lessened, the soils are dominated by deep cracking clay and duplex soils, that often have mottled subsoils, and there are often many gilgai. In areas with better moisture, such as along the wide, shallow watercourses, these streams were often lined with softwood scrubs, tracking through the landscape dominated by Brigalow vegetation.
There were drier scrubs in which the diversity of both plants and animals were lower as a result of lower rainfall, at the headwaters of the Maranoa River and Warrego River, in the southern section of the Central highlands. In these scrubs, nutrient status and moisture content dropped with distance from the eroding basalt. The species diversity also drops with distance from the basalt.
The softwood scrubs in the Brigalow are restricted in their distribution both by fire, and being excluded from the sandstone plateaux by the unsuitable free-draining sandy soil.
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