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Bloodwoods over annual and perennial grasses in
Kakadu National Park
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Most Australian savannas are eucalypt open forests and
woodlands, but they vary considerably because of variation in
annual rainfall and soil texture. Broadleaf trees and shrubs such
as Terminalia (billy goat plum), Brachychiton
(kurrajong) and Erythrophleum (ironwood) may also occur
together with the eucalypts in both the wetter and drier savannas.
In the higher rainfall areas on the lighter textured soils, open
forest is typically dominated by eucalypts 10–20 m tall with
a canopy cover of 40–60%. The understorey consists of annual
or perennial tall grasses. In the Kimberley, Top End and Cape York,
such forests are dominated by Eucalyptus tetrodonta
(stringybark) and E. miniata (woollybutt); open
Melaleuca forests and some patches of monsoonal vine forest
fringe the treeless flood plains. Heaths, with a few trees, and
hummock grasses (spinifex) grow on the ‘stone
country’.
In the semi-arid regions, the savannas are woodlands and open
woodlands. The eucalypts are shorter (5–15 m tall) and have
lower cover (5–30%) than those of the forests. There are
numerous species that dominate, but common ones in north-western
Australia are bloodwoods and boxes, e.g. E. tectifica (grey
box), E. pruinosa (sliver box) and Corymbia
terminalis (bloodwood). Common species in north Queensland
include the iron-barks and boxes, e.g. E. crebra
(narrow-leaved ironbark), E. melanophloia (silver-leaved
ironbark) E. brownii (Reid River box), and bloodwoods, e.g.
Corymbia erythrophloia (red bloodwood). Perennial grasses
here include kangaroo grass ( Themeda triandra ), black
spear grass ( Heteropogon contortus ), ribbongrass(
Chrysopogon fallax ) and white grass ( Sehima
nervosum ) on the lighter textured soils. On the poorest and
most shallow soils in the lower rainfall areas, tree cover is
sparse (1–2%) and spinifex ( Triodia spp.)
predominates.
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Open woodland of boabs over ribbon grass
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Some Acacia -dominated woodlands provide the exception to
the rule of eucalypt predominance in northern Australia. Notable
examples were the extensive areas of brigalow open forest (
Acaciaharpophylla ) on clay soils in eastern
Queensland, large areas of which have been cleared and sown with
pasture grasses such as buffel ( Cenchrus ciliaris ) and
Rhodes ( Chloris gayana ); lancewood (A. shirleyi )
on lateritic soils of the\ Sturt Plateau in the Northern Territory,
and pindan ( A. eriopoda and A. tumida ) on sandy
soils in the Dampierland region of Western Australia. Woodlands and
open woodlands dominated by gidgee ( A. cambagei and A.
georginae ) also occupy substantial areas of fine-textured
soils in central and western Queensland.
The grassy understoreys also vary with rainfall and soil. Annual
sorghums (also often called ‘speargrass’ ) are
common beneath the eucalypt woodlands and open forests of the
wetter areas of the western Top End of the Northern Territory and
parts of the Kimberley. In eastern Arnhem Land and Cape York, in
contrast, annual Sorghum intrans is uncommon, and perennial
Sorghum (S. plumosum), firegrass (Schizachyrium
fragile) and other tall grasses predominate.
In the semi-arid savannas, perennial grasses predominate, with
common species being ribbongrass, white grass, kangaroo grass and
black speargrass. Black speargrass communities occur on
free-draining duplex soils (sand over clay), and cover a huge area
of eastern Queensland. As rainfall declines westward in north
Queensland, Aristida , Bothriochloa and
Chrysopogon spp. begin to dominate. In the driest savannas
of northern Australia, where soils are skeletal, the grass layer is
dominated by ‘spinifex’, under either open eucalypt
woodland or mulga (Acacia aneura). Unlike other perennial
grasses, which have a tussock growth form, spinifex has a hummock
growth form.
