The Victoria River region in the Northern
Territory features savanna woodland, sandstone escarpments and
black soil plains. Photo:Tim Schatz
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Landscapes can be complex to describe. This is because they can
be described at different scales using different variables to
describe them. You can describe a landscape by looking at its
geography or topography such as the hills, rivers and gullies. You
may also look for signs and stories about how these places have
been impacted upon by humans. This can be talked about as the
cultural landscape.
The landscape of the tropical savannas can be described in many
ways. The term ‘savanna’ broadly refers to a grassy
landscape, either just grasslands or woodlands with a grass
understorey that occur in tropical areas that have a dry season
(Williams and Cook 2001). More accurately the tropical savannas
region of northern Australia can be described as a series of
landscapes. If you climbed a small hill somewhere in the tropical
savannas and looked all around you as far as your eye could see,
how would you describe what you saw? This would be your description
of the landscape.
The tropical savannas region has many diverse landscapes. In
places the landscape consists of gently rolling hills covered with
high grasses, a few trees and several rocky outcrops, in others
rocky plateaus with deep gorges, trickling creeks and pockets of
rainforest are found. Closer to the coast the landscape of the
tropical savannas may be described during the wet season as
consisting of wide flooded coastal plains dominated by melaleuca
forests, or swampy tidal mangrove forests.
Evolution
Fire has been a part of tropical savanna
landscapes for a long time
Photo: Katherine Thorburn
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Australian savannas have been evolving for many millions of
years. During the early Tertiary period (about 60 million years
ago), the continent was much warmer and wetter than at present and
rainforests were much more extensive. Since then, the continent has
become drier, the rainforests have contracted, and savannas have
expanded. The extensive savannas and warm, wet–dry climate,
key features of northern Australia, have existed for at least the
last 10 million years. The broad geological features of northern
Australia are also many millions of years old.
Because of a long history of evolution, savannas are highly
complex and diverse ecosystems. Although they have a simple
structure, savannas are rich in species, communities of plants and
animals, and habitats. In addition to high species richness, there
are numerous life forms (e.g. trees, shrubs, grasses, sedges,
herbs, vines), and the savanna plants vary enormously in the ways
they cope with life in a strongly seasonal climate.
The savannas also have many different animal species, both
vertebrate and invertebrate. Indeed, savannas are richer in many
plant and animal groups than are the monsoon rainforests.
Fire has influenced the nature of the savannas over the course
of their evolution. Fire has become more frequent as the continent
has dried out. Indigenous people have used fire in the savannas for
tens of thousands of years, and people continue to use fire for
many types of land management purposes. However, there have been
many changes to the fire regimes—the extent, frequency,
severity and timing of fires—over evolutionary, prehistoric
and contemporary times.
Landforms
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Savanna woodland
Photo: Jean-Charles Perquin
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Most of the savanna region is less than 500 m above sea-level,
with local relief generally less than 100 m. Soil types may vary
considerably, and the current structure and composition of savannas
reflects variations in annual rainfall and in soil texture.
In both higher rainfall and drier savannas there are three broad
landforms:
- flat to hilly savanna woodlands
- ‘stone country’—see sandstone country
below
- ‘black soil plains’—see floodplains
below
Within each, there are also ‘riparian areas’ along
river and stream banks; these are important components of the
savannas.
Most of the savanna woodlands are flat to gently
undulating—the ‘lowland plains’ country—but
there are steeper ranges associated with the Great Dividing Range
in eastern Queensland.
Sandstone outcrop
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The ‘stone country’ consists of rocky escarpments,
slopes and plateaux, and occurs, for example, in eastern Arnhem
Land and parts of the Victoria River District in the NT, the
Kimberley in WA and northeastern Cape York in Queensland.
The ‘black soil plains’, with their cracking clays,
are found on the geologically recent flood plains of the major
river systems in the wetter regions of the savannas, and on more
recent (Quarternary) deposits, or older (pre-Tertiary) fine
sediments and basalts in the semi-arid savannas. The riparian areas
usually have a narrow, but relatively dense band of trees, often
with a grassy understorey.
Black soil plains
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The soils include sands, loams (e.g. red and yellow earths) and
heavy cracking clays (black soils). Each may be locally extensive;
loams and clays tend to be more common in the drier savannas,
although sands and loams are more extensive in the wetter savannas.
The soils of the stone country tend to be shallow and poorly
developed, although deep sandy soils may occur on the outwash
slopes at the base of slopes and plateaux.
