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The Landscapes of Australia's Tropical Savannas

Victoria River Disctrict

The Victoria River region in the Northern Territory features savanna woodland, sandstone escarpments and black soil plains. Photo:Tim Schatz

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

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Fire has been a part of tropical savanna landscapes for a long time
Photo: Katherine Thorburn

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

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.

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Sandstone outcrop

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.

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Black soil plains

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.