Australia: The Land Where Time Began

A biography of the Australian continent 

Tropical Savannas

The tropical savannas of Australia of the wet-dry tropics switch between wet and arid conditions seasonally, the Wet and the Dry. In summer, the wet season brings monsoonal rain and the occasional cyclone that change to low pressure systems after they cross the coast, dropping huge quantities of rain along their path. In the wet season the understorey vegetation grows rapidly. After the end of the wet, throughout the dry season, the conditions become progressively more arid, requiring the biota to switch to arid mode. The eucalypt woodlands, that have an understorey of tall grass, have adapted to the alternating climate and associated seasonal fires, often started by lightning when the first storms of the wet season arrive.

It has been estimated that Australia has been drying for about 20 million years (at least 15 million), which is the time when northern margin of Australia arrived in the tropics as it drifted from Antarctica. A combination of factors have led to the unique nature of the Australian grassy eucalypt woodland savanna vegetation type. Among the factors are the age of the continent, its impoverished soils, the biota that originated in Gondwana, and the changing climate and environment as the continent drifted from the South Pole to the equatorial regions. On other continents the fauna associated with the savannas are mammals, but uniquely in Australia, the same niches are filled by insects. More than 90 % of grazing and nutrient recycling is carried out by insects, with the exception of a few places, such as patches of floodplains that are relatively fertile, supporting sedgelands and better quality grasslands, where the grazing in taken over by mammals, including feral animals such as buffalo and cattle.

It is believed that the dominance of insects in the tropical savannas of Australia is a result of the nutrient deficient soils, on which the grass is of low nutrient value. Such grass can't maintain large populations of mammalian herbivores. In the wet-dry tropics the growing season is short, making it even more difficult for mammals. The African savannas that support vast heads of mammalian herbivores, unlike the Australian variety that are on heavily leached infertile soils, are on young fertile volcanic soils. In Africa there are also many species of dung beetle that improve the nutrient levels of the soil by burying dung, even in areas that don't support large populations of mammalian herbivores. The populations of herbivores on the impoverished soils of Australia have probably never been high, as the soil over much of the continent was impoverished long before the evolution of large mammals, so there was probably never enough dung to support the evolution of large numbers of dung beetles or equivalent insects.

The nutrient-poor nature of the savanna soils make it essential that what nutrients are available are recycled as rapidly as possible. On the Australian tropical savannas about 20 % of organic material is recycled by termites. Unlike their destructive role in buildings, termites are essential parts of the environment, especially in the savannas. In the wet season soil microbes play a large part in nutrient recycling, termites work throughout the year. Fire also has a role in nutrient recycling. When lightning was the main cause of fires, they were spaced in both place and time. According to White (2003), the burn-offs in tropical Australia are now widespread and frequent, especially in the Top End. CSIRO research has found that though fire recycles nutrients, it can also destroy them, indicating that the frequency of man-made fire should be reduced, the suggested regime being 1 in 3 years, though preferably 1 in 5 years. The Aboriginal People maintained their environment sustainably for thousands of years with their fire-sticks, but White suggests that some of the knowledge of the ancestral fire-stick farmers may have been lost, leading to the overburning that now takes place.

Nutrients are mainly deposited as ash when they are released by fire, fine particles going into the atmosphere. Hot fires volatise nitrogen and phosphorus, which are lost from the system. It has been found that in tropical savannas nutrients are moved to the roots from the above-ground plant parts at the end of the growing season, reducing the loss of nutrients in dry season fires. Research at Katherine in the Northern territory has found that 90 % of the nitrogen was lost when the native grass was burnt. It is believed that this loss was probably offset by nitrogen in rain and also by being fixed by non-symbiotic organisms in the surface soil. The study found no definite evidence that nutrient levels were depleted, even over long periods of annual burning. These results suggest that, at least in some tropical savannas, nutrient losses by fire are subsequently replaced by rainfall, indicating that the processes may be closely related, and may possibly be seen as a subcycle within the overall nutrient cycle.

Links

  1. Soil biota, nutrients and water in savannas
  2. Fire
  3. Frequent fires reduce tree growth in northern Australian savannas: implications for tree demography and carbon sequestration
  4. CSIRO Tropical Ecosystems Research Centre, research for northern Australia

Sources & Further reading

  1. Mary E. White, Earth Alive, From Microbes to a Living Planet, Rosenberg Publishing Pty. Ltd., 2003
  2. Listen...Our Land is Crying
Biological Duricrust
Botanical History
Brigalow Belt
Chenopod  Shrublands
Claypans
Cooper Creek Floodplain
Ecosystems
Insects in the Tropical Savanna
Mound Springs
Mt Poole
Mulga Woodland
Nitre Bush
Saltbush
Softwood Scrubs
Spinifex Grasslands
Flood Pulse Concept
Simpson Desert Flora
Tropical Savannas
Tropical Savannas-Ants
Tropical Savannas - Insects
Tropical Savanna-Cape York- Fire
Tropical Savannas-Small Mammals & Fire
Tropical Savannas & Termites
Rainforest
Land Types of the Arid Zone
Floodplains & Fire, Cape York
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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading