Australia: The Land Where Time Began

A biography of the Australian continent 

Spinifex Alliances

In Australia there are 3 main vegetation types associated with spinifex, related to rainfall and soil nutrient levels. In the wettest regions with better soils the Triodia pungens alliance is found. In the mid-rainfall regions with medium soils it is the Plectrachne shinzii alliance. In the lowest rainfall areas with a range of soil nutrient levels the Triodia basdowii alliance is present.

Triodia basdowii and a few other less common species form spinifex rings, the original plant growing in the middle and spreading out as the inner parts die. Kangaroos often camp in these rings, apparently for protection from the wind. In some low rainfall areas with local areas of moister soils Triodia pungens can sometimes be found. Many of these places are where dunes overlie palaeochannels with high watertables. Because of their deep root system these plants can reach deep enough to tap into the moist soil in an otherwise desolate area. High watertables also occur around the base of hills where the water runs off during rain.

Over most of the range of spinifex it is associated with shrubs, but occasionally it can be associated with trees, such as the desert oak (Casuarina decaisneana) and the desert marble gum (Eucalyptus gonglyocarpa). Among the many shrubs usually associated with spinifex are Acacia, Grevillia, Hakea, Thryptomene. (Myrtaceae).

Soft sub-shrubs such as 'rattle pods, parrot beaks' Crotalaria), From the family FABACEAE, 'lamb's tails' (Dicrastylis) CLOANTHACEAE, and native tomatoes (Solanum) (SOLANACEAE), are usually soft perennials with dense hairs covering their leaves and stems. Apart from spinifex, grasses are usually annuals or short-lived perennials. Forbs are usually short-lived perennials or ephemerals. After significant rain there is a flush of ephemerals. These encourage the spread of fire through the systems, which is not possible when the clumps of spinifex are separated by bare ground.

The arid ecosystems where spinifex is present carry a large number of plant species. 180 species have been collected from the Simpson Desert, and 154 in the Central Australian Sandplain. This demonstrates the high diversity of habitats present and the large changes in species composition that occurs during the succession after the system is disturbed. After a fire passes through an area or a drought is broken, many species only occur in the first year or 2. Such plants depend on the presence of resistant, long-lived seeds to re-establish after fire or drought. This is a mechanism many plants in Australia use to survive the Australian climatic extremes of fire, drought and flood. Opportunism is very common in Australian biota.

Spinifex grasslands have a post-fire succession that appears to be more regular than many other ecosystems. The first to appear are short-lived grasses and forbs. Most shrubs and trees resprout but for the first few years form only a small part of the cover of  the area. The area covered by spinifex and woody plants increases after rain for the succeeding years until the community reaches maturity. This final stage of succession can be reached in less than 10 years in the monsoonal regions of the north. It may take 20 years for the ecosystems in the southern, non-monsoonal, part of the range to reach maturity. Spinifex seed germination occurs after fires and La Nina events, but it mainly resprouts.

On the northern coast of New South Wales there is an isolated population of spinifex growing in toxic soils, overlying igneous rock that is rich in magnesium and other elements (serpentine) and in a rainfall zone of 1250-1500 mm/yr. This demonstrates that aridity is not an essential requirement for spinifex to grow. It can  also grow in areas where the conditions preclude competing species from growing. Spinifex species have evolved different levels of salt-hardiness, some being salt tolerant and others have developed the ability to excrete salt from their leaves.

See Simpson Desert Flora

Sources & Further reading

• Mary E. White, The Greening of Gondwana, the 400 Million Year story of Australian Plants, Reed, 1994
• Mary E White, After the Greening, The Browning of Australia, Kangaroo Press, 1994
• Penny Van Oosterzee, The Centre - The Natural history of Australia's Desert Regions, Reed Australia, 1993