Australia: The Land Where Time Began

A biography of the Australian continent 

Simpson Desert Flora

The flora of the Simpson Desert is not evenly distributed, forming distinct zones. A total of 780 species of plant from 75 families have been recorded for the Simpson Desert and its margins. 44 % of the species come from 4 families, Poaceae (grasses), Chenopodiaceae (saltbushes and bluebushes), Asteraceae (daisies or composites), Fabaceae (with pea-like flowers). The low diversity of the flora of this desert is believed to be the result of low topographic relief and water shortage. The northeastern and northwestern parts of the Simpson Desert are where the highest concentration of species are found. It has been shown that the species distribution follows gradients, from north to south, from east to west and from northwest to southeast. Smaller scale gradients of species distribution has also been found to exist, based on substrate characteristics such as soil texture, soil moisture, nutrient status gradient between the crest of a dune and the adjacent swale (Buckley, 1983). The composition and structure of the flora can be drastically altered by events having a large impact on the flora, such as rainfall and wildfire. Introduced grazers, particularly camels and rabbits can severely impact the flora. As is the case of the other arid regions of Australia, but to an even more extreme degree in the Simpson Desert, the infrequent and unpredictable rain is distributed in a patchy fashion. In parts where rainfall is ineffective the flora that survive are the long-lived perennials that have the greatest adaptation to drought.

The various members of the Simpson Desert flora have evolved a number of different ways to survive in such an extreme environment. One adaptation is pointed leaves that are needle-like, greatly reducing water loss from the leaf surface. This method has been adopted by the Hakeas. Eucalypts have the edges of their flat leaves facing the sun, reducing the leaf surface exposed to heating. In Acacias the leaves have been replaced by phyllodes (enlarged leaf stalks), that carry out the same functions as leaves, but with a lower surface to volume ratio. They also have fewer stomates than would be expected, which are sunken below the phyllode surface. Cassias have modified their leaves, dividing them into segments (leaflets). Another method is to have leaves that are leathery and thickened, as well as having a waxy coating or downy hairs, as is found in bluebushes. All these mechanisms reduce water loss from the leaves. The hairy leaves reduce water loss by slowing down the wind in the surface layer of the air across the stomates, reducing the amount of water that is absorbed by the air, as the dryness of the air passing a stomate has a large effect on the amount of water vapour lost from the stomate, the stiller air among the hairs having more time to come closer to saturation, which limits the amount of vapour that can be taken up by the air.

Plants such as samphires have adapted to store water in the manner of cacti, having succulent stems. Mulga has adapted to the arid conditions by having branches that slope towards the trunk, any rain falling on the branches being directed to the stem where it is channelled into the root zone around the trunk. This allows even light rainfall to be made use of.

Any effective rain induces a bloom of the ephemeral plants that have adapted to the desert by avoiding the drought times by surviving as seeds. The result can be a spectacular display of flowers of many colours that set seed as rapidly as possible before the water is gone and they die. There is also a burst of life of the fauna at the time of the bloom of the plants. Among these ephemeral plants are daisies or composites, and species of Eragrostis, Stipa, Ptilotis, Sida, and plants from the pea family.

The desert vegetation has been found to be dynamic, in that after some episodes of heavy rain, as occurred in 1973 and 1976, it was found that there was more tall shrubland, especially of Acacia murrayana and Acacia ligulata.

The ecosystems of the Simpson Desert are also affected by fire, as are those of other arid areas, the structure and functioning of the ecosystems being changed as a result. The various plant types have evolved different mechanisms to survive fire. Thick, fire-resistant bark is the strategy used by corkwood, eucalypts resprout, while acacias produce large amounts of seeds that lie on the ground in capsules that require fire to open. The species mix of particular communities change as fires change in frequency, intensity or with the season, the various plants often adapting to suite a particular regime of fire.

Plants of the dunefields

Acacias

Acacias and wattles are also found among the 2 species of hummock grass on dunes. Acacia murrayana, the colony wattle or sandplain wattle is considered one of the most attractive shrubs of the desert. It has long leaves, the trunk is grey to powdery white, and the golden-yellow flowers stand out against the red sand of the desert. It grows in sandy soils throughout the desert. Loose colonies grow from suckers.

Acacia victoriae, prickly wattle or elegant wattle, gundabluie, have a wide distribution in the desert, especially concentrated in the drier central region. It is a gnarled shrub with a rough barked trunk. The flowers are a pale cream colour, and the short, rounded leaves have a sharp spine at their base. Most of the elegant wattles of the Simpson Desert are more stunted than those found other parts of the continent.

Dead finish is the name of Acacia tetragonophylla, so named because it is often the only plant still living at the end of a particularly severe drought. Its death-like appearance has been suggested as a reason for its common name of 'last finish'. Its short needle-like leaves are arranges in clusters of 4. 

Acacia dictyophleba is often found in sandy sites on crests and flanks of dunes. It is an open straggly shrub with leaves that have white waxy patches on them, feeling a bit like sandpaper when they are rubbed.

 

Sources & Further reading

  1. Shephard, Mark, 1992, The Simpson Desert: Natural History and Human Endeavour, Reed
  2. Mary E White, After the Greening, The Browning of Australia, Kangaroo Press, 1994

Links

Desert Walker

Author: M. H. Monroe
Email:  admin@austhrutime.com
Last updated 30/09/2011 
Home
Journey Back Through Time
Geology
Biology
     Fauna
     Flora
Climate
Hydrology
Environment
Experience Australia
Aboriginal Australia
National Parks
Photo Galleries
Site Map
                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading