Australia: The Land Where Time Began

A biography of the Australian continent 

Saltbush                                        Saltbush Species & distribution

The saltbush genera Atriplex is one of the most important genera in the arid zone. It forms dominant perennial shrublands and has annual and perennial members growing in a wide range of communities. The genera Atriplex is not endemic to Australia. It is believed that it probably was widespread across Gondwana. However, 70 % of Australian species are endemic to Australia. A. vesicularia, the common bladder Saltbush has many variants, both geographic and edaphic (varies according to soil type). It changes shape under different degrees of grazing pressure.

Some scientists believe the genus arrived on the Australian continent around the Geraldton-Carnarvon area of Western Australia, then evolved endemic species as it spread across the arid zone of the continent, in isolation from the other branches of Atriplex. The presumed parental species is still found in the Shark Bay area of Western Australia. It is a bladder saltbush that lacks bladders. Bladder saltbush are so named because of the hollow, inflated bladder that encloses their seeds. Small bladders seem to have evolved as the parental bladderless saltbush spread east across the Nullarbor Plain. The bladders of the species inhabiting the southern arid lands with a heavier type of soil, are of both small and large. They are also found on stony and saline clay soils. There are a number of centres of diversity of Atriplexin in Australia. These include the area around Shark Bay, believed to be the site of its landfall on the continent, which has a particularly high species diversity, and in the ranges that rise from the plains of South Australia. Winter  is the time of year when these get most of their rain, though it sometimes rains in summer, this summer rain is often heavy but is not a reliable as the winter rain. Like other plant types, chenopod species have a evolved a variety of strategies for coping with the erratic nature of the arid zone climate.

Among these, A vesicularia, the Bladder Saltbush, grows on salty clay soils that slows the infiltration of water into the soil. It has evolved shallow fan-like roots that spread up to 2 m around the plant, intercepting the near-surface water before it accumulates too much salt from the saline soil. This plant grows rapidly when there is sufficient water, then as the soil dries out it shuts down. By closing its stomata it reduces evapotranspiration by 50 %. If drought becomes more severe it drops its leaves, and sheds fine roots, a feature that is not common among Australian arid zone plants. The Bladder Saltbush flowers and sets seed every time sufficient rain falls, whatever the time of the year. The seeds are adapted to germinate when the soil is likely to stay moist for long enough for them to become established, that is, when the temperature is low after winter rain that has persisted for several days. The seeds may remain viable for a number of years, having a number of germination inhibitors. The envelope surrounding the fruit is impregnated with an inhibiting salt that requires 50 mm of rain to be leached from the envelope. The seed has a hard coat and also a light inhibitor, contributing to the difficulty of germination when the conditions aren't right for establishment. Once established, the plant lives for about 25 years, which is considered a relatively short life span.

The Black Bluebush, Maireana pyramidata, has extremely deep roots as it grows on sandy soils, as in the spinifex grasslands, which allows water to easily drain through the soil. Because of its deep roots it has access to a more reliable water supply than the Bladder Saltbush so grows more steadily. Also because of this reliable water supply it can withstand higher temperatures and a greater temperature range than the Bladder Saltbush. It only drops its leaves when it dies. Black Bluebush mostly flowers after autumn or winter rain. Its short-lived seeds, only a few months, are set in summer, germinate after a couple of heavy falls of rain, which usually occurs in summer. The seeds lack the germination inhibitors of the Bladder Saltbush. Once germinated, the plant lives for 150 years or more, much longer than the Bladder Saltbush. A difference in the survival strategy of the Bladder Saltbush and the Black Bluebush is that it places its main hope for species survival in the adult plants, whereas the Bladder Saltbush makes its seeds the main hope for its species survival.

Chenopod shrublands are relatively simple, compared with other arid zone ecosystems, mainly because they contain very few trees and not much fire. They appear to the observer, at least European observers, to be desolate places, but actually support many herbivores, kangaroos, sheep, etc, as well as vast numbers of herbivorous insects. The Aboriginal People would have viewed them totally differently from white settlers, seeing the abundance of food when the white man sees only desolation. They support large populations of wildlife.

Old man saltbush (Atriplex nummularia), has a number of advantages over other plants in the arid areas. One of the main advantages is that it possesses a 3-tiered root system. It puts roots down to as much as 5 m and up to 10 m wide, tapping into underground water and minerals that are deficient in the surface layers. Its surface roots collect water from light rain and bind the soil, reducing erosion. They can thrive in extremely saline soils, and can utilise saline underground water, dropping the watertable away from the root zone of less salt tolerant plants. When grazed, whether by native or introduced animals, or even insects, the nutrients brought to the surface are deposited on the soil surface by the grazers, making it more easily available to less deep-rooted plants. It is widespread in the arid zone on heavy clay soils. It has a subspecies, A. spathulata, on the arid loam soils of the Nullarbor. The protein content is high (22 %), making them a good source of food for native or introduced animals.

It is a grey, rounded, annual sub-shrub up to 35 cm tall, with thick, soft, tooth edged leaves. Globular or top-shaped fibrous, spongy fruit. It grows well after winter rain in areas where perennial plants have been removed. It is useful as a coloniser of scalded areas. Stock eat it if no other feed is available.

Saltbush plants are often difficult to identify, as it appears to be a very 'plastic' plant, an example is Atriplex vesicularia. This species has many geographic and edaphic (related to soil type) varieties. It even undergoes considerable changes in shape under different levels of grazing.

Sources & Further reading

  1. Mary E White, After the Greening, The Browning of Australia, Kangaroo Press, 1994
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Chenopod Shrublands
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                                                                                           Author: M.H.Monroe  Email:     Sources & Further reading