Australia: The Land Where Time Began

A biography of the Australian continent 

The Kwongan Sandplain Flora of Western Australia (The Kwongan is a fynbos)

This flora of the South-West Province of Western Australia is highly adapted to the soils of the region which are extremely poor, even compared with the impoverished soils of much of the rest of Australia. The plant species of the region are of such extreme diversity that this part of western Australia has been called mega-diverse, in which 3000 species (53 %) of the 5710 species of the flora are endemic to the Province. Another 1524 (26%) are endemic to Western Australia. Relatively few families are represented by most of the species, with the result that very similar plants coexist. The Kwongan is a fynbos mixed shrubland assemblage, sclerophylly being a dominant character. Together with habitat specialisation, this is a response to the impoverished nature of the soil. 88 % of the species on these highly impoverished sandplain soils have evolved special mechanisms for enhancing nutrient uptake. The same situation exists in the southwestern Cape in South Africa, where many of the Gondwanan families present in the Kwongan and also found in the southwest Cape region.

Because the various species have become specialists, having evolved different ways to exploit subtle variations in the levels of the critical resources of water and nutrients in different places and probably microclimates. The problems for the flora in the Kwongan are compounded by the Mediterannean-type climate, with winter rain and long, hot, dry summers. Under this regime water is most readily available when temperatures are low, but not during the hot summers, that are long, when evaporation is high.

The poor soils have been a characteristic of much of Australia for a very long time, adaptation to the low nutrient soils and water stress has been  developing in some plants for about 125 million years. The continent had been progressively drying almost since it broke from Antarctica about 45 Ma. These pre-adaptations, that included sclerophylly, were a starting point for the large amounts of adaptation that were required in places like the Kwongan as conditions of climate and soil continued to deteriorate, especially since the start of the Pleistocene ice age about 2.6 Ma. In both southwest Australia and southwest Africa, the Mediterranean climate became established only about 5 Ma. These same plants that had been adapting to dry climates on poor soils for millions of years were well suited to take advantage of the worsening conditions after the start of the last ice age. Along with the adaptations to dry conditions they also developed a level of fire tolerance, as fire is associated with dry times in Australia.

The microbiological activities that are the basis of the macroscopic ecosystems in these areas is completely unknown, as is the case over most of the continent.

According to Mary White, "When one realises how much adaptation has been involved in creating a sustainable natural ecosystem that is in balance with the environment, and admits one knows nothing of the microbiological realm that supports the macrobiological assemblages with which one is familiar, perhaps one can see clearly why monocultures of introduced plants, like wheat, are not sustainable substitutes" (1).

During the last 6-5 million years the Kwongan has become geographically isolated, and over this period seasonal aridity has been intensifying. This has allowed the high levels of endemism to develop, as well as the wide variety of adaptations, together with some novel mechanisms for coping with the unique conditions in this part of the continent where the combination of seasonal drought is combined with the poorest soils on a continent with the poorest soils. 

Some of the adaptations made by the various species to cope with the severe conditions

More efficient water uptake
This adaptation involves the development of a finer root system on which are long root hairs. The proteoid, capillaroid, and dauciform roots are the forms in which this trend is most highly developed. Most of the new season rootlets and the specialised roots occur in the top 15 cm of the soil profile near the zone of decomposing leaf litter. The few nutrients in these soils are mainly in the top few centimetres. The microorganisms that recycle nutrients are attracted to the roots by substances exuded from the roots. The presence of mycorrhizal symbiotic fungi increases the absorptive area of the roots and allow access to pore spaces to get more water. These symbionts also make phosphorus available by solubilising it, and host-specific mycorrhizae selectively uptake nitrogen.
 
Prolonging water uptake

This is accomplished by the tendency of rootlets to cluster. Water is trapped by capillary action, that prolongs nutrient release.

Utilising novel nutrient sources

As in other habitats low in nitrogen, such as waterlogged ground, a number of carnivorous plants are present in this habitat. The small invertebrates trapped by the sticky substance produced by glands on their leaves are digested by enzymes produced by the glands and those produced by the bacteria that colonise the glands. Legumes, casuarinas and some other plants have symbionts that fix atmospheric nitrogen. The fires that are common in the region make micronutrients, such as molybdenum and cobalt etc., more available to the plants. There are also a number of plants that have become root parasites that attach to the roots of other plants by haustoria, structures that attach to the roots and penetrate them, to absorb phosphorus and potassium preferentially. An unusual example is Kingia australis, the grass tree, that produces aerial roots that penetrate its own persistent leaf bases to extract nutrients that are spread around the plant through the vascular system. In effect, it eats its own older tissues to feed the younger tissues.

Other mechanisms employed to survive in this difficult environment

Proteoid roots
Dauciform roots
Nitrogen fixing
Carnivory
Parasitic plants

Delayed shoot growth & elevated taproot growth

Cotyledons of some plants maximise nutrient supply to the shoot in order to elongate rapidly and start sending carbon to the roots. This mechanism operates in places where there is plenty of sunlight, the shoot remains small, and because of the good light conditions can manufacture carbohydrates at maximum rate to power the rapid root extension taking place. It has been found in studies of Hakea that in the first year after germination the shoot remains a few cm high, but the taproots grow down more than 1 m. In the second growing season, the lateral roots and and hairy root clusters develop as an extensive system. It is then that the shoot begins its rapid elongation.

Mineral loading of seeds

Protaeaea (8 species studied) send extremely large amounts of phosphorus, and lesser, but still large amounts of nitrogen, copper and zinc, to their seeds. This enables rapid establishment of deep roots, because the poor soil would otherwise make it very difficult for the plant to become established quickly enough.

Aestivation

Some plants, geophytes, aestivate, growing during short winters then the surface parts die off in summer, surviving as below ground structures such as corms and stem tubers. These plants grow in small areas of shallow soil in granite outcrops, the sort of place where heat and desiccation are are very severe in summer. The underground structures are specialised, acting somewhat like seeds, storing large amounts of carbohydrate and protein, and like many seeds, they can withstand desiccation. Adventitious roots sprout and a shoot begins growing as soon as the structure is wet. An example is Borya nitida, a resurrection plant, that is found widely on exposed outcrops. This plant survives desiccation of the complete plant, rehydrating and carrying on with its life as soon as the winter rain arrives, growing, flowering and setting seed.

Micro-stilt roots

Some plants develop micro-stilt roots that hold the surface plant parts above the hot ground. This habit, unique to the Kwongan flora, is found in 70 species of small herbs from 9 genera, in 3 monocot families and 2 dicotyledons. Most plants with micro-stilt root habit are species of the genus Stylidium. Species of this genera grow mostly on gravel or rocky outcrop fringes in open spaces, and are vulnerable to fire. The stilt habit is an adaptation to high surface temperatures of very dry soil in which most or all of the above-ground shoot is raised above the surface of the ground on aerial prop-roots. Their seeds germinate in early winter, sprouting like ordinary seeds, forming a rootlet and a shoot. As growth continues, the shoots put down aerial roots and the seedling root is discarded. The internal tissue of these aerial roots are insulated from the heat and drought of their surroundings by their outer parts being heavily lignified. By raising the shoot as little as 2 cm above the surface has been shown to lower the temperature it is subjected to by between 6o and 10o C.

Links

  1. Tolerance of Borya nitida, a poikilohydrous angiosperm to heat, cold and high-light stress in the hydrated state
  2. Desiccation-induced changes in the protein component of soluble extracts from leaves of resurrection plants and related desiccation-sensitive plants

Sources & Further reading

  1. Mary E. White, Earth Alive, From Microbes to a Living Planet, Rosenberg Publishing Pty. Ltd., 2003
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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading