Australia: The Land Where Time Began
A little-recognised feature of parts of the the southern half of the arid Australian outback, this crust performs a similar function to that performed by iron and siliceous duricrusts that protect underlying strata, of often softer rock, from erosion. They are composed of cryptogams, spore-forming plants that reproduce by spores, e.g., algae, ferns, liverworts, lichens, mosses, bacteria and cyanobacteria (blue-green algae). They form the microscopic to barely visible flora in areas of winter rain. The soils they grow on are mostly ancient, well sorted, and low in nutrients. The landscapes that developed theses soils are usually highly weathered and flat.
In times of drought, they are often the only protection the soil has when the rains finally return, preventing raindrop splash that causes erosion, particularly of fine particles. They increase the small-scale heterogeneity of landscapes, forming patches where infiltration is higher and concentrations of nutrients occur. The microbiotic crusts assist with infiltration of water into the soil by changing the physical and chemical nature of the surface, and also by increasing the roughness of the soil.
This flora is destroyed by frequent fires, and the sharp hooves of introduced livestock, much more than by the padded feet of the native wildlife. When this occurs it exposes the underlying soils to erosion by wind, a much more common threat than water in these arid areas.
Prior to the introduction of hoofed animals to Australia, it is estimated that this biological duricrust covered about 17 % of the continent. It has now been virtually destroyed over vast areas of the continent, leading not only to erosion, but also to soil degradation as well as reduction of the flora and hence the fauna of the degraded areas.
Another function of this crust, where lichen is the predominant cover, is the fixing of and conservation of nitrogen. They also reduce water loss by evaporation and erosion by wind and water.
In the arid zone of the winter rain areas of Australia soil lichens occur in patches of a few mm in diameter. Their colour can vary, pink, brown, yellow-green, white, black. While they can withstand long periods of heat during drought, so long as they are dry, if they are wet they are much less heat-resistant. Hence their distribution being restricted to the more southern parts of the continent that are subject to winter, non-monsoonal, rain.
They aren't found in the more extremely arid parts of the arid zone, as there is a limit to the time they can survive in the dormant, dry state. They survive in the less-extreme arid areas because, while dormant, and photosynthesis has stopped, their respiration is vey slow, but if the drought continues too long they exhaust their energy reserves and die.
These arid region lichens absorb dew during the night and begin photosynthesising as soon as there is enough light, but as the temperature rises they dry out and photosynthesis stops as they return to the dormant state. This strategy allows them to survive much longer periods between rain than they otherwise would.
The distribution patterns of the genera and species of lichen indicate that a number of environmental factors determine the make-up of the lichens in a particular area. Some factors influencing this distribution are mean annual rainfall and its distribution throughout the year, and soil type.
The result is that these associations form a mosaic over their range similar to the distribution of vegetation communities.
A study of the soil lichens was carried out in an area near Koonamore in the arid part of South Australia. In this area lichens form a continuous carpet on the soil, even covering scattered stones. In this area there were 25 genera containing 38 species. Of these, 19 species were endemic. Of the remaining species, 48 % were also found in North American deserts, 38 % in southern Africa, 28 % in western Asian deserts, and 21 % in the Sahara.
It was found that where there was a higher number of species comprising a community, it indicated diversity of the vegetation. Climax vegetation, with the highest number of species, was associated with highest soil coverage by the lichen. Areas with no shrub cover also had no lichen. Regenerating areas where shrubs were establishing also had pioneer lichen species on the soils. The soils of the studied area were calcareous, so it appears on these soils at least, lichen only establishes when the shrub cover begins to appear. They do not seem to be able to get a foothold on eroding soils, maybe a function of the their slow growth, not being able to establish before they are removed by the erosion.
An experiment at Coolibah in New South Wales studied the effects of fire on the cryptogram soil cover. This area is a eucalypt woodland on massive red earth soil. Woody shrub cover was greatly increased as a result of grazing and fire regimes since European management began. The relationship between lichens and soil minerals was displayed by the mature cryptogram communities.
Each lichen patch displayed a similar structure to the other patches. A dark-coloured greyish-black surface cap of continuous lichen overlying a zone of amorphous material in which the sediment was cemented by a layer of organic matter, with fungal hyphae forming a dense mass that penetrated into the soil. Between the lichen patches were micro-mounds of soil cemented with a gelatinous material, with associated algal sheaths. These also contributed to soil stabilisation.
Fire was found to reduce the cryptogam cover, increasing the amount of bare, unprotected soil for erosive forces to work upon. A single fire affected the stability of the top cm of sediment in the soil by altering the gelatinous material produced by the microorganisms. Increased frequency of fires increased the amount of unstabilised soil that was exposed to erosion. This also led to decreased soil layer mixing by soil fauna.
Water infiltration rate is also affected by the removal of cryptogam cover. The exposed soil is then subject to raindrop splash, resulting in increased erosion as the loosened soil is washed away. The biotic crust was completely destroyed by annual fires occurring for 7 consecutive years, including the cementing gels and mucilaginous sheaths that had accumulated in the soil. When the fires were stopped, the biotic crust recovered to about normal levels after 4 years without fire. The combination of hoofed grazing animals, fire, drought and floods combine to have a devastating effect on the biotic crust.
The period of time required for full recovery of the biological duricrust varies with soil type. When the algal soil crusts that are characteristic of Western Australian spinifex grasslands are damaged by fire there is an increase in wind erosion.
The rate of recovery after frequent burning in the mallee country was found to be much slower than on the red earth soils. It took 13 years after the cessation of fires for the ground cover of the biological duricrust to reach its maximum area cover of 40 %, that is believed to be its upper limit. This was the same time it took the ecosystem to reach its maximum fuel load. With increasing time since the last fire the cover level of crustose lichen increased, whereas the cover of algae decreased over this time. It has been found that the components of the biological duricrust varies according to the frequency of burning. In areas burnt more frequently than every 10 years algae are favoured.
The role of cyanobacteria in the cryptogam flora and living duricrust is not completely known. What is known is that they all produce gelatinous sheaths that stabilise soil. Because of the arid habitats where they form part of the living duricrust, they need to be capable of surviving desiccation without damage in dry times, and revival when water returns, whether it is short-term as with dew, or more long term with rain.
Some types fix nitrogen, producing heterocysts, chlorophyll-lacking cells that produce nitrates from atmospheric nitrogen. The groundwater under much of arid central Australia is high in nitrates, in places so high it is unfit for drinking, the nitrate levels can go above the WHO maximum levels for drinking water. As no other source has been identified for the levels found, it seems likely that it is produced by cyanobacteria in the arid country soils.
On the desert soils they look like felt-like mats. When pulled apart, the cyanobacterial filaments associated with soil fungal hyphae can be seen. Pebbles are also often covered with cyanobacteria. Their preferred habitat is the underside of translucent quartz and chalcedony. Situated under these pebbles, they receive sunlight transmitted through the pebble and moisture that condenses out of dew on the pebble surface and runs down to the soil around the base of the pebble. The cyanobacteria can flourish in this protected habitat. It has been estimated that in a paddock near Broken Hill, 8 % of the surface area is covered by pebbles covering these microbes.
It is believed this habit probably developed early in the history of life on Earth, when the cyanobacteria that originally released oxygen to the atmosphere, leading to the formation of ozone high in the stratosphere, that filtered the dangerous levels of UV radiation, allowing living cells to survive on the surface of land or water, without being destroyed by the UV radiation.
It has been found that cyanobacteria and nitrogen-fixing bacteria are widespread in the soils of central Australia, unexpectedly, free in the soil, not always as symbionts with plant roots or in lichens. The cyanobacteria have developed very effective mechanisms to survive in very difficult conditions, such as desiccation and the ability to survive long periods of dormancy. Some have special proteins to allow them to withstand water stress.
Mosses, lichens and liverworts
There are more than 120 species of mosses, lichens and liverworts known from western New South Wales that form part of the biotic crust. Woodlands in the semi-arid country between Burke and Cobar has a high concentration of biotic crusts. High concentrations also occur on the Mallee plains (Eucalyptus spp.) and Beulah Plains (Casuarina cristata) in the far southwest of the state. There are about 15 species that are common to most sites, some only occur in parts of their range where introduced livestock have been absent for at least 50 years, while others have a limited, localised distribution. There is a higher number of species on good sites than on more degraded sites, but they are more important for infiltration on degraded sites.
Some are believed to have evolved the structures to minimise heat absorption and make maximum use of any rain that falls on them by draining water hitting the thallus to its centre. These have the appearance of groups of pyarmid-shaped crystals on their surface. The other types, leafy or shrubby, are mostly from the less arid areas. There are a range of chemicals in their tissues that cause them to have different colours, according to the particular chemicals they contain.
In arid regions, the mosses and liverworts in the crust have evolved mechanisms that allow them to survive drought, they are resurrection plants, they desiccate in dry times then reactivate as soon water becomes available again. A survival mechanism has been evolved by Asterella drummondii, a common liverwort in many arid zone crusts, it has a Y-shaped thallus with plate-like scales on its lower surface. On drying, the thallus sides curl up to enclose the upper surface, the scales protecting the plant until water returns to reactivate it.
The living duricrust is thought to have probably been the method life used to make its first venture on to land. The organisms making up the present-day living duricrust were all already in existence in the oceans at the time life first emerged from the seas.
Among the cryptogams making up the living duricrust are species of mosses that have evolved to survive desiccation, or that reproduce by spores that germinate as soon as water is available. In semi-arid areas there are sometimes also small ferns that develop rapidly when water becomes available.
One of the unfortunate side effects of global warming is that the temperature zones are expected to move further south, in the Southern hemisphere, (and further north in the Northern hemisphere) as the atmosphere warms. This is expected to move the northern limit of the living duricrust further south, leaving large areas of presently protected soils vulnerable to erosion, and also an associated reduction of soil nitrates available to the vegetation.
As the temperatures rises and the living duricrust is destroyed, the bare soils would become invaded by ephemeral grasses. This would open up more land for grazing, but as more hooves churned up the soil the erosion and soil degradation would increase as the nitrogen fixed by the duricrust was used up, leading to ever greater desertification.
Ediacaran Life on Land
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|