Australia: The Land Where Time Began
Spinifex Grasslands (hummock grasslands)
Compared to the grassland ecosystems of the world this is probably the weirdest. Whereas other grasslands have large mammalian grazers and large predators to prey on them the spinifex grassland has termites as herbivores and lizards as the predators of the termites. The food web is dominated by spinifex, termites and lizards.
Spinifex grows on the most infertile soils in Australia, which means it has a vast area to occupy in the arid central parts totalling 22 % of the entire land surface of the continent. It is the main vegetation type of the sand plains and dune fields of the 5 main Australian deserts, the Great Sandy Desert, the Great Victoria Desert, the Gibson Desert, the Tanami desert and Simpson Desert. It also dominates the vegetation of most of the mountain ranges of the arid zone.
Spinifex is the characteristic grass of the deserts of Australia, and is well adapted to the combination of extremely poor soils and great aridity. The grass grows in the form of hummocks and the root system goes down to at least 3 m and is evenly spaced and diffuse. An unusual characteristic of the growth form that is particularly well adapted to life in arid, infertile soils is the way the roots grow from the same node as the stem, so that each stem has its own water and nutrient supply. The roots are stiff, which makes the hummocks rigid. The leaves are like "normal" grass until the hummock experiences its first dry spell, then the leaves roll in on themselves and remain rolled for the life of the plant. The rolling of the leaf drought-proofs the plant is several ways, it reduces the amount of water lost to the atmosphere through open stomata, they are inside the rolled leaf, so away from the wind blowing past the leaf that would increase the rate of water lost by transpiration, it reduces the leaf area exposed to the sun, so reducing the heat absorbed, leading to less water loss than would otherwise occur.
There are 2 genera of spinifex, both endemic to Australia, Triodia and Plectrachne. Among grasses generic level endemism, especially on a continent-wide scale, is unusual. It is believed both the genera descended from a Gondwanan ancestral grass.
At the family level they share characteristics with a number of families but have now been assigned to the Eragrostoideae, which originated in Gondwana. There is a total of 35 species of Triodia, and 11 comprise Plectrachne. Based on their leaf anatomy they can be divided in 2 types, the soft and the hard. The soft species have stomatal grooves on the lower surface of the leaves. This type produce leaf resin. Hard species spread their stomatal grooves over both surfaces, and don't produce leaf resin. The leaves roll forming hard cylinders with a sharp tip. Their epidermal cells contain silica grains.
The name Spinifex is not strictly correct, but is the name commonly used for this hummock grass and is unlikely to change outside the ranks of the plant scientists. Spinifex is a genus of plant belonging to a different family, the Paniceae, which grows on coastal dunes in Asia and Australia.
In limited areas such as rocky ranges and on some dunefields it forms a separate vegetation type not associated with any shrubs or trees. It covers about 25 % of the entire continent as an understorey associated with shrubs of varying density.
There is a variation of its usual form in a large area east of Lake Eyre. Here it is part of an association with Acacia. Spinifex and dune canegrass (Zygochloa paradoxa) are sparsely distributed over low dunes with the occasional Acacia ligulata.
Spinifex understorey - shrubland and open woodland
In these vegetation types it is found as far north as the Kimberleys in Western Australia and the Northern Territory's Arnhem Land. Isolated patches of its present distribution indicate that it was probably even more widely spread during glacial periods.
Spinifex thrives on change, because its competitors are disadvantaged, so any change, such as fire or the drying that occurred in the Late Quaternary, when rapid climatic oscillations brought infertile sand sheets and windblown dunes allowed it to expand.
Because of the presence of fire in this type of grassland the plants making up the community need to be able to recover from fire quickly, either by seed germination or by re-sprouting from protected buds.
Spinifex grows as bristly cushions, the soft grey-green and yellow-grey clumps dotted on the sand or gibbers. in Aboriginal art spinifex is depicted as fields of dots.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|