Australia: The Land Where Time Began
Mallee see Eucalyptus
The name mallee refers to eucalypts that have a multi-stemmed habit. It derives from the aboriginal word mali meaning water. This name of water would refer to the fact that mallee roots contain fresh drinking water, a valuable resource when there is no surface water available.
An extensive woody roots are produced by some mallee. These oversized lignotubers contain water that allows them to regenerate quickly after fire or to survive longer in drought.
Prior to European settlement, much of the semi-arid parts of southern Australia were covered by vegetation dominated by mallee. In the mallee zone, the transitional zone between the wetter southern parts, with sclerophyll woodlands, and the arid country to the north. Rainfall in the mallee zone is mostly in the winter.
In Australia mallee shrublands and woodlands are equivalent to vegetation types in other countries. In South Africa the fynbos, in Chile the matorral, in France the garrigue and in North America the Chaparal. The mallee is extremely flammable, as are the overseas equivalents. It is believed that these plant communities have evolved to promote episodic fires necessary for the maintenance of the biota.
The herbaceous floras of these Mediterranean ecosystems are almost entirely dependent on fire for their propagation, the dormant seeds needing the canopy of understorey shrubs to be opened, as by fire, before they sprout, establish and set seeds before the canopy of shrubs eventually close the gaps again. The seedbank is then replenished to await the next cycle of fire, growth and reproduction, then dormancy of the herb storey again.
In the first 1 or 2 years after a fire in the mallee heathlands of western Victoria and south-western South Australia, there is a pulse when species such as speargrass (Stipa macalpinei), paper daisy, (Helichrysum obtusifolium), and rice flowers (Pimelia spp.) and Stackhousia monogyna, flourish before the canopy closes.
More than 100 eucalypt species have the mallee habit. Most of them are in Western Australia. Among these are red mallee (Eucalyptus oleosa), yorrell (E. gracilis), slender-leafed mallee (E. foecunda), lerp mallee (E. incrassata).
Some species can grow as either tree or shrub form. In some parts of South Australia it is a response to the difficult high country terrain. E. diversifolia grows to more than 10 m on Kangaroo Island, though mallee doesn't get much above 3-9 m.
The number of trunks arising from a single lignotuber varies widely. Bull mallee has only a few trunks, whereas the whipstick mallee grows many stems. Depending on soil type, rainfall, suitability of local conditions, the associated understorey can vary. In higher rainfall areas broombush mallee can form thickets with Melaleuca or Casuarina. On heavier soils the understorey can be composed of sclerophyll heath, sparse grasses, forbes and chenopods. On their preferred soil type, sand, their understorey is composed of Spinifex hummock grasses.
The hottest part of the year, spring and summer, are the times when most vegetative growth occurs. Flowering mostly occurs in summer and in some species flowering can continues for 2 months, though they can flower after rain at any time of year. Hybridisation between compatible species is usually avoided by the different species flowering at different times. Pollinators are insects, bird and bats. Mallee produces large numbers of small seeds that can survive for up to a year, though most are usually eaten by harvester ants, until a fire causes the release of so many seeds that enough are left by the ants to germinate after the next rain. Seedlings that have reached a sufficiently advanced stage of growth can stop growing and wait for a gap in the canopy to open, in the dry times before the next rain, then commence growing again.
Large quantities of litter accumulates around the bases of the trees, and bark is soughed off in strips, many strips hanging from the trees for long periods. When fire returns, as well as the combustible litter, burning strips still attached to the tree by one end can be carried aloft in the thermals that form above the fire and are carried downwind to start spot fires ahead of the main fire front. These burning attached strips also spread the fire to the crown, where the high levels of flammable oil in the leaves bursts into fire when it get hot enough to burn. Spinifex hummock grasses often forms an understorey, and when they do they add to the flammability. Spinifex has high levels of resin, making it highly flammable, so add to the fire-promoting nature of the mallee.
In the arid lands, mallee-spinifex communities occur as a patchwork pattern, not a single continuous community. The result is that when they burn the fires are usually restricted to 1 or a few patches at a time. After a La Nina event, however, there is a lot of ephemeral cover joining the patches, when fire comes much larger areas are burnt in a single fire event. Thousands of square kilometres can be burnt by a single large fire that continues to burn for weeks.
In The Mallee region, in the Murray Basin, the mallee covered the dunes derived from the Parilla Sands, as well as other marine and freshwater sediments, after Lake Bungunnia dried up in the Middle Pleistocene. About 500,000 BP linear dunes first formed in the region and were stabilised by vegetation. These were then covered by dunes formed by wind-blown sand during the last glacial stage, and parna (dust blown from dry salt lakes and saltpans), that were spread across the area. Mallee is mainly restricted to the dunes, the swales (the space between dunes) are covered by grasses and forbs that grow on the clay-rich soil containing parna, not mallee's preferred substrate. Controlled fires that burn the grass cover in the swales with low-intensity fire can be done without affecting the mallee covering the dunes
In The Mallee there are pockets of different vegetation types. Communities of Rosewood (Heterodendrum oleifolium) and belah (Casuarina cristata), that occasionally has a bluebush understorey. These vegetation types can form a patchwork pattern among the mallee.
At the time Europeans first settled in Australia The Mallee covered an area of 11,000,000 Ha. 200 years later on 15 % remain. The loss of 85 % of the deep-rooted mallee makes this enormous area susceptible to salinisation on a vast scale, as most land use for agriculture involves shallow-rooted plant species. For a long period ending in the Miocene, when the sea retreated, the Murray Basin was covered by the Murryian Gulf, a very large incursion of the sea over the continent. Beneath the soils of The Mallee are the Parilla Sands that are marine sands. They now form a water-mound, an unconfined aquifer that is mostly saline. Saline groundwater often reaches the surface in lower parts, especially in the northern part of The Mallee, resulting in saline soils.
Down-wind of salt lakes and saltpans that occur at these places of saline water discharge, salty dust is deposited across the land surface. In these areas mallee is replaced by saltbush. Since so much of The Mallee has been cleared from such a vast area the saline watertable has been rising and salination of the entire region is only a matter of time. This problem has been exacerbated by addition of irrigation water over large areas.
The fragile nature of this ecosystem that can be so easily destroyed by wind and water erosion when disturbed, results from the unreliability and distribution of the rainfall in this arid environment, and the soil they grow in, being thin on top of sand that was spread by the strong winds of the earlier glacial phases.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|