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Mallee

Mallee is found on flat to undulating landscapes on sandy soils, old sand dunes, heavy clay or rocky calcareous soils in north-western Victoria.  The altitude range is generally between 50 and 200 m above sea level and the rainfall varies from 250 to 400 mm a year.  About 45% of the area once supporting Mallee in Victoria falls on public land while a little under one third is represented in conservation parks and reserves.  About 35% of all Mallee has been permanently cleared for agriculture, urban development or mining.

Mallee ecosystems are characterised by, and take their name from, small, multi-stemmed mallee trees mostly 5-8 m tall, over a variable understory of shrubs, and perennial, annual and ephemeral grasses and herbs.   Eucalyptus dumosa (Dumosa Mallee) is the most widespread mallee and is present, although not necessarily abundant, throughout the ecosystem range in Victoria.  Where the landscape is flat, and soils are calcareous and alluvial, the commonest mallees are Eucalyptus dumosa and Eucalyptus oleosa (Oil Mallee) and the understory may be dominated by chenopod shrubs and herbs.  This form often grows close to Black Box Woodland and shares many species with it.  Where the soils are on deep sands Eucalyptus costata (Yellow Mallee) is often the commonest species over an understory of small-leafed shrubs. dryland sedges, and Triodia scariosa (Porcupine Grass).   Where the sands are shallower the main mallees are often Eucalyptus leptophylla (Slender-leaf Mallee) and Eucalyptus socialis (Grey Mallee) over an understorey of small chenopods and ephemeral herbs and grasses.  In some areas, where the soils are shallow and rocky, the mallee understory is dominated by the narrow-leafed shrub Melaleuca uncinata (Broombush), often to the exclusion of all other species.

Mallee
Mallee 


The mammalian fauna consists of large grazers (kangaroos), small ground insectivores and herbivores (ningaui, native mice, hopping mice), small arboreal marsupials (pygmy-possums), and bats.  Most of the birds are common woodland and dryland species of northern and western Victoria (thornbills, treecreepers, whistlers, rosellas, currawongs, robins, pardalotes, cuckoos, parrots, wrens) although some groups are in greater abundance than other ecosystems (honeyeaters, babblers, choughs) and there are mallee specialists (Malleefowl, Apostlebird).  The reptiles are mostly small ground-dwelling or burrowing skinks, legless lizards, geckoes, dragons, goannas and snakes.  Frogs are uncommon and occur mostly near watercourses; some species have a burrowing lifestyle.

The Mallee is found in South Australia and New South Wales but the name itself is Victorian.  The Wergaui people of north-western Victoria, near Swan Hill, used the word to denote Eucalyptus dumosa and it has since been applied to all other small, multi-stemmed eucalypts, the ecosystem they characterise and the region itself.  The key environmental features of Mallee are high summer temperatures, relatively infertile soils and low, unreliable rainfall.  The 'unreliable' qualifier to rainfall is perhaps the most important of these as it appears to have shaped the behavioural and physiological adaptations of many of the plant and animal species of the ecosystem.

The mallee themselves display one of the most impressive adaptive features in the form of the confusingly named mallee root.  The mallee root - also known as a lignotuber - is not a root at all but is essentially a very contracted underground trunk which grows just below the soil surface and from which the stems arise.  It acts as both a storage and reproductive organ.  Its storage function is principally of carbohydrates, essential nutrients (such as phosphates) and some water so that the mallee, once mature, can survive long periods without rain.  Its reproductive function takes the form of multiple growth points from which new shoots can rapidly develop (drawing on the carbohydrates and nutrients) should the above-ground parts of the plant be destroyed by fire or other natural catastrophes.  Most other species of Eucalyptus have similar epicormic shoots on their trunks and larger branches from which new shoots can develop after fires but the mallee protects the trunk from fire and so is much more energy and water efficient.    

In some places many of the understory species are succulents (particularly members or the families Chenopodiaceae and Aizoaceae), others are very short-lived annuals or ephemerals which will germinate, flower and set seed in a very short time after rain, some of the shrubs are simply slow-growing and develop their own lignotubers like the mallees, tuberous herbs are abundant in some areas, and dryland sedges and the spiny Triodia scariosa are very efficient with water use.

Mallee kangaroos (Western Greys and Reds) are both well-known for their ability to manipulate breeding times by use of a reproductive strategy that allows the female to hold a barely formed embryo in its undeveloped state until conditions are suitable for sustaining young.  Consequently in times of drought and food shortage the females will not breed but as soon as conditions improve then most of them will become pregnant without the need for a male.

The small ground-dwelling mammals are all nocturnal and utilise burrows for breeding and protection for heat, fires and predators.

Many of the Mallee birds are not permanent residents in any one place, instead they are nomadic and follow food and water over often large distances.  Some of the frog species employ a burrowing lifestyle where adults can remain in a low-energy state beneath ground for long periods between rains and then emerge to breed during wet weather.

Reptiles are particularly well-adapted to conditions in the Mallee where they make up the highest proportion of the vertebrate fauna of any Victorian ecosystem - nearly 20% of all Mallee vertebrates are reptiles.  Many of the species are nocturnal (geckoes, some skinks and snakes), several of the skinks and snakes are small burrowing species (some snakes feed exclusively on ants and are themselves fed upon exclusively by other snakes), larger lizards are high-speed insectivores and carnivores, and most are long-lived and can survive for long periods without food.

The adaptations of the Mallee flora and fauna are also related to how the Mallee has been exploited by humans.  For example, the oils in the leaves of mallee eucalypts act as a defence against herbivores but are also a significant reason for their cultivation.  Eucalyptus oil 'crops' have been harvested for nearly 100 years in the mallee, with Eucalyptus oleosa (Oil Mallee) and Eucalyptus polybractea (Blue Mallee) being amongst the most commonly used.  The harvesters take advantage of the regenerative properties of the mallee roots that allows them to repeatedly cut the stems for oil extraction, none of which are allowed to grow very large after cropping so as to maintain a large ratio of leaves to woody material.  Mallee roots have also been used as a high-quality firewood when harvested from areas that have been cleared for agriculture.  Most of the agricultural areas are on the more fertile alluvial soils with the consequence that these forms of Mallee are poorly represented in parks and reserves (less than 20%) by comparison to the well-reserved Mallee on deep, infertile sands (over 65%).

The clearing of Mallee from alluvial soils has lead to a widespread problem in these ecosystem called dryland salinity.  In the native Mallee ecosystems the mallees draw slightly salty water from deep below the surface for respiration and in doing so keep the water table well below the level of the upper soils.  When the mallees are removed the water table begins to rise and after it reaches the surface, a process which may take decades, it will evaporate during the summer months and leave a salty crust.  This process will be repeated many times and with each regeneration a more concentrated crust is left behind.  The result is that the only species that can survive in the soils are extremely salt-tolerant plants - usually succulent species such as glassworts (Halosarcia) and pigface (Carpobrotus).   Dryland salinity has become a major topic for agriculture in the Mallee and and enormous research and monitoring effort has been put into the problem over the past couple of decades.  Ironically the best ecological solution (in some areas at least) would appear to be replanting mallees, thus removing any agricultural benefit, so that the water table balance can be restored.

Of all the ecosystems that have been successfully converted to agricultural the Mallee stands out as the one which has caused the greatest amount of ecological change.  Along with the dryland salinity caused by clearances there are other salinity problems related to irrigation, soil erosion problems caused by overgrazing on fragile soils in drought years, high-level weed invasion of both crops and pasture land, and some the worst House Mouse (Mus musculus) plagues in the state's history.  The Mallee was never a natural area for agriculture and its productivity is low by comparison to the more fertile and higher rainfall areas to the south.  Today the word sustainability has become part of most planning programs for the future of Mallee agriculture with problems such as those mentioned here being high on the list waiting for viable long-term solutions. 

© Paul Gullan, Viridans Biological Databases