Black Box Woodland
Black Box Woodland (BBWood) is found on flat to slightly undulating landscapes on alluvial soils in north-west Victoria. The altitude range is generally between 50 and 150 m above sea level and the rainfall varies from 250 to 450 mm a year. About 30% of the area once supporting BBWood in Victoria falls on public land while a little over 15% is represented in conservation parks and reserves. About 65% of all BBWood has been permanently cleared for agriculture, urban development or mining. In most areas BBWood is bordered by the Red Gum, Mallee or Grassland ecosystems.
BBWood is characterised by a dense to sparse canopy of Eucalyptus
largiflorens (Black Box), usually 15-20 m tall, over a sparse to
dense understory of
small to medium-sized saltbushes and
short-lived herbs and grasses, with occasional patches of lignum (Muehlenbeckia).
In some areas Eucalyptus largiflorens are widely-spaced and a
chenopod shrubland develops between the patches of woodland.
|Black Box Woodland|
The most common understorey species, and those that make up the bulk of the biomass, are members of the Chenopodiaceae (Enchylaena, Einadia, Muehlenbeckia, Sclerolaena, Salsola, Atriplex, Maireana). Chenopods are successful in the salty, changing and uncertain environment of BBWood because they are tolerant, adaptive and conservative in their approach to growth and reproduction. Their tolerance is in the form of mechanisms for dealing with saline conditions, which include succulence in some and the excretion of salt in others. They deal with both drought and periodic flooding by methods such as slow growth rates (minimising the need for respiration) and dense protective hairs or waxy scales on leaves and stems of some species (to keep water in and out). They put relatively little energy into flowering structures which are generally small and lack attractive agents such as colourful petals, perfumes and nectar. Their fruits are sometimes succulent, to encourage animal dispersal, or spiny, to adhere to fur or feet of passing mammals, or have papery wings, to enhance wind dispersal. Both spiny and winged fruiting structures are usually dry and made largely from lignin, a material which requires only small amounts of energy to grow and maintain. The presence of spines, hairs, wax and salt provide them with effective (although not complete) deterrents to browsing by mammals and the large proportion of nutritionally low lignin and cellulose in the plant structure make them poor fodder.
A consequence of the adaptive features of the chenopods has been that in areas where BBWood has been subjected to heavy grazing by stock, rabbits and kangaroos (most of the area the ecosystem now occupies), the more nutritious and less coarse grasses and herbs have been reduced in number and the chenopods have increased.
The fauna that inhabits BBWood often display behavioural and reproductive adaptations, similar to animals of Mallee and Pine-Buloke ecosystems. The females of Western Grey and Red Kangaroos can manipulate breeding times by use of a reproductive strategy (delayed implantation) that allows them 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.
Many of the BBWood birds are not permanent residents in any one place, instead they are nomadic and follow food and water over often large distances.
The botanical components of BBWood understory were those that made it an attractive ecosystem for agriculture when the Mallee region was first settled. The irregular inundation of the soils meant that periodic flushes of grasses (particularly wallaby-grasses and spear-grasses) and soft-leafed herbs (a range of daisies) offered excellent fodder in good years while the perennial chenopod shrubs, and occasional Cattle Bush (Alectryon oleifolius), provided subsistence browse at other times. The rough sowing of northern hemisphere grasses (rye grasses, barley grasses) and herbs (medics and clovers) to improve the pasture for sheep and cattle, while initially successful, ultimately lead to the introduction of less desirable species (bromes, fescues, wild turnip, wild mustards, thistles, horehound) which came as contaminants with the original seeds.
The undesirable species have now come to dominate much of the ground cover at the expense of both natives and the useful non-native fodder plants. Today the proportion of the understory flora that is made up of non-native species is, in Victoria, second only to the most disturbed versions of Grassland ecosystems. As a result the ecosystem has limited value for both native and domestic grazing and browsing animals.
Agricultural use of BBWood also affected the Eucalyptus largiflorens trees themselves. Eucalyptus largiflorens is a slow-growing species that was seen by Mallee farmers as useful in a variety of ways. Its timber was valued for construction, fencing and firewood but it was recognised very early on that it didn't regenerate well after harvesting and adult trees often died after fire or in areas where grazing was heavy. Loss of trees was found, in many cases, to be more or less permanent. The leaves were found to contain quantities of oils which were enough to make harvesting a commercial proposition. And the prolific flowering (the specific epithet largiflorens refers to the quantity of flowers rather than their size) quickly made it a target for bee-keepers, a quality that also makes it significant as a food source for honeyeaters, lorikeets and butterflies.
Regeneration of Eucalyptus largiflorens trees is reliant upon an adequate water supply after seed fall and extended periods of drought, or control of water flow to the BBWood ecosystem by water and irrigation authorities, has limited the opportunities for such moist conditions to be sustained very often. When suitable conditions do arise the seedlings are highly susceptible to trampling and browsing by stock and rabbits.
© Paul Gullan, Viridans Biological Databases