DUAL DUNES

 

Comments about the monotony of the mallee are a cultural commonplace. There are, however, a surprising variety of mallee trees, with distinct patterns of distribution across the whole region known simply as "the Mallee".

To begin at the beginning, there are two foundational events in our geological past that explain some of this unexpected diversity. The first is a marine incursion into the western part of the Murray-Darling Basin (ca. 6 MYA). Late Miocene and Early Pliocene retreats by the Southern Ocean left its seabed of sedimentary deposits called the Parilla Sand Formation. 

The second is a freshwater submergence of much of the same area by the palaeo-Lake Bungunnia (ca. 3.5 MYA). Late Pliocene and Early Pleistocene faults and uplifts blocked the drainage of the Murray and Darling Rivers to the sea. The lake overlaid the Parilla Sand with siltier sediments of Blanchetown Clay and some Bungunnia Limestone, before finally breaching the tectonic barriers and emptying out.

Coinciding with the final draining of Lake Bungunnia was the onset of aridity in southern Australia. The modern semi-arid environment of "the Mallee" formed in the last 700,000 years, as cycles of glacial and interglacial periods failed to return the climate to previous hot and humid levels. 

Expansive dunefields developed in the new cooler and drier conditions. Deposits of Parilla Sand and Blanchetown Clay were re-worked and windblown. Today's land surface of Lowan Sand and the Woorinen Formation, the grey Mallee and the red Mallee, is the result of this lengthy mixing and moving process.

Although the Mallee dunefield has long since stabilised with vegetation cover from the Holocene, the powerful legacy of the prevailing westerly winds can still be seen. The major phase of most recent aeolian activity centred around the Last Glacial Maximum which peaked about 20,000 years ago (ka)

Significant mobilisation started in the period from 64 ka to 32 ka, when a pervasive dust flux was recorded from southeastern Australia, coincident with advances by the Snowy and Kosciusko Glaciers. Environmental conditions deteriorated further during the lead up to the Last Glacial Maximum, reaching its most intense between 25 ka and 15 ka.

Sea level dropped to 120 metres below present (bpsl), increasing land mass and dryness. Continental temperatures were between 6 and 10 degrees Celsius lower than present averages. The last advance of the Kosciusko Glacier was in 19 +/- 1.6 ka, its final retreat in 16.8 +/- 1.4 ka. Decreased vegetation cover and increased wind speeds promoted high parna (dust) dispersal and dune transport.

Sufficient rain did not begin to fall in the broad reaches of the Murray Valley until approximately 17 ka to 14.5 ka, allowing some revegetation by low eucalypt shrubs. However, it was not before the higher temperatures and wetter conditions of Early and Mid Holocene that denser mallee woodlands became fully established.

Pollen samples from cores drilled at Darling Anabranch (NSW) and Lake Tyrell (Vic) indicate a moist time from 7.8 ka to 5.2 ka, with significant rises of Eucalypt and Casuarina populations. This was followed by a drying period before another recovery in rainfall and eucalypt density between 2.2 ka and 1.1 ka. It is thought that the climate has become marginally drier in the Mallee bioregion since then, a gradual trend that may have lifted its pace a little over the past 200 years after European settlement.

The palaeo-competition between siliceous grey sand with little carbon or clay content and calcareous red sand with higher clay or carbon content, has created two mallee zones in northwestern Victoria. To the west and south the land is left uncleared in large 'desert parks'. To the east and north the land is considered more fertile and has been extensively cleared for agriculture.

While it is sensible to assume that differences in the north-south gradient of rainfall have a marked impact on where mallee species situate themselves, the same is also true for the different soil types. This is demonstrated by the current distributions of three types of mallee inhabiting the topmost corner of the state.

Eucalyptus oleosa, or Oil Mallee, Eucalyptus socialis, or Grey Mallee, and Eucalyptus gracilis, or Yorrell Mallee, all endure the same extreme summer temperatures of 35 - 40 degrees Celius, the same low winter rainfall at 200-300 mm. And they settle the boundaries of acceptable sandy soil with similar precision.

This sort of map-making has problems with accurate representation. In addition, outcrops of old Parilla Sand in ridgelines, or old Blanchetown Clay near salt lakes or soaks, interrupts the clear delineation of grey and red soil. Nevertheless, there is a clear enough impression that these three red mallee species do their best to avoid the protruding tongues of Lowan Sand from out of the Sunset and Big Deserts.

The pattern of attraction and avoidance is repeated on a broader scale by other mallees, in a roughly west-east divide. Yellow Mallee, E. costata (or incrassata, a former and more familiar classification); the mallee form of Desert Stringybark, E. arenacea; and Slender-leaf Mallee, E. leptophylla; grow on the Lowan side. Dumosa or White Mallee, E. dumosa; Red Mallee, E. calycogona; and Bull Mallee, E. behriana; are found on the Woorinen side.