Red Gum

Red Gum ecosystems in Victoria are found in flat to gently undulating country at low altitudes (generally below 500 m) in low to medium rainfall areas (250-1000 mm a year) near watercourses or on alluvial soils subject to periodic floods.   About 25% of the area once supporting Red Gum in Victoria falls on public land while a little over 10% is represented in conservation parks and reserves.  While the trees are still widespread, the native understory for over 70% of Red Gum has been cleared or substantially altered for agriculture and urban development. 

The Red Gum ecosystem varies considerably over its large climatic and geographic range, from riverine sites amongst grasslands of the south-west, to the broad floodplains of the north-east and south-east, the alluvial flats of the Grampians, and the banks of the Murray River in the north.  In all places, however, the large and spreading Eucalyptus camaldulensis (River Red Gum) is the principal, and often the only, tree species, which grows to 40 m or more.  In lower rainfall areas of the north-west, populations of Eucalyptus largiflorens (Black Box) may grow with Red Gum, in the Grampians and near Melbourne Eucalyptus melliodora (Yellow Box) is a common associate, and in the Wimmera and north-central plains Eucalyptus microcarpa (Grey Box) grows on the overlap between Red Gum and the Grassland ecosystem.

The understorey seldom supports a typical shrub layer (although there are often scattered shrubs and, in wetter sites, clumps of lignum - Muehlenbeckia florulenta) and the ground layer typically consists of annual, ephemeral and perennial grasses, sedges and herbs, as well as small and prostrate shrubs, many of which are adapted to periods of inundation by floodwaters.

Red Gum
Red Gum 

The mammalian fauna is sparse in Red Gum and consists primarily of large grazers (kangaroos), arboreal species (possums, gliders and occasional Brush-tailed Phascogale) and bats.  Most of the birds are common species of grasslands, grassy woodlands (magpies, fantails, thornbills, treecreepers, whistlers, honeyeaters, rosellas, parrots, robins, pardalotes, corellas, Galahs) and wetlands (ducks, ibis, spoonbills, herons, egrets, cormorants, waders, Brolgas, Pelican, raptors, lapwings).  The reptiles are mostly small arboreal skinks with occasional snakes (Tiger Snake is most common).  Frogs are common and widespread and mostly riverine and open wetland species.

Most areas of the Red Gum ecosystem are superficially similar with a landscape of large, often widely-spaced, pale-barked trees and little between their canopy and the low, grassy-looking ground-cover.  Nevertheless, within this theme, it is one of the most variable ecosystems in the state.  The most obvious reason for the variability is geographic and climatic; so it would be expected that representatives of Red Gum near Melbourne would share few understory species with those of the banks of the Murray in the north-west.  It would also be expected that the flora would be much more likely to support a substantial non-native element in agricultural country than it would in national parks such as the Grampians or Wyperfeld.  However, while both these statements are at least partially true they don't adequately explain the variation.

The understory of Red Gum on the banks of Murray in the far north-west often supports a large range of chenopods (Atriplex, Chenopodium, Maireana, Sclerolaena) and other succulents, whose presence is related to the climatic conditions and the proximity to the related Black Box Woodland ecosystem which supports these species.  In southern regions, grasses, sedges and rushes are more prominent, along with a few, scattered shrubs (Melaleuca, Leptospermum, Hakea, Acacia) which are indicative of the surrounding Grassland ecosystems.  These geographic and climatic extremes apart, however, there is a remarkable consistency of species, and if not, genera and families, across a wide expanse of Red Gum.  Some native species of Austrostipa (spear grass), Austrodanthonia (wallaby grass), Poa (tussock grass), Atriplex, Einadia (saltbush), Juncus (rush), Lachnagrostis (blown grass), Lythrum (loosestrife), Marsilea (nardoo), Muehlenbeckia (lignum), Oxalis (wood-sorrel) and Rumex (dock) are found almost throughout the full geographic range of Red Gum.  And Themeda triandra (Kangaroo Grass), the second most widespread native grass in Victoria and the mainstay of the early grazing industry, is found almost everywhere except the far north-west.

A similar pattern emerges with respect to non-native species, as even the stands of Red Gum in parks and reserves have usually been subjected to grazing by domestic stock up until the recent past, and rabbits are still abundant in these areas.  Of the 50 most commonly encountered plants species in Red Gum, 27 have been introduced since European settlement, indeed the three most regularly recorded understory species (Hypochoeris radicata, Sonchus oleraceus and Cirsium vulgare) are all non-natives.

The variability in the composition of the understory in Red Gum is most commonly associated with the distance from the nearest watercourse and the time since, and extent of, the most recent flood.   Clearly sedges, rushes, wetland grasses, lignum and other wetland specialists (mud-mats, water-mats, nardoos, marsh-flowers, water-ribbons, azollas) will be present where there is at least some standing water, and the depth of the water will determine which species will be where.  Immediately after a flood, particularly if there hasn't been one for some decades, many of the existing plant species will die as a result of the inundation and new species will become established while the water is still deep or as it recedes.  Often these early colonisers (for example, water milfoils, water blinks, water star-worts, nardoos, loosestrife, pondweeds, knotweeds) will form dense stands and then make way for larger perennial species (sedges, rushes, grasses) in one or two years.   As the water table drops further, species which cannot tolerate wet conditions will come to dominate the groundcover (especially grasses but also orchids, lilies, woodruffs, violets, daisies, buttercups, bindweeds, blue devils, geraniums).

The most significant species which is affected by floods is Eucalyptus camaldulensis itself.   Its presence in this ecosystem is dictated by the fact that its seeds will only germinate in standing water.  Floods on river banks, ephemeral lake beds and alluvial flats will, under ideal conditions, cause a flush of new seedlings which will grow into a dense, single species stand of saplings, very like an impenetrable monoculture crop.  Over decades the population will thin dramatically and after 100 years or more a single tree will remain where perhaps a million seeds had originally germinated.

Flood conditions for most of the Red Gum area has for some time now been regulated by state and federal water authorities who can determine, at least to some extent, when and where floods will occur.  About 70% of the Red Gum in Victoria is found within the Murray-Darling Basin and the federal Murray-Darling Basin Authority (MDBA), established under the Water Act 2007, has a very large say in how water will flow.  The principal reasoning behind the control of otherwise natural processes is the need for Australian farmers to access water for irrigation (the grape, cotton, rice and citrus industries in particular) and the need for clean drinking water for the rural towns in the basin.  As a consequence there are many reservoirs, dams, locks and river re-directions (for example, water from the Snowy Mountain Hydro-electric Scheme in southern NSW is re-directed to the Murray and away from the Snowy itself) which allow the MDBA and others a wide range of options for control of the water movement. 

The MDBA faces significant problems with developing a long-term strategy due to recent droughts severely limiting the available water, the regular outbreaks of toxic algal blooms in many parts of the river system and a range of soil salinity and other deterioration processes in farms due to inappropriate irrigation management.  The water supply to Adelaide, which is almost entirely dependant upon the Murray-Darling system, has also been in jeopardy because of poor outflows into South Australia.

Behind all of these agricultural and commercial problems there are ecological ones too and possibly the most adversely affected Victorian ecosystem is the Red Gum.  Many areas which currently support Red Gum are now effectively unsustainable as the land which supports it is unlikely to flood again.  As the old trees die they are not being replaced, at least not naturally, and the normal process of succession and vegetation renewal which comes from inundation will cease.  In addition to the loss of plants in the Red Gum, frogs and wetland birds, are becoming less abundant, as are arboreal mammals such as possums and phascogales, and any species that require tree hollows for breeding or roosting (for example parrots, kingfishers, treecreepers, pardalotes, owls and bats).  Breeding sites for egrets, ibis, herons, spoonbills and Brolgas are also matters of major concern for biologists.

The longevity of Eucalyptus camaldulensis and the superficial similarity of sites supporting disturbed and healthy Red Gum vegetation has meant that the deterioration of this ecosystem in Victoria has gone effectively unnoticed for some time.  Biologists have been warning government agencies for many years that the problems discussed here were imminent but because there have been no dramatic visual triggers little urgency for action has accompanied the warnings.

© Paul Gullan, Viridans Biological Databases