Trees of Australia the Mulga

Acacia aneura (Mulga or True Mulga) is a member of the Mimosaceae family. ‘Mulga’ was the name given by an aboriginal tribe to a small, flat shield. Mulga is a generic term given to a number of different species. Acacia aneura is the most common although there are a number of recognised varieties. Mulga woodlands cover 20% of Australia. Mulga needs a minimum of 50 to 60 mm of rain a year and also needs summer and winter rainfall.

There are over 800 species of acacia in Australia. Some of the species are shared with Africa. Many of the acacias have a relatively short life but the mulga is the exception. Although it can grow at the rate of one metre every ten years, it may also have periods of little or no growth in years of drought. Mature years are generally over a century old. During no growth periods, there will be no growth ring formed yet trees in arid areas with diameters of only 25cm may display 150 to 240 growth rings. Undisturbed by fire the typical life span is around 200 to 300 years.

Terms used:
Petiole – the small stalk attaching the leaf to the stem
Phyllode – a modified petiole, the petiole becoming flattened and widened.
Stomata – the openings or pores on the leaf surface that open or close to allow exchange of gases and/or water.
Recurve – to bend back or down

Acacia aneura comes in several forms, depending on its habitat. It sandhill areas, it is rounded. There is also a weeping form. On red infertile soils the tree looks like an umbrella turned inside out. This helps funnel water to the roots. It is a shrub or tree which branches about one metre from the ground. Branches may reach out horizontally or point upwards. The trunk has a diameter of 20 to 30 cm.

What appear to be leaves are actually phyllodes. This modification of the petiole is common in acacias. The phyllodes are a greyish-green with a yellowy margin and are very variable in size, ranging from 3 to 25 cm long and 1 to 10 mm in width. The shape is also variable, ranging from cylindrical, to narrow linear or elliptic and from straight to curved with a blunt or slanted point. The surface has very fine lines which are sometimes only visible when magnified. When young the phyllodes may be sticky.

Flowering occurs at any time through the year following a period of adequate rain. One or two 3 to 10 mm long stalks grow out of the leaf axils. The stalks are covered in short white hair. The stalks support the spikes which are 15 to 30 mm long and are a bright yellow.

The fruit is a flat, broad pod which may be oblong or narrowed towards the base. The pod ranges from 20 to 35 mm long and 5 to 14 mm wide and has inconspicuous wings to facilitate wind dispersal. The surface is slightly sticky.

Drought adaptation
The mulga is highly adapted to cope with little rainfall. The phyllodes and branches are arranged to divert any precipitation down the stem and trunk to the ground to feed the deep taproot.  Seedlings with a height of only 10cm have been found to have taproots of nearly 3 metres. The phyllodes have thick skins and stand erect to lessen the surface area exposed to midday sun. Mulga has a high oil content and sunken stomata. The tiny hairs on the stems help reduce transpiration. The mulga will drop much of its foliage during dry spells thus providing its own extra mulch which will eventually break down to provide nutrients. The mulga also fixes atmospheric nitrogen, just as clover does and this has a fertilising effect on the soils on which it grows and benefits all vegetation in the area.

The mulga has a multitude of uses. To the aboriginals living in the outback it was one of their most important food sources. Once separated from the pods by rubbing, threshing and drying, the seeds would be dampened and ground to an edible paste or used to make seedcakes. Attack by the lerp scale causes the plant to exude a sweet substance which was made into a sweet drink or sucked as ‘bush lollies’. A white, powdery substance on the phyllodes and small branches was used as resin. Mulga wood was used for spear-throwers, boomerangs and other tools. Insect activity causes swellings on the mulga but most of these are inedible. The ash of the mulga was mixed with pituri to form a tobacco which was chewed.

The healing qualities of the mulga were also utilised. Young phyllodes and twigs were boiled and the brown, scented water used as a wash to ease fevers, or were heated and softened and used as a poultice to ease headaches.

Graziers and settlers used the mulga for a multitude of purposes from bullock yokes to dray making. During drought periods, settlers would fell mulga trees to provide forage for starving livestock. The high protein content of the mulga could cause digestive problems after long periods. The timber is resistant to termites and vast areas of mulga were cleared to provide fence-posts and general construction timber. It is a good firewood and yields excellent charcoal.

The long-living nature of the mulga makes it an ideal host for parasitic forms such as mistletoe and sandalwood and it is now planted in commercial projects as a sandalwood host. It is favoured by bees providing water is available. Such is the value of mulga that efforts are now being made to protect it from rabbits and feral grazers. In areas where the mulga has been allowed or assisted to re-establish itself, the natural landscape with all its associated grasses and herbs is returning.