Mulga; face of the inland

This is another in an irregular series on (mostly Australian) trees. It's been a while since I posted one of these, but you can find some past ones here, here, and here. 

It is a remarkable fact that some 20-25% of the entire continent comprises semi-arid woodland dominated by just one tree - the hardy Mulga, Acacia aneura. 

Map of Mulga range from Atlas of Living Australia; in fact this map implies a greater even than
25% coverage, but I think the significant word is 'dominated'. This map includes more scattered stands,
in addition to the vast areas with just one dominant species.
(And I think we can discount records from Brisbane, Victoria and Adelaide, as well as up towards
the Top End, as being either errors or plantings.)

A drive through the inland of Australia can involve hundreds of kilometres of this habitat; these three photos show Mulga plains in different parts of Australia.

From Chambers Pillar, south-east of Alice Springs, central Australia.

Near Quilpie, south-west Queensland.

Idalia National Park, central Queensland.

Having made the claim about 'one species' however, it may not be quite that neat, as nature is wont to do to our attempts to create tidy little boxes. It is certainly a very variable species (which is perhaps not surprising over such a vast range), with very different foliages.

Mulga near Windorah, south-west Queensland; note the slender foliage.

Mulga near Cobar, central western New South Wales.
Here the foliage is much broader, giving a quite different aspect.

One thing that is consistent however is the distinctive form, which has been described as like an upside-down umbrella. 

Mulga near Windorah, south-west Queensland.

The erect phyllodes (modified leaf stems in acacias, which function as leaves) direct rain down the stem and ultimately down the trunk, so that virtually all the water which hits the canopy enters the soil less than 50cm from the trunk in old trees, and even closer in small ones. 25mm of rain can, in this fashion, be equivalent to a fall of 140mm where the water enters the soil; it follows the trunk down through the soil to the roots, which take the water deeper and thus hold it longer than can be done by other more shallow-rooted plants. In addition of course any plant which grows under the Mulga canopy is going to be robbed of rain by the tree.

Another constant, as we'd expect from a species, or even a species complex, is the flower, which forms a spike of clustered little flowers, as opposed to the sphere of many acacias.

Mulga flowers, near Windorah, south-west Queensland.
Mulga produces heavy crops of flowers following spring and summer rain,
but only produces seed after late summer rains.
Moreover for actual seed set to occur, follow-up winter rains are needed too. The seeds drop after October-November,
then need more summer rain to germinate. This set of circumstances may only come along once a decade or so.

Mulga grows in a range of habitats (so the long drive through the Mulga lands can never be boring) but one constant factor seems the iron-rich red soils, as you've probably already noticed in the photos above; here are some more examples.

In sandy clay plain, Idalia NP, central Queensland.

Growing on a harsh stony plateau, above Trephina Gorge, East MacDonnell Ranges, central Australia.
Below them, in the relatively sheltered creek bed, is a River Red Gum E. camaldulensis.

On a stony escarpment above Palm Valley, central Australia, with spinifex hummock grassland Triodia sp.

On a lunette sand dune above a dry salt lake, Great Sandy Desert.

I'm sure that you've noticed too that Mulga is tough, with regard to its ability to survive very harsh dry conditions. Drought is part of its every-day existence, but somewhat counter-intuitively it doesn't grow where either summer or winter are always dry - perhaps here other species are better adapted, though I've not worked through this one properly yet. 

Mulga in severe drought, near Lightning Ridge, northern New South Wales.
The Mulga is surviving, where not much else is.

 One thing that Mulga does not cope with well however is regular fire; it can regrow to some extent from underground buds, but not from trunk buds, as eucalypts do so well. It regenerates well from seed, but if the next generation of seedlings is killed by another fire before setting seed the population is in trouble.

The next two scenes of burnt Mulga are from central Australia.

Plenty Highway; some regeneration from underground buds.

Watarrka (Kings Canyon) National Park; a very intense fire and no regeneration here yet.

The term 'mulga' is also used widely as a name for the habitat; 'mulga lands' is a common descriptor. We've stressed the prevalence of dry conditions in these lands, especially when El Niño grips the lands, but there are good times too, when the rains do come and the mulga lands burst with life.

Massed flowering in (and of) the Mulga, Paynes Find, inland Western Australia.

A reflection of this use of the term is in the other organisms with mulga in the name; here are a couple.

Mulga Parrot Psephotus varius; a beautiful bird, which in fact can be found in pretty much
every arid habitat across southern Australia.

Mulga Ant nest, Polyrachis sp.
These big nests, protected from cross-ground flooding and erosion by carefully placed mulga litter,
are common in the mulga lands. Other Polyrachis species built turret nests for the same reason.

And just in passing, other acacia species are also referred to as 'mulga', with an adjective, including Red Mulga A. cyperophylla.

Red Mulga, Mount Magnet, inland Western Australia; also known as Miniritchie, along with
other species with similar bark.

I hope this has brought back some good Mulga memories for you, but if they are still in your future, perhaps this can encourage you to bring your trip forward in time.

To assist with it, here are the start and end of a day in the mulga lands, this time in the Great Sandy Desert, in eastern Western Australia.

Sunrise on Mulga...

... and sunset through it.

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