In the arid Australian outback, plants must be tough and resilient to survive, and although there are some 60 species of the genus Allocasuarina, all of which are endemic to Australia, only one thrives in the vast deserts that lie in the heart of the country.
Allocasuarina decaisneana, known colloquially as the desert oak, is a tree that bares a resemblance to the more familiar casuarinas – the she oaks and river oaks that are common in regions closer to the coast, and its name reflects this similarity with the prefix ‘allo’ meaning ‘like’. Its species name honours Joseph Decaisne, but this 19th century Belgian botanist never visited Australia, and never saw the tree, which has become his living memorial, thriving in the wild. If he had done so, he might not have been overly impressed by its somewhat bedraggled appearance, but he would have undoubtedly been astounded to learn of its intricate connection to the lives of indigenous people.
Its hard and durable timber was used for making tools and weapons, and individual trees were well known to desert dwelling aborigines for they played a vital role in the story of human survival. Heavy dew and the lightest shower of rain deposits water in small cavities and hollows in the trees’ gnarled trunks, and indigenous nomads, using a fine tube of bark as a straw, sucked it out to quench a desperate thirst. The tree’s roots, some of which often lie close to the surface of the soil, also contain a considerable amount of water, and these were cut and sucked to provide additional moisture. It’s a survival strategy that might save a life when death is the only other option, but it’s a practice that modern day nomads would contemplate only in the most dire of circumstances.
The desert oak, which ultimately reaches a height of six to eight metres, can be found in the deserts of eastern Western Australia, in the southern section of the Northern Territory, and in northern South Australia, and is often the only tree species to be found in this arid and sandy region where spinifex is the dominant vegetation.
To preserve vital moisture, the trees have evolved with long segmented branchlets, known as cladodes, that function as leaves and that resemble olive green pine needles. The leaves themselves are nothing more than inconspicuous scales that encircle each joint of the branchlets. As the cladodes fall from the tree, they form a dense brown mat of material that prevents other plants, which would compete for scarce nutrients and water, from growing in the desert oak’s shadow.
Like legumes and their close relatives the she oaks, desert oaks have roots with nodules that contain nitrogen fixing bacteria, and it is this feature that enables them to thrive in sandy soils that have a low level of this essential plant nutrient. The other survival tactic of this slow growing tree is that, in its early years, it develops a vigorous tap root that can reach a depth of 10 metres and is thus able to access any sub-surface water.
Fluffy red or brown flowers are produced in spring, and although these are small and inconspicuous, the cylindrical seed cones that subsequently develop are hard to miss. They are similar to those of casuarinas, but being up to 10cm in length, they are far larger.
When the wind blows across the outback plains, the desert oak’s shaggy foliage mutters with a subtle voice. With fossils revealing that a closely related species thrived in the region when Australia was part of the great land mass of Gondwana, it’s a sound that’s whispered across this ancient land since before the dreamtime, and one that, for many years to come, will alert travellers to the presence of one of nature’s great survivors.
FACTS ABOUT ALLOCASUARINA DECAISNEANA
- It played a vital role in allowing aboriginal people to survive in arid regions.
- Its roots can penetrate the soil to a depth of 10 metres.
- It is the only species of its genus that’s found in Australia’s central desert region.