Trapping Desert Water; mulga and devil

To survive in an arid land where it might not rain meaningfully for years requires strategies. Many larger animals are nomads, following the rains across the landscape; Red Kangaroos and many desert birds do this. Many smaller ones escape into burrows, even going into a torpor until the day is right - desert frogs are specialists, spending most of their life dozing in a muddy cocoon while waiting for the rains. 

Plants don't have that option - though many actually do something similar, surviving the dry times as a buried seed, and sprouting and flowering quickly when the rains come. Trees don't have this luxury - it takes too long to grow to maturity and flower - and in Australia none is more typical of the inland vastnesses than the Mulga Acacia aneura, whose woodlands dominate an astonishing 20% of the huge country, well over 1.5 million square kilometres.

Mulga plains to the horizon; from Chambers Pillar, Northern Territory (above)
and near Quilpie, south-west Queensland (below).

The red soils in both photos above are typical of the mulga lands, ancient leached infertile material eroded over vast periods of time from old ranges, now gone entirely or ground down to their bases. Mulgas, like all acacias, can deal with the lack of nutrient in the soil with the help of colonies of symbiotic bacteria in lumpy nodules on the roots; these bacteria can convert atmospheric nitrogen directly into soluble salts that the plant can use. No plant can perform that trick unaided. 

Fighting for the scarce rains is another matter however, and Mulga is an excellent fighter. The phyllodes (modified leaf stems which are better at conserving water than true leaves, a useful trick by many species of wattles and apparently some peas) are held vertically, to reduce overheating; the silver-grey colour is another heat-reducing factor.

Flowering Mulga near Windorah, south-west Queensland; it takes a reasonable water
supply to allow a tree to flower on soil like this.
Note the erect foliage.

However, this erect foliage plays a role in water harvesting as well, in conjunction with the typical Mulga form, which is like an inverted cone.

Mulga and spinifex on sandstone, plateau above Palm Valley.

This form is an excellent one for trapping rain, which is directed down phyllodes and along branches to the ground near the base of the tree - in the case of a large tree, to within 50cm of the base, closer for younger plants. In this way a 25mm rainfall can deliver the equivalent of 150mm of rain to the Mulga's roots! A Mulga tap root can go down for metres to access deep cool moist soil, where the water it has harvested can be stored. In addition there is a net of shallow roots stretching at least to the diameter of the canopy; I've heard it said that a 10cm sapling can have a root spread of three metres. There's not much future for plants growing under a Mulga's canopy!

One animal, also a truly deserving symbol of the Australian drylands, also uses its structure to bring water to where it can absorb it - which in its case means its mouth, rather than roots!

Never was animal more inappropriately name - both in the vernacular and scientifically - than the gentle utterly harmless Thorny Devil Moloch horridus. Moloch was a particularly unpleasant god of the ancient middle east, who required child sacrifices. 'Harmless' might be contested if you're a small ant however, as the 10cm long lizard can scoff up to 5,000 of them at a time; perhaps it was an ant who named it. With its upcurved tail, slow rocking gait and rose-like thorns it is a truly odd-looking little animal, found right across the dry inland.

Thorny Devil, Alice Springs Desert Park.

One of its most remarkable characteristics though is not readily visible without closer examination. Its whole body is covered with a network of fine canals, arranged so that any moisture landing anywhere on its skin permeates slowly but inevitably to its mouth. This may be from rain, or dew, or just standing with a foot in a puddle! However none of these water sources would seem to require such an elaborate mechanism; in fact the system really comes into its own when there is no surface water, but simply moisture in the air. Then the thorns pay their way - water droplets condense on their tips, and end up via the skin channels in the lizard's mouth.

Two remarkable strategies for survival in a very harsh land. Isn't it a wonderful world??

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