How water works in Australia
Don Burke says recent research proves that everything from lowly weeds thriving through to ancient gum trees dying is affected by how we are using water on properties close to our bushland areas.
With the recent catastrophic floods, all of us have been put in our place by Mother Nature. Dorothea Mackellar clearly nailed Australia when she described it as follows:
“I love a sunburnt country,
A land of sweeping plains,
Of ragged mountain ranges,
Of droughts and flooding rains.”
Peter Andrews (featured on ‘Australian Story’ on ABC-TV some time ago) who wrote the book ‘Back from the Brink’ has brought forward a new understanding of our sunburnt country. He states that our dry, flat land has evolved weird water movements. During rains, water spreads out naturally like a blanket over the land and slowly soaks in. The water pressure created by all of that surface water forces the water down and sideways in the soil, wetting huge areas of soil. Recent scientific research has proven Peter to be correct.
So, shallow floods are a necessary part of survival for our normally parched land. We mucked it all up by bringing in cattle that damaged the surface soil, leading to soil erosion and water being confined to deep ruts in the land (aka rivers). Out at Bourke, the river passes by the land, down about 10m in a steep, eroded rut.
This is really bad for our land, leading to greater aridity, poorer crops and bad times for farmers, native plants and animals. We urgently need a new National Strategy to manage water movement across Australia.
Recent work that I have done sheds even more light on our weird country. It is so weird that you probably won’t believe my observations.
I live in a dry open forest full of scribbly gums (Eucalyptus haemastoma); grey gums (E. punctata), peppermint gums (E. piperita) and red bloodwoods (Corymbia gummifera).
The soil is sandy, made from eroded sandstone and it is always dust-dry: it is water repelling. When organic matter (dead leaves, etc) is broken down by fungi in the soil, the residue from the breakdown coats the sand particles with a waxy coating that causes the water-repellency. So water pools on the surface of the soil and then runs off. No matter how much rain you get or how much watering you do, the soil stays dry.
This pattern seems to occur widely across the dry forest areas of Australia. You would estimate that this potentially translates to as much as 80% or more of our continent. But this doesn’t make sense: soils that shed water like a duck’s back. Yet this is clearly a normal state of affairs in our dry land.
So I set up some experiments to see what happens (see pics below). I hand-watered the ground in undisturbed bushland areas, waited several minutes then dug holes to see what happened. The soil was dust-dry immediately after watering with no signs of where the water went. So I set up sprinklers and followed the rivulets of water to discover that they disappeared down cracks in the soil, ant holes, etc. So the water was bypassing the soil, rather than soaking in. This is a rather ‘Alice in Wonderland scenario’ – disappearing down holes.
Deeper digging led to the discovery that under the dry, sandy soil was a clay layer and it was a layer that seemed to absorb and hold the water. These clay layers under sandy sandstone-derived soils are normal since sandstone rock contains lots of clay. Obviously the sandstone rocks break down to produce sand and clay: the clay then settles lower in the soil horizon.
So in the sandstone areas of open forests, a fungus works to keep the soils dust-dry! This is weird since both fungi and forests need water to survive. How and why would plants and fungi get involved in such a disaster? Yet this disaster is widespread over huge areas in Australia. It does not appear to happen commonly in the rest of the world. How are we different?
My thoughts are that the dust-dry water repelling soils are there on purpose. The fungi and the local plants (trees, shrubs, grasses, etc) must have a symbiotic relationship. It must be that these dry soils act like a thick mulch overlying the moist clay subsoil. I think that this dry, sandy ‘mulch’ prevents the germination and growth of new seedlings of the local native plants. In normal years most young seedlings would perish for lack of water, leaving the sparsely distributed existing trees able to survive. In dry, open forests too many trees, shrubs or grasses would lead to an unstable, over-populated forest that would collapse during dry years.
Presumably, after a tree dies, the water-repellency declines, leading to successful germination of seedlings during a wet year. Those that get their roots down to the moist clay subsoil have a really good chance of survival.
So the dry, sandy soil ‘mulch’ keeps an open forest open. That is, it keeps a sparse forest sparse. These dry soils act as a brake on over-population of plants. This fits in well with what’s called ‘allelopathy’.
Allelopathy is a proven situation where trees drop toxic chemicals into the soil areas around them. These chemicals inhibit the successful germination of competing seedlings around the parent tree. After the tree dies, the chemicals break down and new seedlings emerge to fight for their place in the forest.
So far as I know, these water-repelling soils do not exist in the wetter forest areas with better soils. One would never expect this situation in Victoria’s Dandenongs, for example.
The Bad News
Once I thought about this sort of a dry, water-repelling forest floor, so much more came together. When people build a house in dry, open forests, they immediately start enriching their soils and adding heaps more water. This creates a more evenly moist soil – eventually, after wetting agents, soil cultivation and irrigation are applied. If my theory is correct, you are now creating moist conditions around trees that only ever grow in dust-dry conditions. This neatly explains why existing scribbly gums, bloodwoods, peppermints and other eucalypts get root rot and die in bushland gardens: they simply aren’t adapted to normally moist soil conditions. That is, they have no resistance to root-rotting fungi.
As an arborist and tree surgeon, I spent years of my life giving the last rites to many dry scherophyll plants: eucalypts such as E. haemastoma and E. piperita, corymbias such as C. gummifera and C. eximia plus assorted angophoras. I watched as many banksias, persoonias and grevilleas carked it. All of these were local bushland plants that died after houses and gardens were built around them. Obviously moist, fertile soils are poison for many of these plants. Runoff from gardens, streets or septic tanks kills them all.
The worst of all these things is septic runoff in sandstone soil areas. A plume of toxic soil forms downhill from the absorption trench and it’s ‘goodnight native bushland’. I have seen this many many times. Then, of course, the septic, moist soil guzzlers move in: privet, lantana, asparagus weed, Pittosporum undulatum, etc.
This clearly points to the lunacy of weed groups who try to eradicate the weeds. There is no point in doing this. The idea is to identify and cut off the source of water and nutrients. After some years, native plants can be re-established.
Dry is Normal
So, my theory is that dry soils are part of nature in Australia. They underpin the very stability of many of our forests in dry zones.
If this sounds too weird to accept, consider this: desert spinifex grasses which grow around 60cm tall send roots down as much as 30 metres to get water in the deserts.
What a weird country we live in. It’s much more topsy-turvy than just “Down Under”.
I am indebted to Simon Leake of Sydney Environmental and Soil Laboratory for his help in developing this theory.