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Wildfire, water and eucalypts

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Dr Rachael Nolan, Research Associate in the School of Life Sciences at the University of Technology Sydney, looks at how bushfires affect our water supply.On the east coast of Australia the water supply reservoirs are mostly found in eucalypt forests, forests that are prone to burning from bushfires. Some 70% of the rain that falls within these forests can go back into the atmosphere, either by evaporation or plant transpiration. What’s left will enter creeks, streams and rivers, making its way to dams and reservoirs.
In 2009, the year of Dr Nolan’s research at The University of Melbourne, Melbourne got most of its drinking water this way. At that time there was severe drought, which prompted Melbourne Water to fund Dr Nolan’s research. The history of that research goes back to the 1939 bushfires that swept across much of Victoria, one consequence being that the water flow from the dams and reservoirs was less than previously.

In the decades following the 1939 bushfires it was thought that the forest cover was sucking up almost twice the usual water, resulting in a dramatic loss of water. That research had a focus on the Mountain ash forests. After the bushfires the mountain ash forest produced masses of seedlings which proved to be the cause for the drop in water feeding into the water supply system. They are the world's largest flowering plant - one reason why they have such an impact on water supply, especially given it can take 100 years for trees to reach full size.


However, Mountain ash forests are not the dominant forest outside of Melbourne’s catchments, so Dr Nolan’s research focused on the more common mixed species eucalypt forests. To measure the plant water usage of trees in this forest type, 1-2mm wide probes were inserted into the part of the tree that conducts water. The sensors put a pulse of heat into the tree which can be tracked. This tracking was timed, giving a measure or how long it took the heated water to travel up the tree, hence its speed.

That speed multiplied by the area of water-the conducting sapwood gives a measure of how much water the whole tree used. In mixed species eucalypt fores, trees are not killed by bushfires, with the thick insulating bark protecting the tree from fire.
The trees recover by growing new foliage, and there is also some seedling germination triggered by fire. This strategy of recovery after fire means that those forests used less water over the 10-15 years that it took for the area to regenerate.

So severe fires that burn these forests do not impact on water supply, unlike mountain ash forests. Surprisingly, less severe fires may have an impact on water supply. This is because much of the canopy is left but the fire still triggers seedling germination and the growth of new foliage, resulting in the forests using more water.

Bushfires are a part of life in Australia, which eucalypts have adapted to. Indeed these trees actually require fire to assist in their regeneration.

Dr Rachael Nolan was interviewed for A Question of Balance by Ruby Vincent. All images are from Dr Nolan and are from her Doctoral research study. Summary text by Victor Barry, August 2015.

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