Wungong Catchment Forest Thinning
Killing trees not a good way to get more water
Beth Schultz Conservation Council of WA - September 2009
The Water Corporation is killing trees in the Wungong catchment in an attempt to increase run-off into the Wungong Dam. It hopes to get at least 4-6 GL of water a year from Wungong at a cost of 20 cents/kL.
The Wungong catchment, some 40 km south-east of Perth, has an area of 13,000 ha. It contains 300 ha of pine plantations and plots of exotic trees; 1100 ha of bauxite mining rehabilitation planted with exotic species; 1000 ha of dieback rehabilitation sites planted with exotic species; 1400 ha of bauxite mine rehabilitation planted with native species, mainly jarrah; and 5100 ha of regrowth jarrah forest.
There is no problem with Water Corp replacing pine and other exotic trees with local species or thinning mine site rehabilitation. But there are serious problems with its treatment of regrowth jarrah forest.
Water Corp has started ‘thinning’ in the catchment. But what Water Corp is doing is in fact partial clearing because the intention is to reduce the basal area of retained trees (the area of a cross section of tree trunks at breast height) to 12 m2 per hectare or less. That’s about one-third of the tree density of the original forest that grew there. It had a basal area of 35 m2 per hectare. And the basal area will be kept at the reduced level by repeated killing of unwanted trees.
The killing is done either by felling the trees and poisoning the stumps with Roundup or, mainly, by injecting Roundup into the trunks because that method is cheaper. There are real concerns about the use of Roundup, especially in a water catchment. Felling is only done along roadsides and recreation areas, for aesthetic reasons, where it may be seen by the public. It’s not just jarrah trees that are killed. Marri, sheoak and banksia are also killed. And it’s not just stunted regrowth that gets poisoned. Giant veteran jarrah and marri that are centuries old are poisoned too. Trees from 100mm to over a metre in diameter are targeted.
The regeneration that has taken place in the thinned areas is now growing well – more ‘stems’ than the original forest that has been destroyed. However, these young, fast growing plants use more water than the trees that have been killed. To add insult to injury, the trees are just killed and left standing in the forest. It is a waste of native forest trees and a potential serious fire hazard. There will soon be so much dry ‘fuel’ that a future wildfire could be so intense it would kill the retained trees.
Research shows that thinning jarrah forest increases the impact of Phytophthora dieback. If the disease is already present, its rate of spread accelerates. If it’s not present, it will almost certainly be introduced. Furthermore, while the small increase in salinity that often follows logging and burning is not a problem for human use of water, it may be deadly for the creatures that live in forest streams. All this is bad news for biodiversity in an area within one of the world’s 34 internationally recognised biodiversity hotspots.
The Wungong trial is the thin end of the wedge because Water Corp wants to use this method in other catchments to get up to 10 times the amount of water it hopes to get in Wungong. It has engaged in a deceitful propaganda campaign to try to convince people that killing trees to get more water is a good solution to Perth’s water shortage. A consultant’s report called the project “Forest enhancement through catchment management” and a TV commercial and newspaper advertisements spoke of “removing trees” (they won’t be removed) and “overgrown forest” (the forest is trying to recover from repeated logging). Claims of positive effects on the environment are trotted out with little or no evidence that they will eventuate.
WA has already lost half the pre-European area of native forests. We cannot afford to permanently deplete any of what is left, especially in the face of climate change. The rainfall in WA’s South-West has decreased significantly since the 1970s and will probably keep on decreasing. If it doesn’t rain, there won’t be more run-off no matter how many trees they kill.
We have to look at all our uses of water and make sure we are using this precious gift of nature wisely, carefully and sustainably. Killing trees is not a good way to provide people with more water. The Water Corporation must find more ecologically sustainable sources of water and the community must use much less.
Photographs above: ‘Cut stump’ method of ‘thinning’. The trees are felled and the stumps are treated with roundup. Photo 2: Jarrah forest after it has been ‘thinned’ using the cut stump method.
Last Updated (Wednesday, 16 September 2009 09:21)