Why some willows can make you weep
Wednesday, 28 February 2001
Summertime: cricket season; the welcome crack of a willow bat on a leather ball. Pick the wrong willow, however, and all you'd get would be the crack of the bat.
The wrong willow! It seems an improbable place to start a career in ecological research, but there is much in Australian ecology that once caused little concern but is now emerging as an important, even urgent, problem. Willows are among them.
Willows line the banks of the River Murray. For those who live in river towns, fish the Murray's waters or moor their houseboats to the banks in summer, willows are cool and beautiful That can't deny their insidious invasion of the ecology of Australia's largest River.
"Willows are attractive and ornamental, but they tend to dominate river banks," says Susan Gehrig, who is completing a Ph D on river ecology in the Department of Environmental Biology.
"The story goes that willows were planted for navigation, to mark the main channels for river boat navigation back in the 1800s," says Ms Gehrig. "Willows were certainly considered good for shade and shelter, they are a good feed stock, and they stabilise the banks and prevent them from eroding," she says.
"But because they are so shady, they displace the native vegetation completely, and dominate the river banks. If you were to take a boat down the Murray, all you might see is willows along the banks; not the great river red gums - willows displace them, too," she says.
This loss of native vegetation means loss of habitat. Unlike gum trees, willows don't tend to form hollows, so the natural hollows needed by mammals and birds vanish as well. So do food sources, such as gum flowers on which native animals depend. Native and introduced bees are believed to be among the few animals to benefit from willows.
Below water level the story is similar. "Fisherman like the willow roots as they provide shelter for fish," says Ms Gehrig, "And they do attract some fish species which like shady refuges, but when we look at total biodiversity, we lose animals like platypus, tortoises and other fish," she says.
"Many native fish depend on snags provided by river red gum branches," says Ms Gehrig. "Murray Cod need snags for spawning sites. In fact, radio tracking shows that even adults spend about 80% of their time near these woody snags."
Water quantity and quality dominate current concerns about the river. The willows are implicated in both. Because they are shallow-rooted, they take most, if not all, of their water from the topsoil or the river itself, and they appear to take a lot.
"We hope to find out the amount and rate of their water uptake, compared to that of native vegetation," says Ms Gehrig, "But it seems significant, and my research aims to determine whether the amount of water they consume is comparable to that used by irrigators," she says.
Willows add to salinity problems, too. Their shallow roots don't reach down into the saline aquifers which supply the deeply rooted red gums. Red gums usually take 40-50% of their water from these regions, helping to keep the saline water table low. Willows skim only the upper, fresh water layers, allowing salt to rise and spill into the river.
There are actually two kinds of willows. The weeping willow, Salix babylonica, is the tree made familiar by willow-pattern plates. It has a drooping aspect, whereas Salix fragilis is the more upright 'crack' willow. "As the name suggests, it is easily broken," says Ms Gehrig. "If you bend a twig of it, it breaks with a clear crack, which the weeping willow tends not to do," she says.
"River boat captains learnt not to tie their boats to the crack willow because it was fragile and very shallow rooted," says Ms Gehrig. "They would wake to find they were drifting downstream with a large willow in tow." In fact, photographs of river boats tied to willows have helped to date the introduction of both kinds of trees, and identify a clump at Mannum as the oldest on the river.
The earliest weeping willow cuttings are believed to have come from a tree planted by Napoleon's grave. It is the ease by which they can be spread by cuttings that has helped both species dominate the lower Murray.
"If a branch breaks off, floats downstream and sticks in the mud, it takes root and you have another willow that easily," says Ms Gehrig. "Fishermen often break off a twig and stick it in the river bank to hold their line, and that will grow, too," she says.
The ease with which willows spread makes them hard to eradicate, but total eradication is not on the agenda in any case. "It can be very expensive and time consuming," says Ms Gehrig, "and other weedy species can simply replace them."
"It needs a lot of follow-up and revegetation with native species," she says. "Also, lots of towns like to retain the scenic element of their willows, so we need to know the areas of high biodiversity and significance, and concentrate our efforts there."
Ms Gehrig's research will be one of many to be featured in a 30-part radio series on the River Murray to be broadcast in September.
Photos available at: /pr/media/photos/2001/
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