Receding gums: What ails Australia’s iconic trees?

By Wendy Zukerman 

Eucalyptus trees are dying all over Australia. To save them, we might have to learn to play with fire

A friendly fire might help (Image: Melanie Stetson Freeman/Getty)  A friendly fire might help (Image: Melanie Stetson Freeman/Getty)

Playing with fire

PICTURE an Australian landscape and the scene you conjure up will almost certainly be one graced by gum trees. Eucalyptus has colonised just about every corner of the country, from the forests that fringe the sandy beaches of Australia’s southern shores, to the baking heat of the outback.

Which makes it all the more alarming that across the country, swathes of gum trees are dying without obvious cause. In forests from Western Australia to the island state of Tasmania, trees that should be living for more than 400 years are mysteriously dying before reaching their 100th birthday.

If the gums are lost, the outcome would be dire for a whole suite of plants and animals. "Old trees are keystone species that support a disproportionate amount of species," says Dugald Close, a plant ecologist at the University of Tasmania. Gum leaves feed local birds and their hollows provide shelter for endangered species such as Leadbeater’s possum.

The premature death of gum trees was noted as far back as the 1970s, when eucalypts started succumbing to a disease known as Mundulla Yellows. The leaves of affected trees would gradually turn yellow, until the tree finally died. It took until 2004 for the culprit to be identified – lime, or calcium hydroxide, used in road-making, was washing into nearby eucalypt forests, making the soil alkaline (Australasian Plant Pathology, vol 36, p 415). Eucalyptus tree roots need neutral or acidic soil to suck up nutrients such as iron and manganese. When iron was injected back into affected trees, they made a full recovery.

Today’s dieback extends far beyond just a few roadside trees. By 2000, large tracts of eucalypts were dying along Australia’s east coast. In many places, the decline coincided with a booming population of native birds called bell miners, and their partners in crime, sap-sucking native insects called psyllids.

These insects sit on the vein of tree leaves, sucking their sap and excreting a sugary solution known as lerp. "The bell miners suck off the lerp, but leave the insect, which keeps working its way around the leaf," says Paul Meek, executive officer of the bell miner associated dieback (BMAD) working group. In return for their sugary meal, the aggressive bell miners ward off any other bird or insects that would eat the psyllid itself.

This duo are long-time inhabitants of Australian forests, but disturbances to the ecosystem, such as removing trees through logging, can seemingly tip the balance dramatically in their favour. Logging opens up the forest canopy, allowing growth of the understorey of shrubs and other low-level plants where the bell miners nest.

Meek’s team has confirmed that 187,000 hectares of eucalyptus forest are dying from BMAD, but the real figure could much higher. In New South Wales alone, up to 2.5 million hectares of forest are wasting away.

However, Vic Jurskis of the Institute of Foresters of Australia says tree foliage can start looking sick before any sign of psyllids or bell miner outbreaks – or, indeed, any other obvious factor that might kill the trees. So it looks like the problem runs deeper than bell miners.

The mystery has revived a radical old theory, proposed over 40 years ago by ecologist William Jackson of the University of Tasmania. According to one of his former students, David Bowman, an ecologist also at the University of Tasmania, Jackson presented his work to the Ecological Society of Australia in 1968. Bushfires had ripped through his home town of Hobart, Tasmania, in 1967, prompting questions over frequency of natural fires.

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