Receding gums: What ails Australia’s iconic trees?
16 September 2010 by Wendy Zukerman
New Scientist Magazine issue 2777
Eucalyptus trees are dying all over Australia. To save them, we might have to learn to play 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.
Jackson proposed that the number of fires sweeping through the bush dictate what type of vegetation grows there. Very frequent fires favour grassland, an intermediate number would support eucalyptus forest, while infrequent fires produce a forest of dense, shade-tolerant plants, an ecosystem dubbed the “dry rainforest” for its structural similarity to the classical tropical rainforest. So, according to Jackson’s theory, without regular bushfires, many eucalyptus forests will die out, leaving species-poor dry rainforests devoid of gums.
The idea might have seemed radical at the time, but four decades later, the scenario that Jackson predicted is exactly what now seems to be happening across Australia, says Close. In parts of Australia’s south coast, free of forest fires for at least 90 years, she-oaks (Allocasuarina) have choked out the manna gum (Eucalyptus viminalis) and swamp gum (Eucalyptus ovata). The same is happening to the Tuart trees (Eucalyptus gomphocephala) in the Yalgorup National Park in Western Australia. Since fires were suppressed in 1976, the West Australian peppermint tree (Agonis Flexuosa) has flourished at the expense of the gums (Botanical Review, vol 75, p 191).
Why would fires be so important to lowland gum forests? Without low-level fires certain gum trees can’t thrive, so the theory goes, because those are the conditions in which they have evolved. In the past, lightning strikes would have triggered fires, and we know from anecdotal and archaeological evidence that Australia’s indigenous people also practised regular burning for millennia. Across swathes of the country, indigenous Australians used fire to improve access across forests, encourage plant growth, and remove leaf litter to avoid uncontrollable wildfires during long hot summers. But since European settlement these practises have largely stopped, and the gums are beginning to pay the price.
There are many reasons why gums rely on fire. In the absence of regular fires, dry rainforest species grow up beneath the eucalyptus canopy, competing with the gums for water – and winning. These species also contribute a thick litter layer that changes the soil chemistry, further stressing the gums.
This situation is not unique to Australia – a similar phenomenon is happening to the North American pine, Pinus ponderosa. Before 1900 there was a long history of frequent, low-intensity fires in Yosemite National Park, but after 1900 the fires stopped. As a result, pines in this region are dying prematurely – coinciding with a growth in “midstorey” vegetation.
The question is, should we intervene in the process by reintroducing regular burning, or let nature take its course? That is a question for policy-makers, not scientists, says Close. “We aren’t saying [Jackson’s theory] is good or bad, just that it’s an explanation for the forest decline that we are seeing across Australia.” However, the dry rainforests emerging throughout Australia are not thriving with biodiversity, Meek points out. “You are getting particular rainforest elements coming in,” he says, resulting in “very simple ecosystems.”
Reintroducing regular burning should also help control bell miners, says Meek. His BMAD working group is now experimenting with low-intensity fires on small plots of land around south-east Australia, hoping to remove the dry-rainforest species in which the bell miners nest.
Over-burning can be just as damaging as not burning enough, though, says David Lindenmayer at the Australian National University in Canberra. “There are different forests, with different climates and different historical fire regimes,” he says. Over-burning is responsible for gum dieback in some areas of Australia, such as the extremely dry Kakadu region, where frequent low-level fires are killing savannah gums which haven’t adapted to these blazes. In southern Australia’s wet alpine forests it’s a similar story. There is one natural major fire every 100 to 300 years, says Lindenmayer. “It is extremely detrimental to be burning there.”
So saving the eucalypt will mean finding the fire regime that was in place before white settlement, and mimicking it (Biological Conservation, vol 143, p 1928). In many regions, this knowledge has been lost, but Lindenmayer and other ecologists are taking a multi-pronged approach to reconstruct it, from examining archaeological evidence, to theoretical modelling, to experimenting with different burning patterns.
After all, who can picture an Australia without its forests of gum trees?
Wendy Zukerman is an Asia Pacific reporter for New Scientist