A PROBLEM IN THE TREE-TOPS
One-time Grafton resident, Henry Kendall, wrote those lines in his poem ‘Bellbirds’, in celebration of the beautiful call of the Bell Miner. This call is one of the distinctive sounds of the Australian bush in South East Australia, and it used to be only heard in moist gully forests. Bell Miners are honeyeaters, which live in large family groups, similar to their close relatives, the Noisy Miners.
But something is wrong in the forests – magnificent trees are dying back, often without any apparent reason. And the range of the Bell Miner is expanding.
In 1995, when Toonumbar National Park was created west of Kyogle, conservationists and park rangers were shocked to see that the park contained graveyards of dead trees standing above a sea of lantana. And all around Bell Miners were calling. Similar reports came from the Watagan Mountains and elsewhere.
This decline became known as Bell Miner Associated Dieback (BMAD). The Bell Miners are incriminated by this name but are they part of the problem or merely the messenger?
This is the one of the question that a multi-agency working group has been set up to investigate. The other key question is how the forests can be managed back to health.
It is a large-scale problem identified throughout the Bell Miner’s range – from south-west Victoria to south-east Queensland. An estimated 2.5 million hectares of tall forest are at risk on both public and private lands – Blue Gums, Ironbarks, White Gums and Grey Gums are highly susceptible, but even Spotted Gum can be affected. Because all of the forest values are under threat, the working group is a unique alliance of timber, conservation and beekeeping interests, united by the common aim of restoring forest health.
BMAD is a dieback of the tree canopy thought to be directly due to increased numbers of scale insects known as psyllids. These and other sap suckers literally suck the life force from the trees, first depleting the canopy foliage and then attacking the softer epicormic growth which the stressed trees produce (this is the short, bushy growth you often see on gum trees after fire). A tree may have enough energy stores to support two or three cycles of epicormic growth but then they give up and die.
The psyllids are native and are doing what they have always done – so what has created the ecological imbalance to prompt dieback, and why has it only been a serious problem since the 1980s?
This is where things become unclear – there are a lot of unknowns (both known and unknown) in this complex problem. What is known is the strong correlation, not only between dieback and high numbers of Bell Miners, but also with high soil nitrogen levels and a thick understorey (of lantana, blackberry or native vines).
High soil nitrogen fertilises the trees, producing more nutrition for the psyllids feeding on the leaves. The psyllids produce a honey-like ‘lerp’ which is a large part of the diet for the Bell Miners. These aggressively exclude other birds from the area, including those that would eat the psyllids (as well as the lerp). The thick understorey gives good nesting habitat for the Bell Miners as well as increasing soil nitrogen. Positive feedback loops are evident, which escalate the problem.
The thick understorey often grows after disturbances such as logging and fragmentation. As its growth can be checked by fire, the lack of burning is often claimed to be the problem. But, since there are no old-growth forests suffering BMAD, disturbance may be the root cause. Certainly, in recent decades, forestry agencies have devoted fewer resources to weed control and rehabilitation of logged areas.
The working group, assisted by a scientific reference committee, has started adaptive management programs to work out how to solve the problem. Lantana control trials, using chemical herbicides and fire, are currently underway on the north coast. On the south coast, Bell Miners are being culled from an area of forest. The results will be monitored and used to inform future management options.
So, next time you hear Bell Miners, look up and see how the canopy is going. It may be time to start worrying. Further information can be found here.
-Janet
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