Part 3 – The climate killer
‘I have gathered you all here today, to at last reveal the identity of the killer in this intriguing case of Manna Gum murder. Agricultural practices and fire exclusion have each been investigated and ruled out, leaving one final suspect – the climate. It seems that the climate has become drier and more variable, causing stress to trees and making them more susceptible to attack by the eucalyptus weevil. This deadly weapon was used to defoliate the trees, slowly but surely leading to mass decline and death.
However, much of the evidence is circumstantial, and further investigation will be required to uncover exactly how the changes in climate and the insect infestation are interacting to cause the dieback. Unfortunately, we haven’t yet been able to apprehend and arrest the climate – so the killer walks free and could well strike again.’
The Monaro dieback may represent one of the first cases of significant vegetation shift due to climate change in eastern Australia, and this presents some unique problems for managers. Given that there is no evidence of recovery and little regeneration, it seems likely that the Manna Gum will disappear entirely from an area where it has been dominant, which will have a profound effect on the ecosystem.
Much of the Monaro is considered to be naturally treeless or has been cleared since European settlement, so the remaining patches of Manna gum woodland are important refuges for wildlife to move across the landscape. The patches of woodland are also very important as shelter for stock and to prevent erosion, not to mention their aesthetic value.
What can we do?
Current management of dieback usually involves changing land management practices to limit disturbance and increase diversity and resilience. For example, the exclusion of stock to limit nutrient input and soil compaction, reintroduction of native understorey species to attract insect predators, or burning to address nutrient imbalances and encourage regeneration. These strategies are very important for maintaining biodiversity, but the evidence from my research suggests that they would be unlikely to prevent or improve dieback symptoms in this case.
Treatment with stem-injected pesticides has been shown to be successful in preventing insect damage and allowing trees to recover in the short term. This method is very expensive and labour intensive, and is only effective for four years, making it impractical on the large scale of the dieback. However, it may still be useful for protecting selected trees that have particular importance e.g. large old trees that provide important habitat, or seed sources which could contribute to regeneration if conditions become more favourable in future.
One possible approach might be to replace the Manna Gums with another tree species that is better adapted to the new (and future) climatic conditions; for example a species that currently occurs in more arid areas to the north and west. This strategy of ecosystem rehabilitation is known as ‘assisted migration’ or ‘assisted colonisation’, and is a controversial topic of debate amongst ecologists and land managers. Introducing species always carries the risk of causing damage to the ecosystem, but in an already very degraded landscape the benefits may far outweigh the risks. I will explore the concept of assisted migration further in a future post.
A novel landscape
Due to the actions of humans, the Monaro has changed dramatically and irreversibly, creating an entirely new and unfamiliar landscape. Unfortunately, with future climate change this kind of ‘novel landscape’ will become more common. In many cases it will be our decision what kind of landscape we want to create, and this is quite a daunting responsibility. So what do you think? What do we want the Monaro landscape to look like in 50 or 100 years, and how do we get there?
This research is now published:
Ross & Brack (2015) Eucalyptus viminalis dieback in the Monaro region, NSW. Australian Forestry, published online 16 Sep 2015 (http://www.tandfonline.com/eprint/nxuybQ4uQWh2DHvPXgdc/full).