By John Eddy

LA TROBE University Honours student Evie Cowling presented her findings from a recently completed 12-month research project at a public meeting at the Koala Conservation Reserve, 16th July 2026. 

In introducing Evie to the audience of about 60 people, Phillip Island Landcare’s Lisa Wangman, and one of Evie’s supervisors, Dr Keith McDougall, explained how concern among the local community about die-back among eucalypts on the Island over recent years had prompted the formation in 2019 of a committee of land managers, scientists and farmers to seek some answers.  

While many theories as to the cause of this mortality had arisen, nothing definite had emerged. Evie’s research, culminating in her Bachelor of Science (Hon) thesis, aimed to clarify what is happening with eucalypt decline, and what factors may be contributing.

She confined her attention to Oswin Roberts Reserve (as representative of much of the Island’s woodlands) and to the 3 main species of eucalypts found there: E. viminalis (Manna Gum), E. ovata (Swamp Gum), and E. globulus (Blue Gum). 

A national problem

Eucalypt decline is a problem across the state, and, indeed, across Australia, with much research being done. Often, findings show not just a single cause for the mortality, but a number of causes. 

There were three phases in Evie’s research: 

a. Desktop mapping of the Reserve using satellite images 2010-2025 

b. Field surveys in the Reserve, covering 60 circles of radius 10m, recording species, status, settings, height and canopy assessment 

c. A glasshouse experiment studying the effects of 7 weeks of drought on seedlings of the 3 species. 

Continued decline

The analysed satellite images, when presented consecutively and with dead trees marked, showed a dramatic but gradual dying-off of eucalypts in the reserve over the 15 years. There was a 17.5% decline in live eucalypt trees over that period. 

Equally disturbing was the finding that no new tree canopy was appearing, so no natural regeneration (recruitment) from seedlings germinating and establishing. Not just big old trees, but more often medium-sized ones, are dying. Often, the dead trees are in clusters, suggesting a local source of stress, such as insect damage, local disease or possum browsing. 

There seemed to be no clear link between the mortality and environmental settings, such as elevation, slope, or aspect confirming findings of earlier investigation, though trees in higher, more exposed sites had less healthy canopies. 

In the glasshouse study of response to drought, Manna Gum seedlings fared the best of the three species. However, in the Reserve canopy health of Manna Gum was rated as poorest of the three, while Swamp Gum and Blue Gum were about equal. 

All three species showed signs of being eaten by insects and possums, but Manna Gums were worst affected. 

Clear plastic collars around the trunks of established eucalypts are used as possum guards. Evie evaluated their effectiveness in reducing mortality, and found they were only of minor benefit for Manna Gums and did not reduce mortality rates in Swamp Gums. 

Recommendations 

1. Results support the view that mortality of eucalypts is resulting from predisposing stresses such as climate change, and other stresses, e.g. herbivory, operating at a local scale. 

2. Tree decline at Oswin Roberts Reserve is on-going, not the result of any one stressful event, such as a drought year, with no clear correlation with topographical settings. 

3. Supplementary planting in Oswin Roberts Reserve will be necessary to ensure eucalypts remain a dominant component of the woodland.

4. For revegetation projects, choose species that are less vulnerable to insects and herbivores such as possums and wallabies. At Oswin Roberts Reserve, Blue Gums seem to be more robust than Manna Gums or Swamp Gums. Peppermint (E. radiata) and Messmate (E. obliqua) are the other indigenous species that may be worth trying in certain areas. Perhaps other nonindigenous eucalypt species would be worth trialling too. 

5. Seedling protection after planting, including adequate guarding against vertebrate herbivores, is essential for preservation of the woodland. 

6. Undertake research into reasons for the lack of natural recruitment of the three main eucalypt species, including the impact of herbivores and nutrient run-off from adjacent farmland. 

7. A lack of diversity in the understorey at Oswin Roberts Reserve was noted and may be contributing to eucalypt mortality in some way, e.g. favouring tree-damaging insects via an ecological shift away from insectivorous birds and predatory invertebrates. It may also impact the availability of soil moisture. 

8. Researching the effect of the depth of the water table on eucalypt survival may also be useful. 

9. Continue to monitor survival of the eucalypts at Oswin Roberts Reserve, using the mapped tree distribution from Evie’s thesis. 

The talk was well received and prompted a series of questions from the audience to conclude the presentation by Evie.