Ensuring Victorian Forests Survive a Changing Climate

By Dr Catriona Nguyen-Robertson MRSV

This article revisits Professor Patrick Baker’s presentation to the Royal Society of Victoria in March 2021 and discusses where we are two years on.

Approximately 6.4 million hectares of public land in Victoria are covered by forest.¹ But we started out with more – since European Invasion, over 14 million hectares have been cleared, and Victoria’s urban expansion continues to increase the pressure on our forests.¹ We need to protect what we have left.

Forests are essential for the health and wellbeing of all Victorians. They filter the water we drink, purify the air we breathe, buffer the effects of climate change by storing carbon, nourish soil, and are an essential part of history and culture for Victoria’s Traditional Owners and other communities. They are also home to entire ecosystems, providing shelter to Victoria’s diverse array of plants and animals, as well as providing essential resources from timber to food and medicinal plants.

Recently, the Victorian government committed to ending native forest logging in early 2024. In a world that is warming rapidly and relentlessly, the forests, once threatened by overharvesting, are also threatened by fires and droughts. As a Professor of Silviculture and Forest Ecology, Patrick Baker investigates how better forest management practices can ensure our forests are resilient to global warming and extreme weather events.

Threats to our forests

Australia has warmed by just over 1°C since the early 20th century, with most of the warming having occurred within the last 50 years.² This warming has seen an increased frequency of extreme heat events and severity of drought conditions. Patrick warns that the oceans are also steadily heating, which will contribute to future warming of the planet in decades to come – even if we stop all emissions today.

Droughts are becoming longer and more severe. Water availability in the Australian landscape has varied greatly over the past 500 years – and Patrick and his colleagues measure tree rings to determine just how much. The colour and width of tree rings provide snapshots of past climate conditions, dating back centuries. Periods of drought are evident in narrower rings, as trees grow less with less water. Trees in Southeast Australia have experienced drier and wetter periods, with the worst drought in the past 550 years being back in the early 1500s. But Patrick warns that future droughts will inevitably be worse – even of greater magnitude than the Millennium Drought of 2003-2009.³ And, he cautions, we need to prepare our forests for these.

While climate itself is increasingly seen as a disturbance to the landscape, it also drives additional threats such as fire. Intense fires can scar the wood of trees, leaving a historical record of past fires. Patrick studies this record to understand the history of the landscape. In the Australian Alps, for example, fires were not as widespread in the past, nor were they hot enough to put charcoal into the landscape. The charcoal from more recent intense, catastrophic fires is now evident in sediment cores, and trees that were once resilient are now being killed. This has flow-on effects for native animal populations whose habitats are being consumed in the fires, and even if the forests bounce back, it is not fast enough. In addition, climate-sensitive animals retreat to higher elevations and suitable climates. Threatened animals, such as the greater glider, are being forced to retreat to increasingly limited areas.⁴

^‍Another factor to consider in forest management is native forest logging. In November 2022, the Supreme Court of Victoria found that the state-owned logging company VicForests had failed to protect threatened greater gliders and yellow-bellied gliders – and that its methods to carry out surveys for them prior to logging an area were inadequate.⁵ In a separate case a month earlier, the Court ruled that the harvesting and regeneration burning practices of VicForests had likely led to the loss of ‘significant numbers of mature Tree Geebungs’.⁶ These judgements triggered a chain of events, including the shutdown of Australia’s last white paper mill in the Latrobe Valley and the Victorian Government announcing the end of native forest logging in May this year, six years earlier than originally planned. However, Patrick is concerned as to what this will mean for the future of forest management given that this decision to stop native forest logging did not articulate whether there will still be active forest management in those areas going forward.

Managing our forests to withstand these threats

Patrick investigates ways that forest management can help buffer threats. He compared forest areas with different tree coverage and found that areas with lower tree density produced larger, more vigorous trees that were better able to survive limited water availability during the Millennium drought (2003-2009).⁷ When packed closely together, there are fewer resources available per tree, which limits the availability of any individual tree to invest in new roots and leaves. This makes the trees more vulnerable to extreme climatic conditions.

A tree’s size and the age of a forest stand (a community of sufficiently uniform trees) also influence resilience to bushfires. One study revealed that older Mountain Ash stands have a reduced risk of losing their canopy in fires, while those between the ages of 7-36 were at the highest risk of scorching and having their canopies being consumed by flames.⁸ Many studies from Australia and elsewhere show that larger trees tend to fare better than smaller trees in fires.

However, tree species also differ in their susceptibility to fire. Studying native tree species responses after the 2009 bushfires, Patrick’s team found that Eucalyptus obliqua trees essentially cannot be eliminated by fire once their diameter is greater than ~30 cm.⁹ Species that are more susceptible to fire have to be larger – often much larger – to have a chance of surviving fires. Patrick advises that we need to ensure that at least some of the trees in a stand will grow faster so that they can reach “fire-safe” sizes faster. Shifting to a landscape that supports a greater proportion of more fire-resistant species and thinning denser population may help to reduce the impacts of fires.

Keeping track of progress

In an era of rapidly changing landscapes and climate, it is important to observe forests over time. Patrick’s research team spent 15 months documenting the size, species, and location of 51,324 individual trees over 16 hectares - “The Great Tree Census of 2023”.¹⁰ The project established a forest dynamics plot near Starvation Creek, the first Australian plot in the Smithsonian Institution’s Forest Global Earth Observatory (ForestGEO), a network of large-scale, long-term studies that monitor the dynamics of forests around the world.Victoria’s unique tree flora and its variable – and at times extreme – climate have created complex forests with complex histories that had not yet been monitored at this scale and level of detail.

Our forests are dominated by Acacia and Eucalyptus species, and the Starvation Creek plot in the Central Highlands of Victoria included 47 species of trees and woody shrubs.¹⁰ The eucalypts were particularly diverse, with 13 species occurring in close proximity – and in some places as many as eight eucalypts within a 20 x 20 m² area. This raises a new question of why so many eucalypt species coexist in the Australian landscape, as well as other questions about the dynamics of our remarkable forests that Patrick and his research team will continue to investigate.

Patrick also specifically studies old-growth forests. These support many plant, animal, and insect species; they provide hollows for nests and coarse, woody debris as food, they protect soils, and they can supply more water runoff than younger forests.

Old-growth forests are defined differently depending on the forest type and the region. As they are defined in Victoria, old-growth forests are rare: the area of old-growth Mountain Ash forests has declined to 1.16% of ash-type forests in the Central Highlands.¹¹ Patrick’s team recently used remote sensing data to map all old-growth and high-conservation value forests in the Central Highlands and East Gippsland. This revealed that there is more than twice as much old-growth forest than previously estimated – although at approximately 2.5% of the landscape, this is still much less than pre-Invasion levels.

Furthermore, each vegetation type has its own ecological characteristics and age structures that need to be considered rather than simply using a one-size-fits-all approach. These variations raise important issues about how current definitions of old-growth forest may distort how and what we protect.

To ‘spread risk across the landscape’, Patrick believes we need more forests with large, old trees, rather than large areas of dense trees that are all the same age. This would more closely align with the impacts of natural fires and would increase the resilience of Victoria’s forests to many of the threats that climate change will throw at them in the coming decades. Climate events are shaping our forests, and with better forest management, we can help make them more resilient.

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You can watch Professor Patrick Baker’s presentation at: youtu.be/gRVJWXt__Vo

‍Patrick Baker is a Professor of Silviculture and Forest Ecology at The University of Melbourne. This piece appears in the September 2023 edition of Science Victoria magazine. All issues can be read online for free at rsv.org.au/Science-Victoria.

‍References:

1. Commissioner for Environmental Sustainability. (2018). State of the Forests Report 2018. ces.vic.gov.au/sites/default/files/publication-documents/State%20of%20the%20Forests%202018%20Report.pdf
2. GISTEMP Team. 2023. GISS Surface Temperature Analysis (GISTEMP), version 4. NASA Goddard Institute for Space Studies. Accessed 12 August 2023. data.giss.nasa.gov/gistemp/
3. Cook, B.I., et al. (2016). The paleoclimate context and future trajectory of extreme summer hydroclimate in eastern Australia. JGR Atmospheres, 121(21), 12,820-12,838. doi.org/10.1002/2016JD024892
4. Wagner, B., et al. (2020). Climate change drives habitat contraction of a nocturnal arboreal marsupial at its physiological limits. Ecosphere, 11(10). doi.org/10.1002/ecs2.3262
5. Environment East Gippsland Inc v VicForests (No 4) [2022] VSC 668 (4 November 2022). (2022). Supreme Court of Victoria. austlii.edu.au/cgi-bin/viewdoc/au/cases/vic/VSC/2022/668.html
6. Warburton Environment Inc v VicForests (No 5) [2022] VSC 633 (25 October 2022). (2022). Supreme Court of Victoria. austlii.edu.au/cgi-bin/viewdoc/au/cases/vic/VSC/2022/633.html
7. Horner, G. J., et al. (2009). Mortality of developing floodplain forests subjected to a drying climate and water extraction. Global Change Biology. 15(9), 2176-2186. doi.org/10.1111/j.1365-2486.2009.01915.x
8. Taylor, C., et al. (2014). Nonlinear Effects of Stand Age on Fire Severity. Conservation Letter. 7(4), 355-370. doi.org/10.1111/conl.12122
9. Trouve, R., et al. (2021). The effect of species, size, and fire intensity on tree mortality within a catastrophic bushfire complex. Ecological Applications, 31(6). doi: 10.1002/eap.2383
10. Baker, P. (3 June 2023). The Great Tree Census of 2023. Pursuit. pursuit.unimelb.edu.au/articles/the-great-tree-census-of-2023
11. Lindenmayer, D. & Bowd, E. (2022). Critical Ecological Roles, Structural Attributes and Conservation of Old Growth Forest: Lessons from a Case Study of Australian Mountain Ash Forests. Frontiers in Forests and Global Change, 5. doi.org/10.3389/ffgc.2022.878570