The 2009 Black Saturday fires burned 437,000 hectares of Victoria, including tens of thousands of hectares of Mountain Ash forest.
As we approach the tenth anniversary of these fires, we are reminded of their legacy by the thousands of tall Mountain ash “skeletons” still standing across the landscape. Most of them are scattered amid a mosaic of regenerating forest, including areas regrowing after logging.
But while we can track the obvious visible destruction of fire and logging, we know very little about what’s happening beneath the ground.
In a new study published in Nature Geoscience, we investigated how forest soils were impacted by fire and logging. To our surprise, we found it can take up to 80 years for soils to recover.
Decades of damage
Soils have crucial roles in forests. They are the basis for almost all terrestrial life and influence plant growth and survival, communities of beneficial fungi and bacteria, and cycles of key nutrients (including storing massive amounts of carbon).
To test the influence of severe and intensive disturbances like fire and logging, we compared key soil measures (such as the nutrients that plants need for growth) in forests with different histories. This included old forests that have been undisturbed since the 1850s, forests burned by major fires in 1939, 1983 and 2009, forests that were clearfell-logged in the 1980s or 2009-10, or salvage-logged in 2009-10 after being burned in the Black Saturday fires.
We found major impacts on forest soils, with pronounced reductions of key soil nutrients like available phosphorus and nitrate.
A shock finding was how long these impacts lasted: at least 80 years after fire, and at least 30 years after clearfell logging (which removes all vegetation in an area using heavy machinery).
However, the effects of disturbance on soils may persist for much longer than 80 years. During a fire, soil temperatures can exceed 500℃, which can result in soil nutrient loss and long-lasting structural changes to the soil.
We found the frequency of fires was also a key factor. For instance, forests that have burned twice since 1850 had significantly lower measures of organic carbon, available phosphorus, sulfur and nitrate, relative to forests that had been burned once.
Sites subject to clearfell logging also had significantly lower levels of organic carbon, nitrate and available phosphorus, relative to unlogged areas. Clearfell logging involves removing all commercially valuable trees from a site – most of which are used to make paper. The debris remaining after logging (tree heads, lateral branches, understorey trees) is then burned and the cut site is aerially sewn with Mountain Ash seed to start the process of regeneration.
Logging compounds the damage
The impacts of logging on forest soils differs from that of fire because of the high-intensity combination of clearing the forest with machinery and post-logging “slash” burning of debris left on the ground. This can expose the forest floor, compact the soil, deplete soil nutrients, and release large amounts of carbon dioxide into the atmosphere.
Predicted future increases in the number, frequency, intensity and extent of fires in Mountain Ash forests, coupled with ongoing logging, will likely result in further declines in soil nutrients in the long term. These kinds of effects on soils matter in Mountain Ash forests because 98.8% of the forest have already been burned or logged and are 80 years old or younger.
To maintain the vital roles that soils play in ecosystems, such as carbon storage and supporting plant growth, land managers must consider the repercussions of current and future disturbances on forest soils when planning how to use or protect land. Indeed, a critical part of long-term sustainable forest management must be to create more undisturbed areas, to conserve soil conditions.
Specifically, clearfell logging should be limited wherever possible, especially in areas that have been subject to previous fire and logging.
Ecologically vital, large old trees in Mountain Ash forests may take over a century to recover from fire or logging. Our new findings indicate that forest soils may take a similar amount of time to recover.
Giant eucalypts play an irreplaceable part in many of Australia’s ecosystems. These towering elders develop hollows, which make them nature’s high-rises, housing everything from endangered squirrel-gliders to lace monitors. Over 300 species of vertebrates in Australia depend on hollows in large old trees.
These “skyscraper trees” can take more than 190 years to grow big enough to play this nesting and denning role, yet developers are cutting them down at an astounding speed. In other places, such as Victoria’s Central Highlands Mountain Ash forests, the history of logging and fire mean that less than 1.2% of the original old-growth forest remains (that supports the highest density of large old hollow trees). And it’s not much better in other parts of our country.
David Lindenmayer explains how these trees form, the role they play – and how very hard they are to replace.
State governments are poised to renew some of the 20-year-old Regional Forest Agreements (RFAs) without reviewing any evidence gathered in the last two decades.
The agreements were first signed between the federal government and the states in the late 1990s in an attempt to balance the needs of the native forest logging industry with conservation and forest biodiversity.
It’s time to renew the agreements for another 20 years. Some, such as Tasmania’s, have just been renewed and others are about to be rolled over without substantial reassessment. Yet much of the data on which the RFAs are based are hopelessly out of date.
Concerns about the validity of the science behind the agreements is shared by some state politicians, with The Guardian reporting the NSW Labor opposition environment spokeswoman as saying “the science underpinning the RFAs is out of date and incomplete”.
New, thorough assessments are needed
What is clearly needed are new, thorough and independent regional assessments that quantify the full range of values of native forests.
Much of the information underpinning these agreements comes largely from the mid-1990s. This was before key issues with climate change began to emerge and the value of carbon storage in native forests was identified; before massive wildfires damaged hundreds of thousands of hectares of forest in eastern Australia; and before the recognition that in some forest types logging operations elevate the risks of crown-scorching wildfires.
The agreements predate the massive droughts and changing climate that have affected the rainfall patterns and water supply systems of southwestern and southeastern Australia, including the forested catchments of Melbourne.
It’s also arguable whether the current Regional Forest Agreements accommodate some of the critical values of native forests. This is because their primary objective is pulp and timber production.
Yet it is increasingly apparent that other economic and social values of native forests are greater than pulp and wood.
To take Victoria as an example, a hectare of intact mountain ash forests produces 12 million litres more water per year than the same amount of logged forest.
The economic value of that water far outstrips the value of the timber: almost all of Melbourne’s water come from these forests. Recent analysis indicates that already more than 60% of the forest in some of Melbourne’s most important catchments has been logged.
The current water supply problems in Cape Town in South Africa are a stark illustration of what can happen when natural assets and environmental infrastructure are not managed appropriately. In the case of the Victorian ash forests, some pundits would argue that the state’s desalination plant can offset the loss of catchment water. But desalination is hugely expensive to taxpayers and generates large amounts of greenhouse emissions.
A declining resource
Another critical issue with the existing agreements is the availability of loggable forest. Past over-harvesting means that much of the loggable forest has already been cut. Remaining sawlog resources are rapidly declining. It would be absurd to sign a 20-year RFA when the amount of sawlog resource remaining is less than 10 years.
This is partially because estimates of sustained yield in the original agreements did not take into account inevitable wood losses in wildfires – akin to a long-distance trucking company operating without accident insurance.
Comprehensive regional assessments must re-examine wood supplies and make significant reductions in pulp and timber yields accordingly.
The inevitable conclusion is that the Regional Forest Agreements and their underlying Comprehensive Regional Assessments are badly out of date. We should not renew them without taking into consideration decades of new information on the value of native forests and on threats to their preservation.
Australia’s native forests are among the nation’s most important natural assets. The Australian public has a right to expect that the most up-to-date information will be used to manage these irreplaceable assets.
Great cities need trees to be great places, but urban changes put pressure on the existing trees as cities develop. As a result, our rapidly growing cities are losing trees at a worrying rate. So how can we grow our cities and save our city trees?
Tree bonds have recently been proposed by Stonnington City Council as a way to stop trees being destroyed in Melbourne’s affluent southeastern suburbs.
Tree bonds are a common mechanism for protecting trees on public land, but have so far had limited use on private land. A tree bond requires a land developer to deposit a certain amount of money with the local authority during development. If the identified tree or trees are not present and healthy after the development, the funds are forfeited.
The size of the bond can be established based on estimated tree replacement costs, and/or set at a level that is likely to achieve compliance (likely to be thousands or tens of thousands of dollars).
Why are trees important in cities?
The concept of an “urban forest” includes all the trees and plants in cities. This includes tree-lined city streets as well as parks, waterways and private gardens. The urban forest contributes substantially to the quality of life of all urban dwellers, both human and non-human, and is increasingly used to adapt cities to climate change.
There is growing research evidence for the physical, mental and social health benefits of urban trees and green spaces. Many local councils such as Brimbank and Melbourne are investing substantially in tree planting to increase these benefits.
However, despite new tree planting on public land, tree canopy on private land is declining.
What can we do to protect trees?
There are a range of existing policy and land use planning measures focused on landscaping requirements for new development. Recently, the Victorian government introduced minimum mandatory garden area requirements. Some Melbourne councils, including Brimbank and Moreland, have also included planning scheme requirements for tree planting for multi-dwelling developments.
Other mechanisms for protecting urban trees on private land include heritage and environmental overlays within local planning schemes, and listings of significant trees and heritage trees.
However, penalties, monitoring and enforcement of tree protection bylaws have not kept pace with the pressures of urban change.
If penalties are insignificant relative to development profits, developers can easily absorb the costs. If monitoring is weak and removal has a good chance of going undetected, tree protection is more likely to be ignored. And if enforcement is weak, or there is a history of successful appeal or defeat of enforcement, many trees may be at risk of removal.
Even when it is successfully pursued, after-the-fact planning enforcement action is a particularly unsatisfactory recourse for tree removal. Replacement trees may take decades to match the quality of mature trees that were removed. What is needed, then, are mechanisms that prevent tree removal in the first place.
Increasing use of tree bonds
The advantage of tree bonds is that they place the onus of proof of retention on developers, rather than the onus of proof of removal on local councils. If a tree is removed, the mechanism is already in place to monitor (the developer needs to demonstrate the tree is still there) and penalise (the financial penalty is already with the enforcing body).
However, tree bonds still do not guarantee tree protection. Some mechanisms used to impose tree bonds may be vulnerable to challenge. For example, historically in Victoria, the planning appeals body VCAT has struck out conditions imposing tree bonds, arguing that punitive planning enforcement measures should be used where trees are removed.
Even where bonds can be imposed and enforced, developers may still be able to demonstrate that trees are unsafe or causing infrastructure damage, and thus need to be removed. In these circumstances, it is often hard to prove otherwise once the tree has been removed.
Nurturing an urban forest
Ultimately, if a landowner is hostile to a tree on their land, that tree’s health and survival can be imperilled, whether through illegal removal, neglect, or applications for removal based on health and safety grounds. It is therefore important that building layout and design realistically allow space for trees to flourish and be valued by landowners.
The urban forest needs protecting and enhancing. This calls for a range of policy mechanisms that work together to retain mature trees, maintain adequate spacing around them, and encourage residents to value and protect the trees around their homes.
Tree bonds provide an attractive solution for local governments in the absence of a strong land use policy framework for protecting trees.
Joe Hurley, Senior Lecturer, Sustainability and Urban Planning, RMIT University; Dave Kendal, Senior Lecturer in Environmental Management, University of Tasmania; Judy Bush, Postdoctoral Research Fellow, Clean Air and Urban Landscapes Hub, University of Melbourne, and Stephen Rowley, Lecturer in Urban Planning, RMIT University
A new global analysis of the distribution of forests and woodlands has “found” 467 million hectares of previously unreported forest – an area equivalent to 60% of the size of Australia.
The discovery increases the known amount of global forest cover by around 9%, and will significantly boost estimates of how much carbon is stored in plants worldwide.
The new forests were found by surveying “drylands” – so called because they receive much less water in precipitation than they lose through evaporation and plant transpiration. As we and our colleagues report today in the journal Science, these drylands contain 45% more forest than has been found in previous surveys.
We found new dryland forest on all inhabited continents, but mainly in sub-Saharan Africa, around the Mediterranean, central India, coastal Australia, western South America, northeastern Brazil, northern Colombia and Venezuela, and northern parts of the boreal forests in Canada and Russia. In Africa, our study has doubled the amount of known dryland forest.
With current satellite imagery and mapping techniques, it might seem amazing that these forests have stayed hidden in plain sight for so long. But this type of forest was previously difficult to measure globally, because of the relatively low density of trees.
What’s more, previous surveys were based on older, low-resolution satellite images that did not include ground validation. In contrast, our study used higher-resolution satellite imagery available through Google Earth Engine – including images of more than 210,000 dryland sites – and used a simple visual interpretation of tree number and density. A sample of these sites were compared with field information to assess accuracy.
Given that drylands – which make up about 40% of Earth’s land surface – have more capacity to support trees and forest than we previously realised, we have a unique chance to combat climate change by conserving these previously unappreciated forests.
Drylands contain some of the most threatened, yet disregarded, ecosystems, many of which face pressure from climate change and human activity. Climate change will cause many of these regions to become hotter and even drier, while human expansion could degrade these landscapes yet further. Climate models suggest that dryland biomes could expand by 11-23% by the end of the this century, meaning they could cover more than half of Earth’s land surface.
Considering the potential of dryland forests to stave off desertification and to fight climate change by storing carbon, it will be crucial to keep monitoring the health of these forests, now that we know they are there.
Climate policy boost
The discovery will dramatically improve the accuracy of models used to calculate how much carbon is stored in Earth’s landscapes. This in turn will help calculate the carbon budgets by which countries can measure their progress towards the targets set out in the Kyoto Protocol and its successor, the Paris Agreement.
Our study increases the estimates of total global forest carbon stocks by anywhere between 15 gigatonnes and 158 gigatonnes of carbon – an increase of between 2% and 20%.
This study provides more accurate baseline information on the current status of carbon sinks, on which future carbon and climate modelling can be based. This will reduce errors for modelling of dryland regions worldwide. Our discovery also highlights the importance of conservation and forest growth in these areas.
The authors acknowledge the input of Jean-François Bastin and Mark Grant in the writing of this article. The research was carried out by researchers from 14 organisations around the world, as part of the UN Food and Agriculture Organization’s Global Forest Survey.
Kelp forests along some 100km of Western Australia’s coast have been wiped out, and many more areas damaged, by a marine heatwave that struck the area in 2011.
The heatwave, which featured ocean temperatures more than 2℃ above normal and persisted for more than 10 weeks, ushered in an abrupt change in marine plant life along a section of Australia’s Great Southern Reef, with kelp disappearing to be replaced by tropical species.
As we and our international colleagues report in the journal Science, five years on from the heatwave, these kelp forests show no signs of recovery.
Instead, fish, seaweed and invertebrate communities from these formerly temperate kelp forests are being replaced by subtropical and tropical reef communities. Tropical fish species are now intensely grazing the reef, preventing the kelp forests from recovering.
Assessing the damage
We and our team surveyed reefs along 2,000km of coastline from Cape Leeuwin, south of Perth, to Ningaloo Reef between 2001 and 2015.
Up until 2011, temperate reefs were clearly defined by the distribution of kelp forests which formed dense, highly productive forests as far north as Kalbarri in WA’s Mid West.
Since 2011, the boundary between these temperate reefs of southern WA and the more tropical reefs (including Ningaloo) to the north has become less clear-cut. Instead, the sharp divide has been replaced by an intermediate region of turf-dominated reefs.
This has implications for the Great Southern Reef (GSR), which extends more than 8,000km around the southern half of Australia from the southern half of WA all the way to southern Queensland – a coastline that is home to around 70% of Australians.
Kelp forests are the GSR’s “biological engine”, feeding a globally unique collection of temperate marine species, not to mention supporting some of the most valuable fisheries in Australia and underpinning reef tourism worth more than A$10 billion a year.
But our research shows that on the GSR’s western side, kelp forests are being pushed towards Australia’s southern edge, where continued warming puts them at risk of losses across thousands of kilometres of coastline because there is no more southerly habitat to which they can retreat.
While the 2011 marine heatwave affected some 1,000km of Western Australia’s temperate coastline, it was a stretch of roughly 100km extending south of Kalbarri on the state’s Mid West coast that was most severely affected.
In this area alone an estimated 385 square km of kelp forest have been completely wiped out.
Further south, from Geraldton to Cape Leeuwin, the extent of kelp loss was less severe, despite an estimated total area of 960 square km having been lost in the region.
Northern regions towards Kalbarri were more severely affected because these kelp forests were closer to their limit, and also because this area is closer to the tropical regions like Ningaloo Reef, meaning that tropical species could more easily move in.
The problem was exacerbated by the southward-flowing Leeuwin Current, which helps tropical species move south while making it harder for temperate species to move north and recolonise the affected areas of the GSR.
The combination of these physical and ecological processes set within a background warming rate roughly twice the global average, compounds the challenges faced by kelp forests in the region.
The plight of WA’s kelp forests provides a strong warning of what the future might hold for Australia’s temperate marine environment, and the many services it provides to Australians.
Scott Bennett, Marie Curie Fellow at the Spanish National Research Council (CSIC), Curtin University; Julia Santana-Garcon, Postdoctoral research associate in Marine Ecology, and Thomas Wernberg, ARC Future Fellow in Marine Ecology, University of Western Australia