Marisa Stone, Griffith University; David Lindenmayer, Australian National University; Kurtis Nisbet, Griffith University, and Sebastian Seibold, Technical University of MunichIf you’ve wandered through a forest, you’ve probably dodged dead, rotting branches or stumps scattered on the ground. This is “deadwood”, and it plays several vital roles in forest ecosystems.
It provides habitat for small mammals, birds, amphibians and insects. And as deadwood decomposes it contributes to the ecosystem’s cycle of nutrients, which is important for plant growth.
But there’s another important role we have little understanding of on a global scale: the carbon deadwood releases as it decomposes, with part of it going into the soil and part into the atmosphere. Insects, such as termites and wood borers, can accelerate this process.
The world’s deadwood currently stores 73 billion tonnes of carbon. Our new research in Nature has, for the first time, calculated that 10.9 billion tonnes of this (around 15%) is released into the atmosphere and soil each year — a little more than the world’s emissions from burning fossil fuels.
But this amount can change depending on insect activity, and will likely increase under climate change. It’s vital deadwood is considered explicitly in all future climate change projections.
An extraordinary, global effort
Forests are crucial carbon sinks, where living trees capture and store carbon dioxide from the atmosphere, helping to regulate climate.
Deadwood — including fallen or still-standing trees, branches and stumps — makes up 8% of this carbon stock in the world’s forests.
Our aim was to measure the influence of climate and insects on the rate of decomposition — but it wasn’t easy. Our research paper is the result of an extraordinary effort to co-ordinate a large-scale cross-continent field experiment. More than 30 research groups worldwide took part.
Wood from more than 140 tree species was laid out for up to three years at 55 forest sites on six continents, from the Amazon rainforest to Brisbane, Australia.
Half of these wood samples were in closed mesh cages to exclude insects from the decomposition process to test their effect, too.
Some sites had to be protected from elephants, another was lost to fire and another had to be rebuilt after a flood.
What we found
Our research showed the rate of deadwood decay and how insects contribute to it depend very strongly on climate.
We found the rate increased primarily with rising temperature, and was disproportionately greater in the tropics compared to all other cooler climatic regions.
In fact, deadwood in tropical regions lost a median mass of 28.2% every year. In cooler, temperate regions, the median mass lost was just 6.3%.
More deadwood decay occurs in the tropics because the region has greater biodiversity (more insects and fungi) to facilitate decomposition. As insects consume the wood, they render it to small particles, which speed up decay. The insects also introduce fungal species, which then finish the job.
Of the 10.9 billion tonnes of carbon dioxide released by deadwood each year, we estimate insect activity is responsible for 3.2 billion tonnes, or 29%.
Let’s break this down by region. In the tropics, insects were responsible for almost one-third of the carbon released from deadwood. In regions with low temperatures in forests of northern and temperate latitudes — such as in Canada and Finland — insects had little effect.
What does this mean in a changing climate?
But given the vast majority of deadwood decay occurs in the tropics (93%), and that this region in general is set to become even warmer and wetter under climate change, it’s safe to say climate change will increase the amount of carbon deadwood releases each year.
It’s also worth bearing in mind that the amount of carbon dioxide released is still only a fraction of the total annual global deadwood carbon stock. That is, 85% of the global deadwood carbon stock remains on forest floors and continues to store carbon each year.
We recommend deadwood is left in place — in the forest. Removing deadwood may not only be destructive for biodiversity and the ability of forests to regenerate, but it could actually substantially increase atmospheric carbon.
For example, if we used deadwood as a biofuel it could release the carbon that would otherwise have remained locked up each year. If the world’s deadwood was removed and burned, it would be release eight times more carbon than what’s currently emitted from burning fossil fuels.
This is particularly important in cooler climatic regions, where decomposition is slower and deadwood remains for several years as a vital carbon sink.
The complex interplay of interactions between insects and climate on deadwood carbon release makes future climate projections a bit tricky.
To improve climate change predictions, we need much more detailed research on how communities of decomposer insects (such as the numbers of individuals and species) influence deadwood decomposition, not to mention potential effects from insect diversity loss.
But insect diversity loss is also likely to vary regionally and would require long-term studies over decades to determine.
For now, climate scientists must take the enormous annual emissions from deadwood into account in their research, so humanity can have a better understanding of climate change’s cascading effects.
Marisa Stone, Adjunct Research Fellow, Centre for Planetary Health and Food Security, Griffith University; David Lindenmayer, Professor, The Fenner School of Environment and Society, Australian National University; Kurtis Nisbet, Scientific Officer, Griffith University, and Sebastian Seibold, Adjunct Teaching Professor, Technical University of Munich
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In recent decades, has New Zealand lost forest (both native and exotic) or gained it, courtesy of the One Billion Trees programme? What about natural habitats like wetlands?
Apart from wetlands, land above the treeline, coastal dunes and a few other exceptions, New Zealand was once covered in forests from Cape Reinga to Bluff.
So was Europe, which basically consisted of a single forest from Sicily in southern Italy to the North Cape in Norway, before human intervention.
But since people arrived in New Zealand some 850 years ago, about three quarters of the country’s native forest area has been lost. About half of the loss happened before Europeans arrived, mostly through burning to clear large areas of native bush.
In recent decades, the loss of native forest has slowed down. For example, in the first decade of the 21st century, we lost roughly 16,000 hectares of native forest, which translates to a loss of about 0.2% of the remaining total area covered in native forest (about 7.5 million hectares). The error associated with such estimates is considerable, though, because land cover is complex and highly fragmented.
A billion trees
According to Global Forest Watch, the drivers behind the more recent losses of native forests include exotic plantation forests, urban developments and wildfires. Indeed, the total land area dedicated to exotic plantation forests increased by about 200,000 hectares per decade between 1990 and 2017.
So what has the One Billion Trees Programme achieved in comparison to these changes?
The project’s aim is to double the current planting rate and plant one billion trees between 2018 and 2028. The latest report shows about a quarter of this goal has been achieved in terms of the number of trees planted. In regards to forest area, 25,557 hectares have been reforested, about half of it with natives.
This is a remarkable achievement in light of the losses cited above and the short duration of the programme.
Saving remaining peat bogs
We think of forests as our guardians of carbon — and indeed, an aged New Zealand forest can hold about 350 tonnes of carbon per hectare. But intact peat bogs, such as the Kopuatai dome in the Waikato region, can hold up to 1,400 tonnes of carbon per hectare.
But peat bogs only store carbon if they remain wet. Once drained, they begin to emit carbon dioxide. Almost half of New Zealand’s peatlands are in the Waikato, but of a total of 89,000 hectares only 19,400 hectares remain in a natural state.
The Kopuatai bog itself is surrounded by dairy farms operating on drained peat. Collectively, the Waikato’s drained peatlands produce 10-33 tonnes of CO₂-equivalent emissions per hectare each year.
The draining of peatlands in the Waikato region did far more damage, in terms of carbon emissions, than a small loss of forest area.
But nevertheless, planting trees and increasing our forest area is an important and necessary contribution to climate mitigation, and often comes with a myriad of other benefits, far beyond carbon sequestration.
Sometimes it’s as easy as planting your own fruit trees around your house. They will capture carbon for years to come, and keep you from buying fruit that has been transported thousands of kilometres.
They might even motivate you to reduce food waste. Globally, about 25-30% of food goes to waste. If we reduced food waste, we could save agricultural land multiple times the size of New Zealand and plant trees there instead.
Philip Zylstra, University of Wollongong; Grant Wardell-Johnson, Curtin University; James Watson, The University of Queensland, and Michelle Ward, The University of QueenslandThe Black Summer bushfires burned far more temperate forest than any other fire season recorded in Australia. The disaster was clearly a climate change event; however, other human activities also had consequences.
Taking timber from forests dramatically changes their structure, making them more vulnerable to bushfires. And, crucially for the Black Summer bushfires, logged forests are more likely to burn out of control.
We believe these findings are too narrowly focused and in fact, misleading. They overlook a vast body of evidence that crown fire – the most extreme type of bushfire behaviour, in which tree canopies burn – is more likely in logged native forests.
Crown fires vs scorch
The Black Summer fires occurred in the 2019-20 bushfire season and burned vast swathes of Australia’s southeast. In some cases, fire spread through forests with no recorded fire, including some of the last remnants of ancient Gondwanan rainforests.
Tragically, the fires directly killed 33 people, while an estimated 417 died due to the effects of smoke inhalation. A possible three billion vertebrate animals perished and the risk of species extinctions dramatically increased.
Much of the forest that burned during Black Summer experienced crown fires. These fires burn through the canopies of trees, as well as the undergrowth. They are the most extreme form of fire behaviour and are virtually impossible to control.
Crown fires pulse with such intense heat they can form thunderstorms which generate lightning and destructive winds. This sends burning bark streamers tens of kilometres ahead of the fire, spreading it further. The Black Summer bushfires included at least 18 such storms.
And to our knowledge, every empirical analysis so far shows logging eucalypt forests makes them far more likely to experience crown fire. The studies include:
- A 2009 paper suggesting changes in forest structure and moisture make severe fire more likely in logging regrowth compared to undisturbed forest
- 2012 research concluding the probability of crown fires was higher in recently logged areas than in areas logged decades before
- A 2013 study that showed the likelihood of crown fire halved as forests aged after a certain point
- 2014 findings that crown fire in the Black Saturday fires likely peaked in regrowth and fell in mature forests
- 2018 research into the 2003 Australian Alps fires, which found the same increase in the likelihood of crown fire during regrowth as was measured following logging.
The findings of these studies are represented in the image below. The lines a, b and c refer to the 2013, 2014 and 2018 studies respectively.
Crown fires take lives
The presence of crown fire is a key consideration in fire supression, because crown fires are very hard to control.
However, the study released last week – which argued that logging did not worsen the Black Summer fires – focused on crown “scorch”. Crown scorch is very different to crown fire. It is not a measure of how difficult it is to contain the fire, because even quite small flames can scorch a drought-stressed canopy.
Forestry studies tend to focus more on crown scorch, which damages timber and is far more common than crown fires.
But the question of whether logging made crown scorch worse is not relevant to whether a fire was uncontrollable, and thus was able to destroy homes and lives.
Importantly, when the study said logging had a very small influence on scorch, this was referring to the average scorch over the whole fire area, not just places that had been logged. That’s like asking how a drought in the small town of Mudgee affects the national rainfall total: it may not play a large role overall, but it’s pretty important to Mudgee.
The study examined trees in previously logged areas, or areas that had been logged and burned by fires of any source. It found they were as likely to scorch on the mildest bushfire days as trees in undisturbed forests on bad days. These results simply add to the body of evidence that logging increases fire damage.
Managing forests for all
For example during the Black Saturday fires in 2009, the Kilmore East fire north of Melbourne consumed all before it as a crown fire. Then it reached the old, unlogged mountain ash forests on Mount Disappointment and dropped to the ground, spreading as a slow surface fire.
The trees were scorched. But they were too tall to ignite, and instead blocked the high winds and slowed the fire down. Meanwhile, logged ash forests drove flames high into the canopy.
Despite decades of opportunity to show otherwise, the only story for eucalypt forests remains this: logging increases the impact of bushfires. This fact should inform forest management decisions on how to reduce future fire risk.
We need timber, but it must be produced in ways that don’t endanger human lives or the environment.
Philip Zylstra, Adjunct Associate Professor at Curtin University, Honorary Fellow at University of Wollongong, University of Wollongong; Grant Wardell-Johnson, Associate Professor, Environmental Biology, Curtin University; James Watson, Professor, The University of Queensland, and Michelle Ward, PhD Candidate, The University of Queensland
David Bowman, University of TasmaniaThe Black Summer bushfires shocked the world and generated enormous global media interest. Fire scientists like myself found themselves filling a role not unlike sport commentators, explaining the unfolding drama in real time.
Scientists who engaged with the media during the crisis straddled two competing imperatives. First was their duty to share their knowledge with the community while knowing their understanding is imperfect. Second was the ethical obligation to rigorously test hypotheses against data analysis and peer review – the results of which could only be known long after the fires were out.
One area where this tension emerged was around the influential idea that logging exacerbated the bushfire disaster. During the fire crisis and in the months afterwards, some scientists suggested logging profoundly affected the fires’ severity and frequency. There were associated calls to cease native forestry and shift wood production to plantations.
But there is no scientific consensus about the possible effects of logging on fire risk. In fact, research by myself and colleagues, published in Nature Ecology and Evolution today, shows logging had little if any effect on the Black Summer bushfires. Rather, the disaster’s huge extent and severity were more likely due to unprecedented drought and sustained hot, windy weather.
These findings are significant for several reasons. Getting to the bottom of the bushfires’ cause is essential for sustainable forest management. And, more importantly, our research confirms the devastating role climate change played in the Black Summer fires.
Looking for patterns
Our research focused on 7 million hectares of mostly eucalyptus forests, from the subtropics to temperate zones, which burned between August 2019 and March 2020.
There is some evidence to suggest logged areas are more flammable that unlogged forests. Proponents of this view say logging regimes make the remaining forests hotter and drier, and leave debris on the ground that increases the fuel load.
In our research, we wanted to determine:
- the relative roles logging and other factors such as climate played in fires that destroyed or completely scorched forest canopies
- whether plantations are more vulnerable to canopy scorch than native forests.
To do so, we used landscape ecology techniques that could compare very large areas with different patterns of land use and fire severity. We sampled 32% of the area burnt in three regions spanning the geographic range of the fires.
What we found
Fire intensity is classified according to the vertical layer of vegetation burnt. A scorched tree canopy suggests the most intense type of fire, where the heat extended from the ground to the treetops.
We found several predictors of canopy damage. First, completely scorched canopy, or canopy consumed by fire, typically occurred across connected swathes of bushland. This most likely reflected instances where the fire made a “run”, driven by localised winds.
Extreme weather fire conditions were the next most important predictor of canopy damage. The drought had created vast areas of tinder-dry forests. Temperatures during the fire season were hot and westerly winds were strong.
Southeast Australia’s climate has changed, making such extreme fire weather more frequent, prolonged and severe.
Logging activity in the last 25 years consistently ranked “low” as a driver of fire severity. This makes sense for several reasons.
As noted above, fire conditions were extraordinarily extreme. And there was mismatch between the massive area burnt and the comparatively small areas commercially logged in the last 25 years (4.5% in eastern Victoria, 5.3% in southern NSW and 7.8% in northern NSW).
Fire severity is also related to landscape features: fire on ridges is generally worse than in sheltered valleys.
Our research also found timber plantations were as prone to severe fire as native forestry areas. In NSW (the worst-affected state) one-quarter of plantations burned – than 70% severely. This counteracts the suggestion using plantations, rather than logging native forest, can avoid purported fire hazards.
A challenge awaits
Our findings are deeply concerning. They signal there is no quick fix to the ongoing fire crisis afflicting Australia and other flammable landscapes.
The crisis is being driven by relentless climate change. Terrifyingly, it has the potential to turn forests from critical stores of carbon into volatile sources of carbon emissions released when vegetation burns.
Under a rapidly warming and drying climate, fuel loads are likely to become less important in determining fire extent and severity. This will make it increasingly difficult, if not impossible, to lower fuel loads in a way that will limit bushfire severity.
A massive challenge awaits. We must find socially and environmentally acceptable ways to make forests more resilient to fire while the also produce sustainable timber products, store carbon, provide water and protect biodiversity.
The next step is a real-world evaluation of management options. One idea worth exploring is whether the fire resistance of native forests can be improved in specific areas by altering tree density, vegetation structure or fuel loads, while sustaining biodiversity and amenity.
Commercial forestry could potentially do this, with significant innovation and willingness to let go of current practices.
Through collective effort, I’m confident we can sustainably manage of forests and fire. Our study is but a small step in a much bigger, zig-zagging journey of discovery.
Forests are thought to be crucial in the fight against climate change – and with good reason. We’ve known for a long time that the extra CO₂ humans are putting in the atmosphere makes trees grow faster, taking a large portion of that CO₂ back out of the atmosphere and storing it in wood and soils.
But a recent finding that the world’s forests are on average getting “shorter and younger” could imply that the opposite is happening. Adding further confusion, another study recently found that young forests take up more CO₂ globally than older forests, perhaps suggesting that new trees planted today could offset our carbon sins more effectively than ancient woodland.
How does a world in which forests are getting younger and shorter fit with one where they are also growing faster and taking up more CO₂? Are old or young forests more important for slowing climate change? We can answer these questions by thinking about the lifecycle of forest patches, the proportion of them of different ages and how they all respond to a changing environment.
The forest carbon budget
Let’s start by imagining the world before humans began clearing forests and burning fossil fuels.
In this world, trees that begin growing on open patches of ground grow relatively rapidly for their first several decades. The less successful trees are crowded out and die, but there’s much more growth than death overall, so there is a net removal of CO₂ from the atmosphere, locked away in new wood.
As trees get large two things generally happen. One, they become more vulnerable to other causes of death, such as storms, drought or lightning. Two, they may start to run out of nutrients or get too tall to transport water efficiently. As a result, their net uptake of CO₂ slows down and can approach zero.
Eventually, our patch of trees is disturbed by some big event, like a landslide or fire, killing the trees and opening space for the whole process to start again. The carbon in the dead trees is gradually returned to the atmosphere as they decompose.
The vast majority of the carbon is held in the patches of big, old trees. But in this pre-industrial world, the ability of these patches to continue taking up more carbon is weak. Most of the ongoing uptake is concentrated in the younger patches and is balanced by CO₂ losses from disturbed patches. The forest is carbon neutral.
Now enter humans. The world today has a greater area of young patches of forest than we would naturally expect because historically, we have harvested forests for wood, or converted them to farmland, before allowing them to revert back to forest. Those clearances and harvests of old forests released a lot of CO₂, but when they are allowed to regrow, the resulting young and relatively short forest will continue to remove CO₂ from the atmosphere until it regains its neutral state. In effect, we forced the forest to lend some CO₂ to the atmosphere and the atmosphere will eventually repay that debt, but not a molecule more.
But adding extra CO₂ into the atmosphere, as humans have done so recklessly since the dawn of the industrial revolution, changes the total amount of capital in the system.
And the forest has been taking its share of that capital. We know from controlled experiments that higher atmospheric CO₂ levels enable trees to grow faster. The extent to which the full effect is realised in real forests varies. But computer models and observations agree that faster tree growth due to elevated CO₂ in the atmosphere is currently causing a large carbon uptake. So, more CO₂ in the atmosphere is causing both young and old patches of forest to take up CO₂, and this uptake is larger than that caused by previously felled forests regrowing.
The effect of climate change
But the implications of climate change are quite different. All else being equal, warming tends to increase the likelihood of death among trees, from drought, wildfire or insect outbreaks. This will lower the average age of trees as we move into the future. But, in this case, that younger age does not have a loan-like effect on CO₂. Those young patches of trees may take up CO₂ more strongly than the older patches they replace, but this is more than countered by the increased rate of death. The capacity of the forest to store carbon has been reduced. Rather than the forest loaning CO₂ to the atmosphere, it’s been forced to make a donation.
So increased tree growth from CO₂ and increased death from warming are in competition. In the tropics at least, increased growth is still outstripping increased mortality, meaning that these forests continue to take up huge amounts of carbon. But the gap is narrowing. If that uptake continues to slow, it would mean more of our CO₂ emissions stay in the atmosphere, accelerating climate change.
Overall, both young and old forests play important roles in slowing climate change. Both are taking up CO₂, primarily because there is more CO₂ about. Young forests take up a bit more, but this is largely an accident of history. The extra carbon uptake we get from having a relatively youthful forest will diminish as that forest ages. We can plant new forests to try to generate further uptake, but space is limited.
But it’s important to separate the question of uptake from that of storage. The world’s big, old forests store an enormous amount of carbon, keeping it out of the atmosphere, and will continue to do so, even if their net CO₂ uptake decreases. So long as they are not cut down or burned to ashes, that is.
It’s tempting to think that our forests would be fine if we could simply stop trees being felled or burnt. But forests – particularly tropical ones – are more than just trees. They’re also the animals that skulk and swoop among them.
Worryingly, these furry and feathered companions are rapidly disappearing – and our new research indicates that this will have grave repercussions for the role forests play in combating climate breakdown.
Healthy tropical forests swarm with life. Beyond myriad invertebrates there are seed-eating rodents, a range of leaf eaters, birds of all kinds, and often primates. However, many forests have already lost most of their largest animals, mainly as a result of hunting to supply a growing bushmeat trade.
Hunting isn’t the only reason. Thanks to deforestation for farmland and logging, many forests today are highly fragmented. The small, unconnected patches that remain aren’t big enough to support populations of the largest species, which tend to need more space.
The disappearance of animals from otherwise intact habitats is known as defaunation, and it is leading to a growing number of empty forests not just in tropical countries, but around the world. The UK has already lost most of its largest species (think lynx, wolf, and wisent), while woodland bird numbers have declined by a quarter since 1970.
The impacts of this defaunation have attracted the attention of the world’s conservation scientists, but studies to date have usually been carried out at single locations. Consequently, we lack a worldwide picture that takes into account different types of forest and the diversity of animals that are disappearing.
To fill this gap, we worked with William Baldwin-Cantello, chief adviser on forests at the World Wide Fund for Nature UK, to gather together all the existing research and perform a meta-analysis – an analysis of analyses – on the available data.
Forest flora need flourishing fauna
Our findings reveal a worrying trend. The loss of animals compromises the ability of forests to reproduce. This effect is particularly severe when primates and birds disappear, because of the key role they play in seed dispersal. Trees make fruit to entice animals to transport their seeds, because they are more likely to germinate and grow successfully if they fall further from their parent tree. So when fruit-eating animals disappear, fewer seeds are dispersed and the trees struggle to reproduce.
This animal absence will slowly change how forests look. Most tropical forests today are dominated by trees whose seeds are dispersed by animals. Over time, they are likely to be gradually replaced by trees that use the wind to reproduce. Naturally, these usually have small seeds, and therefore produce smaller trees that store less carbon for the same area of forest. As a result, forests will store less and less carbon, even if we completely halt deforestation.
This is particularly concerning because roughly 20% of the carbon dioxide we emit is absorbed by the world’s vegetation and soils, and half of this is due to tropical forests alone.
Rethinking forest health
Conserving forests is essential for the fight against climate breakdown – and, we do have a global tool at our disposal to help. Known as Reducing Emissions from Deforestation and forest Degradation, or REDD+ for short, it allows wealthy countries with large carbon footprints to pay poorer, tropical countries to protect their forests.
Of course, REDD+ is only an effective tool if the forests countries pay to protect continue to store the same amount of carbon. We usually monitor this by taking satellite images of the quantity of forest canopy remaining. But what satellite imagery can’t do is measure aspects of forest quality beneath the canopy.
Our research strongly suggests that one aspect of forest quality – defaunation – is a vital early warning sign of future losses in the carbon storing capacity of forests. In light of this, policies for managing forest carbon around the world may need a rethink.
We need to pay more attention to what’s going on beneath global forest canopies through research on the ground, though this will be difficult in remote areas. More importantly, we must make sure we’re doing all we can to conserve the full complement of animal species that live in our forests. For example, we need to heavily invest in conservation actions that help communities accustomed to hunting bushmeat to meet their dietary protein needs without harming wildlife. We must also enforce existing rules better, such as those that outlaw hunting within parks and reserves.
Preventing defaunation in forests won’t be easy. But given what we know about the critical role forest animals play, doing so will be essential if we hope to retain diverse and carbon-rich forests in the tropics and around the world. If the beauty and wonder of the forest’s animals wasn’t enough reason to protect them, we now have another: by conserving wildlife, we will be helping to save ourselves from the catastrophic effects of climate breakdown.
Charlie Gardner, Lecturer in Conservation Biology, University of Kent; Jake Bicknell, Lecturer in Conservation Biology, University of Kent; Matthew Struebig, Senior Lecturer in Biological Conservation, University of Kent, and Zoe Davies, Professor of Biodiversity Conservation, University of Kent