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
Large trees are the living, breathing giants that tower over tropical forests, providing habitat and food for countless animals, insects and other plants. Could these giants also be the key to slowing climate change?
The Earth’s climate is changing rapidly due to the buildup of greenhouse gases, like carbon dioxide, in the atmosphere as a result of human activities. Trees absorb carbon from the air and store it in their trunks, branches, and roots. In general, the larger the tree, the more carbon it stores.
Globally, tropical forests remove a staggering 15% of carbon dioxide emissions that humans produce. Africa’s tropical forests – the second largest block of rainforest in the world – have a large role to play in slowing climate change.
But large trees are in trouble everywhere. I carried out research to examine the distribution, drivers and threats to large trees in Gabon. Gabon has 87% forest cover and is the second most forested country in the world.
By carrying out this project, I was able to identify areas with a wealth of large trees (and therefore key carbon stores and sinks), what needed to be done to better protect them and eventually recommend those areas as a priority for conservation.
In 2012, the government of Gabon began a national inventory of its forests to measure the amount of carbon stored in its trees – one of the first nationwide efforts in the tropics.
An inventory of this scale isn’t easy, especially in a heavily forested country. Technicians from Gabon’s National Parks Agency travelled to every corner of the country, sometimes hiking more than two days crossing swamps and traversing rivers, to measure the diameter and height of trees in plots a bit larger in size than a soccer field.
Using Gabon’s new inventory of 104 plots, we calculated the amount of carbon in 67,466 trees, representing at least 578 different species. We did this by applying equations to the tree measurements.
The results indicated that the density of carbon stored in Gabon’s trees is among the highest in the world. On average, Gabon’s old growth forests harbour more carbon per area than old growth forests in Amazonia and Asia.
Most of this carbon is stored in the largest trees – those with diameters bigger than 70cm at 1.3 meters from the ground. Just the largest 5% of trees stored 50% of the forest carbon. In other words, 3,373 trees out of the 67,466 measured trees contained half of the carbon.
Drivers of forest carbon stocks
Next, we examined the drivers of carbon stocks. What determines whether an area of forest holds many large trees and lots of carbon? Do environmental conditions or human activities have the largest impact on forest carbon stocks?
Environmental factors – such as soil fertility and depth, temperature, precipitation, slope and elevation – often influence the amount of carbon in a forest. During photosynthesis, trees harness energy from the sun to convert water, carbon dioxide, and minerals into carbohydrates for growth. Therefore, forests with low levels of soil minerals or that receive little rainfall should store less carbon than areas with abundant minerals and water.
Human activities – like agriculture and logging – also influence carbon stocks. Cutting down trees for timber, to clear land for farming, or for construction reduces the amount of carbon stored in forests.
We examined the amount of carbon in each tree plot in relation to the environmental factors and human activities associated with the plot. Surprisingly, we found that human activities, not environmental factors, overwhelmingly affect carbon stocks.
The impact of human activities on forest carbon was largely unexpected because of Gabon’s high forest cover (the second highest of any country) and low population density (9 people per square kilometer), 87% of which is located in urban areas. If human impacts are this strong in Gabon, what must their effects be in other tropical nations?
Although we don’t know for sure, we believe past and present swidden (slash-and-burn) agriculture is the principle cause for low carbon stocks in some areas. Forests close to villages had lower levels of carbon, probably because forest clearing for farming converts old growth forest to secondary forest.
Interestingly, forests in logging concessions held similar amounts of carbon as old growth forests. It is too early to conclude that timber harvest doesn’t reduce carbon levels by cutting large trees, but this finding gives hope that logging concessions can be managed sustainably to conserve carbon stocks.
Importantly, forests in national parks stored roughly 25% more carbon than forests outside of parks. Thus, protecting mostly undisturbed forests can effectively conserve carbon and biodiversity.
Saving Gabon’s giants
The critical role of humans in diminishing carbon stocks is both a blessing and a curse. One one hand, the future of forests are in our hands, giving us the power to choose our fate. On the other hand, we cannot ignore the responsibility to act collectively to secure these resources while considering the interests of the countries that host them.
Gabon is taking laudable actions to conserve its forests, including a protected area network of 13 parks. In addition, Gabon is reforming its logging sector and developing a nationwide land use plan. These actions are a great start, yet continued action is necessary to curb the effects of swidden agriculture and ensure that growing industrial agriculture does not reverse Gabon’s achievements.
Intact forests can pay returns. Norway recently committed to paying Gabon $150 million for stewardship of its forests. Conservation of forests requires sacrifice by the Gabonese people. Yet, this payment demonstrates that Gabon’s large trees are a national asset that can contribute to its development as well as an international resource requiring collective action to conserve.
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.
Calls from industry and unions for increased thinning in forests to reduce bushfire risks have been met with concern from conservation scientists. They suggest forest thinning makes forests more fire prone.
So who’s right? Well, it’s complicated. The short answer is forest thinning is a good way to lower the risk of fire and is a widely-used strategy to improve forest health. However, there are potential downsides. Thinning needs to be carefully planned to avoid effects on soil, water or sensitive habitats.
Unlike clearfell logging and selection harvesting, mechanical thinning for timber involves felling about half the trees in even-aged, uniformly structured forests. Recently, forest managers are using the practice more for ecological outcomes.
If we look to the future, the recent fires have created conditions for forest regeneration on a large scale. These regenerating forests will thin naturally over time, creating more fuel and increased risk of more large-scale fires. Mechanical thinning can remove this potential flammable vegetation.
Forest thinning should be one of the ways we tackle fire management and forest resilience in future, but we need more research to understand the best way to go about it. Here’s what the evidence says.
What is thinning?
Thinning is a natural forest process, where tree numbers in most even-aged forests reduce through competition over time. For example, Mountain ash forests regenerating naturally after a severe fire might have hundreds of thousands of new seedlings per hectare that self-thin to a few thousand after 20 years, and a few hundred after 80 years.
Mechanical thinning for producing timber is a long-standing commercial forestry practice that uses herbicides, chainsaws or mechanical harvesters. It reduces tree numbers and concentrates growth on fewer trees so they reach a valuable size more quickly. This is to improve commercial timber quality, or to more quickly remove trees that would die through natural thinning.
Thinning for ecological outcomes, on the other hand, is a relatively recent practice being tested in many parts of Australia. It can produce more rapid development of “old-growth” forest features, such as large trees, branches, hollows and coarse woody debris – all important wildlife habitats.
Forest managers are using thinning for other reasons, too. For example, to adapt to climate change by reducing stresses on individual trees from increased drought, heat, insects, disease or wildfire because, among other things, thinning takes away the added stress of competition.
The case for thinning to reduce fire risk
Thinning to reduce fire risk is intended to slow the rate fire spreads, lower flame heights and improve recovery after wildfire hits. This was shown in a 2016 extensive review of US research, which found thinning and prescribed burning helped reduce fire severity, tree mortality and crown scorch. A 2018 study on Spanish pine forests had similar results.
Our own research on Australian forests also supported these findings. We found mechanical thinning plus burning in silver top ash reduces fire fuel hazard, with major reductions in dead trees, stumps and understory.
We compared thinned and unthinned alpine ash forests using computer modelling, simulating severe to extreme weather conditions. And we found modelled fire intensity decreased by 30% and the rate of fire spread and spot fires moving ahead of the main fire decreased by 20% with thinning.
Reducing tree density and fuel through thinning can also make it easier and safer for fire-suppression activities, like direct attack with fire hoses, litter raking or back burns, increasing our chances to control the size of wildfires.
Another study from 2015 in East Gippsland forests found that while overall fuel hazard was lower at thinned sites than nearby unthinned sites, larger woody debris from thinning persisted for 15 years or longer.
This is both a good and bad thing. More logs or woody debris may slow fire spreading, but can make it harder to completely extinguish fires after the fire front passes through.
Thinning is potentially costly, but selling the wood or other organic matter may offset the cost. Timber harvesting machines can also disturb soils or wildlife habitat, but these can be minimised with modern equipment and careful planning.
What’s more, forests store carbon. Thinning can, in the short term, release carbon dioxide into the atmosphere. The overall effect on carbon emissions in the long term, however, depends on the extent thinning reduces fire risk and intensity. In some cases, we may need to accept decreased forest carbon storage in return for reduced risks.
We’ve seen in the media arguments about using thinning to manage bushfire risks. It’s important conservation and bushfire scientists, the timber industry and government bodies understand all concerns and create space for inclusive dialogue to identify where thinning and prescribed burning are best practised.
In any case, whether you’re for or against the practice, more research is needed to determine how much we should use it. In 2017, the Federal Government funded mechanical fuel reduction trials in three states. But these trials must be expanded to a national program.
This can be done in using adaptive management – trialling the practice at larger scale and monitoring the outcomes.
The evidence from Australia and overseas is compelling, but we need careful planning and thoughtful discussion about how to use thinning to its full potential as part of our strategy in addressing the escalating risks of bushfires in a changing climate.