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.
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
Tomorrow a special report on how land use affects climate change will be released by the Intergovernmental Panel on Climate Change.
Land degradation, deforestation, and the expansion of our deserts, along with agriculture and the other ways people shape land, are all major contributors to global climate change.
Conversely, trees remove carbon dioxide and store it safely in their trunks, roots and branches. Research published in July estimated that planting a trillion trees could be a powerful tool against climate change.
However, planting new trees as a climate action pales in comparison to protecting existing forests. Restoring degraded forests and expanding them by 350 million hectares will store a comparable amount of carbon as 900 million hectares of new trees.
Natural climate solutions
Using ecological mechanisms for reducing and storing carbon is a growing field of study. Broadly known as “natural climate solutions”, carbon can be stored in wetlands, grasslands, natural forests and agriculture.
This is called “sequestration”, and the more diverse and longer-lived the ecosystem, the more it helps mitigate the effect of climate change.
Research has estimated these natural carbon sinks can provide 37% of the CO₂ reduction needed to keep the rise in global temperatures below 2℃.
But this research can be wrongly interpreted to imply that the priority is to plant young trees. In fact, the major climate solution is the protection and recovery of carbon-rich and long-lived ecosystems, especially natural forests.
With the imminent release of the new IPCC report, now is a good time to prioritise the protection and recovery of existing ecosystems over planting trees.
Forest ecosystems (including the soil) store more carbon than the atmosphere. Their loss would trigger emissions that would exceed the remaining carbon budget for limiting global warming to less than the 2℃ above pre-industrial levels, let alone 1.5℃, threshold.
Natural forest systems, with their rich and complex biodiversity, the product of ecological and evolutionary processes, are stable, resilient, far better at adapting to changing conditions and store more carbon than young, degraded or plantation forests.
Protect existing trees
Forest degradation is caused by selective logging, temporary clearing, and other human land use. In some areas, emissions from degradation can exceed those of deforestation. Once damaged, natural ecosystems are more vulnerable to drought, fires and climate change.
Recently published research found helping natural forest regrow can have a globally significant effect on carbon dioxide levels. This approach – called proforestation – is a more effective, immediate and low-cost method for removing and storing atmospheric carbon in the long-term than tree planting. And it can be used across many different kinds of forests around the world.
Avoiding further loss and degradation of primary forests and intact forest landscapes, and allowing degraded forests to naturally regrow, would reduce global carbon emissions annually by about 1 gigatonnes (Gt), and reduce another 2-4 Gt of carbon emissions just through natural regrowth.
Research has predicted that protecting primary forests while allowing degraded forests to recover, along with limited expansion of natural forests, would remove 153 billion tonnes of carbon from the atmosphere between now and 2150.
By the numbers
Tree planting carries more limited climate benefits. The recent Science paper focused on mapping and quantifying increases in tree canopy cover in areas that naturally support trees. However, increasing canopy cover through natural forest regeneration can sequester 40 times more carbon over the course of the century than establishing new plantations.
We need to think very carefully about how we use land that has already been cleared: land is a finite resource, and we need to grow food and resources for a global population set to hit 9 billion by 2050.
Any expansion of natural forest area is best achieved through allowing degraded forests to naturally recover. Allowing trees to regenerate naturally, using nearby remnants of primary forests and seed banks in the soil of recently cleared forests, is more likely to result in a resilient and diverse forest than planting massive numbers of seedlings.
Instead of planting entirely new areas, we should prioritise reconnecting forested areas and restoring the edges of forest, to protect their mature core. This means our carbon-storing forests will be more resilient and longer-lasting.
For forests to effectively help avert dangerous climate change, global and regional policies are needed to protect, restore and regenerate natural forests, alongside a carbon-zero energy economy.
A version of this article was co-published with Pursuit.
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.