Logged tropical forests are a ‘substantial’ carbon source for at least 10 years

Logged tropical forests are a net source of carbon emissions for at least a decade after being degraded, according to new research.

Tropical forests that have been logged or otherwise degraded in the past are generally thought to absorb more carbon than they emit through rapidly regrowing trees.

But new research shows that this carbon uptake is overshadowed “by the high emissions from soil organic matter and from deadwood from logging,” the paper’s corresponding author told Carbon Brief.

By examining both intact and degraded forest areas, the researchers quantify CO2 emissions and uptake from forests in Malaysian Borneo – an area severely affected by deforestation and forest degradation.

Deforestation refers to the clearing of a forest. A degraded forest, on the other hand, has not been completely cleared, but has been seriously affected by issues such as logging and climate change.

Although the study, published in the Proceedings of the National Academy of Sciences, focuses on just one area, the researchers write that “the potential implications are serious” for the extent of the overall carbon sink in the tropical forest.

The researchers believe that the findings mean that the amount of carbon sequestered in tropical forests around the world “may be significantly lower than currently estimated”.

Effects of Logging

About 30% of worldwide greenhouse gas emissions are absorbed by the soil – making it an important “carbon sink”.

Forests are crucial components of the global carbon sink. The Amazon, the world’s largest tropical rainforest, stores a huge amount of carbon and is home to at least 10% of the world’s biodiversity.

The new study says tropical forests are “particularly important” to the global carbon budget – the highest level of net global emissions allowed to limit global warming to a certain level.

Tropical forests absorbed 15% of all anthropogenic CO2 emissions between 1990 and 2007, according to a paper published in 2011.

But recent research shows that the carbon sequestration capacity of tropical forests is rapidly declining, even those undisturbed by logging or fires.

A 2017 study also found that tropical forests are no longer carbon sinks due to deforestation and soil degradation.

The new study uses two primary methods to quantify CO2 exchange between the ecosystem and the atmosphere in the Malaysian part of Borneo.

Both methods the researchers used confirm that the landscape is a “significant net carbon source to the atmosphere for at least a decade after logging,” the study says.

The researchers compared data from moderately and heavily logged forests with those that had not experienced logging.

By measuring plant growth and respiration rates, the researchers found that the logged ecosystem was a source of carbon on 99% of the 455 days sampled.

The diagram below shows that heavily logged plots (brown shaded) are a greater source of CO2 compared to more moderately logged (brown striped) and unlogged (green). While the brown bars show the net emissions from logged sites, the green bar highlights that intact, old-growth sites are typically carbon sinks.

The average net ecosystem CO2 exchange.
The average net CO2 exchange in the ecosystem of six unlogged (green) and logged (brown) sites, measured as millions of grams of carbon per hectare per year. The logged plots are further divided into moderately logged (striped) and heavily logged (shaded). A positive value indicates a net source of CO2 to the atmosphere. Source: Mills et al. (2023)

The study points out that other research into the recovery of logged and otherwise degraded forests has focused on the trajectory of the forests’ ability to store carbon, but has generally not assessed the total carbon budget, including losses from sources such as soil and dead trees.

The researchers write that the plots they studied share characteristics with other tropical forests, but acknowledge that the heavily logged areas “more represent an extreme and unsustainable approach to logging,” adding that the study “highlights a worst-case scenario” .

“However, such high degradation is – unfortunately – not unique to our study site”, the paper continues, referring to similar areas in Indonesia and Myanmar.

“Overall, we believe that our study site and our data are broadly representative of the wider tropical forest landscape,” the paper concludes.

A ‘complete picture’

Dr. Terhi Riutta, the corresponding author on the new study and a postdoctoral researcher at the University of Exeter, says that most previous studies on this topic looked at “individual components” of the ecosystem, such as soil loss or tree growth, but that approach “doesn’t give you the complete picture of what is happening”. She tells Carbon Brief:

“We know from previous studies that the trees grow very quickly after logging because there are more holes, more resources available [and] most importantly, light.

“So we expected these forests to be net carbon sinks based on the growth of the trees, but it turns out that the forests are actually net carbon sources. We found this rapid tree growth, [but] it was offset by the high emissions from soil organic matter and from dead wood as a result of logging.”

Riutta says that logging results in damage beyond just the trees that are “actually extracted” – that “many of the neighboring trees either die at the time of logging or are damaged at the time of logging and then die later”.

Borneo – an island politically divided between three countries, including Malaysia – was chosen as a focus area in part because it receives less attention than other tropical forests in studies, she says, adding:

“It’s a hotspot for emissions from deforestation and forest degradation, so it was important to try to quantify those emissions.”

Timber concession in Sabah, Malaysia.
Timber concession in Sabah, Malaysia. Credit: Horizon International Images / Alamy Stock Photo.

Riutta adds that she and her team do not want the findings to be “used as a justification for converting the cleared forests to other harmful land uses,” such as oil palm plantations. She tells Carbon Brief:

“We don’t want that because they hold enormous biodiversity value compared to plantations and they are rich ecosystems.”

Riutta would also like to see more focus put on “the health and well-being of these felled forests and how we can deal with these emissions – rather than just using it as a case of felled forests on some way are bad”, she says.

Further investigation around the world

Academics not involved in this research say the findings support studies conducted in other parts of the world.

Dr. Celso HL Silva-Junior, a Brazilian postdoctoral researcher at the University of California, Los Angeles, says the research “reaffirms a concern that Amazonian scientists have raised in the region: the contribution of emissions from forest degradation (e.g. logging, fire, edge effect) to the region’s carbon budget”.

Silva-Junior says the research “makes an important advance by considering the total carbon budget of timber forests and demonstrating that these areas act as a carbon source for the atmosphere”. He tells Carbon Brief in an email:

“Although previous studies in the tropics have shown a rapid recovery of carbon stocks due to rapid regrowth of trees, in the Amazon, for example, depending on the intensity of selective logging, forests take decades to recover the original carbon stocks.”

Dr. Jamilah Mohd Salim, associate professor at Universiti Malaysia Terengganu, says the Malaysian Borneo economy “is still heavily dependent[ent] on the income” from the forest and palm oil plantations. She tells Carbon Brief in an email:

“Such research findings need to be communicated from the scientific community to the local government and larger public to correct the misleading fact about the effect of logging or removal of forest trees.”

“Something greater than revenue from logging (e.g. carbon finance) should be offered by the market to the state with the forested land, so [there is] more incentive to preserve their forest for carbon and climate.”

Salim says the research findings will not surprise people in her field, but among non-experts she believes there is still a belief that tropical forests that are cut down continue to absorb more carbon than they emit. She adds:

“The newly sequestered carbon will not be fair to the original carbon sink and could not compensate for carbon loss due to logging. On the other hand, logging can increase heterotrophic activities such as decomposition and soil degradation, which can worsen the carbon balance of a forest ecosystem.”

Silva-Junior says this kind of research helps improve estimates of carbon emissions from decomposition and can help plan to mitigate and prevent emissions from these forests. He adds:

“We still need to go beyond the humid tropical forests; little is known about the effect of the various driving forces for degradation in e.g. tropical dry forests. In addition, researchers also need to focus on other gaps, such as understanding the impact of forest degradation on biodiversity.”

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Mills, MB et al. (2023) Post-deforestation tropical forests are a persistent net source of carbon to the atmosphere, Proceedings of the National Academy of Sciences, doi:10.1073/pnas.2214462120

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