Even in recovery, previously logged tropical forests are carbon sources: Study

Tropical forests play a key role in our efforts to stabilize the global climate. As a readymade means of ameliorating our continued greenhouse gas emissions, tree-clad tropical landscapes provide us with some much-needed hope. But as we relentlessly degrade and disturb them, how are we affecting their crucial capacity to lock away carbon dioxide from the atmosphere?

This is a question scientists have investigated for decades. Recent studies, for instance, have revealed that intense deforestation and fires have transformed parts of the Brazilian Amazon from massive carbon sinks to sources. While the situation in the Amazon is cause for concern, frustratingly little is known about the true forest carbon balance in other parts of the world, such as Southeast Asia.

Now, a new study indicates that logged forests in Malaysian Borneo are a “significant and persistent” net source of carbon emissions for at least one decade after disturbance.

Based on their results, the authors say that rates of carbon sequestration in recovering tropical forests around the world could be “considerably lower than currently estimated.”

The findings, published in Proceedings of the National Academy of Sciences, counter the commonly held belief that recovering and degraded forests become net carbon sinks due to the rapid regrowth of trees and other vegetation on newly vacated land. This reasoning is premised on studies that have typically concentrated on quantifying only the aboveground biomass in recovering forests.

In contrast, the new study accounts for both aboveground and belowground carbon emissions and uptake. This approach enabled the research team to detect emissions stemming from organic matter in disturbed soils and the decay of deadwood, factors that offset any carbon gains from new tree growth in recovering forests.

The research represents one of the most robust calculations of the full carbon balance in disturbed forests to date, according to Thomas Pugh, an associate professor from Lund University in Sweden and the University of Birmingham in the U.K, who was not involved in the research. “This is one of the first studies I’ve noticed that really carefully quantifies what’s happening with the release of emissions from dead material following logging events,” Pugh told Mongabay.

Between 2011 and 2017, the international team of researchers led by Terhi Riutta, a postdoctoral researcher at the University of Exeter, U.K., continuously monitored a series of forest plots in the Malaysian states of Sabah and Sarawak.

The team used two distinct methods to quantify CO2 exchange between the forest and the atmosphere: one comprised detailed on-the-ground observations of plant growth and respiration rates; the other involved measurements taken from the top of an eddy covariance tower — a mast that rises high above the forest canopy equipped with tools to measure gas exchange.

The former method allowed them to obtain fine-tuned details on where emissions were coming from, and the latter allowed them to calculate net carbon source and sink values over time.

By comparing measurements taken in unlogged forests with those gathered in forests that had undergone various levels of logging over the previous several decades, they demonstrated that logged forests were unequivocally net carbon sources. On the other hand, unlogged forests were more or less carbon neutral.

Maria Mills, a doctoral candidate at the University of Leicester in the U.K. and lead author of the study, said that while logged forest plots did accumulate more woody biomass compared to unlogged forests, as was expected, these carbon gains were negated by larger losses from deadwood and soil.

“The deadwood losses could be due to damage and mortality associated with logging — trees get damaged [by machinery], and they will eventually die and rot away on the ground, releasing carbon as they degrade,” Mills told Mongabay. Crucially, they found that emissions increased with logging intensity and continue for at least one decade after logging has ceased.

Riutta said a move toward less intensive logging levels and improved practices, such as minimizing the logging footprint by carefully cutting liana vines that can entangle and pull down adjacent trees, could help to reduce emissions. “We need to recognize that improving logging practices and sticking to sustainable logging levels is important.”

According to Riutta, the findings shouldn’t lead people to believe that degraded tropical forests aren’t important. “Although logged forests don’t harbor as many species as unlogged forests, they are really important for biodiversity [and] they absolutely have conservation value compared to having them converted into different land uses, like oil palm or plantation,” she said.

Indeed, a recent study by some of the same researchers and focused on several of the same forest plots in Malaysian Borneo found that logged forests had more abundant birds and mammals than patches of pristine forest, demonstrating their value for biodiversity conservation and the need to safeguard them.

Kristina Anderson-Teixeira, leader of the ForestGEO ecosystems and climate program at the Smithsonian Institution, who was not involved in either study, said the new findings emphasize the carbon costs of logging and the pitfalls of relying on natural ecosystems to absorb our continued greenhouse gas emissions.

“[The study] shows that we are overestimating the capacity of logged tropical forests to take CO2 out of the atmosphere and thereby help mitigate climate change,” Anderson-Teixeira told Mongabay in an email. She added that the results also spotlight the urgency of reducing fossil fuel emissions.

With tropical landscapes the world over increasingly dominated by logged and otherwise degraded forests, building a clearer picture of precise carbon losses and gains is paramount to the development of effective global climate policy. For Pugh, this means investing in on-the-ground research like the Borneo study, that takes account of both aboveground and belowground carbon emissions and uptake.

“We absolutely have to understand the whole forest,” Pugh said. “If we have our assumptions wrong, then we’re making our calculations on a false premise.”

Banner image: The research team used an eddy covariance flux tower pictured here rising above a heavily logged tropical forest landscape within the Stability of Altered Forest Ecosystem (SAFE) Project in Malaysian Borneo. Photo courtesy of Maria Mills

Citations:

Mills, M. B., Malhi, Y., Ewers, R. M., Kho, L. K., Teh, Y. A., Both, S., … Riutta, T. (2023). Tropical forests post-logging are a persistent net carbon source to the atmosphere. Proceedings of the National Academy of Sciences, 120(3). doi:10.1073/pnas.2214462120

Malhi, Y., Riutta, T., Wearn, O. R., Deere, N. J., Mitchell, S. L., Bernard, H., … Struebig, M. J. (2022). Logged tropical forests have amplified and diverse ecosystem energetics. Nature, 612(7941), 707-713. doi:10.1038/s41586-022-05523-1

Carolyn Cowan is a staff writer for Mongabay. Follow her on Twitter @CarolynCowan11

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