Logged tropical forest areas found to be a carbon source

A new study has found that, contrary to previous assumptions, tropical forests recovering from logging are carbon sources for years afterward.

Previously, researchers believed that recovering logged tropical forests were carbon absorbers as the new trees grow quickly. However, in a new study, led by Imperial College London researchers, it has been revealed that these tropical forests are actually a carbon source, as carbon released by soil and rotting wood outpaces the carbon absorbed by new growth.

The work, ‘Tropical forests post-logging are a persistent net carbon source to the atmosphere,’ which monitored carbon in forests in Malaysian Borneo as part of the Stability of Altered Forest Ecosystem (SAFE) Project, is published in the journal Proceedings of the National Academy of Sciences.

The new study detected that recovering forests are carbon sources

Previous studies of recovering forests focused on measuring tree growth to estimate the amount of carbon taken from the atmosphere, but the new study also measured how much carbon was coming from the ground. From this, the team was able to calculate the carbon budget from the incoming and outgoing carbon flows for the logged and unlogged (old-growth) forests.

In the study, logged forest plots had experienced logging at different stages over the prior few decades. The measurements were taken between 2011 and 2017.

The researchers measured the carbon released from the ground by using a portable carbon dioxide monitor to test patches of ground and pieces of deadwood in several plots monthly for several years. A 52-metre-tall tower above the forest canopy was also set up by the team to measure the ‘flux’ of carbon into and out of the forest to see whether it was a net source or sink of carbon.

It was found that unlogged forested areas are generally carbon neutral, but a moderately and heavily logged tropical forest area is a carbon source. The team estimates an average carbon source of 1.75 +/- 0.94 tonnes of carbon per hectare within moderately logged plots and 5.23 +/- 1.23 tonnes of carbon per hectare in severely degraded plots. These carbon emissions continue at these rates for at least one decade after logging.

© shutterstock/Maksim Safaniuk

Co-author Professor Rob Ewers, from the Department of Life Sciences at Imperial, said: “The measurements from the tower show us whether the forest area is a source or a sink of carbon, and the soil monitoring tells us why this is. From these measurements, we know logged forests are still a source of carbon up to a decade after they have been logged, and that this primarily comes from organic matter in the soil or from rotting wood.”

A reassessment of the role of recovering logged forests in the global carbon budget is required

The team concluded that carbon monitoring should be done in other forests to create a more accurate picture of how logged forests contribute to global carbon budgets.

First author Maria Mills, who began the work at Imperial and completed it at the University of Leicester, said: “Our results show that for the tropical forest we studied, logged areas are a source of carbon even a decade after logging has occurred. This means we need to reassess their role in global carbon budgets – we can no longer apply the blanket assumption that they are carbon sinks.”

Lead researcher Dr Terhi Riutta, now at the University of Exeter, said: “A lot of the carbon released in recovering forests is from collateral damage – trees that have died as a result of damage during the logging operations left to rot, and from disturbed soil. Logged forests still have value – we know they have a unique biodiversity – so making sure they are also not releasing extra carbon through better logging practices will boost their sustainability.”

The result of the study highlights the necessity of logging practices that minimise collateral damage to improve the sustainability of the industry.

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