Researchers find the emissions released by soil and decaying wood outweigh the CO2 absorbed by new trees in logged forests.
(Bloomberg) — When carbon-storing trees are cut down for timber and new ones grow in their place, those young trees also absorb carbon. That might seem to suggest the new trees make up for the loss of the old ones.
Not so fast, says a new study.
A paper published today in Proceedings of the National Academy of Sciences finds that logged tropical forests are net producers of carbon for years as they recover from tree removal. Logged forests emit carbon from soil matter and decaying wood. Those emissions outweigh the amount of carbon that the regrowing forests absorb for at least a decade, according to the six-year study.
Previous research has focused on how much carbon new trees remove, but did not necessarily account for the carbon emissions from the soil and decaying wood, according to lead author Terhi Riutta of the University of Exeter. The new findings hint that the absorption potential of logged tropical forests may, in fact, be smaller than previously estimated.
“That’s how logging has often been justified: that you are replacing something that is old, that is not growing very much, by something younger that is growing very fast,” said Riutta. “But it’s not the whole truth at all.”
The team studied a tropical forest landscape in Malaysian Borneo, a region characterized by forest degradation, and compared the net carbon exchange of a logged area with nearby unlogged areas. Using two different methods, the scientists concluded that the logged area had produced a substantial amount of carbon for at least 10 years. The research was initiated at Imperial College London as part of the Stability of Altered Forest Ecosystem (SAFE) project.
The researchers found that a major source of carbon emissions in the studied area was rotting trees or branches, many of which are initially damaged in the logging process.
More targeted logging practices that reduce damage to surrounding trees could decrease CO2 emissions, says Riutta, although there’s still much to be learned about the impacts of logging practices on emissions, and the impacts of cutting down trees may vary by site.
The study highlights the need for further research in other tropical areas as well as more accurate estimates regarding global carbon emissions from logged areas, Riutta says. She adds that it’s unclear whether plots of land that are logged more sustainably could act as carbon sinks, meaning they absorb more carbon than they generate. But her best guess is that better practices could at least yield a carbon neutral environment.
“If fewer trees die, then the emissions will decrease,” she said. “That’s very simple.”
Knowing whether or not logged tropical forests are net carbon emitters “is crucial to our net zero ambitions,” said Thomas Pugh, a scientist who researches forests and climate change at Lund University and the University of Birmingham and who was not involved in the study.
“Measuring the whole-ecosystem carbon change, such as this team have done, is painstaking work, but incredibly important,” Pugh said. “Policymakers should take note of this.”
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