Deforestation Could Be Worse for Carbon Emissions Than We've Calculated

Deforestation is often seen as a culprit for rising levels of carbon emissions in our atmosphere: Logic states that the fewer trees we have, the fewer there are to take in excess carbon dioxide via photosynthesis. But what's been relatively unaccounted for up until now is that dead and decaying trees left behind as a by-product from logging also emit a CO2-rich swan song.

In a stu​dy published in Environmental Research Letters today, researchers at Imperial College London have suggested that global carbon emissions from forests may have been miscalculated, as they have not accounted for the CO2 being produced from dead and decaying wood left behind by loggers.

The researchers surveyed a large area of rainforest in Malaysian Borneo at the Stab​ility of Altered Forest Ecosystems (SAFE) site—one of the largest ecological studies in the world, spanning 8,000 hectares of rainforest. Kickstarted in June 2011, the ten-year project aims to investigate the impact of deforestation on the ecosystem.

The researchers assessed the contributions of the dead wood to CO2 emissions across a "range of landscapes including pristine forest, logged forests of increasing severity, and oil palm plantation."

Dead wood is usually created through natural process in forests that aren't logged, where it makes up less than 20 percent of the total aboveground biomass of the forest. Previously, researchers had based their carbon emission calculations from logged rainforests on the assumption that when logged trees were taken out of the forest, the amount of dead wood was also taken away with it.

But when loggers rip through the forest, natural dead wood production cycles are destroyed. "Selective logging"—where loggers only pick and remove the best parts of the forest—is a growing trend that has caused significant damage to trees and caused dead wood, which can account for up to 64 percent of the total aboveground biomass of the forest that the researchers surveyed.

Author of the study Marion Pfeifer, from the Department of Life Sciences at Imperial College, noted in a statement that the large and diverse landscape surveyed by the team produced field data that supported their conclusions. However, she added that the research group now needed to focus on applying their results to carbon emissions calculations, for example, "determining how fast the wood is decomposing."

Pfeifer noted that selectively-logged tropical forests make up 30 percent of rainforests worldwide, leading global calculations to be wrong "at least 30 percent of the time."

"That such logged forests are not properly accounted for in carbon calculations is a significant factor," said Pfeifer. "It means that a large proportion of forests worldwide are less of a sink and more of a source, especially immediately following logging, as carbon dioxide is released from the dead wood during decomposition."

Changes in agriculture, forestry, and land-use are said to account for close to 25 percent of global greenhouse emissions. Living trees absorb carbon dioxide for pho​tosynthesis, a process through which they take in light and CO2 to make glucose, release oxygen as a by-product. Dead trees release carbon dioxide as they decay.

Understanding the proportion of living trees and decaying ones will allow researchers to determine whether vast swathes of forest act as "sinks", which help absorb CO2 from the air, or are in fact CO2 producers.