Partially logged forests may be emitting more carbon than was previously assumed, raising doubts about the ability of selective logging to reduce global warming.
A study, published on 28 April in Environmental Research Letters, shows that conventional assessments of emissions in partially logged forests are wrong as they dramatically underestimate the prevalence of deadwood, which releases carbon as it decays.
Marion Pfeifer, an ecologist at Imperial College London, United Kingdom, and the study’s lead author, tells SciDev.Net that, based on estimates from intergovernmental body the International Tropical Timber Organization, more than a third of tropical humid forests are being logged, with the vast majority in developing countries. “For these forests, traditional methods underestimate the deadwood carbon pool considerably,” she says.
Current estimates of global carbon emissions from forests are based on the assumption that all forests contain the same amount of deadwood.
However, the study found that this plant debris makes up less than 20 per cent of total above ground biomass in pristine forests, but up to 64 per cent in partially logged forests, where high-value trees had been cut down.
The research involved surveying a large area of rainforest in Malaysian Borneo at the Stability of Altered Forest Ecosystems (SAFE) Project site, one of the largest ecological studies in the world.
The research team found that global carbon emissions from forests may be incorrectly accounted for, as they assume that all forests contain the same amount of deadwood.
“Many studies of carbon stocks in tropical humid forests do not measure the carbon stored in deadwood, but rather assume that deadwood carbon stocks are a percentage of live tree carbon stocks,” says Pfeifer. Selective logging is on the rise, and is often encouraged in poorer countries by environmental watchdogs in an effort to reduce both the damage done to forests by logging and the industry’s carbon emissions. As a result, selective logging features increasingly in international climate negotiations as a pressure point to force governments to better manage and protect their forests.
“Using literature estimates won’t be enough to get an accurate picture of what’s happening in terms of carbon storage and emissions on the ground, especially in logged forests stands,” says Pfeifer. “You need to account for potentially high carbon emissions in the near future of selectively logged forests after the logging has occurred.”
Timothy Pearson, an ecosystem service expert at Winrock International, an NGO in the United States, says the UN Framework Convention on Climate Change is moving towards a carbon accounting approach that assumes committed emissions occur at the time of deforestation or forest degradation. This would mean that all deadwood stocks produced at the time of logging are assumed to immediately go up into the atmosphere.
But Pearson points out that, if left untouched, those stocks may take ten or even 20 years to fully decompose and so for emissions to be complete.