Sebastiaan Luyssaert and colleagues at Antwerp University reckon that 15% of the world’s old forests, which are not usually considered when offsetting carbon dioxide emissions, provide at least 10% of the global terrestrial carbon sink. So disturbing these forests would release huge amounts of carbon into the atmosphere and seriously contribute to climate change.

Luyssaert and colleagues say old forests continue to store carbon over time periods of centuries, mainly in live woody tissues and decomposing leaf litter and soil. Although young forests admittedly store more carbon each year, they contain less biomass. As a result, the total amount of carbon captured from the atmosphere in these younger forests is lower.

The researchers obtained their results by studying existing measurements of how much carbon is absorbed by and released from old forests in temperate and boreal regions around the world. These measurements included biomass studies combined with simple ecological modelling; productivity ratio (the amount of carbon added each year to forests compared to that released from the decomposition of dead plant matter); and air flow in and out of forests.

The team concluded that forests between 15 and 800 years of age are not carbon neutral as previously believed but can sequester around 1.3 gigatonnes of carbon per year. They are therefore crucial long-term carbon sinks and disturbing them would release vast quantities of carbon into the atmosphere. Most larger old forest landscapes are located in Russia, Canada, Alaska and the US Pacific Northwest, with smaller ones found in northern Scandinavia.

Luyssaert suggests that old forests should perhaps be protected under carbon trading schemes and other measures to combat climate change. Money being invested in newer forests could instead be directed towards protecting older ones, he says.

"We also need to create awareness," he told environmentalresearchweb, "because deforestation is not just a tropical problem."

Luyssaert's team concentrated on forests in boreal and temperate regions since the data the researchers needed for tropical regions is lacking.

The researchers reported their work in Nature.