That's according to researchers in the US and Brazil who have been looking at the fragmentation of the Amazon rainforest. Previous work had shown that biomass collapse at the edge of the remaining forest is significant, so Izaya Numata and his colleagues from South Dakota State University, US, and IMAZON in Brazil aimed to quantify this effect in terms of carbon emissions.

When deforestation occurs, tree mortality at the edge of the remaining forest increases dramatically due to changes in temperature and microclimate. This in turn boosts carbon emissions when the biomass decomposes.

"We found that edge-released carbon accounted for 2.6–4.5% of deforestation-related carbon emissions between 2001 and 2010," Numata told environmentalresearchweb. "However, the two halves of the decade experienced different deforestation rates. While more forest was lost in the first half of the decade, the second half saw a more fragmented loss, which created a higher proportion of forest edge. This resulted in the relative importance of carbon emissions from forest fragmentation increasing from 1.7–3.0% (2001–2005) to 3.3–5.6% (2006–2010)."

While Numata admits that these figures may not seem large, he points out that, when added to other types of forest degradation such as selective logging and fires, the combined carbon emissions from these effects may exceed 10% of deforestation-based carbon flux estimates.

Numata and his colleagues hope that this work will help increase the accuracy of carbon accounting for REDD projects. They also recommend that larger contiguous blocks of forest should be maintained wherever practicable. "A highly fragmented forest landscape will have a proportionally higher forest edge and therefore relatively higher emissions," said Numata.

The researchers published their study in Environmental Research Letters (ERL).