The remaining undisturbed rainforests in Borneo, some of the world’s tallest and most carbon-dense, have been removing carbon dioxide from the atmosphere over the past 50 years, a new study shows.
The research, led by the University of Leeds, found that while the intact forest area has declined, those that remain have increased in biomass. The study quantifies for the first time the role of South East Asian tropical rainforests in the fight against climate change.
An international team of more than 50 scientists monitored tens of thousands of trees in over a dozen locations across Borneo for up to half a century, revealing the rainforest’s steady increase in biological material – biomass – as well as its vulnerability to climate and land-use changes.
Lead author Dr Lan Qie, who carried out the work whilst at the University of Leeds’ School of Geography, said: “Borneo’s remaining rainforests are increasing in size, adding to their already high carbon stocks.
“The average increase we saw in Borneo is equivalent to adding 700 household Christmas trees to each 100 metre by 100 metre forest plot, each year. There is of course both growth and death in these towering and dynamic tropical forests, but on the whole they are absorbing more carbon year after year, storing it as wood.”
The team’s research indicates that Borneo’s average increase in forest biomass carbon, 430 kg per hectare per year, is consistent with the increases shown across the tropical African and Amazon forests in the past.
Co-author Professor Simon Lewis, also from the University of Leeds, said: “After conducting dozens of field campaigns across the tropics over the past two decades, we can now finally say that the world’s remaining intact tropical forests, across the Amazon, Africa and Asia, are all acting as carbon sinks – absorbing more carbon than they are releasing.
“It is now clear that undisturbed tropical rainforests across the world are all providing an important service to humanity in removing carbon from the atmosphere, adding a further reason to protect these vulnerable forests.”
The international forest monitoring network of researchers that contributed to this work was developed with support from a European Research Council grant awarded to Professor Oliver Phillips and Professor Simon Lewis, both at the University of Leeds.
The study highlights two threats to continued carbon uptake by rainforests: droughts and forest fragmentation. Co-author Professor Oliver Phillips said: “By monitoring forests before, during and after the 1997-1998 El Niño drought we were able to show how the drought killed trees, so returning carbon to the atmosphere and halting the forest’s ability to act as a carbon sink. This illustrates the risk posed by future droughts, which climate models suggest will become more severe, as rainforests across the globe may temporarily pause in their roles as carbon sinks.”
The study also highlights concerns about the carbon uptake in fragmented patches of forests – sections of forest that are isolated from the larger forest. Monitored forest areas close to the edge of a patch, which may be adjacent to burnt land, oil palm plantations or farmers’ fields, tended to lose carbon to the atmosphere. Trees were more likely to die if they were closer to the edge, and the tree species that replace them tended to be those that store less carbon.
Dr Lan Qie, who is now at Imperial College London, added: “Our calculations indicate a minimum size that a patch of forest must be in order for it to be a carbon sink – where its interior is absorbing enough carbon to outweigh its edges, which may be losing carbon. A forest reserve of 300 hectares, a little over one square mile, is just about big enough.
“We must aim to preserve larger areas of continuous forest, and protect against further fragmentation, in order to maintain its ability to uptake carbon emitted by human activity.
“Effective buffer zone management can also reduce carbon loss along forest edges and very small forest fragments can still help protect carbon. Preserving existing fragments of forest of any size remains important, for both carbon storage and biodiversity conservation.”
Professor Simon Lewis added: “This study can provide a foundation for future long-term observations of Asia’s tropical forests, delivering essential baseline information on how these globally important ecosystems are responding to rapid global environmental change.”