"We provide the first on-the-ground evidence of a large sink in trees in undisturbed African tropical forests, removing 1.2 billion tonnes of carbon dioxide from the atmosphere each year over recent decades," Simon Lewis of the University of Leeds, UK, told environmentalresearchweb. "The increase could be caused by recovery from past large-scale disturbances across African and Amazonia, or due to global environmental changes giving trees better conditions for growth."
Lewis reckons that global change is the more likely explanation, because of the similar response on both continents, the difference in likely disturbance histories, and the lack of known historical disturbances in the fairly recent past. "Given what we know about photosynthesis, fertilisation by increasing atmospheric carbon dioxide may be the cause," he said.
To carry out the study, Lewis and colleagues measured the change in diameter of more than 70,000 trees in 10 African countries over time. Combining these measurements with the height and density of the trees (which depends on the species), indicated how much the amount of carbon storage was changing. Between 1968 and 2007, above-ground carbon storage increased by 0.63 Mg of carbon per hectare of tropical forest per year.
But whatever lies behind it, the increase in carbon storage will stop at some point as trees cannot continue increasing in size forever. What's not clear is when this will happen. "Some models predict a continuing sink for many decades, others that the sink will turn off in the decades to come, while others still predict a switch to a source of carbon as the forests are affected by high temperatures and occasional but severe droughts," said Lewis.
As a result, the emissions cuts needed to stabilize atmospheric carbon dioxide levels must take into account a likely diminishing sink in tropical forest trees, say the researchers.
Next Lewis and colleagues would like to understand what is causing the sink in tropical forest trees. That should enable them to develop robust predictive models of when the sink will begin to diminish, and whether the forests will become sources of carbon to the atmosphere, which they reckon is a real possibility as air temperatures rise.
The researchers reported their work in Nature.