All trees help to mitigate CO2 build-up by absorbing carbon from the atmosphere and depositing it in their growing tissues. How effectively a whole forest does this depends on how many trees are growing and how many are dying at any one time.
In total, the world’s forests suck up 33 per cent of anthropogenic emissions; of that figure, 66 per cent is performed by tropical forests. Those tropical forests that remain intact – portions of the Amazon, and the Congo rainforest, for example – cover 10 per cent of the planet’s total land area, yet represent 60 per cent of the entire terrestrial carbon sink.
At the more granular level, the largest one per cent of trees within these intact tropical forests (those of more than 60cm in diameter in breast height) typically contain 50 per cent of the forest’s aboveground biomass, and are therefore some of the most precious living vessels of terrestrial carbon.
Small alterations to the numbers of large tropical trees living and growing at any one point therefore have planetary impacts on the world’s terrestrial carbon sink. ‘The net carbon balance depends on the two components of tree growth and tree death,’ explains University of Birmingham forest ecologist Adriane Esquivel Muelbert. This is because tree death is the beginning of a decomposition process that unlocks carbon and releases it back to the atmosphere. ‘We need to understand this dynamic and the causes of tree death to be able to properly know and model how forests will sequester carbon in the coming years.’
Surprisingly, however, modern science has little understanding of what naturally kills trees in tropical forests. What is known, is that mortality of trees in the tropical forests of Amazonia and Africa has been rising. This has some serious knock-on effects. A 2020 study demonstrated that, due to rising tree mortality, the carbon-storage potential of the world’s tropical forests is waning. Sometime during the 1990s, Amazonian tropical forests reached a saturation point in the amount of carbon being stored. African tropical forests reached the same point later in the 2000s.
Some ecologists believe there is a lag phenomenon occurring here: anthropogenic emissions initially caused a spike in tree growth, which has now ended and come back to bite us. ‘There is a hypothesis that rising CO2 levels have been fertilising forests, making trees grow big and allowing them to accumulate more carbon,’ says Muelbert. ‘But because of this accelerated growth, trees are also likely to die younger because they’re less likely to invest in their supporting structures or defence systems.’
The researchers stress that this type of natural tree mortality needs to be considered alongside deforestation. ‘Deforestation is a critical problem that we have in the tropics. We need to keep forests and carbon in the ground,’ says Muelbert. ‘But even if we do, if mortality is still high, we will continue losing that carbon-storage potential. So we must improve our understanding of the natural processes of tree mortality for us to better predict what will happen in the future.’ Filling this research gap, she says, is a critical piece of the carbon-sequestration puzzle.
It’s a difficult scientific pursuit, however. Trees’ long lifespans complicate data collection and the exact moment of death is abstruse (a tree may narrowly survive a lightning strike one year, only to be invaded by decomposing fungi the next, before finally succumbing to a windstorm the following year) and tropical forests are extremely complex, with many mutualistic and parasitic relationships between tree species, animals and other plants. Muelbert, like many other forest researchers, has issued a clarion call: keep research flowing into forests, solve the riddle of tree mortality and better understand the carbon sinks on which we depend.