The fact that trees in forests die off is part of the natural cycle. But since the 1980s, it's been happening faster and more frequently in Australian rainforests than ever before. The forests are thus changing from a sink to a source of carbon.
In the Australian rainforests, tree mortality has doubled since the 1980s. According to the analyzes of an international research team, climate change is responsible for this, more precisely a combination of rising temperatures and drier air. The team writes in the journal "Nature" that this turns the former carbon sinks into carbon sources. This trend, which has also been observed in other rainforests, is jeopardizing the goal of the Paris climate agreement to keep global warming well below two degrees Celsius compared to pre-industrial times.
"It was a shock to find such a pronounced increase in tree mortality, and also that the trend extended across the variety of species and areas examined," said lead author David Baumann from the University of Oxford in England. "A permanent doubling of the death rate means that the carbon stored in trees returns to the atmosphere twice as fast."
There are now indications that other tropical forests are also evolving from carbon sinks to sources - such as the world's largest rainforest in the Amazon region. According to the researchers, tropical rainforests absorb about 12 percent of man-made CO₂ emissions and thus slow down the greenhouse effect.
In the study, the team - including those from Australia, Great Britain, France and the USA - examined the growth of 81 tree species in various areas of the Australian state of Queensland from 1971 to 2019, measuring more than 70,000 trees. Accordingly, the mortality of the trees increased significantly from the mid-1980s, for almost all species and in almost all of the more than 20 areas examined.
According to their analyses, the researchers consider two developments to be particularly significant: increased maximum daily temperatures and - related to this - a reduced moisture saturation of the air (vapor pressure deficit). Both factors increased over the 49-year period, and mortality increased most where this trend was most pronounced. Tropical cyclones may have contributed to the trend, but would not explain it on their own, the researchers write.
In general, the increased tree mortality is due to the fact that the warmer atmosphere removes more moisture from the plants. This increases their susceptibility to other adverse influences, such as the consequences of storms or diseases. The loss of biomass was therefore not offset by new trees or stronger growth.
The research team warns that this process may also affect the tropics outside of Australia. "If that is the case, tropical forests may soon become sources of carbon, and the task of limiting global warming to well below two degrees Celsius becomes both more urgent and more difficult," says co-author Yadvinder Malhi.