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Rainforest trees in Australia's rainforests have been dying at twice the rate since the 1980s, apparently due to climate impacts. This is what the findings of a long-term international study published this Wednesday in the popular science journal Nature show.
Rainforest trees
Research has found that Rainforest trees mortality rates have doubled in the last 35 years, as global warming increases the drying effect of the atmosphere. The deterioration of these forests reduces biomass and carbon storage, making it increasingly difficult to keep global maximum temperatures well below the 2 degree Celsius target, as required by the Paris Agreement.
The study, led by researchers from the Smithsonian Environmental Research Center and the University of Oxford, and the French National Research Institute for Sustainable Development (IRD), used exceptionally long data records from Australia's rainforests. It finds that the average rates of tree death in these forests have doubled in the last four decades. The researchers found that the trees live about half the time, which is a consistent pattern across all species and sites in the region. In addition to that impacts can be seen since the 1980s, according to scientists.
David Bauman, a tropical forest ecologist at the Smithsonian, Oxford and IRD, and lead author of the study, says: "It was shocking to detect such a marked increase in tree mortality and a consistent trend in the diversity of species and sites that we study. A sustained doubling of mortality risk would imply that carbon stored in trees returns twice as fast to the atmosphere ."
Climate change as a factor
Sean McMahon, the Senior Research Scientist at the Smithsonian and the study's lead author, says: "Many decades of data are needed to detect long-term changes in long-lived organisms, and the signal of a change can be overwhelmed by the noise of many processes." Drs. Bauman and McMahon write in the report: "A remarkable result of this study is that we not only detected an increase in mortality, but this increase appears to have started in the 1980s, indicating that the natural systems of the Earth may have been responding to climate change for decades."
Oxford Professor Yadvinder Malhi, the co-author of the study, says: "In recent years, the effects of climate change on corals in the Great Barrier Reef have become well known. Our work shows that if you look inshore from the reef, Australia's famous rainforests are also changing rapidly.
Furthermore, the likely driving factor we identified, the increasing drying effect of the atmosphere caused by global warming, suggests similar increases in tree mortality rates in the world's tropical forests. "
It is because of this suspicion that scientists fear that forests will go from being carbon sinks to emitters: "If that is the case, tropical forests could soon become sources of carbon, and the challenge of limiting global warming to well below 2 degrees Celsius becomes more urgent and more difficult.
water stress
Recent studies in the Amazon have also suggested that tropical tree mortality rates are increasing, weakening the carbon sink. But the reason is not clear. Intact forests are major carbon stores and have so far been carbon sinks, acting as moderate brakes on the rate of climate change by absorbing about 12% of carbon dioxide emissions caused by human activity.
By examining the climatic ranges of the tree species that show the highest mortality rates, the team suggests that the main driver of climate is the increasing drying effect of the atmosphere. As the atmosphere warms, it draws more moisture from plants, leading to greater water stress on trees and ultimately a higher risk of death.
When the researchers analyzed the numbers, they further showed that the biomass loss from this increased mortality in recent decades has not been offset by the biomass gains from tree growth and the recruitment of new trees. This implies that increased mortality has translated into a net decrease in the potential of these forests to offset carbon emissions.
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