Tropical trees in Australia’s rainforests have been dying at double the previous rate since the 1980s, seemingly because of climate impacts, according to the findings of a long-term international study published in Nature today. This research has found the death rates of tropical trees have doubled in the last 35 years, as global warming increases the drying power of the atmosphere.
Deterioration of such forests reduces biomass and carbon storage, making it increasingly difficult to keep global peak temperatures well below the target 2 °C, as required by the Paris Agreement. Today’s study, led by researchers from the Smithsonian Environmental Research Center and Oxford University, and French National Research Institute for Sustainable Development (IRD), has used uniquely long data records from across Australia’s rainforests.
It finds average tree death rates in these forests have doubled over the past four decades. Researchers found trees are living around half as long, which is a pattern consistent across species and sites across the region. And the impacts can be seen as far back as the 1980s, according to the team.
Susan Laurance, Professor of Tropical Ecology at James Cook University, adds, ‘Long-term datasets like this one are very rare and very important for studying forest changes in response to climate change. This is because rainforest trees can have such long lives and also that tree death is not always immediate.’
Recent studies in Amazonia have also suggested tropical tree death rates are increasing, thus weakening the carbon sink. But the reason is unclear.
Intact tropical rainforests are major stores of carbon and until now have been ‘carbon sinks’, acting as moderate brakes on the rate of climate change by absorbing around 12% of human-caused carbon dioxide emissions.
Examining the climate ranges of the tree species showing the highest death rates, the team suggests the main climate driver is the increased drying power of the atmosphere. As the atmosphere warms, it draws more moisture from plants, resulting in increased water stress in trees and ultimately increased risk of death.
When the researchers crunched the numbers, it further showed the loss of biomass from this mortality increase over the past decades has not been offset by biomass gains from tree growth and recruitment of new trees. This implies the mortality increase has translated into a net decrease in the potential of these forests to offset carbon emissions.
The research team included colleagues from Oxford University, James Cook University (Australia), and other institutions (UK, France, USA, Peru).
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