Tropical trees dying twice as fast from climate change

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Climate change, according to a new study, may have caused rainforest trees to die more quickly beginning in the 1980s.

The findings of a long-term worldwide research published in Nature on May 18, 2022, demonstrate that since the 1980s, tropical trees in Australia's rainforests have been dying at a pace twice as fast as previously, likely owing to climate changes.

According to this study, as the drying influence of the environment has grown owing to global warming, tropical tree death rates have more than quadrupled over the previous 35 years.

The degradation of such forests reduces biomass and carbon storage, making it more difficult to meet the Paris Agreement's obligation to keep global peak temperatures far below the objective of 2 degrees Celsius. The latest research, led by scientists from the Smithsonian Environmental Research Center, Oxford University, and the French National Research Institute for Sustainable Development (IRD), examined comprehensive data sets from Australia's rainforests.

It discovers that during the previous four decades, the average tree death rate in these woodlands has more than doubled. Researchers determined that trees are surviving about half as long as they used to, and this is consistent across species and places throughout the region. The researchers believe the impacts may be traced back to the 1980s.

According to Dr. David Bauman, a tropical forest ecologist at Smithsonian, Oxford, and IRD, and lead author of the study, “It was a shock to detect such a marked increase in tree mortality, let alone a trend consistent across the diversity of species and sites we studied. A sustained doubling of mortality risk would imply the carbon stored in trees returns twice as fast to the atmosphere.”

The senior research scientist at Smithsonian and senior author of the study Sr. Sean McMahon stated, “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.”

Bauman and McMahon highlight that one of the most amazing results from the study is that not only has the mortality rate increased, but it has been increasing since the 1980s, which suggests that Earth has been suffering due to climate change for several decades.

Oxford Professor Yadvinder Malhi, a study co-author, highlighted that corals in the Great Barrier Reef have also suffered from climate change recently.

“Our work shows if you look shoreward from the Reef, Australia’s famous rainforests are also changing rapidly. Moreover, the likely driving factor we identify, the increasing drying power of the atmosphere caused by global warming, suggests similar increases in tree death rates may be occurring across the world’s tropical forests. If that is the case, tropical forests may soon become carbon sources, and the challenge of limiting global warming well below 2 °C becomes both more urgent and more difficult.”

Read more: Australia's Great Barrier Reef suffering 'widespread' bleaching

Susan Laurance, Professor of Tropical Ecology at James Cook University, added that “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 Amazonian research has also found that tropical tree mortality rates are rising, decreasing the carbon sink. However, the reason remains unknown.

Intact tropical rainforests are important carbon sinks, functioning as mild brakes on the rate of climate change by absorbing around 12% of human-caused carbon dioxide emissions.

Examining the temperature ranges of the tree species with the greatest mortality rates, the team concludes that the key climate driver is increased atmospheric drying power. As the atmosphere heats, it takes more moisture from plants, causing increasing water stress and, eventually, an increased risk of mortality in trees.

When the researchers crunched the statistics, they discovered that the loss of biomass from the recent spike in mortality has not been compensated by biomass gains from tree growth and recruitment of new trees. This suggests that the increased mortality has resulted in a net loss in the ability of these forests to offset carbon emissions.