Scientists say rainforest trees dying twice as fast as four decades ago

By Mark Waghorn via SWNS

Trees in the rainforest are dying twice as fast as four decades ago, according to new research.

Life expectancy has halved in the wet tropics of North Queensland - an area stretching 280 miles along the north east coast of Australia.

Global warming is increasing the drying power of the atmosphere. More extreme weather events such as cyclones are also being blamed.

A similar phenomenon has been identified in the Amazon, Indonesia, Malaysia and Papua New Guinea.

It is speeding up climate change, scientists warn. Trees act as a carbon sink - storing greenhouse gas from the air as they grow.

Lead author Dr David Bauman, an ecologist at Oxford University, said: "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 international team identified mortality patterns dating back half a century by tracking 74,135 trees belonging to 81 different species on two dozen forest plots.

They found annual death risk doubled, on average, across all sites and types between 1984 and 2019. Trees in drier locations were most prone.

Atmospherics water stress may be the main cause - increasing premature demise in moist tropical forests.

When trees die, they give up their stored carbon gradually, in the form of methane, another greenhouse gas.

It adds to evidence that climate change models overestimate the benefits of forests - because trees die off sooner.

Co-author Professor Yadvinder Malhi, also from Oxford, said: "In recent years the effects of climate change on the corals of the Great Barrier Reef have become well known.

"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."

They are critical to the global carbon cycle - influencing the pace of climate change.
Previous research has suggested tree deaths are accelerating.

The study sheds light on the mechanisms - and if particular species are especially vulnerable.

Senior author Dr Sean McMahon, of Smithsonian Environmental Research Center, Maryland, said: "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.

"One remarkable result from this study is that, not only do we detect an increase in mortality, but this increase seems to have started in the 1980s, indicating the Earth's natural systems may have been responding to changing climate for decades."

Rainforests absorb around 12 percent of manmade CO2 emissions - helping to slow global warming.

Examining climate ranges of tree species with the highest death rates found the atmosphere is getting drier.

As it warms it draws more moisture from plants, resulting in more water stress - and ultimately death.

When the researchers crunched the numbers, they also found loss of biomass over the past decades has not been offset by gains from growth and new trees.

It implies the mortality increase has translated into a net decrease in the potential of these forests to offset carbon emissions.

Professor Susan Laurance, of James Cook University, Queensland, added: "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."