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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 the journal Nature today.
Susan Laurance, Professor of Tropical Ecology at James Cook University, was part of a study involving researchers from the Smithsonian Environmental Research Center, Oxford University, and the French National Research Institute for Sustainable Development (IRD).
She said the researchers found the average death rates of tropical trees in Australia have doubled in the last 35 years, as global warming increases the drying power of the atmosphere.
“We 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,” said Professor Laurance.
Dr David Bauman is a tropical forest ecologist at the Smithsonian, Oxford and the 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,” said Dr Bauman.
Dr Sean McMahon, Senior Research Scientist at Smithsonian and senior author of the study 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.”
The scientists said that one remarkable result of the study is that the increase in mortality seems to have started in the 1980s, indicating the Earth’s natural systems may have been responding to changing climate for decades.
Oxford Professor Yadvinder Malhi, a study co-author 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,” said Professor Malhi.
“Long-term datasets like this one, which looked at more than 8300 trees over 50 years’ of data in 24 forests, 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,” said Professor Laurance.
Associate Professor Lucas Cernusak from James Cook University was a co-author of the study. He said 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,” said Dr Cernusak.
The team believe the main climate driver is the increasing 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,” said Dr Cernusak.
Expert Reaction
These comments have been collated by the Science Media Centre to provide a variety of expert perspectives on this issue. Feel free to use these quotes in your stories. Views expressed are the personal opinions of the experts named. They do not represent the views of the SMC or any other organisation unless specifically stated.
Dr Russell Barrett is a Senior Research Scientist at the Australian Institute of Botanical Science, Australian Botanic Garden, Mount Annan
Baseline data on which to assess long-term trends in tree health is scarce in Australia, and especially so in the tropics, so this dataset provides invaluable insights into what forests may look like under future climate change scenarios. The study assessed 81 dominant tree species, so was not unduly influenced by the particular responses shown by individual species.
Most significantly, the team found a doubling in the risk of tree death since 1984 primarily due to Vapour Pressure Deficit, a result of thermal stress (which may include increasing temperatures or reduction in rainfall).
Shifts in Vapour Pressure Deficit are reported as a global issue facing tropical forests, so these findings are significant at a global scale.
Other stresses, such as cyclonic winds, can also increase tree death, having a compound effect when trees are already stressed due to Vapour Pressure Deficit or other environmental factors.
Tree death can ultimately be caused by many factors, so teasing apart the significance of Vapour Pressure Deficit is a very important result for this study, and future studies can build on this work to more accurately predict changes that may take place under fluctuating or extreme climatic conditions.
With large implications for global carbon storage, a doubling of tree death risk dramatically changes our calculations for the quantity of carbon stored in our forests, and how long it is likely to stay there before being released back into the atmosphere.
While this study focussed on tropical moist forests in North Queensland, the phenomenon of Vapour Pressure Deficit is global, and affects all Australian plant communities. These results therefore highlight the need to similar studies in a broader range of habitats, including very detailed monitoring, so we can understand both the types of changes and the rate of change in plant community structure across our unique and biodiverse landscapes.
This need is especially great for plant communities that are already at the edge of their climatic windows, such as alpine vegetation and wet rainforests.
The catastrophic fires of 2019-2020 burnt wet rainforests that many scientists thought were too wet to burn, demonstrating the risks that are present under extreme conditions, beyond most predictions.
This study will help us to quantify the increased risk that our forests are facing due to global climate shifts that influence available atmospheric moisture in particular.
Long-term environmental health is underpinned by long-term resistance – largely the ability to withstand extreme events – and this research shows that this ability is declining rapidly in tropical forests.
An increase in tree mortality will increase carbon release, result in dryer forests that are more fire-prone, and increase runoff, decreasing the amount of moisture captured in ecosystems, a crucial factor for nutrient cycling.
One of the most fundamental factors in ecosystem health is soil moisture and associated micro-biomes, so this study suggests that our forests are under increasing threat. As multiple threats build or converge, ecosystems can reach critical points, sometimes resulting in ecosystem collapse, with new ecosystems emerging when conditions change too much for original ecosystems to recover.
The increasing rate of tree death serves as a warning sign for climate change that should be as clear and stark as mass coral bleaching events on the Great Barrier Reef – it is just much harder to see and document – a key achievement of this paper.
Distinguished Professor Bill Laurance is Director of the Centre for Tropical Environmental and Sustainability Science at James Cook University
I find this study eminently believable. It really looks like something peculiar is happening to rainforests in North Queensland—and possibly globally. We’ve found similar trends in the Amazon basin, where rates of tree death have also risen markedly in recent decades.
Importantly, the authors of the study tried hard to nail down the specific mechanisms killing trees. They conclude that rising drought stress to trees from increasingly dry, warm weather is the main cause—and that global warming is probably responsible.
This explanation resonates with me. For decades, we’ve studied how habitat fragmentation affects rainforest trees, and we found that many trees die near fragment margins where conditions are often hot and dry.
Sadly, it’s really not that hard to kill a rainforest tree—just warm things up a bit and quite a few species will just drop their leaves and die standing.
The authors also suggest the cyclones that pummel North Queensland rainforests might be growing in intensity over time and thereby killing more trees. Again, global warming might be the villain, as warmer conditions tend to spawn bigger cyclones, but I think more research on this is needed.
Overall, this study is a testament to the importance of long-term research, especially when one lives on a planet whose atmosphere, land and seas are changing in many different ways all at once.
