Scientists warn these signs are an early warning indicator that these areas are approaching a threshold to a die-off event
Between 2001 and 2019, the Amazon has witnessed three, “one-in-a-century” droughts. Here, small boats can be seen stranded in the nearly dry bed of the Tapajos river in Alter do Chao, Santarem, Brazil, during the Amazonian drought in 2023. Photo: iStock
More than a third of the mature Amazon rainforest — 37 per cent — is struggling to recover due to frequent droughts over the past two decades.
A new study has highlighted that the world’s largest tropical forest, which is a biodiversity hotspot with over 15,000 tree species and serves as the largest carbon sink, is showing signs of ‘critical slowing down’ of vegetation activity.
The paper warned these signs are an early indicator of dynamic systems approaching a critical threshold, beyond which the forest would function differently. It further suggested that climate change may be the potential driver of large-scale ecosystem collapse.
“This increasing trend could be considered an early warning indicator that these areas are approaching a threshold for a die-off event, leading to an ecosystem state with qualitatively different functioning,” it noted.
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The study, Critical slowing down of the Amazon forest after increased drought occurrence, was published in journal Proceedings of the National Academy of Sciences on May 20, 2024. It noted that between 2001 and 2019, the Amazon has witnessed three, “one-in-a-century” droughts and these extreme events are estimated to become more severe owing to climate change.
The researchers raised concerns that increased drought severity and frequency would result in reduced forest recover rates due to gradual decrease in tree growth, eventually leading to tree mortality and potential forest dieback.
The team did not include future projections in the study, Johanna Van Passel, lead author of the paper told Down To Earth (DTE). Van Passel is a PhD researcher at the Division of Forest Nature and Landscape at KU Leuven. “We do see that the current slowing down of forest recovery is linked to drought occurrences, so we expect an increase in the vulnerable areas, but this might not be a linear increase,” she said.
The researchers identified two main reasons why the trees will die: Carbon starvation and hydraulic failure.
They wrote in the paper:
High intensity droughts are more likely to cause irreversible dehydration, resulting in hydraulic failure, while longer low-intensity droughts can cause mortality through carbon starvation. Hydraulic failure has been proposed as the major trigger for tree mortality in tropical rainforests, which aligns with our findings that drought intensity increases the slowing down response more than drought duration.
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Hydraulic failure results when water loss from transpiration exceeds a tree’s intake, causing its xylem vessels to rupture and lose their capability to channel water. Carbon starvation occurs when a tree, in an attempt to avoid hydraulic failure, stops photosynthesis for a prolonged period by closing its stomata, resulting in a negative carbon balance and eventual death.
Van Passel clarified that all of the Amazon is not approaching a regional tipping point, as two thirds of the forest did not show slowed recovery.
“The risk is higher that small forest patches within the Amazon forest might be approaching local tipping points. This study was done using relatively coarse satellite images, so our results only tell us something about how the forest canopy is behaving, and not how individual plants are dealing with these increased droughts. Therefore, the situation could be worse when you look at the scale of these individual plants,” the author told DTE.
The study observed that Amazon areas experiencing the lowest rainfall since the 2000s have suffered significantly, with a notable decline in their stability.
The scientists found that the trend of slowing recovery and increasing drought intensity progresses from the northwest to the southeast, with the southeastern Amazon showing higher vulnerability and being close to a tipping point.
The scientists also noted that extreme rainfall events negatively affect tree growth and survival. Their analysis did not show any changes in the slowing down of Amazon forest vegetation due to severe, extreme wet periods.
The results indicated the intensity of extreme drought events is more crucial than their duration for a critical slowdown and that a combination of intensity and duration would exacerbate the situation.
“More future droughts are expected to cause changes in forest structure and functioning by increasing forest mortality and can potentially bring more areas in the Amazon closer to a tipping point, especially those areas already more affected by climate change and land use changes,” the researchers wrote.
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According to the analyses, the scientists predict that an increase in drought intensity will further expand the areas of the forest experiencing critical slowdown, especially in the seasonal north and southeastern regions.
“The internal rain cycle in these areas may trigger a cascading effect, potentially leading to further slowing down in other parts of the Amazon forest, with implications for global effects on other tipping points,” it noted.
The study also recognised the role of indigenous peoples and traditional communities in preserving these forests to mitigate global warming and calls for the safeguarding of these forests.
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