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In 2023, the Amazon rainforest experienced its worst recorded drought since records began. River levels dropped dramatically and vegetation at all levels deteriorated due to intense heat and water shortages. In such conditions, plants release increased amounts of monoterpenes—small, volatile organic compounds that act as a defense mechanism and help communication with their environment. Some molecules, such as α-pinene, which smells like pine, occur as mirror-image pairs, known as enantiomers.

The ratio of these two forms changes measurably when plants are under stress, for example due to heat or water shortage. Researchers at the Max Planck Institute for Chemistry investigated how this ratio changed in the Amazon before, during, and after the drought period. Their research is in Communications Earth & Environment.

The results show that under normal conditions a clear ratio was consistently measured, but with increasing drought stress it shifted to ever higher values. In the most extreme phase of the drought, the usual ratio of the two α-pinene variants even reversed. Thus, the mirror molecules of α-pinene can tell us how much stress an ecosystem is currently under.

Giovanni Pugliese, a scientist from the Max Planck Institute for Chemistry who was on site during the measurement campaign, recalls, "The heat was unbearable when collecting the samples. The forest was clearly suffering; its leaves were yellowing and the dry clay soil was cracking".

The problem in 2023 was that the September to October dry season coincided with an El Niño event. This is part of the global climate oscillation ENSO, and in El Niño mode, it brings extremely low rainfall and high temperatures to the Amazon basin.

Measurements deep in the rainforest

At the measuring station of the Amazon Tall Tower Observatory (ATTO), 150 kilometers northeast of Manaus, the researchers collected air samples at a height of 24 meters directly in the forest canopy. In the laboratory in Mainz, they later determined the ratio of the two α-pinene forms using chiral gas chromatography-time-of-flight-mass spectrometry.

"First, we determined the ratio in which the two variants occur under normal conditions," explains Joseph Byron, researcher at the Max Planck Institute for Chemistry and first author of the study. "We then observed how this ratio shifted during the El Niño-impacted dry season and slowly returned to normal afterwards."

Plants in survival mode

Project leader Jonathan Williams is impressed by these vegetation responses and explains further, "It is amazing that we can read directly from the air how the rainforest is reacting to current conditions. During the worst part of the drought, when the ratio flipped at midday, we knew that the vegetation had had enough, it had stopped photosynthesizing and closed up its pores to stop losing precious ground water ."

This work builds on an earlier experimental drought study conducted in an enclosed forest grown within a greenhouse. There the Max Planck Research team then showed that the two mirror-image molecules are released via different processes in the plant. While one form of α-pinene is released immediately after photosynthesis, the mirror molecule comes from storage pools within the plant. The indoor experiment revealed this relationship and now this behavior has been recorded in the real-world extreme drought situation in the Amazon rainforest.

The Amazon rainforest is the world's largest source of biogenic volatile compounds. Using the ratio of α-pinene molecules, these emissions and their changes under drought conditions can now be represented more realistically in climate models. This is crucial because researchers expect more frequent and severe El Niño-related droughts in the future.