Back in 2023, the Amazon rainforest experienced its most extreme drought on record – drying out soils, yellowing leaves, and drastically lowering river levels. But according to a new study, the canopy was also sending out subtle chemical signals.
Researchers from the Max Planck Institute for Chemistry have shown that monoterpenes, particularly the mirror-image forms of α-pinene, serve as reliable molecular indicators of ecosystem stress. Their study, published in Communications Earth & Environment, reveals that shifts in the ratio of α-pinene enantiomers can be used to track plant responses to extreme environmental conditions in real time.
Using chiral gas chromatography coupled to time-of-flight mass spectrometry, the team analyzed volatile organic compound (VOC) samples collected from the Amazon Tall Tower Observatory (ATTO), 150 km northeast of Manaus. Air samples taken directly from the canopy during and after the drought period showed that the typical enantiomeric dominance of (−)-α-pinene flipped at midday during peak drought – signaling that photosynthesis had stalled and plants were relying on internal VOC stores instead of newly synthesized compounds.
“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 groundwater,” said project leader Jonathan Williams in the team’s press release.
The technique builds on previous greenhouse work demonstrating that (−)-α-pinene is typically emitted de novo during photosynthesis, while the (+) enantiomer is released from storage pools. The new field data validate this mechanism under real-world conditions. By correlating enantiomeric ratios with environmental metrics such as ozone levels, soil moisture, and carbon dioxide flux, the researchers defined “low-stress,” “high-stress,” and “recovery” zones. Drone-based sampling in October 2024 across different forest types further confirmed the method’s sensitivity to soil water availability and edge effects.
“First, we determined the ratio in which the two variants occur under normal conditions,” said first author Joseph Byron. “We then observed how this ratio shifted during the El Niño-impacted dry season and slowly returned to normal afterwards.”
With the Amazon being the world’s largest source of biogenic VOCs, the researchers suggest that the α-pinene enantiomer ratio could be incorporated into climate and vegetation models to better simulate the impact of increasing drought frequency. “It is amazing that we can read directly from the air how the rainforest is reacting to current conditions,” Williams noted.
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