Mirror image molecules expose state of rainforest stress

Monoterpenes are key to plant communication and defence against biotic (e.g., herbivory) and abiotic (e.g., heat, drought) stress. Chiral monoterpenes, like alpha-pinene, exist as mirror image pairs, known as enantiomers. Enantiomers have the same atmospheric reactivity, but are produced and emitted by different enzymes and internal leaf mechanisms. Abiotic stress can alter their relative emissions, suggesting enantiomer ratios could indicate stress severity. Here we present (−)- and (+)-alpha-pinene and methyl salicylate measurements from the Amazon rainforest over time-of-day, season, and the 2023-24 El Niño. Correlations between alpha-pinene enantiomers shifted with stress, aligning with weakening carbon dioxide uptake by vegetation and transition between de novo and storage emissions. Low- and high-stress zones, along with a recovery zone, were defined through alpha-pinene enantiomer correlations, revealing a metric for ecosystem stress. This chiral ratio reveals diel, seasonal, El Niño, and edge effect stresses, offering a method to gauge drought severity due to climate change.