Mirror image molecules and low volatility organic compounds emissions expose state ofrainforest stress

Climate change is increasing the frequency and severity of Amazonian droughts, and El Niño events are predicted to become more intense and persistent. Despite this, the effects of drought on biogenic volatile organic compound (BVOC) emissions from tropical rainforests remain poorly understood. Chiral BVOCs 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 ambient concentrations from within the Amazon rainforest canopy of methyl salicylate, isoprene, monoterpenoids, and sesquiterpenoids from the Amazon rainforest spanning the 2023–2024 El Niño, the most severe drought ever recorded in the basin. 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. Isoprene and total monoterpenoid abundances showed little influence from El Niño, while total sesquiterpenoids increased by 122% across the El Niño duration. Unexpected emissions of lower-volatility sesquiterpene alcohols, including beta-eudesmol, alpha-eudesmol, and gamma-eudesmol, occurred during the wet season following the peak drought revealing an adaptation to adverse conditions linked to oxidative stress defence. Our results show how severe drought drives shifts in enantiomer ratios and isoprenoid composition in the atmosphere, reflecting underlying physiological changes as vegetation responds to abiotic stress.