The Amazon rainforest responded to the most severe drought ever recorded in the basin with an unexpected defense mechanism. Researchers at the Max Planck Institute for Chemistry in Mainz, Germany, found that during and after the intense 2023-2024 El Niño cycle, the most intense drought ever recorded in the region, vegetation significantly changed its chemical emissions to cope with environmental stress. The study was published in Nature Communications Earth & Environment.
Sesquiterpenes act as stress signals and protective compounds
The research team measured the forest’s release of biogenic volatile organic compounds, or BVOCs, which are carbon-based molecules naturally emitted by vegetation. The results were striking. While isoprene and monoterpenoid levels showed little influence from El Niño conditions of drought and heat, emissions of sesquiterpenes increased by 122% over the course of the event. Sesquiterpenes are reactive airborne molecules that trees produce as stress signals and protective substances. A well-known example is caryophyllene, a peppery-smelling compound found in cloves and black pepper.
Even more surprisingly, the study detected unexpected emissions of less volatile sesquiterpene alcohols, including beta-eudesmol, alpha-eudesmol, and gamma-eudesmol, during the wet season after the drought peak. These findings suggest an adaptive response to oxidative stress, revealing how vegetation metabolically adjusts to adverse conditions. Interestingly the change persisted long after the immediate stressor has passed. "Our results show that severe drought shifts the atmosphere toward lower-volatility and more reactive compounds”, explains Joseph Byron, the study’s first author and a researcher at the Max Planck Institute for Chemistry. "This reflects underlying metabolic changes as the rainforest attempts to mitigate damage from abiotic stress." Project leader Jonathan Williams added “between El Nino events, which occur every 2-7 years, the rainforest can revert to the original non-stressed emissions. However, climate models suggest that El Nino events will increase in frequency and intensity in this century, so these emissions may become a permanent feature of the region, altering the overlying atmospheric chemistry”.
Air samples collected directly above the forest canopy
The researchers collected canopy air samples at the Amazon Tall Tower Observatory (ATTO), 150 km northeast of Manaus, Brazil. Using sorbent cartridges, they gathered samples every 1.5 to 3 hours at 23 meters on an 80-meter tower. In the laboratory in Mainz, they later analyzed them offline using gas chromatography and mass spectrometry.
The findings build on earlier work by the same team on chemical indicators of stress in the Amazon rainforest. In their previous research, they identified specific mirror molecules, or enantiomers, that can serve as indicators of stress within the rainforest ecosystem (see related press release: Mirror image molecules reveal drought stress in the Amazon rainforest ). The current study expands this understanding by showing which specific reactive volatile compounds are produced directly as part of the forest’s defense response during extreme climate events.
Implications for climate change and rainforest resilience
Understanding these chemical responses is very important, as climate change is predicted to make El Niño events more intense and persistent. The shift toward more reactive compounds could have significant implications for atmospheric chemistry and the overall resilience of the Amazon rainforest.
Background ATTO
ATTO is a German-Brazilian joint project which was launched in 2009. It is managed by the Max Planck Institutes for Biogeochemistry in Jena and for Chemistry in Mainz, as well as by the Brazilian INPA and the Amazon State University (UEA) in Manaus. The project is funded by the German Federal Ministry of Education and Research (BMBF), the Ministério da Ciência, Tecnologia e Inovações (MCTI), the Max Planck Society and the Brazilian organizations including FAPEAM and individual researchers bring funding from other scientific funding agencies.
More on ATTO: https://www.mpic.de/3538403/ATTO
Communications Earth & Environment
Experimental study
Not applicable
Intense El Niño provokes production of new reactive volatiles as stress defences in Amazon rainforest