An Amazon canopy study spanning the 2023-2024 El Niño has found that extreme drought pushed the rainforest toward a different volatile chemistry, with sesquiterpene levels rising by 122 percent while isoprene and monoterpenes changed far less. The study also detected sesquiterpene alcohols in the wet season after peak drought, suggesting that stress-related metabolic changes can persist well beyond the driest phase itself.
The work addresses a persistent blind spot of rainforest atmospheric chemistry. Sesquiterpenes are highly reactive, low-volatility compounds that can influence ozone chemistry and secondary aerosol formation, but they are far harder to measure than isoprene or monoterpenes under field conditions. To follow those shifts through the El Niño cycle, the Max Planck-led team sampled canopy air before, during, and after the 2023-2024 event at the Amazon Tall Tower Observatory near Manaus.
Air was collected every 1.5 to 3 hours at 23 m on an 80 metre tower using sorbent cartridges, then analyzed offline by gas chromatography time-of-flight mass spectrometry (GC-ToF-MS) and proton transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS). The setup allowed the researchers to track speciated isoprene, monoterpenoids, and sesquiterpenoids across four sampling periods, rather than relying on bulk hydrocarbon totals alone.
The clearest shift came in the sesquiterpenoid fraction. Beta-selinene rose during peak drought, while beta-eudesmol, alpha-eudesmol, and gamma-eudesmol appeared during the following wet season. Correlation analysis linked several of these compounds through a shared eudesmane-skeleton pathway, which the authors interpret as an upregulated metabolic response to severe drought and oxidative stress.
“Our results show that severe drought shifts the atmosphere toward lower-volatility and more reactive compounds,” co-corresponding author Joseph Byron said in the team’s press release. “This reflects underlying metabolic changes as the rainforest attempts to mitigate damage from abiotic stress.”
The analysis also revealed a delayed response. Sesquiterpene alcohols including beta-eudesmol, alpha-eudesmol, and gamma-eudesmol were detected in the wet season after peak drought, suggesting that the forest’s stress-defence chemistry remained active after conditions had begun to ease.
Jonathan Williams, the study’s other co-corresponding author, said the shift may not be confined to a single drought cycle. “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.”
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