Scent molecules reveal the stress levels of the rainforest

New study shows: The ratio of α-pinene mirror molecules reveals the extent to which the Amazon rainforest is suffering from drought stress. Measurements taken at the Amazon Tall Tower Observatory (ATTO) documented these changes during the record drought of 2023

A new study by the Max Planck Institute for Chemistry and the Max Planck Institute for Biogeochemistry, with contributions from Michelle Robins and Eliane Gomes Alves, shows that the health of the Amazon rainforest can be determined from the air. The decisive factor here is the ratio of two mirror-image forms of the fragrance α-pinene. During the extreme drought in 2023, this ratio changed significantly – a direct indication that the plants were under severe drought stress and were changing their metabolism.

Normally, plants release α-pinene into the ambient air in a stable ratio of both mirror molecules. However, during periods of extreme drought, this balance shifts: instead of the normally dominant form, the variant derived from the plant's reserves becomes increasingly prevalent. At the height of the drought, the ratio was even completely reversed. The measurements thus demonstrate for the first time in the field what had previously only been observed in greenhouse experiments: plants switch to survival mode, largely cease photosynthesis and primarily release stored substances.

The data for the study was obtained at the Amazon Tall Tower Observatory (ATTO) – a unique research platform in the middle of the Brazilian rainforest. There, a 325-metre-high measuring tower enables the analysis of atmospheric processes over an area of around 100 square kilometres. Air samples from the canopy provided the basis for the precise laboratory analyses in Mainz, which detected the tiny differences between the two forms of α-pinene.

ATTO is a German-Brazilian joint project in which the Max Planck Institute for Biogeochemistry (MPI-BGC) in Jena also plays a key role. The institute contributes its many years of expertise in carbon and material cycles and works closely with partners in Brazil and Germany to better understand the interactions between vegetation, the atmosphere and the global climate. Without the infrastructure of ATTO – which is jointly operated by MPI-BGC, the Max Planck Institute for Chemistry and Brazilian research institutions – these unique insights into the ‘inner workings’ of the rainforest would not be possible.

The current results are also of great importance for climate research. The Amazon rainforest is the world's largest source of biogenic volatile compounds, and their behaviour under drought stress influences both regional and global climate processes. With the help of the new data, climate models can now more realistically depict how more frequent and intense droughts affect vegetation and what feedback effects on the climate can be expected. In this way, the researchers are not only contributing to our understanding of the Amazon, but also to a better assessment of the consequences of climate change worldwide.

This news item is based on information provided by the Max Planck Institute for Chemistry.