Changes in chiral monoterpenes during drought in a rainforest reveal distinct source mechanisms

Abstract

Monoterpenes (C10H16) are emitted in large quantities by vegetation to the atmosphere (>100 TgC year−1), where they readily react with hydroxyl radicals and ozone to form new particles and, hence, clouds, affecting the Earth’s radiative budget and, thereby, climate change[1-3]. Although most monoterpenes exist in two chiral mirror-image forms termed enantiomers, these (+) and (−) forms are rarely distinguished in measurement or modelling studies[4-6]. Therefore, the individual formation pathways of monoterpene enantiomers in plants and their ecological functions are poorly understood. Here we present enantiomerically separated atmospheric monoterpene and isoprene data from an enclosed tropical rainforest ecosystem in the absence of ultraviolet light and atmospheric oxidation chemistry, during a four-month controlled drought and rewetting experiment, the Biosphere 2 Water, Air and Life Dynamics campaign (B2WALD) in 2019[7, 8]. The measurements were obtained with an on-line gas chromatograph-mass spectrometer over five time periods: pre-drought, early drought, severe drought, deep rewet and rain rewet.Surprisingly, the emitted enantiomers showed distinct diel emission peaks, which responded differently to progressive drying. Isotopic labelling established that vegetation emitted mainly de novo-synthesized (−)-α-pinene, whereas (+)-α-pinene was emitted from storage pools. As drought progressed, the source of (−)-α-pinene emissions shifted to storage pools which would favour cloud formation since the peak concentration became more aligned with temperature. Pre-drought mixing ratios of both α-pinene enantiomers correlated better with other monoterpenes than with each other, indicating different enzymatic controls. These results show that enantiomeric distribution is key to understanding the underlying processes driving monoterpene emissions from forest ecosystems and predicting atmospheric feedbacks in response to climate change.1. Jokinen, T., et al., Production of extremely low volatile organic compounds from biogenic emissions: Measured yields and atmospheric implications. Proceedings of the National Academy of Sciences, 2015. 112(23): p. 7123-7128.2. Engelhart, G.J., et al., CCN activity and droplet growth kinetics of fresh and aged monoterpene secondary organic aerosol. Atmos. Chem. Phys., 2008. 8(14): p. 3937-3949.3. Laothawornkitkul, J., et al., Biogenic volatile organic compounds in the Earth system. New Phytologist, 2009. 183(1): p. 27-51.4. Yáñez-Serrano, A.M., et al., Monoterpene chemical speciation in a tropical rainforest:variation with season, height, and time of dayat the Amazon Tall Tower Observatory (ATTO). Atmos. Chem. Phys., 2018. 18(5): p. 3403-3418.5. Jardine, K.J., et al., Monoterpene 'thermometer' of tropical forest-atmosphere response to climate warming. Plant Cell Environ, 2017. 40(3): p. 441-452.6. Guenther, A., et al., The Model of Emissions of Gases and Aerosols from Nature version 2.1 (MEGAN2. 1): an extended and updated framework for modeling biogenic emissions. 2012.7. Byron, J., et al., Chiral monoterpenes reveal forest emission mechanisms and drought responses. Nature, 2022. 609(7926): p. 307-312.8. Werner, C., et al., Ecosystem fluxes during drought and recovery in an experimental forest. Science, 2021. 374(6574): p. 1514-1518.