Chiral monoterpenes reveal forest emission mechanisms and drought responses

Monoterpenes exist in mirror image forms called enantiomers, but their individual formation pathways in plants and ecological functions are poorly understood, as enantiomers are usually measured and modelled together. Here we present enantiomerically separated atmospheric monoterpene and isoprene data from an enclosed tropical rainforest ecosystem without photo-chemistry during a four-month controlled drought and rewetting experiment. Surprisingly, the enantiomers showed distinct diel emission peaks, which responded differently to progressive drying. Isotopic labelling established that vegetation emitted (-)-α-pinene mainly de novo while (+)-α-pinene was emitted from storage pools. As drought stress increased, (-)-α-pinene emis-sions shifted to storage pools, which are released later in the day, favouring cloud formation. The α-pinene enantiomers each correlated better with other monoterpenes than with each other, indicating different enzymatic controls. These results show that enantiomeric distribution is key to understanding the processes driving monoterpene emission from ecosystems and to predict-ing atmospheric feedbacks in response to climate change.