Monoterpene ‘thermometer’ of tropical forest‐atmosphere response to climate warming

Tropical forests absorb large amounts of atmospheric CO2 through photosynthesis but elevated temperatures suppress this absorption and promote monoterpene emissions. Using 13CO2 labeling, here we show that monoterpene emissions from tropical leaves derive from recent photosynthesis and demonstrate distinct temperature optima for five groups (Groups 1–5), potentially corresponding to different enzymatic temperature‐dependent reaction mechanisms within β‐ocimene synthases. As diurnal and seasonal leaf temperatures increased during the Amazonian 2015 El Niño event, leaf and landscape monoterpene emissions showed strong linear enrichments of β‐ocimenes (+4.4% °C−1) at the expense of other monoterpene isomers. The observed inverse temperature response of α‐pinene (−0.8% °C−1), typically assumed to be the dominant monoterpene with moderate reactivity, was not accurately simulated by current global emission models. Given that β‐ocimenes are highly reactive with respect to both atmospheric and biological oxidants, the results suggest that highly reactive β‐ocimenes may play important roles in the thermotolerance of photosynthesis by functioning as effective antioxidants within plants and as efficient atmospheric precursors of secondary organic aerosols. Thus, monoterpene composition may represent a new sensitive ‘thermometer’ of leaf oxidative stress and atmospheric reactivity, and therefore a new tool in future studies of warming impacts on tropical biosphere‐atmosphere carbon‐cycle feedbacks. High temperatures threaten the ability of tropical forests to absorb large amounts of atmospheric CO2 by photosynthesis, but promote the emissions of monoterpenes (C10H16). Here, we show that as record high leaf diurnal and seasonal temperatures were experienced in the central Amazon during the 2015 El Niño event, leaf and landscape monoterpene emissions showed strong linear enrichments of β‐ocimenes (+4.4% °C−1) at the expense of other monoterpene isomers. The results demonstrate consistent temperature sensitivities of five monoterpene groups that were reproducible across large temporal (minutes to seasons) and spatial (leaves to landscape) scales but are not accurately simulated in current Earth Systems models. We suggest that the shift to highly reactive β‐ocimenes (Group 1) at high leaf temperatures facilitates forest‐atmosphere response to warming by functioning as: (1) effective antioxidants in plants, enhancing thermotolerance of photosynthesis and (2) as efficient a