Monoterpene ‘thermometer’ of tropical forest response to climate warming

Abstract:

Tropical forests represent a large terrestrial carbon sink, but are highly sensitive to climate warming. To improve predictions of terrestrial carbon and water cycling, a better understanding of the physiological mechanisms underlying forest warming response is required. Monoterpenes are known to protect photosynthesis during abiotic stress, modify aerosol and cloud lifecycles. Although assumed to be of constant composition and dominated by α-pinene, little is known about the sources and composition of monoterpenes in tropical ecosystems. In this study, we show that as temperatures increased and soil moistures in the upper 100 cm layers strongly decreased during the 2015 El Niño in the central Amazon, both diurnal and seasonal leaf and ecosystem monoterpene emission patterns demonstrated a strong linear increase in the relative abundance of linear β-ocimenes (+4.4% ºC^-1) at the expense of the cyclic and bicyclic monoterpenes observed. These temperature-sensitive changes in the monoterpene composition could not be reproduced using state-of-the-art coupled MEAGAN 2.1 - CLM 4.5 emission models at either the ecosystem or regional scales. Given that β-ocimenes are highly reactive with respect to both atmospheric and biological oxidants, these observations suggest an important functional role at high temperatures and a diminishing importance of α-pinene in future global warming scenarios. Further, this work demonstrates that monoterpene emission composition is a new highly relevant signal for future studies on the warming impacts of terrestrial carbon and water cycling in the tropics.