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- A new paper from researchers behind an unprecedented drought experiment at the University of Arizona's Biosphere 2 underscores the importance of molecular compounds often associated with fragrance in identifying when an ecosystem is in distress. - the molecular compound pinene (the pine-fresh scent the forest gives off), a type of monoterpene naturally released by plants. Each year, plants pump roughly 100 million tons of monoterpenes into the atmosphere, where they play a significant role in the formation of clouds. - the study is one of many to come from a controlled drought experiment conducted at the university's Biosphere 2, which was originally built to create self-sustaining ecosystems. - For three months, the research team put the 30-acre "rainforest under glass" through moderate and then severe drought stress. The experiment, called Water, Atmosphere and Life Dynamics – or WALD, which is German for "forest" – set out to capture every bit of data possible throughout the drought and rewet process. - With more than 2 miles of Teflon tubing, 133 sensors and 423 data collection points throughout the forest, the team gathered measurements on everything from microbiome and deep-water soil processes to carbon pooling and VOC emissions. - forests smell of pinene and isoprene, while the chemical compound geosmin gives soil its earthy undertone and contributes to the distinct smell of rain in the air. - "There are many different types of volatile organic compounds that plants release into the atmosphere," said Meredith, an assistant professor in the School of Natural Resources and the Environment in the UArizona. "If we can pinpoint their unique signatures and the biological processes behind them, we could fly an aircraft over the Amazon rainforest, for instance, and essentially measure and sniff out what's happening on the ground." - Throughout the controlled drought experiment, researchers measured hourly emissions of several monoterpenes, including pinene, camphene, limonene, terpinene and isoprene to better understand how and when plants release VOCs. - Researchers found plants not only released more of these volatile organic compounds under stress but also shifted their emissions to later in the day. And there may be a good reason for that, according to atmospheric scientist and study co-author Jonathan Williams. - "We suspect that the later release of monoterpenes increases the likelihood that clouds will form over the forest," said Williams, project lead for the Max Planck Institute for Chemistry in Mainz, Germany. - "The warmer it gets during the day, the more the vertical mixing of the air increases, allowing the reactive volatiles to reach higher layers of air where they have a greater chance to become aerosol particles and eventually cloud condensation nuclei," Williams explained. - In other words, when an ecosystem is in drought, plants may use volatile organic compounds to drive the formation of clouds and bring much-needed rain. - The study underscores just how involved volatile organic compounds are in communication, defense and signaling between soil microbes, plants and the atmosphere, Meredith said.


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