Biosphere drought reveals scent of troubled ecosystem

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 volatile organic compounds (VOCs). (Credit: Tim Stahmer/Flickr)

Findings from an unprecedented drought experiment in a biosphere underscore the importance of molecular compounds often associated with fragrance in identifying when an ecosystem is in distress.

Ever wonder what gives a forest its signature pine-fresh scent? The answer is the molecular compound pinene, 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 published in Nature explores how and under what conditions plants emit these volatile organic compounds, or VOCs, into the atmosphere. The results may help scientists sniff out when an ecosystem is in distress and better understand how Earth may try to adapt in the face of a hotter, drier future.

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.

“There’s no other place in the world where you can encapsulate a rainforest, subject it to a drought, and then bring it out of that drought on a schedule that you dictate,” says John Adams, Biosphere 2 deputy director and chief of operations at the project’s start. “This gives scientists a really unique opportunity to have everything well-poised so they can monitor and collect data that oftentimes is very difficult or impossible to get in the field.”

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 two 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.

Many volatile organic compounds have a unique scent, explains Laura Meredith, who helped lead the B2 WALD project and is a coauthor of the most recent study. For instance, 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,” says Meredith, an assistant professor in the School of Natural Resources and the Environment in the university’s College of Agriculture and Life Sciences and a member of the BIO5 Institute.

“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.”

During the B2 WALD project, Meredith served as director of the rainforest at Biosphere 2 and brought together 90 scientists from five different countries to monitor the resiliency and vulnerability of plants and microbes. The scientific expertise of the project spanned all aspects of environmental stress, including hydrology, vegetation, soil, and atmospheric science.

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 coauthor Jonathan Williams.

“We suspect that the later release of monoterpenes increases the likelihood that clouds will form over the forest,” says 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 explains.

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 says.

Meredith will continue her exploration of VOCs with a recent grant from the Department of Energy. Along with ecosystem genomics expert Malak Tfaily, an associate professor and environmental scientist in the College of Agriculture and Life Sciences, Meredith will explore carbon sequestration through soil, microbe, and plant interactions.

“After exploring the impact of VOCs on the atmosphere, we will now turn our attention to how these carbon-carrying molecules influence soil and its capacity to sequester and store carbon,” Meredith says. “Do plants and microbes pump VOCs into the ground, like they do in the atmosphere, and what processes help ensure the carbon stays below ground?”

Source: University of Arizona