How a forest’s smell could help scientists sniff out drought stress

By Rosemary Brandt, College of Agriculture and Life Sciences


The rainforest at Biosphere 2 has received rain after three months of drought

An unprecedented experiment currently underway at Biosphere 2 forced the “world’s hottest tropical rainforest” through a controlled drought and recovery experiment to paint a clearer picture of how global climate change will affect Earth’s ecosystems.
Rosemary Brandt

Ever wondered what gives a forest its signature pine-fresh scent? The answer is the molecular compound pinene, a type of monoterpene naturally released by plants. Plants pump around 100 million tons of monoterpenes into the atmosphere every year, where they are instrumental in cloud formation.

A study published this month in Nature examines how and under what conditions plants release these volatile organic compounds, or VOCs, into the atmosphere. The findings could help scientists figure out when an ecosystem is in distress and better understand how Earth might try to adapt in the face of a hotter, drier future.

Exterior shot of Biosphere 2

Biosphere 2, originally built to house self-sufficient ecosystems
Rosemary Brandt

Forcing an artificial rainforest through drought

Co-author with University of Arizona researchers Laura Meredith and Joost van HarenThe study is one of many to come from a controlled drought experiment conducted at the university’s Biosphere 2, originally built to create self-sustaining ecosystems.

“There’s no other place in the world where you can encapsulate a rainforest, subject it to drought, and then bring it out of that drought on a schedule that you set,” said Biosphere 2 Deputy Director and Chief of Operations John Adams at the start of the project. ‘This gives scientists a truly unique opportunity to have everything well prepared so they can monitor and collect data that is often very difficult or impossible to obtain in the field.’

For three months, the research team subjected the 30-hectare “rain forest under glass” to moderate and then severe drought stress. The experiment, called Water, Atmosphere and Life Dynamics – or WALD, which is German for “forest” – set out to collect all sorts of data during the drought and rewetting process.

Using more than 2 miles of Teflon tubing, 133 sensors, and 423 data collection points throughout the forest, the team collected measurements on everything from microbiome and deep-water soil processes to carbon storage and VOC emissions.

Biosphere 2 terrestrial biome manager Jason Deleeuw climbs the space frame above the rainforest floor to collect leaves and samples that will be placed overnight to link leaf climate, microbiome and volatile organic compound emissions.

Biosphere 2 terrestrial biome manager Jason Deleeuw climbs the space frame above the rainforest floor to collect leaves and samples that will be placed overnight to link leaf climate, microbiome and volatile organic compound emissions.
Laura Meredith

Get a whiff of those VOCs

Many volatile organic compounds have a unique odor, explains Meredith, who helped lead the B2 WALD project for UArizona and is a co-author of the latest study. For example, forests smell of pine and isoprene, while the chemical compound geosmin gives soils their earthy notes 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 at the UArizona 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, for example, fly a plane over the Amazon rainforest and essentially measure and sniff out what’s happening on the ground.”

Meredith has made a career out of studying the potential of VOCs and was recently recognized by the American Geophysical Union with the Thomas Hilker Award, given annually to a young scientist who advances unusually creative work in the field of biogeoscience.

“My background is in atmospheric chemistry and I’ve been very interested in how soil microbes affect the atmosphere,” Meredith said. “So I bridged that into microbiology, ecology, microbial and soil science to study ecosystems and global change at large scale.”

During the B2 FOREST project, Meredith served as the Rainforest Director at Biosphere 2, bringing together 90 scientists from five different countries to monitor the resilience and vulnerability of plants and microbes. The project’s scientific expertise covered all aspects of environmental stress, including hydrology, vegetation, soil and atmospheric science.

Joost van Haren, Assistant Professor of Research at Biosphere 2, measures rainfall as part of the B2 WALD drought recovery experiment.

Joost van Haren, assistant professor of research at Biosphere 2 and professor of practice at Honors College, measures rainfall as part of the B2 WALD drought recovery experiment.
Rosemary Brandt

Talking to Clouds

During 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.

The researchers found that under stress, plants not only release more of these volatile organic compounds, but also delay their emissions until later in the day. And there could be a good reason for that, according to atmospheric researcher and study co-author Jonathan Williams.

“We suspect that the later release of monoterpenes increases the likelihood of clouds forming over the forest,” says Williams, project leader at the Max Planck Institute for Chemistry in Mainz.

“As the day gets warmer, vertical air mixing increases, allowing the reactive volatiles to reach higher layers of air where they have a greater chance of becoming aerosol particles and eventually cloud condensation nuclei,” Williams explained.

In other words, when an ecosystem is in a state of drought, plants can use volatile organic compounds to drive cloud formation and bring much-needed rain.

The study highlights how volatile organic compounds are involved in communication, defense and signaling between soil microbes, plants and the atmosphere, Meredith said.

Unlocking the potential of VOCs

Meredith will continue her research into VOCs with a recent grant from the Department of Energy. Together with an expert in ecosystem genomics Malak Tfailyan associate professor and environmental scientist at the College of Agriculture and Life Sciences, Meredith will study carbon sequestration through soil, microbial and plant interactions.

“Having studied the effects of VOCs on the atmosphere, we will now focus on how these carbon-bearing molecules affect soil and its ability to fix and store carbon,” Meredith said. “Do plants and microbes pump VOCs into the soil like they do in the atmosphere, and what processes help keep the carbon underground?”

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