The forest for the trees

May 20, 2020
An aerial photo of a forest of pine and deciduous trees with hazy air above

Harvard scientists investigate how trees, from pre-industrial to modern forests, impact pollution and climate change


By Caitlin McDermott-Murphy



Everyone knows that telltale pine forest smell. Candles and deodorants try to duplicate the scent. The most iconic air fresheners are even shaped like little pine trees. But that perfume may not be so innocent.

“The plants don’t do this because we find it pleasurable to walk around in a pine forest,” said Frank Keutsch, Professor of Chemistry and Chemical Biology at Harvard. Although scientists have demystified much of plant chemistry, some questions—like why pine forests smell so darn good—still don’t have a clear answer.  

For Keutsch, an atmospheric chemist, this question is intriguing for reasons far more important than fragrance. Pine scent comes from a collection of molecules known as volatile organic compounds (VOCs). When trees emit these chemicals, they can react with oxidants to form other pollutants like particulate matter and ozone, both of which can impact climate change and also respiratory diseases, potentially even COVID-19.

To figure out exactly how forests impact air quality and human health, experts like Keutsch first need to understand the complex oxidation chemistry behind that pine forest smell. To do that, he and two chemistry PhD candidates in the Graduate School of Arts and Sciences—Joshua Shutter and Joshua Cox, or "Team Plant"—are studying what happens when specific VOCs interact with plant leaves and how these interactions are different now, in modern air, compared to pre-industrial.


A black and white photo of Keutsch lab members with only Frank Keutsch, Josh Shutter and Josh Cox in color

A recent photo of the Keutsch lab members with "Team Plant" in the bottom row (Joshua Shutter is left of Joshua Cox) and Frank Keutsch in the center. Photo courtesy of the Keutsch lab


“Under pre-industrial conditions, the world was entirely different,” Keutsch said. “From an environmental perspective, we want to get back to pre-industrial conditions, right? We want cleaner air; we want this romanticized picture of the pre-industrial world that was so wonderful, but what were the details of atmospheric chemistry like?”

Since scientists have primarily studied modern conditions, they don’t yet know how a return to pre-industrial air might affect the modern world. Now, this gap is becoming more critical: During the COVID-19 pandemic, air pollution and greenhouse gas levels have dropped significantly, enabling many places to approach these pre-industrial conditions.

To start unraveling these mysteries, the Joshes chose to track one specific VOC: gas-phase formaldehyde (HCHO). When VOCs are oxidized in the atmosphere, HCHO is almost always a byproduct, Shutter said. “If you measure HCHO and can postulate the chemical reactions that lead to its formation, atmospheric chemists can backtrack the amount of VOC that was originally emitted.” That could help scientists understand how particulate matter and ozone are produced from VOC oxidation.

In the summer of 2016, Shutter visited a Michigan forest that has returned to near-pre-industrial conditions and took HCHO measurements. His data showed something unusual: There was far more HCHO floating around than atmospheric models predicted. Keutsch came up with a hypothesis: maybe HCHO is not just a byproduct; maybe trees emit it from their leaves.

Back in the lab, the Joshes tested the theory. In a basement room the size of a gardening shed, they grew red oak saplings, a common species in that Michigan forest. They placed the leaves in a softball-sized glass enclosure with precisely controlled temperature, humidity, carbon dioxide, and lighting and introduced the tiniest amount of HCHO, 100,000 times less than the concentration of carbon dioxide in outdoor air. The Keutsch lab’s laser-based instrument is precise enough to pick up even these trace concentrations.


A photo of the plant lab, which contains a large metal table with three computers, many wires, and white tubes with red oak saplings A photo of a large red oak leaf pinned underneath a metal ring
LEFT: The basement lab where "Team Plant" grows red oak saplings like the ones pictured here and analyze how individual leaves interact with atmospheric chemicals. RIGHT: A red oak leaf ready to be analyzed. Photos courtesy of the Joshes


Contrary to Keutsch’s hypothesis, the Joshes discovered their leaves did not emit HCHO under ambient conditions in the forest—rather, they absorbed it. That meant the models accounted for even less of the high levels of HCHO Shutter measured in Michigan. “Something in our understanding is wrong,” Keutsch said. “What can possibly account for this level of HCHO in pristine forests?”

He doesn’t have an answer, but he does have a new hypothesis.

Trees may absorb HCHO, but other gases perform more complex interactions with trees. Some, like organic peroxides, react with leaves and return to the atmosphere as different, sometimes more harmful, molecules, Keutsch said. Maybe, he said, the most abundant of those peroxides—isoprene hydroxy hydroperoxide, or ISOPOOH—reacts with leaves to form HCHO.

If not for the campus shutdown due to COVID-19, the Joshes would be testing this hypothesis. Since the Keutsch lab and NASA demonstrated that ISOPOOH is converted to HCHO on metal surfaces, leaf surfaces could potentially do the same.

Still, plants perform chemical reactions—like those responsible for that rich pine aroma—that scientists don’t yet understand. And a lab, of course, cannot replicate the chemical chaos of the outdoor environment. So, once the Joshes collect enough data in a controlled space, they will take their experiment outside, partnering with plant scientists to investigate how different species react to pre-industrial versus modern atmospheric chemicals.

Our main goal, Cox said, is to collect precise data on atmosphere and plant interactions from pre-industrial to modern conditions. “It's really about trying to understand all these fundamental processes,” Cox said, “so that we know what chemistry is going on that drives air pollution and climate.”


Cover photo: Filip Zrnzević on Unsplash