When wildfire smoke is in the air, doctors recommend that people stay indoors to avoid breathing in harmful particles and gases. But what happens to trees and other plants that can’t escape the smoke?
It turns out they react a bit like we do: some trees essentially close their windows and doors and hold their breath.
As atmospheric scientists and chemists, we study the air quality and ecological effects of wildfire smoke and other pollutants. In a study that began quite by accident when smoke engulfed our research site in Colorado, we were able to observe in real time how the leaves of living pine trees responded.
How plants breathe
Plants have pores on the surface of their leaves, called stomata. These pores are a lot like our mouths, except that while we inhale oxygen and exhale carbon dioxide, plants inhale carbon dioxide and exhale oxygen.
Humans and plants inhale other chemicals in the air around them and exhale chemicals produced inside them—coffee breath for some people, pine smell for some trees.
Unlike humans, leaves inhale and exhale at the same time, constantly taking in and releasing atmospheric gases.
Clues from over a century of research
In the early 1900s, scientists studying trees in heavily polluted areas discovered that those chronically exposed to pollution from burning coal had black granules that clogged the pores in the leaves through which plants breathe. They suspected that the substance in these granules was partly created by the trees, but due to the lack of instruments available at the time, the chemistry of these granules was never explored, nor was their effect on plant photosynthesis.
Most modern research on the effects of wildfire smoke has focused on crops, and the results have been conflicting.
For example, a study of several crops and wetlands in California showed that smoke diffuses light in a way that makes plants more efficient at photosynthesis and growth. However, a lab study in which plants were exposed to artificial smoke found that plant productivity dropped during and after exposure to the smoke — although the plants recovered after a few hours.
There are other clues that wildfire smoke can negatively impact plants. You may have tasted one already: When grapes are exposed to smoke, their wine can be affected.
What makes smoke toxic, even away from fire?
As smoke from a wildfire travels long distances, it bakes in the sun and undergoes chemical changes.
The mixture of volatile organic compounds, nitrogen oxides and sunlight produces ground-level ozone, which can cause respiratory problems in humans. It can also damage plants by degrading leaf surfaces, oxidizing plant tissues and slowing photosynthesis.
While scientists generally consider urban areas to be major sources of ozone that affect downwind crops, wildfire smoke is a growing concern. Other compounds, including nitrogen oxides, can also harm plants and reduce photosynthesis.
Studies suggest that wildfire smoke interacts with plants, but in ways that are poorly understood. This lack of research is because it is difficult to study the effects of smoke on the leaves of living plants in the wild: wildfires are hard to predict, and being in smoky conditions can be dangerous.
Accidental search – in the middle of a forest fire
Our goal wasn’t to study how plants respond to wildfire smoke. Instead, we were trying to understand how plants emit volatile organic compounds—the chemicals that give forests their smell, but also impact air quality and can even change clouds.
Fall 2020 was a bad season for wildfires in the western United States, and thick smoke billowed through a field site where we were working in the Colorado Rocky Mountains.
On the first morning of heavy smoke, we did our usual test to measure photosynthesis in the leaves of ponderosa pines. We were surprised to find that the pores of the tree were completely closed and photosynthesis was almost non-existent.
We also measured the emissions of volatile organic compounds (VOCs) from the leaves and found very low values. This means that the leaves are not “breathing”: they are not inhaling the carbon dioxide they need to grow and are not exhaling the chemicals they normally release.
Faced with these unexpected results, we decided to try to force photosynthesis and see if we could “defibrillate” the leaf so that it returned to its normal rhythm. By changing the temperature and humidity of the leaf, we cleared the leaf’s “airways” and saw a sudden improvement in photosynthesis and an explosion of volatile organic compounds.
Our months of data tell us that some plants respond to heavy wildfire smoke by cutting off their exchanges with the outside air. They actually hold their breath, but not until they’ve been exposed to the smoke.
We hypothesize that several processes could have caused the leaf pores to close: Smoke particles could have coated the leaves, creating a layer that prevented the pores from opening. Smoke could also have penetrated the leaves and clogged their pores, keeping them sticky. Or, the leaves could have physically responded to the first signs of smoke and closed their pores before they were hit.
It is probably a combination of these answers and others.
The long-term impact is still unknown
It is not yet clear how long the effects of wildfire smoke last and how repeated smoke episodes will affect plants, including trees and crops, in the long term.
As wildfires increase in intensity and frequency due to climate change, forest management policies and human behavior, it is important to better understand their impact.
Delphine Farmer, Professor of Chemistry, Colorado State University and Mj Riches, Postdoctoral Researcher in Environmental and Atmospheric Sciences, Colorado State University
This article is republished from The Conversation under a Creative Commons license. Read the original article.
