Wildfire smoke tends to have harmful particles and gases in it. People are often told to stay indoors to avoid breathing in those things. Trees, which have no way of going inside or moving somewhere else, have their own way of holding their breaths to reduce smoke intake.
In an Aug. 5, 2024 article in Atlas Obscura, authors Delphine Farmer and MJ Riches explained that plants have pores on the surface of their leaves called stomata. Delphine is a professor of chemistry at Colorado State University, and Riches is a postdoctoral researcher in environmental and atmospheric science at CSU.
The authors explained, "These pores are much like our mouths, except that while we inhale oxygen and exhale carbon dioxide, plants inhale carbon dioxide and exhale oxygen. … Unlike humans, however, leaves breathe in and out at the same time, constantly taking in and releasing atmospheric gases."
A scientific study of trees in heavily polluted areas in the early 1900s revealed something interesting about the tree leaves. Those that had been chronically exposed to pollution from coal burning had pores clogged with black granules.
Because there weren't instruments available to explore the chemistry of those granules back in the early 1900s, scientists weren't able to study the granules. And they weren't able to discover what affects the granules had on the plants' photosynthesis.
Then, as now, wildfire smoke sometimes travels long distances. When it does, exposure to the heat of sunlight can chemically change the smoke. In addition to the effects of sunlight, smoke becomes mixed with volatile organic compounds and nitrogen oxides. Together, they make ground-level ozone which can cause breathing problems in humans, Farmer and Riches wrote.
The ground-level ozone "can also damage plants by degrading the leaf surface, oxidizing plant tissue, and slowing photosynthesis," according to the authors.
All those things make wildfire smoke a growing concern. A study in 2020 of ponderosa pines and other plants in the Rocky Mountains of Colorado focused on the plants' leaf-level photosynthesis. The results showed that the tree's pores were completely closed. That meant they weren't breathing in the carbon dioxide they needed to grow, and they weren't breathing out oxygen along with the chemicals that the oxygen usually released. The trees, basically, were holding their breaths to keep from breathing in the wildfire smoke.
No one is certain how frequent wildfire smoke will affect trees and other plants. Time and additional studies may help to answer those questions. Perhaps the results will help scientists discover ways of protecting trees and people from the smoke.