As smoke from fires hangs in the air, doctors are warning people to stay inside to avoid breathing in harmful particles and gases. But what about trees and other plants that can’t escape the smoke?

Apparently, they respond to us in a similar way: Some trees actually close their windows and doors and hold their breath. As atmospheric and chemical scientists, we study air quality and the environmental effects of wildfire smoke and other pollutants. In a study that began quite by chance when smoke covered our research field in Colorado, we were able to observe the response of living pine leaves in real time.

How do plants breathe?

Plants have pores on their leaves called stomata. These pores are very similar to our mouths, except that while we take in oxygen and release carbon dioxide, plants take in carbon dioxide and release oxygen.

Both humans and plants breathe in other chemicals from the air around them and breathe out chemicals they themselves produce—the smell of coffee to some people, the smell of pine to some trees. Unlike humans, however, leaves breathe in and out simultaneously, constantly absorbing 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 exposed to pollution from continuously burning coal contained black granules that blocked the leaf pores through which plants breathed. They suspected that the material in these pellets was partly created by the trees, but due to the lack of instruments available at the time, the chemical composition of these pellets and their effect on plant photosynthesis were never investigated.

Most current research on the effects of wildfire smoke has focused on agricultural crops, and the results are inconsistent.

For example, a study of various crops and wetlands in California found that smoke scatters 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 showed that plant performance decreased during and after exposure to smoke, but these plants recovered after a few hours. There are other signs that smoke from wildfires can negatively affect plants. You may have even tasted one: Grapes can spoil wine when exposed to smoke.

This makes the smoke toxic even far from the fire.

As smoke from a fire travels long distances, it cooks and chemically changes in sunlight. The mixing of volatile organic compounds, nitrogen oxides, and sunlight will create ground-level ozone, which can cause respiratory problems in humans. It can also harm plants by damaging leaf surfaces, oxidizing plant tissues, and slowing photosynthesis.

While scientists generally think of urban areas as major sources of ozone, which affects crops downwind, smoke from wildfires is a concern. Other compounds, including nitrogen oxides, can also harm plants and reduce photosynthesis.

Taken together, the studies suggest that wildfire smoke interacts with plants, but in ways that are not fully understood. The lack of research is explained by the difficulty of studying the effects of smoke on the leaves of living plants in nature: wildfires are difficult to predict, and exposure to smoke can be dangerous.

Random research – in the middle of a forest fire

We didn’t set out to study plants’ responses to wildfire smoke. Instead, we wanted to understand how plants release volatile organic compounds (the chemicals that make forests smell like forests, but also affect air quality and can even change clouds).

The fall of 2020 was a bad one for wildfires in the western United States, and there was heavy smoke rising from the field we were working in the Colorado Rockies. On the first morning of heavy smoke, we conducted our routine test to measure leaf-level photosynthesis in Ponderosa pines. We were surprised to see that the tree’s pores were completely closed and photosynthesis was almost zero.

We also measured foliar emissions of common volatile organic compounds and found very low values, meaning the leaves were not “breathing” – they were not breathing in the carbon dioxide they need to grow, and they were not breathing out the chemicals they normally release.

After getting these unexpected results, we decided to try forcing photosynthesis and see if we could “defibrillate” the leaf’s normal rhythm. By changing the temperature and humidity of the leaves, we cleared the leaf’s “respiratory tract” and saw an immediate improvement in photosynthesis and the release of volatile organic compounds.

Our data over several months showed that some plants respond to the intense smoke explosions from wildfires by stopping the exchange with outside air. They effectively hold their breath, but not until they are exposed to the smoke.

We assume that there are several processes that could cause leaf pores to close: Smoke particles could be coating the leaves and forming a layer that prevents the pores from opening. Smoke could also be getting inside the leaves and clogging their pores, keeping them sticky. Or the leaves could be physically responding to the first signs of smoke and closing their pores before they reach the worst. It’s probably a combination of these and other answers.

Long-term effects still unknown

The jury is still out on exactly how long the effects of wildfire smoke will last and how repeated smoke events will affect vegetation, including trees and crops, in the long term. Wildfires are becoming more intense and frequent due to climate change, forest management policies, and human behavior, so it’s important to better understand their impacts.