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While scientists usually think about urban regions as being large sources of ozone that effect crops downwind, wildfire smoke is an emerging concern.It can also damage plants by degrading the leaf surface, oxidizing plant tissue and slowing photosynthesis.

Even trees 'hold their breath' to avoid harmful wildfire smoke,
research finds

By Mj Riches published July 31, 2024
Trees don't like to breathe wildfire smoke, either

When wildfire smoke is in the air, doctors urge people to stay
indoors to avoid breathing in harmful particles and gases. But what
happens to trees and other plants that can't escape from the
smoke?
They may respond a bit like us, it turns out: Some trees essentially
shut their windows and doors and hold their breath.
As atmospheric and chemical scientists, we study the air quality
and ecological effects of wildfire smoke and other pollutants. In a
study that started quite by accident when smoke overwhelmed our
research site in Colorado, we were able to watch 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 much like our mouths, except that while we inhale
oxygen and exhale carbon dioxide, plants inhale carbon dioxide and
exhale oxygen.
Both humans and plants inhale other chemicals in the air around
them and exhale chemicals produced inside them
Unlike humans, however, leaves breathe in and out 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 coal-burning had black granules clogging the leaf pores through which
plants breathe. They suspected that the substance in these granules
was partly created by the trees, but due to the lack of available
instruments at the time, the chemistry of those granules was never
explored, nor were the effects on the plants' photosynthesis

Most modern research into wildfire smoke's effects has focused on
crops, and the results have been conflicting
For example, a study of multiple crop and wetland sites in Californi

showed that smoke scatters light in a way that made 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 smoke exposure - though
those plants did recover after a few hours.Mixing volatile organic compounds, nitrogen oxides and sunlight will
make ground-level ozone, which can cause breathing problems in
humans. It can also damage plants by degrading the leaf surface,
oxidizing plant tissue and slowing photosynthesis.
While scientists usually think about urban regions as being large
sources of ozone that effect crops downwind, wildfire smoke is an
emerging concern. Other compounds, including nitrogen oxides,
can also harm plants and reduce photosynthesis
On the first morning of heavy smoke, we did our usual test to
measure leaf-level photosynthesis of Ponderosa pines. We were
surprised to discover that the tree's pores were completely closed

and photosynthesis was nearly zero.
We also measured the leaves' emissions of their usual volatile
organic compounds and found very low readings. This meant that
the leaves weren't "breathing" -- they weren't inhaling the carbondioxide they need to grow and weren't exhaling the chemicals they
usually release.
The long-term impact is still unknown
The jury is still out on exactty how long the effects of wildfire smoke
last and how repeated smoke events will affect plants--including
trees and crops - over the long term.
With wildfires increasing in severity and frequency due to climate
change, forest management policies and human behaviour, it's
important to gain a better understanding of the impact.