Some trees ‘hold their breath’ during wildfires
When wildfire smoke hangs in the air, trees close their windows and doors, choking people’s breath.
When wildfire smoke hangs in the air, doctors urge people to stay indoors to avoid breathing in harmful particles and gases, but what happens to the trees and other plants that can’t escape the smoke?
As it turns out, trees react a bit like us: some close their windows and doors and hold their breath.
As atmospheric and chemical scientists, we study the effects of wildfire smoke and other pollutants on air quality and ecosystems. A coincidental start to our research when our Colorado research facility was blanketed in smoke allowed us to observe in real time how living pine tree leaves responded.
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How plants breathe
Plants have holes on the surface of their leaves called stomata. These holes are very similar to a human mouth, except that whereas humans take in oxygen and exhale carbon dioxide, plants take in carbon dioxide and exhale oxygen.
Both humans and plants inhale chemicals from the air around them and exhale chemicals produced within their bodies, resulting in coffee breath for some people and pine-scented breath for others.
However, unlike humans, leaves also breathe, constantly absorbing and releasing gases from the atmosphere.
Clues from more than a century of research
In the early 1900s, scientists studying trees in highly polluted areas found that the stomata in the leaves of trees chronically exposed to pollution from coal burning were clogged with black particles. They assumed that some of the substances contained in these particles were produced by the trees, but due to a lack of equipment available at the time, the chemical nature of the particles and their effect on plant photosynthesis were not investigated.
Most recent studies on the effects of wildfire smoke have focused on crops, and the results have been conflicting.
For example, a study of several crops and wetlands in California showed that smoke scatters light, making plants more efficient at photosynthesis and growth. However, laboratory studies exposing plants to artificial smoke found that plant productivity decreased during and after exposure to smoke, although it recovered after a few hours.
There are other clues that wildfire smoke can harm plants, and you may have tasted one of them: When grapes are exposed to smoke, it can taint wine.
Why smoke can be toxic even far away from the fire
As wildfire smoke travels long distances, it is heated by sunlight and chemically altered.
A combination of volatile organic compounds, nitrogen oxides, and sunlight creates ground-level ozone, which can cause respiratory problems in humans and can damage plants by deteriorating leaf surfaces, oxidizing plant tissue, and slowing photosynthesis.
Scientists typically think of urban areas as the biggest sources of ozone that affects crops downwind, but wildfire smoke is an emerging concern. Other compounds, such as nitrogen oxides, can also harm plants and reduce photosynthesis.
Taken together, the research suggests that wildfire smoke interacts with plants, but the ways in which it does so are not well understood. This lack of research is due to the fact that it is difficult to study the effects of smoke on living plant leaves in the wild. Wildfires are hard to predict, and being in smoky conditions can be dangerous.
Accidental Research – In the Midst of a Wildfire
We weren’t trying to study plants’ responses to wildfire smoke. Instead, we were trying to understand how plants release volatile organic compounds, the chemicals that not only give forests their scent but can also affect air quality and even alter clouds.
Fall 2020 was wildfire season in the western United States, and thick smoke hung over the site where we were working in the Colorado Rocky Mountains.
On the first morning of the thick smoke, we did our usual test to measure photosynthesis at needle level in ponderosa pines. To our surprise, the trees’ stomata were completely closed and photosynthesis was nearly zero.
They also measured the levels of volatile organic compounds normally emitted by the leaves and found that the levels were very low, meaning the leaves were not “breathing” – taking in the carbon dioxide they need to grow and exhaling the chemicals they normally release.
Given these unexpected results, we decided to try to see if we could force photosynthesis and “tame” the leaves into a normal rhythm. By altering the temperature and humidity of the leaves, we cleared the leaves’ “airways,” which suddenly improved photosynthesis and led to a surge in volatile organic compounds.
Months of data suggest that some plants respond to intense smoke from wildfires by cutting off the exchange of air with the outside world — essentially holding their breath before being exposed to the smoke.
There are several hypotheses as to why leaves close their stomata. It could be that smoke particles coat the leaves, creating a layer that keeps the stomata from opening. It could also be that the smoke penetrates the leaves and blocks the stomata, making them sticky. Or it could be that the leaves have a physical reaction to the first signs of smoke, closing their stomata before the worst happens.
Probably a combination of these and other responses.
Long-term effects still unknown
The jury is still out on exactly how long the effects of wildfire smoke last, and how repeated exposure to smoke will affect plants, including trees and crops, in the long term.
This article was originally published on conversation. read Original Article.