How Wildfire Smoke Affects Trees and Other Plants

When wildfire smoke fills the air, doctors advise people to remain indoors to avoid inhaling dangerous particles and gases. But trees and other plants cannot get away from the smoke, so what happens to them?

It seems they react somewhat as we do: certain trees effectively close their windows and doors, then hold their breath.

As atmospheric and chemical scientists, we investigate air quality along with the ecological consequences of wildfire smoke and other pollutants. A study that began accidentally, after smoke engulfed our Colorado research site, let us observe in real time how living pine-tree leaves reacted.

How plants breathe

Plants have tiny pores across their leaf surfaces, known as stomata. These pores resemble our mouths, but plants take in carbon dioxide and release oxygen, whereas humans breathe in oxygen and breathe out carbon dioxide.

People and plants also take in other chemicals from the surrounding air and emit chemicals made within their bodies: coffee breath in some people, and pine fragrances in certain trees.

Unlike humans, though, leaves inhale and exhale simultaneously, continually absorbing and releasing gases in the atmosphere.

Clues from over a century of research

In the early 1900s, researchers examining trees in severely polluted places found black granules blocking the leaf pores that plants use to breathe in trees regularly exposed to emissions from burning coal. They believed trees themselves partly produced the material in these granules. However, the instruments available at the time could not examine the granules' chemistry or their effects on plants' photosynthesis.

Much of the current research on the consequences of wildfire smoke has concentrated on crops, with inconsistent findings.

For instance, research across several Californian crop and wetland sites found that smoke scatters light, enabling plants to photosynthesise and grow more efficiently. Yet a laboratory experiment exposing plants to manufactured smoke found that productivity declined both during and after exposure, although the plants recovered within a few hours.

Other evidence indicates that wildfire smoke may harm plants. You may even have experienced one effect: smoke exposure can taint grapes and, consequently, their wine.

What makes smoke toxic, even far from the fire

As wildfire smoke travels over long distances, sunlight effectively cooks it and alters its chemistry.

When volatile organic compounds, nitrogen oxides and sunlight combine, they can produce ground-level ozone, which may cause breathing difficulties in people. Ozone can also harm plants, degrading leaf surfaces, oxidising plant tissue and slowing photosynthesis.

Although scientists generally regard urban areas as major sources of ozone that affects crops downwind, wildfire smoke is an increasing concern. Other chemicals, including nitrogen oxides, may likewise damage plants and lower photosynthesis.

Together, these studies indicate that wildfire smoke interacts with plants in ways that remain poorly understood. Research is limited because investigating smoke's effects on leaves of living plants in the wild is difficult: wildfires are challenging to predict, and smoky environments may be unsafe.

Accidental research – in the middle of a wildfire

Our original aim was not to investigate how plants respond to wildfire smoke. We were instead seeking to understand how plants emit volatile organic compounds - chemicals responsible for the smell of forests that also influence air quality and can even alter clouds.

Autumn 2020 brought a severe wildfire season to the western United States, and dense smoke swept through a Rocky Mountains field site in Colorado where we were working.

On the first morning of heavy smoke, we carried out our routine test of leaf-level photosynthesis in Ponderosa pines. To our surprise, the tree's pores had shut completely and photosynthesis was almost zero.

We also tested leaves for emissions of their normal volatile organic compounds and recorded very low levels. In other words, the leaves were not "breathing" - they were neither taking in the carbon dioxide required for growth nor releasing the chemicals they normally emit.

Given these surprising findings, we tried to stimulate photosynthesis to see whether we could "defibrillate" the leaf back into its usual rhythm. Altering the leaf's temperature and humidity cleared its "airways", producing an immediate improvement in photosynthesis and a surge in volatile organic compounds.

Our data collected over several months showed that some plants respond to intense episodes of wildfire smoke by stopping their exchange with the outside air. In effect, they hold their breath, although only after smoke has already reached them.

We propose several possible mechanisms behind the closure of leaf pores. Smoke particles may cover leaves, forming a layer that stops pores opening. Smoke might also enter leaves and block the pores, leaving them sticky. Alternatively, leaves could physically react to the earliest signs of smoke, closing their pores before exposure becomes more severe.

A combination of these responses, and others, is likely involved.

The long-term impact is still unknown

It remains unclear precisely how long wildfire smoke effects persist, or how repeated smoke events will affect plants - including trees and crops - in the long term.

As climate change, forest management policies and human behaviour make wildfires more frequent and severe, developing a clearer understanding of their impact is important.

Delphine Farmer, Professor of Chemistry, Colorado State University and Mj Riches, Postdoctoral Researcher in Environmental and Atmospheric Science, Colorado State University

This article is republished from The Conversation under a Creative Commons licence. Read the original article.