How Wildfire Smoke Affects Trees and Other Plants

When wildfire smoke fills the air, doctors advise people to remain indoors so they do not breathe harmful gases and particles. But trees and other plants cannot get away from the smoke. What happens to them?

It appears that they react somewhat as people do: some trees effectively close their windows and doors, then hold their breath.

As atmospheric and chemical scientists, we investigate the air-quality and ecological consequences of wildfire smoke and other pollutants. Our study began unexpectedly, after smoke engulfed our Colorado research site, giving us the chance to observe live pine leaves reacting in real time.

How plants breathe

Leaves contain tiny surface pores known as stomata. They are comparable to mouths, although the gas exchange works differently: humans breathe in oxygen and breathe out carbon dioxide, while plants take in carbon dioxide and release oxygen.

Humans and plants also take in other chemicals from the surrounding air, and give off chemicals made within their bodies - coffee breath in some people, and pine fragrances in some trees.

Unlike people, though, leaves inhale and exhale simultaneously, continuously absorbing and releasing gases in the atmosphere.

More than a century of clues

In the early 20th century, researchers examining trees in severely polluted places found black granules blocking the leaf pores plants use to breathe in trees repeatedly exposed to pollution from coal burning. They believed trees had created some of the material in these granules. However, the instruments needed to investigate their chemistry did not exist at the time, and neither did researchers examine the consequences for the plants' photosynthesis.

Much of today’s research on the effects of wildfire smoke has concentrated on crops, with mixed findings.

For instance, research across several California crop and wetland sites found that smoke diffused light in a manner that improved plants’ photosynthetic efficiency and growth. Yet a laboratory experiment exposing plants to synthetic smoke showed that their productivity declined both during exposure and afterwards - although those plants recovered within a few hours.

Further evidence suggests wildfire smoke may harm plants. You may even have experienced one outcome: grapes exposed to smoke can produce tainted wine.

Why smoke can be toxic far from a wildfire

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

Volatile organic compounds, nitrogen oxides and sunlight can combine to form ground-level ozone, which can cause breathing difficulties in humans. It may also harm plants, degrading leaf surfaces, oxidising plant tissue and slowing photosynthesis.

Scientists have generally regarded urban areas as major sources of ozone that affect crops downwind, but wildfire smoke is an emerging concern. Nitrogen oxides and other compounds can likewise damage plants and reduce photosynthesis.

Collectively, the research indicates that wildfire smoke interacts with plants in ways that remain poorly understood. One reason for this gap is the difficulty of studying smoke’s effects on leaves of living plants outdoors: wildfires are difficult to predict, and smoky conditions can be unsafe.

Accidental research during a wildfire

Our original aim was not to examine how plants respond to wildfire smoke. We were instead seeking to understand how plants emit volatile organic compounds - chemicals responsible for forests smelling like forests, which also affect air quality and can even alter clouds.

Autumn 2020 was a severe wildfire season in the western United States, and dense smoke passed through a field site where we were working in Colorado’s Rocky Mountains.

On the first morning of heavy smoke, we carried out our standard test of leaf-level photosynthesis in ponderosa pines. To our surprise, the tree’s pores had shut completely and photosynthesis had fallen to almost zero.

We also assessed the leaves’ emissions of their normal volatile organic compounds, finding 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 usually emit.

Faced with these unexpected findings, we attempted to force photosynthesis, to see whether we could “defibrillate” the leaf back into its usual rhythm. We altered the leaf’s temperature and humidity, clearing its “airways”, and observed an abrupt rise in photosynthesis alongside a burst of volatile organic compounds.

Our data, collected over several months, showed that some plants answer intense episodes of wildfire smoke by shutting down their gas exchange with outdoor air. In effect, they hold their breath - but only after the smoke has already reached them.

We propose several processes that might cause leaves to close their pores. Smoke particles may cover leaves and form a layer that stops pores opening. Smoke might instead enter leaves and obstruct their pores, leaving them sticky. Alternatively, leaves may physically detect the earliest signs of smoke and shut their pores before the heaviest exposure arrives.

The explanation is likely to involve a combination of these and other responses.

The long-term effects of wildfire smoke remain uncertain

It is not yet clear precisely how long wildfire smoke effects persist, or how repeated smoke events will influence plants - including trees and crops - in the long term.

As climate change, forest-management policies and human behaviour contribute to wildfires becoming 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.