It’s known that wildfires have adverse effects on humans, but what about the plants, animals and trees?
New research by Mj Riches, a postdoctoral research fellow in the Colorado State University Department of Chemistry, found that poor, smoke-induced air quality affects ponderosa pine trees’ ability to maintain photosynthesis and release natural chemical emissions. Riches estimates a 7-14% decrease in ponderosa pine growth rate due to heavy smoke.
Riches, who came to CSU in 2016 to earn her Ph.D. in chemistry, found inspiration for this project as a not-so-happy accident. In 2020, much of Colorado was ravaged by wildfires — three of the state’s largest recorded fires happened that year — and that smoke traveled onto a field site where she was initially testing leaf-scale photosynthesis and chemical emissions in ponderosa pine trees.
“Given that the direction of the project had to change to accommodate smoke, this research question itself came about as I suspiciously witnessed no photosynthesis on the plants,” she said. “In fact, the results were so curious, I thought that my measurement system was broken.”
In a recent interview, we sat down with Riches to discuss the implications of this research, now published in Geophysical Research Letters.
Question: Can you please describe your research question and methods?
Answer: Our research question was ‘what is the extent to which wildfire smoke impacts the photosynthesis and chemical emissions of ponderosa pine trees?’
To investigate this, we deployed several instruments into a rural forest near Woodland Park, Colorado. As wildfire smoke spread across the region, we measured leaf-level photosynthesis — using an instrument called a portable photosynthesis system — that was coupled with two trace gas analyzers, which measure the chemical emissions from the leaves.
The portable photosynthesis system controls leaf-level carbon dioxide, light, temperature and humidity, so we can alter any of these parameters as needed.
We induced photosynthesis by changing leaf-level environmental conditions using the portable photosynthesis system. When we did, we saw a burst of both plant-related and smoke-related chemicals, meaning that there’s more than one way that smoke chokes the plant and more than one way that the plant tries to recover.
Q: How does wildfire smoke reduce photosynthesis in pine trees?
A: Photosynthesis is regulated by stomatal conductance. Stomatal conductance is how open the stomata, or pores, of the leaf are.
To reduce photosynthesis, then, we hypothesized that those stomata were closed in some way. The most likely mechanism is that the stomata were plugged, likely by wildfire particles and gases depositing around or into the pores. Once those pores can open, photosynthesis resumes.
Q: How does wildfire smoke reduce chemical emissions in pine trees?
A: Plants naturally emit volatile organic compounds, which are a diverse group of chemicals that can easily evaporate into the air and play a part in a complex feedback loop that impacts both air quality and climate. As a plant’s physiology and wellbeing changes, the chemicals emitted may also change.
We saw a reduction of chemical emissions in ponderosa pines due to wildfire smoke, which could be caused by several factors.
Plants can change the synthesis of their chemical emissions to conserve energy and outlast the smoke exposure. Additionally, some chemicals that plants emit are made in advance and stored in case of emergency. Once these reserves are depleted, the plant must take time and spend energy to make more of these. The plant’s emissions may be reduced because its reserves of these chemicals are depleted.
Q: How do these findings vary with smoke intensity and length of exposure to smoke?
A: We know that both smoke intensity and duration matter. The more intense the smoke, the worse the plant fares. Heavy and extended smoke leads to a near-shutdown of both photosynthesis and chemical emissions. However, even long-lasting, light smoke can negatively impact the photosynthesis of these trees. We hope that future projects will quantify how bad smoke exposure must be to really harm these trees in the long run.
Q: What were your key takeaways from this research? How are pine trees affected by wildfire smoke?
A: The most shocking finding was the extent to which heavy smoke events decrease photosynthesis. To see both photosynthesis and chemical emissions reduced to nearly nothing was stunning. If this occurs on a larger scale, that has huge implications for plant productivity and air quality. In the paper, we estimate a 7-14% decrease in ponderosa pine growth rate due to this heavy smoke.
Q: What are the broad impacts of implications of your findings?
A: Extreme wildfire events are becoming more common, and we are underestimating the impact of wildfire smoke on trees and thus the atmospheric implications of wildfire smoke.
Wildfire smoke consists of dangerous chemicals that negatively impact air quality. When photosynthesis is drastically decreased, plants aren’t as efficient at taking up carbon dioxide from the atmosphere, which compounds the problem. Smoke contributes directly to air quality by releasing dangerous chemicals, but also limits plants’ ability to remove carbon dioxide.
This research is just an initial step towards understanding how increasing wildfires will impact plants, and thus our climate. We’re really hoping this paper brings attention to the complexities of wildfire-plant interactions and will inspire future research projects on the subject, both in the lab and in the field.
