The policy over much of the last century to quickly suppress wildfires no matter where they occur has led to a buildup of fuels in forests. This, coupled with some hot and dry conditions, has resulted in more frequent and intense wildfires across much of the U.S. and Canada. As many growers can attest, this increase in wildfires has had impacts on agriculture.
What’s in Wildfire Smoke?
Wildfires discharge large amounts of carbon dioxide, black and brown carbon particles, and ozone precursors like hydrocarbons and nitrogen oxides that can form ozone when they mix with air. Many of these things are toxic pollutants that can be harmful to plants, people and animals when exposed to them in concentration. These emissions also modify solar radiation and other things that change the weather we experience at the ground level.
Ozone Injury Related to Wildfires
Ozone is one of the most damaging air pollutants for plants. It enters the leaf through stomata (pores on the surface of the leaf) during normal gas exchange processes. Once inside the plant, it forms reactive oxygen species that can damage cell membranes and cell contents. Exposure to high concentrations of ozone for several hours (acute exposure) can result in visible injury to leaf tissues. Symptoms may include stippling (small, brown-black spots), flecking (tiny, light-colored spots), bronzing and reddening. These symptoms appear within a few hours or days of exposure. With continued exposure to high concentrations of ozone, symptoms can progress to chlorosis, necrosis and early plant senescence. Exposure to more moderate levels of ozone for prolonged periods does not usually cause visible symptoms, but it can impair plant functions and limit growth.
Ozone injury symptoms are occasionally seen on potatoes, but we don’t see them every time there is smoke because ozone originating from wildfires is highly variable. Despite the presence of ozone precursors in smoke plumes, ozone doesn’t always form. If it does form, it may not stick around for long. Cool and cloudy conditions inhibit ozone production, and windy conditions tend to dissipate ozone when it does form. This explains why I have only observed acute ozone injury symptoms on potatoes (and other plants) when wildfires were very close to the field, it was very hot and sunny, and air inversions kept the smoke low to the ground and limited its dispersal.
How Does Smoke Affect Photosynthesis?
Some potato growers have speculated that wildfire smoke is limiting yields by slowing photosynthesis. They may be right; however, quantifying the effects of smoke on photosynthesis is complex and depends on many details (plant species involved, quality of the smoke, etc.). It is more practical to consider its effects in general terms. The main factors that affect the rate of photosynthesis are light intensity, carbon dioxide concentration and temperature; whichever of these factors is most limiting will determine the rate. This is called the law of limiting factors. Smoke can affect these limiting factors in different ways, some positive for plant productivity and some negative.
- Light intensity is reduced by smoke, but smoke almost never completely blocks the sun’s rays. How dark it gets depends on the components of smoke and how much they are absorbing light or scattering it. Smoke with a lot of soot tends to absorb light, while smoke with ash and other light-colored particles tends to reflect it like clouds.
- Carbon dioxide levels are dramatically increased by wildfires, and increased carbon dioxide usually leads to an increased rate of photosynthesis because normal atmospheric levels tend to be limiting. Ambient levels of carbon dioxide are estimated to be about 400 parts per million (ppm), but many plants will not reach their maximum uptake until closer to 1,000 ppm.
- Air temperatures are often moderated by smoke. Since the optimal temperature for potato photosynthesis peaks around 90 degrees Fahrenheit, the smoke cover may be a positive factor for plant productivity if it helps to curb temperatures when they would otherwise be very hot.
According to this assessment, light is the most limiting factor for photosynthesis when there is smoke. But the degree to which smoke limits photosynthesis and potato yields will depend on how dark it gets and how long the smoke persists. This is an oversimplification of the influences smoke can have on photosynthesis, since we should also consider that the chemical constituents of smoke can damage plant functions at a cellular level. And we need to recognize that plants can modify leaf structure (orientation, chlorophyll content, etc.) to maximize light captured under low light conditions when they are given some time to adjust.
Smoke Changes the Weather
Smoke can change the weather by blocking or scattering light and limiting radiant warming from the sun, much like clouds. These changes will affect the crop, so it’s important to modify management accordingly. For instance, you might find that the crop needs less irrigation when it’s smoky, because evaporative water loss is reduced when it’s cooler and sunlight is less intense.
Smoke, Air Inversions & Herbicide Drift
Smoke tends to prolong thermal air inversion conditions because it inhibits some of the radiant warming of the ground layer that is needed to break up the warm inversion layer. Air inversions are often associated with herbicide drift, especially for lightweight herbicides like 2,4-D. So be careful when spot-spraying weeds when there is a smoky haze, because the herbicide may travel farther than you expect.
Smoke, Prolonged Dew & Potato Diseases
You may notice more dew on your crop in the mornings when it’s smoky, and it may take longer to dry. The particles in smoke promote dew formation; they serve as condensation nuclei or tiny particles in the air on which water vapor can condense. The smoke can also help keep the dew around longer by blocking the sun’s warming rays and slowing evaporation.
The hot and dry summer environment in most potato-growing regions of the western U.S. discourages diseases like gray mold and late blight. But smoke cover can modify the environment so that it favors these diseases, and outbreaks have been known to occur. When it’s smoky, keep a closer eye out for diseases that favor cooler, moist conditions, and be prepared to manage them more aggressively than you usually do.
Smoke & Human Health Concerns
If you want to know how wildfires (and other things) are affecting air quality, visit the EPA’s AirNow website (www.airnow.gov) or your state’s smoke information website (e.g., Idaho Smoke Blog, Washington Smoke Blog). These websites have maps that show current readings from sensors that monitor air quality. These websites also provide information about how to protect yourself when air quality conditions are poor.
Carrie H. Wohleb is an associate professor and specialist of potato, vegetable and seed crops in Washington State University’s Agriculture & Natural Resources Program Unit. She can be contacted at cwohleb@wsu.edu.