Smoke from Canadian wildfires drifted across the Midwest in 2023. When it turned the sky hazy in late June near Johnston, Iowa, it caught Mark Jeschke’s attention. He is the agronomy manager overseeing Pioneer’s long-running plots. After intense research, he concluded that wildfires and smoke will likely increase in the future.

“Growers want to know if smoke affects crop yields,” Jeschke says. “They also want to know if it contains substances harmful to crop growth.

“There is potential for wildfire smoke to impact corn yields, but it likely hasn’t affected yields so far. Likewise, substances within wildfire smoke could harm plants, but this likely isn’t a major factor so far either.”

Smoke and solar radiation

Smoky days result in less solar radiation reaching corn plants, Jeschke says. Partly cloudy days reduce solar radiation by about 25%. Shading studies indicate significant yield reductions, with timing vs. the plant’s reproduction cycle key in quantifying loss.

Less solar radiation can mean less photosynthesis, Jeschke continues. Two separate California-based studies found that smoky conditions reduced total photosynthesis by 8% to 11%, and an Ohio State University study found a 6% to 7% reduction in 2021 with smoky conditions.

However, things become complicated, Jeschke explains. Some effects from wildfire smoke benefit crop growth and perhaps increase yield. Wildfire smoke diffuses light, and diffused light can increase efficiency of photosynthesis for corn.

Related:Expect to see wildfire smoke more often

“Reduced total solar radiation is likely to be negative for growth, but increased diffusion of solar radiation could potentially be positive,” Jeschke says. “However, any benefit derived from increased diffuse radiation could be negated if the reduction in total solar radiation is too great.

“Reduction in solar radiation can reduce surface temperatures, which may be good, bad or neutral for yield. It depends on timing and circumstances. If a crop suffers from drought stress, lower daytime temperatures may help. But if a crop is behind due to below-normal growing degree unit accumulation, further reductions in temperature from smoke could make things worse.”

What is in smoke?

The largest component of smoke is water vapor, Jeschke says. Other elements include carbon dioxide, carbon monoxide and nitrogen oxides. Fine particulate matter is the main cause of the visible haze that reflects and scatters sunlight. All four of these can harm plants. Some evidence points to 5% yield reductions over the past 20 years. However, these things come from many sources besides smoke. Does smoke contribute enough to impact yield?

Thousands of different chemical compounds can be in smoke in small amounts. They can undergo reactions in the atmosphere, changing the composition of smoke over time. Ozone, harmful to plants, forms from reactions in the atmosphere too.

“We tested air quality during the growing season, and on our smokiest days in late June,” Jeschke says. “Pollution crept into the unhealthy level for humans on the worst days, but wildfire smoke did not affect ozone levels or nitrogen dioxide levels on those days.”

What’s the bottom line? “There are pollutants in wildfire smoke, but phytotoxic effects of wildfire smoke are not likely impacting crops,” Jeschke concludes. “If there is any effect, it is not likely significant. We need more research since wildfire smoke events will likely increase.”