Determining the actual impact of wildfire smoke on crop yields is extremely difficult for a number of reasons, including the multiple, competing effects involved and the difficulty in isolating the effects of smoke from other influences. Conducting controlled experiments on wildfire smoke is impractical, so research has often focused on measuring the effects of smoke events as they occur. Experiments such as shading studies can provide important insights into the possible impact of specific aspects of smoke cover on crop yield but cannot replicate the full suite of effects.
Based on what is known about the effects of reduced solar radiation and elevated ozone on crops, it seems very plausible that wildfire smoke could cause reductions in crop yields. The scope of possible outcomes likely ranges from slightly beneficial to significantly harmful. The study by Hemes et al. (2020) probably represents something close to a best-case scenario where the benefit of increased diffuse PAR exceeded the negative effects of slightly lower total PAR and elevated ozone. The heavier the smoke, the more likely reduction in total PAR will be the dominant factor.
In general, corn is likely to be more susceptible to the effects of wildfire smoke than soybeans. Corn has a higher light saturation point due to its C4 photosynthetic pathway, so is more likely to be impacted by reductions in total PAR. Corn may also experience reduced standability if lower solar radiation during grain fill forces plants to remobilize more carbohydrates from the stalk. The risk of yield loss and reduced stalk health is likely greater when smoke imposes an additional stress upon a crop that is already experiencing the effect other stresses, like disease or drought stress. Clearly identifying all contributing stresses can be very difficult, much less being able to precisely quantify the impact each of the those compounding factors may have had on the crop.
Corn and soybean can both be harmed by elevated ozone levels; however, both the production of ozone from wildfire smoke and the intake of ozone through plant stomata can be influenced by a number of different factors. Corn and soybean already experience wide scale reductions in yield from ozone associated with other sources of air pollution (McGrath et al., 2015), so the additional effect of ozone specifically associated with wildfire smoke could be difficult to determine.
Wildfire smoke is not a problem that’s going away anytime soon. Based on what we know about the contributing factors, wildfires in western North America are likely to increase in frequency and intensity in the coming years. The effects on wildfire smoke on both agricultural and natural ecosystems will likely continue to be an active area of research.