Written by: Christopher J. Kucharik
The presence of wildfire smoke in our skies has the potential to impact crops in three primary ways: 1) reduction in amount of solar radiation received by plants, 2) an increase in the ratio of diffuse to direct beam radiation, and 3) supporting the development of ozone in the lower atmosphere. Any significant reduction in the total amount of radiation intercepted by crops would lead to a reduction in photosynthesis and potentially yield if the presence of smoke was sustained for long periods (weeks to months) during the growing season. However, the increase in diffuse radiation created by smoke can actually be beneficial to crops by increasing light use efficiency. Diffuse radiation can increase the amount of light received by canopy leaves that are normally shaded (and darker) and don’t receive direct beam radiation from the sun. This effect of increased diffuse radiation would be most pronounced when canopies are tall and leaf area index is greatest, which typically is towards the end of the vegetative stage and persisting through the reproductive phase. Which one of these effects (reduced total light vs. increased diffuse light) wins out is a difficult question to answer given there is a lack of solid research on the topic.
Given the amount and duration of smoke during the 2023 growing season thus far, it is likely that the fluctuations in light have had a minimal impact on crop growth to this point. The reduction in total radiation from smoke on the worst days (June 27-28) was approximately 5-15% during mid-day hours (when peak photosynthesis occurs) at the Arlington Ag Research Station, but the increase in diffuse radiation and having more canopy leaf area exposed to increased diffuse light could have offset that reduction. Keep in mind that cloud cover associated with precipitation and more moisture in the atmosphere can also greatly diminish solar radiation received by plants, to an extent that may be on the same order of magnitude or greater than the effects of smoke on a given day. During June when smoke was most persistent and air quality was the worst, we were also in the middle of a drought whereby we likely had more radiation due to reduced cloud cover compared to other growing seasons. Thus, it is possible that crops actually intercepted greater solar radiation during June than typically occurs in seasons when we receive normal (around 4-5.5 inches) or above normal precipitation.
Therefore, while wildfire smoke can most definitely have impacts to our crops, there are both positive and negative effects and other confounding factors like cloud cover, temperature, and soil moisture that make it difficult to determine whether smoke is causing a reduction in photosynthesis. But for now it’s more likely that smoke has not had a significant impact to this point of the 2023 growing season and other environmental controls like rainfall and temperature – like any other year – will be the dominant drivers of spatial variability in crop growth and end-of-season yields.