By Matt Reese
From the driest spring weather on record for some areas to some record-setting wheat, corn and soybean yields for farms, the 2023 growing season had its share of ups and downs statewide. There was no shortage of coffee shop conversation fodder with a number of agricultural oddities from the year, but it seems the most discussion, questioning, theorizing, and speculating in 2023 revolved around the hazy, smoggy stretch of days due to the smoke from the Canadian wildfires.
At the time, many farmers had concerns yields were being limited by the filtered sunlight through the smoke, but with some high yielding crops around the state, many are now speculating that the smoke could have somehow improved yields.
There were three major stretches of wildfire smoke in Ohio: June 6 and 7, June 27 through 29, and July 16 and 17. There were several other smoky sky days of less intensity throughout the two-month period as well.
So, with harvest wrapped up for 2023, what’s the verdict? Were the smoky conditions positive, negative or neutral for Ohio’s crop yields?
“That’s a really hard question to answer. I think overall when we look at growth and development, oftentimes it’s more paired to precipitation and temperature. I think those were kind of the big drivers that were affecting our productivity this year again,” said Alex Lindsey, Ohio State University Associate Professor in Crop Ecophysiology and Agronomy. “At the Northwest Agricultural Research Station there is a light sensor on the weather system. Most of our haze incidents were in June, particularly the latter part of that month. Overall, for the month of June, we only saw about one day’s worth of reduction in daylight as compared to the 10-year average from that same station. The month of May, conversely, was a lot less cloudy and so we actually had more light, probably 7 days more light than we normally experience for the 10-year average. So, if we look at the accumulated absorbance of light at that station over the growing season from May through September, we were actually at the 10-year average or slightly above.”
Also affecting the issue of determining the impact of the smoky conditions is the complexity of light absorption by plants.
“Whenever we think about crop yield potential, it starts with when we got planted and how much light was potentially available during the season,” Lindsey said. “Ultimately what the plant is doing is taking light energy and converting it into chemical energy in the form of photosynthesis, which then allows the plant to grow, do different metabolic activities, set seed, and ultimately fill those seeds with protein, oil and starch. Those are the main goals of plant productivity and it starts with external energy. Anytime you limit light availability, either because of delayed planting or potentially because of cloudy conditions, there could be potential to lower productivity because your input energy is not maximized. That’s kind of the baseline starting point.”
In addition, plants do not use all of the light available to them.
“Plants don’t use 100% of the light they absorb. Sometimes the light intensity is too much for the plant to use efficiently. The plant is taking it up but then has to get rid of it in a way that’s not contributing towards photosynthesis, and that’s a natural phenomenon. It’s how plants deal with low light conditions. They can use that light, but then under high light conditions, sometimes that becomes stressful. It’s not only about the light energy that’s coming into the system but it’s how effectively the plant is able to convert the absorbed light into photosynthate that it makes throughout the season. In that situation, too much light has the potential to reduce yields because of the stress it could cause,” Lindsey said. “It could cause photoinhibition, meaning that the plant needs to divert metabolic activity towards dealing with too much light. It needs to protect its photosystems. It changes how the plant can photosynthesize. It may result in reactive oxygen species in the cells and the plant then has to build enzymes to deal with metabolizing those so there is not hydrogen peroxide floating around in the cells. So, yes, too much light can actually cause photoinhibition and can cause problems that are negatively associated with crop yield.”
Plants also need the right type of light.
“The plant specifically needs wavelengths between 400 and 700 nanometers to do photosynthesis effectively. So, if we get a reduction in photosynthetically active radiation that’s reaching the plant canopy, that’s one potential issue,” Lindsey said. “The other issue we have to think about is overall light intensity, because each light, regardless of its wavelength, acts like a particle and plants need those particles of light to do photosynthesis effectively, so we’re thinking about wave spectrum and we’re also thinking about intensity.”
This translates into an important metric to consider when looking at crop yield.
“That one metric that we look at with regard to crop productivity is the idea of photothermal quotient, which is the number of photons received per growing degree day (GDD) accumulated. The idea there is if you have more photons coming into your system per GDD that, in theory, may lead to better productivity because you’re able to get more light absorbed and more photosynthesis to occur,” Lindsey said. “We did see photothermal quotients were trending slightly higher than normal for the month of May and June this year. The months of August and July were fairly similar to the 10-year average. So, per unit, the plants at the Northwest Research Station were actually getting a little bit more light than usual this year.”
In addition to influencing the light, the smoke also changed the air quality. Did that affect the plants?
“In general, when plants are utilizing carbon for photosynthesis, it has to be in the form of CO2. Corn, for example, actually has a mechanism built into its physiology to increase the concentration of CO2 where photosynthesis is happening. So, increasing carbon outside of the plant really won’t have an effect on increasing productivity for the corn because it’s so well built to do it naturally,” Lindsey said. “For soybeans and wheat, those are called C3 plants and they don’t have that carbon dioxide concentration mechanism built in; but again, if the carbon outside of the leaves was in a form other than CO2, it wouldn’t really be contributing directly to increasing the carbon feeding of those plants during the season.”
Though it was a noteworthy oddity, the yields of 2023 probably had less to do with the smoke and more to do with the weather system responsible for bringing the smoke down from Canada.
“I think predominantly when we think about yields this season, we need to think more or less about what we usually do in a regular growing season. I might attribute some of the yield trends that we’re seeing more to the temperature and moisture distribution throughout the season this year. When we had the drought conditions in mid-May into mid-June, one of the things that helped our crops survive to some degree were the temperatures, which were not very warm. That may have helped our crops survive some of those conditions,” he said. “And, overall, the high temperatures this year they weren’t quite as high as in years past, which may have contributed to our really high wheat yields and also potentially some of the less impacted yields that we’re seeing coming off for corn and soybeans this season. We didn’t really have super stressful temperatures throughout much of the state this season and even though we had maybe more scattered light, the intensity wasn’t necessarily declined as much as we may have been perceiving it to be. Our eyes are really good at picking up the visible light spectrum and while changes to that may look really impactful for us, the way the plants experienced it may not have been as severe as some might have feared.”
In the future, a similar smoky situation at different points in the growing season or in different conditions could have different impacts.
“Time will tell with this. Hopefully, fingers crossed, we don’t experience more wildfire smoke, but it’s possible. They’re predicting increases of these kinds of events and if there are, we may be able to start to pull some of these data sets together and look at it from a broader perspective to really start to delve into pros and cons of having this situation in the growing season,” Lindsey said. “The other thing to keep in mind is that these haze windows that occurred did occur in a fairly short span of the growing season. It’s possible that, if these conditions occurred at a different time in the season, maybe during pollination for example or different growth stages, it’s possible the effects may have been different. Also, how it interacts with the temperature at the time and the precipitation at the time may potentially be something that we would want to watch moving forward. But, as far as this season, I think a lot of the yield trends that we’re seeing are more associated with temperature and precipitation patterns.”