Effects of haze and weather in 2023 on crop maturation and yield in Ohio

Wildfire smoke was evident throughout much of the US Midwest from mid-June through July in 2023 (Figure 1) and was attributed as the primary cause of hazy conditions during that period. According to the National Weather Service, haze is “an aggregation in the atmosphere of very fine, widely dispersed, solid or liquid particles, or both, giving the air an opalescent appearance that subdues colors” (NOAA-NWS, n.d.). Many farmers and practitioners had concerns regarding this phenomenon on crop growth and development, and questioned how detrimental these conditions could be to crop yields.

This brief report uses data from Ohio in the period of May to August in 2023 compared to the 10- or 30-year averages to help quantify the effect the 2023 wildfire smoke had on (1) available light, (2) growing degree day (GDD) accumulation, and (3) crop maturation and yield for the growing season. As wildfire smoke is anticipated to become more frequent (Burke et al., 2021; Ostoja et al., 2023), understanding the effects on the growing environment is key to implement potential changes to management to minimize possible stress from these events.

All light, precipitation, and GDD data (using the solar radiation, precip, and GDD columns, respectively) were collected from three Ohio State University College of Food, Agricultural and Environmental Science Weather System stations: Northwest (Custar, OH), Western (South Charleston, OH), and Ohio Agricultural Research and Development Center (Wooster, OH) for 2014–2023. These are three representative locations where corn (Zea mays L.), soybean [Glycine max (L.) Merr.], and wheat (Triticum aestivum L.) are commonly grown in Ohio. Photothermal quotient (PTQ) averages were calculated by dividing the monthly total for accumulated light (MJ m⁻²) by total accumulated GDDs (°F days). Data for aerosol optical depth (AOD), which is a unitless number that quantifies scattering of light due to particles in the atmosphere, were collected from the Dayton location of the NASA Goddard Space Flight Center (AERONET, 2024). Associated maps were generated using the Midwestern Regional Climate Center (MRCC) cli-MATE data portal (MRCC, 2024) and the National Oceanic and Atmospheric Administration (NOAA) Physical Sciences Laboratory (NOAA-PSL, 2024).

In 2023, one major identified concern from farmers and consultants was that light was being reduced and photosynthesis was negatively affected. Overall, average daily light integral (DLI) was above normal in May due to clear skies and abnormally dry weather pattern across all three locations (Table 1). However, DLI was reduced by 2.7–9.1% and 0.8–3.6% from the 10-year average in June and July, respectively. August DLI values ranged from 5% below average to 1% above average in Ohio.

Despite lower overall DLI values observed in June, July, and August, it is possible that diffuse light concentrations increased, though this was not measured by the Ohio State University weather stations. Past research has reported increased diffuse photosynthetically active radiation by 34.7% under wildfire smoke haze despite seeing a 3.5% reduction in direct radiation (Hemes et al., 2020). Gains in photosynthesis from increased diffuse light concentrations or more uniform distribution of light in canopies in corn and wheat have been documented previously (Hemes et al., 2020; Zhang et al., 2022).

Precipitation was below average for May and June at all sites by 18.8–64.1%. However, July precipitation was above average at two sites by 10.0–25.4% and was above average for August at all sites by 15.4–96.3%. Total monthly GDD accumulation was 1.5–21.8% less in 2023 relative to 10-year average for all months and all locations, except the month of July at the Western site (1.4% more GDDs than 10-year average). The greatest reductions (8.2–21.8%) were documented primarily for May and June. Our analyses showed that calculated average daily PTQ values were 3.7–38.1% greater than normal (10-year average) in 2023 for May, June, and August. Generally, greater PTQ values suggest that more photosynthesis can occur in the same thermal period and could lead to greater yields. Greater PTQ values have been associated with higher yields (Fischer, 1985; Veenstra et al., 2021). The dynamics observed in 2023 suggest that despite lower DLI, more days spent at each growth stage from a reduced growth rate (caused by lower GDD accumulation) may have helped to offset potential yield losses by increasing the PTQ.

Other questions were raised about the wildfire smoke and its direct impact on temperatures in 2023 and if these changes could help explain the observed differentials in GDD accumulation from average. David et al. (2018) reported that increases in aerosol optical thickness (AOT), which is another term for AOD (Levy et al., 2009), from wildfires contributed to reductions in daily temperatures in northern California, with each unit increase in AOT resulting in a 1.76°F reduction in daily maximum temperature. Using the AOD data from the NASA Goddard Space Flight Center in Dayton, OH (AERONET, 2024), we determined monthly AOD values averaged across the eight available wavelengths compared to available historical values (ranging from 6–9 years of data collected periodically between 2009 and 2021 as not every month was available every year) (Table 2). The predicted reduction in daily maximum temperature for this location using the factor reported by David et al. (2018) would range from 0.2–0.6°F (Table 2). This reduction was less that what was experienced (1–4°F reduction) in Ohio for May, June, and August in 2023 (Figure 2). Assuming a reduction in average daily temperature of 0.85°F per unit increase in AOD (David et al., 2018), GDD reductions from the 10-year average caused by the increased AOD were calculated to be 0.4–1.4% using data from the nearest weather station (Western, 30 mi east of Dayton) (Table 2). However, GDD accumulation for 2023 at Western was reduced by 4–13% (Table 1). This suggests that approximately 9–12% of the observed GDD reduction experienced in 2023 at Western could potentially be attributable to the increased AOD; other meteorological factors were the predominant influencers on seasonal weather aberrations observed in 2023.

Our analyses identified important surface wind dynamics in the same period, especially in May and June (Figure 3A, B). These dynamics brought lower temperatures, and it is likely that the main contributor for the cooler season (and lower GDD) was from abnormally northerly wind flow near the surface (Figure 3). Wind direction returned to more normal patterns during the months of July and August (Figure 3C, D), and resulted in precipitation patterns and GDD accumulation returning to values closer to the long-term averages (Table 1).

The reduction in GDD accumulation likely had minimal effect on wheat maturation in 2023 compared to the 10-year average (Table 3). However, the lower GDD accumulation in 2023 could have placed a relevant effect slowing on corn maturation in 2023 compared to the 10-year averages through about Week 40 and through about Week 38 for soybean (Table 3). An additional consideration for crop maturation was distribution of rainfall during the year. Below average precipitation in May and June followed by above normal precipitation in July and August for most of Ohio (Figure 4), paired with slower GDD accumulation, may have also contributed to a lengthened grain and seedfill period for corn and soybean in 2023. Delayed crop development in response to later-season rains in corn has been observed in recent years (Agyei et al., 2022; Ortez et al., 2023), and when paired with below-average GDD accumulation, may help explain the delays in maturation observed in 2023 for corn. Soybean senescence was marginally behind the average earlier in the season, but accelerated quickly in latter weeks as changing photoperiod is primarily responsible for progression through the late reproductive stages (Setiyono et al., 2007). Photoperiod responses are induced by changing ratios of daily light and dark hours and are less affected by intensity; reductions in DLI would have minimal effects on photoperiod responses.

Besides crop maturation, an additional factor to consider is grain moisture. The GDD accumulation delays and altered precipitation distribution likely resulted in higher corn grain moisture at harvest between weeks 40 and 47, with greater moisture content in Weeks 40–44 compared to the 10-year average. On the other hand, soybean seed moisture at harvest in 2023 was comparable to the 10-year average (Table 4), likely due to timely maturation in later weeks as described previously (Table 3).

Despite the cooler weather, haze in the atmosphere, wind flows, shifts in precipitation distribution, and lower GDD accumulation in 2023, crop yields were the greatest they have been over the past 10 years for corn, soybean, and wheat (Table 5) resulting in new yield records for all three crops in the state of Ohio. Past modeling work suggests yield increases may be expected in some cases under haze (Durand et al., 2021), but it is recognized improved knowledge of crop responses are needed to verify these predictions. It is possible that if the haze conditions had occurred during corn or soybean grain fill, crop yield may have been affected differently. Simulation work from Nebraska suggested a 46% decrease in light occurring for seven consecutive days in August would result in a 5.2% decrease in corn yield (Elmore et al., 2019). It is also difficult to tease out potential losses on yield as a result of the haze in 2023 without a true comparison where haze was not experienced. However, the high average yields for corn, soybean, and wheat for Ohio relative to the past 10 years suggest that the growing season was overall favorable for crop growth and yield despite the weather limitations experienced.

Based on collected weather data, a measurable impact on reduced light availability and increased AOD was quantified in Ohio in 2023 during periods coinciding with haze from wildfires. Daily light integrals were reduced in June and July by 1–9%, and haze was likely contributing partially to the reduction in GDD accumulation in 2023 (9–12% of the reduced GDD accumulation may have been a result of higher AOD values). However, factors other than wildfire smoke were likely the predominant drivers of weather experienced during the 2023 growing season. Abnormal northerly wind flow brought colder temperatures and below-average precipitation in May and June, though above-normal precipitation occurred in July and August for most of Ohio when winds turned more seasonal out of the south. Regardless of the questions and concerns raised due to hazy skies in parts of 2023, conditions were very favorable for yield and resulted in the greatest average yields for the state for the three main commodity crops in the past 10 years. The weather did affect crop maturation in September and October primarily for corn, and greater than average corn grain moisture was observed during harvest. Future work should quantify the AOD-temperature reduction relationship more aptly for the US Midwest and expand the study to include a historical analysis of the impact of haze on crop yields in the state of Ohio and the US Midwest.

A. J. Lindsey: Conceptualization; data curation; formal analysis; investigation; project administration; writing—original draft; writing—review and editing. A. B. Wilson: Conceptualization; data curation; formal analysis; visualization; writing—original draft; writing—review and editing. O. A. Ortez: Conceptualization; visualization; writing—original draft; writing—review and editing. L. E Lindsey: Project administration; visualization; writing—original draft; writing—review and editing.

The authors declare no conflicts of interest.