Crop Report – Week of August 28th, 2023

What Are Potential Impacts to Field Crops from Wildfire Smoke in 2023?

Smoke from northern Ontario and Quebec wildfires was prevalent across Ontario for several weeks in June and early July. While a smoky haze was often visible during these weeks (Fig. 1), the heaviest days carried a campfire-like smell and reportedly even reduced visibility in parts of Northern and Eastern Ontario.

Figure 1. Smoky haze over a wheat field late June, early July in 2023.

What is the Impact of Wildfire Smoke on Field Crops?

A common question this year has been “what is the impact of wildfire smoke on field crops”? Given the random and infrequent nature of these smoke events, research is limited and it’s difficult to conduct controlled experiments to measure impacts directly.

In absence of research, one approach is to look at characteristics of smoke events (e.g., smoke levels, crop stages affected) to estimate what crop impacts may be (Jeschke, 2021). Several authors have outlined common themes for potential positive and negative impacts of wildfire smoke on crops (Table 1).

Table 1. Negative and Positive Impacts of Wildfire Smoke on Field Crops

Negative Impacts	Positive Impacts
Reduced Light Availability	Diffuse Light
Ozone	Lower Leaf Temperatures
Ash Deposition

Negative Impacts

Reduced Light Availability

Smoke particles reflect a portion of incoming solar radiation potentially reducing the amount available for photosynthesis at plant level.

How Much Does Wildfire Smoke Reduce Light?

Jeschke (2021) summarized results from a couple of research trials investigating light reductions during wildfire smoke in California which ranged from 4-11% over the period smoke was present. Data from Ohio suggested a 6-7% reduction during June/July when wildfire smoke was present compared to the same period for the previous 5 years (Lindsey, 2021).

How Does This Compare to Cloudy Weather?

For comparisons sake, Jeschke (2021) compared measurements of photosynthetically active radiation (PAR) for varying cloud covers relative to a sunny July day in Iowa.

• a partly cloudy day reduced PAR by 23%
• a cloudy day reduced PAR by 52%
• a rainy day reduced PAR by 62%

What are Yield Impacts for Reducing Light?

In corn, small amounts of shading can have limited yield impacts, but significant shading can impact yields. Amount of yield loss depends on crop stage. Yield impacts are minimal during vegetative stages and greatest during pollination and early grain fill (likely because kernel number and kernel fill is highly sensitive to photosynthate production at these times) (Jeschke, 2021).

The amount of light we receive may be greater than what plants require to optimize photosynthesis. Reducing light availability is expected to have a greater impact on C4 plants (corn, sorghum) as they have higher light saturation points (the point where additional light provides no more benefit for photosynthesis). Light quantity must be reduced a greater amount before it starts to impact photosynthesis in C3 plants (most other field crops grown in Ontario) (Lindsey, 2023).

Other Reduced Light Risks

If carbohydrate accumulation during grain fill is lower than normal, another potential risk could be increased remobilization from stalks to help fill grain, as can occur during years with other stresses (drought, disease/leaf area loss, N loss etc.). Compromised stalks can increase risks for stalk rots and/or lodging (Archontoulis and Licht, 2023; Jeschke, 2021).

Ozone

Ozone is produced when certain pollutants produced by wildfires react in the presence of sunlight. Ozone enters plants through stomata and damages plant tissues during respiration which can cause growth issues. Dicots (e.g., soybeans and other non-grasses) are thought to be more susceptible than monocots (e.g., grasses) possibly because injury symptoms may be more clearly observed, but yields can be reduced in both (Jeschke, 2021).

Ash Deposition

Crops that are in the vicinity of wildfires, could experience ash deposits on leaves and potentially block various amounts of light and photosynthesis (Lindsey, 2023).

Positive Impacts

Diffuse Light

Smoke particles scatter light which produces diffuse light (light received indirectly after bouncing off objects rather than directly from the sun). Diffuse light can penetrate deeper into crop canopies and provide light for photosynthesis to lower leaves which are shaded under sunny conditions. Because photosynthesis may not use all light provided (Lindsey, 2023) this may increase total photosynthesis in the crop canopy if the increase in total photosynthesis is greater than any decreases in light received from what is causing diffusion (cloud, smoke). Some greenhouses use diffuse covers to scatter light on sunny days for this purpose. Canopies with greater heights or leaf area indexes likely see the greatest benefits from diffuse light (Jeschke, 2021).

Lower Leaf Temperatures

Leaf temperatures are more related to light intensity than air temperature and lower light intensity can reduce leaf temperatures (Lindsey, 2023). This can reduce plants reliance on transpiration to cool leaves. Preserving moisture can be a benefit under sunny, hot and drought-stressed conditions.

Final Comments

There can be both potentially positive and negative impacts, but they are complex, depend on degree of smoke, crop growth stage, and potentially interact with each other (Jeschke, 2021). It’s difficult to put exact numbers on yield risks. While it can’t be said wildfire smoke has no impact, other factors such as drought or excess rainfall can have much more significant impacts on yields, and other areas of North America have been able to achieve record yields in years where wildfire smoke was apparent during parts of the growing season (Archontoulis and Licht, 2023).

In Ontario, on the favourable side this year, most smoke was encountered during the vegetative stages for crops like corn and soybeans, when impacts from potential negative factors are likely much less.

References

Archontoulis, A. and M. Licht. 2023. Wildfire smoke impacts on crop production. Iowa State University. https://crops.extension.iastate.edu/blog/mark-licht-sotirios-archontoulis/wildfire-smoke-impacts-crop-production. (Accessed 30 Aug. 2023).

Jeschke, M. 2021. Is Smoke from Wildfires Affecting Crop Yields? Pioneer. https://www.pioneer.com/us/agromomy /wildfires-crop-yields.html. (Accessed 30 Aug. 2023).

Lindsey, A., L. Lindsey and A. Wilson. 2021. Hazy days… How does light influence corn and soybean? Ohio State University. https://agcrops.osu.edu/newsletter/corn-newsletter/2021-26/hazy-days%E2%80%A6how-does-light-influence-corn-and-soybean. (Accessed 30 Aug. 2023).

Lindsey, A., L. Lindsey and O. Ortez. 2023. How could the haze of wildfires affect crop growth? Ohio State University. https://agcrops.osu.edu/newsletter/corn-newsletter/2023-21/how-could-haze-wildfires-affect-crop-growth. (Accessed 30 Aug. 2023).

Weather Data – August 21-27, 2023

Location	Year	Highest Temp (°C)	Lowest Temp (°C)	Rain (mm)	Rain (mm) April 1st	GDD 0C April 1st	GDD 5C April 1st	CHU May 1st
Harrow	2023	28.7	12.3	119.2	570	2488	1759	2675
	2022	27.4	11.7	3.1	377	2624	1906	2933
	10 YR Norm (11 – 20)	26.7	15.6	19.2	467	2712	1939	3055
Ridgetown	2023	26.9	9.5	36.2	495	2367	1647	2501
	2022	28.9	8.7	0.3	269	2480	1771	2709
	10 YR Avg. (2011-20)	26.4	13.7	18.6	424	2553	1786	2845
London	2023	25.7	10.1	63.1	540	2346	1626	2484
	2022	28.4	10.4	1.6	307	2417	1716	2645
	10 YR Avg. (2011-20)	25.9	13.5	17.0	439	2531	1767	2826
Brantford	2023	25.9	10.4	23.7	437	2329	1608	2460
	2022	29.6	10.3	5.9	276	2434	1723	2598
Welland	2023	26.2	11.2	44.0	486	2370	1643	2518
	2022	28.4	12.4	2.2	294	2488	1768	2736
	10 YR Avg. (2011-20)	25.9	12.9	12.8	389	2547	1782	2849
Elora	2023	24.0	9.1	38.8	429	2154	1450	2258
	2022	27.8	9.0	15.6	243	2229	1530	2382
	10 YR Avg. (2011-20)	24.5	11.5	11.8	434	2289	1538	2532
Mount Forest	2023	23.3	7.2	21.2	383	2151	1452	2265
	2022	26.7	8.6	25.4	336	2221	1529	2416
	10 YR Avg. (2011-20)	24.1	12.2	10.9	446	2276	1533	2549
Peterborough	2023	24.9	7.7	2.1	410	2206	1490	2279
	2022	28.1	7.8	11.5	316	2243	1531	2406
	10 YR Avg. (2011-20)	25.9	9.7	15.2	389	2322	1571	2533
Kemptville	2023	26.0	10.1	2.2	481	2342	1630	2452
	2022	29.2	8.8	22.3	428	2395	1663	2610
	10 YR Avg. (2011-20)	25.0	12.1	13.0	436	2440	1690	2707
Earlton	2023	24.5	4.9	1.5	311	1997	1333	2117
	2022	26.0	9.6	0.5	327	2023	1360	2230
	10 YR Avg. (2011-20)	23.4	8.9	25.9	386	1991	1325	2252
Sudbury	2023	23.8	7.3	4.5	477	2029	1366	2165
	2022	26.1	11.3	7.8	303	2055	1382	2298
	10 YR Avg. (2011-20)	23.3	10.7	28.2	423	2130	1440	2420
Thunder Bay	2023	25.0	3.4	24.2	266	1849	1206	1991
	2022	27.6	11.1	8.7	453	1818	1182	1974
	10 YR Avg. (2011-20)	23.5	8.8	20.5	384	1909	1230	2108
Fort Frances	2023	26.3	5.7	15.3	307	2014	1365	2194
	2022	28.7	8.4	16.5	571	1900	1266	2127
	10 YR Avg. (2011-20)	24.1	8.1	22.1	389	2033	1336	2256