Over the past week, most of Minnesota has been blanketed by unhealthy concentrations of wildfire smoke from fires burning in Canada and northern Minnesota. Climatologists predict that wildfire smoke frequency and intensity will increase across northern Minnesota and Ontario (Wotton et al., 2005), and so farmers should learn about the impacts to plants and people, and develop strategies for worker safety.
Impacts to plant health
When the sky is thick with smoke, it’s hard to imagine that there’s not an impact to crops. However, the impacts are a bit nuanced.
Wildfire smoke aerosols do create a shadier atmosphere, so you’d think that sunlight isn’t reaching plants as well. However, studies like Hemes et al. (2020) have found that wildfire smoke creates more diffuse radiation (aka light is scattered throughout the atmosphere), and plants can use this light more efficiently than direct solar radiation. Therefore, photosynthesis can actually increase in these conditions. However, this must be balanced with the impacts of other pollutants such as ozone.
Fires generate ozone and other aerosols. Plant stomata are pores on the leaf surface where gas exchange occurs, and while we’ve probably all learned that plants intake CO2 and exhale oxygen, other gases also enter plants through their stomata. When high concentrations of ozone are present in the atmosphere, it enters plants through their stomata, and can interfere with photosynthesis. These impacts can occur hundreds of miles from the area that’s actively burning (Yue & Unger, 2018).
The scale of these impacts differs between ecosystems: the uncertainty is based on ozone damage sensitivity, cloud cover, and aerosol properties. There are very few studies about the impacts of wildfire smoke on crops, and they are mostly on global scales, so we can’t translate this directly to say how much these wildfires will impact vegetable growth.
In places like California and Australia where wildfires are much more common, people have cited problems like ash settling directly on crops, creating a physical barrier on the leaves, which prevents access to light. Grapes in California and Australia have had to be discarded after fires due to flavor changes, but this doesn’t happen all the time, and seems to be most common when wildfires are happening within a couple of miles of farms. The wildfire smoke we experienced last week in Minnesota was likely not thick enough to create these types of issues.
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| Wildfire smoke makes for some beautiful sunrises, but it's damaging to our lungs and can cause problems for plants. |
Plant stress symptoms?
There is not enough research on the impacts of smoke to specialty crops to say for sure what symptoms may be caused. There is also significant variability in the contents of wildfire smoke. Smoke that is high in ozone has the potential to cause plant damage. The APS compendium series includes ozone in the chapters about abiotic stressors in most of their compendia. In Brassicas, ozone exposure can cause chlorotic flecks. In cucurbits, it can cause chloritic flecks, bleaching, stippling, and premature necrosis of leaves. Potatoes can respond with either chlorotic spots or darkly pigmented spots on the leaves. (Sources: APS compendia for potatoes, cucurbits, and brassicas).
Will we see these symptoms in Minnesota? We haven't noticed symptoms so far, so it is unlikely that we will see sufficient impacts to expect damage. Keep in mind that vegetables have long been grown in places like California, Washington, and Oregon where wildfires are common, and the smoke we’re witnessing is from fires much further away than the fires impacting fruits and vegetables on the west coast.
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| Ozone damage on a potato leaf. Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org |
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| Ozone damage on a watermelon leaf. Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org |
Human health and safety
While the potential impacts of smoke on vegetables are speculative, the impacts on people are clear. Know the symptoms: Health effects of wildfires include burning eyes, runny nose, chest pain, fatigue, coughing, difficulty breathing, and rapid heartbeat.Wildfire smoke is a relatively new issue for the Midwest, and so we don’t have a lot of resources to guide farmers in decision-making processes for employee health. However, we can look to states in the west that have navigated wildfires for decades for ideas and guidance. California implemented a “Protection from Wildfire Smoke” rule under CAL/OSHA General Industry Safety Orders in 2019 (Title 8, Section 5141.1) which includes the following rules for employers:
- If the AQI reaches 151, it is recommended that outdoor workers use a respirator. N95 respirators are the minimum recommended respirators. N99, N100, R95, R99, R100, P95, P99, and P100 are all suitable respirators because they provide more protection than N95s. The use of respirators by employees is voluntary, but they should be provided respirators to use by employers. Bandanas and respirators that do not have the above labels (e.g. N95) are not sufficient.
- At an AQI of 151, employers are encouraged to modify work schedules, reduce work intensively, and schedule additional rest periods.
- If the AQI reaches or exceeds 500, work will either stop or respirator use will be required. (This is a legal requirement, but respirator use is recommended well before this point.)
- Respirators should be discarded after 8 hours. They become filled with particles and lose efficacy over time, so they should be considered disposable. Some respirators such 0as half or full face reusable respirators have filters that should be changed out every 8 hours.
- See the UC Davis Western Center for Agricultural Health and Safety’s Wildfire Resources for more information and printable posters.
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Image: UC David wildfire smoke employer training discussion guide (linked above) |
Sources
Hemes, K.S., J. Verfaillie, D.D. Baldocchi. (2020). Wildfire-smoke aerosols lead to increased light use efficiency among agricultural and restored wetland uses in California’s Central Valley. JGR Biogeosciences 125(2). DOI 10.1029/2019JG005380.Wotton, M., K. Logan, R. McAlpine. (2005). Climate change and the future fire environment Ontario: Fire occurrence and fire management impacts. Ontario Ministry of Natural Resources. Climate change research report; CCRR-01.
Yue, X., Unger, N. (2018). Fire air pollution reduces global terrestrial productivity. Nat Commun 9, 5413. https://doi.org/10.1038/s41467-018-07921-4
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