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The Effect of Wildfire Smoke on U.S. Crops

Smoke from Canadian wildfires has shows up in parts of farm country throughout the summer of 2023. Seth Naeve, extension agronomist with the University of Minnesota, says the smoke’s effect on U.S. crops isn’t simple to describe.

“Yes, the overall quantity of light is reduced. But one really interesting thing about crops is that the upper leaves of crop plants can’t absorb all of the light that’s incident upon them,” according to Naeve. “There’s two or three times as much intensity of light than those first leaves can absorb. Therefore, leaves below them receive a little bit more, and then a little bit is transmitted through the leaves. It goes down to the next leaves, and the next leaves, and by the time you get down in the canopy, the leaves are fully shaded.”

He talks about what smoke does to sunlight that the plants need for photosynthesis.

Naeve says, “What happens when we have partially-overcast skies is the light is actually scattered. So instead of having light coming through directly – and that’s like what you see when you get a hard shade around leaves under a tree or something like that – instead when we have some overcast skies, the light is completely scattered. So, it’s coming from all directions, and that allows the light to penetrate down in the canopy, and we’re able to better distribute the light throughout the canopies, instead of it being nailed in the first level of leaves.”

Naeve says the smoke does limit the overall amount of sunlight getting to the crops. However, it spreads that light more evenly throughout the plants.

He says, “It can pass through those leaves and even out the distribution of that light throughout the canopy. So that’s a well-known physiological principle that goes back decades that old physiologists have done work on this, so, there’s some balancing effect on that. And recently, I saw a publication – and I didn’t dive into the citations on it – but there’s also a light quality effect, in that the smoke is taking out some of the green lights and leaving some of the red spectrum that are more useful for the plant. And so, although the light quantity is reduced, the fact that it’s coming through as scattered light and the quality of that light is slightly improved, most of the physiologists agree that there’s probably little or no detrimental effect on crop plants, per se.”

It’s not entirely positive as the burning wildfires create ozone, something harmful to crops and humans. “The only bad piece of this stuff for plants is ozone,” according to Naeve. “So, we’ve all heard of ozone, and that’s this reactive oxide that comes about by burning. And if ozone levels are increased, that has some detrimental effects on plants, so that these little radical oxygens kind of disrupt the individual cells, some parts within cells, within the plants, and so it could have a negative effect.”

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