Do Smoky Skies Reduce Crop Yield?
Dr. Dennis Egli, Professor Emeritus
Smoke from forest fires in Canada recently produced hazy conditions in parts of the Midwest, the East, and the Mid-South. Smoke and haze reduce the solar radiation reaching the earth’s surface. Will this reduction reduce crop yields? This is a logical question, given that photosynthesis produces yield, and the energy to drive photosynthesis comes from solar radiation. Reducing solar radiation should reduce yield – right? It’s not that simple.
Yes, the reduction in solar radiation will reduce photosynthesis, but that doesn’t automatically translate into lower yield. First, the relationship between solar radiation levels and photosynthesis is not a straight-line relationship (to put it another way, the relationship curves over as solar radiation increases), so the decrease in photosynthesis is less than the decrease in solar radiation. For example, in a field experiment with soybean, 63% shade (much much greater than reductions from smoke and haze) from planting to maturity only reduced the yield by 50%. The solar radiation – photosynthesis relationship in corn is closer to a straight line, but still, the reduction in photosynthesis from a 20% reduction in solar radiation, for example, would be less than 20%.
Secondly, smoky conditions increase the proportion of solar radiation that is diffuse (as opposed to direct radiation). Diffuse radiation occurs when the radiation from the sun bounces off the dust, smoke particulate matter, and other pollutants in the air and arrives at the surface from all directions (direct radiation comes in a straight line from the sun). Diffuse radiation penetrates farther into the plant canopy resulting in a more even distribution of radiation over the leaves and higher photosynthesis. The benefits of diffuse radiation may be larger on the relatively compact soybean canopy compared with the more upright leaves in the corn canopy.
Finally, reduced solar radiation will reduce water use (evapotranspiration, ET), which could be a positive effect on fields experiencing drought stress. The first step in the ET process is the conversion of water from a liquid to vapor which requires energy from the sun, so reducing solar radiation could reduce ET. Any reduction in ET would probably be relatively small and may be important only in marginal situations when the crop is just beginning to experience drought stress.
Reductions in photosynthesis during vegetative growth rarely carry over to yield unless there are large reductions in plant size. Lower photosynthesis during vegetative growth will reduce plant size and leaf area; if this reduction is large enough to reduce solar radiation interception during reproductive growth, the yield will be reduced. If not, the smaller plants will not result in a lower yield. When we shaded soybean communities (30 and 63% shade) in the field from planting to growth stage R1 (initial bloom), total plant dry weight at R1 was reduced by 20 (30% shade) and 42 (63% shade) %, but there was no effect on yield. The smaller plants still intercepted all of the solar radiation, so size did not affect yield.
The overall effect of smoky skies on crop yield is the result of one negative effect (less solar radiation and less photosynthesis), one positive effect (more diffusive radiation and higher photosynthesis), and one possible positive indirect effect (reduction in drought stress). The combined effect on yield is hard to predict as it depends on how much smoke and haze is in the sky (how much the solar radiation is reduced and the proportion of diffuse radiation is increased), the water status of the crop, and how long and when (before or after flowering) the smoke and the reduction in photosynthesis occurs.
At this point in the growing season, most of the smoky days occurred during vegetative growth, so my best guess is that the yield potential of corn and soybean has not been affected. I don’t think smoke is worth worrying about unless we get a lot more smoke later this summer, and even then, its usually better not to worry too much about things we can’t control. Practicing up on your rain dance might be a better strategy this year if you are going for maximum yield.