By Ashley Dean

We have read the headline a number of times now warning us that increasing temperatures are threatening global crop production. One need only to recall the drought and heat wave that hit the mid-western United States last summer, damaging corn and soybean production. Higher temperatures are certainly part of the problem, but a new study led by Stanford Woods Institute Center Fellow David Lobell, associate director of the Center on Food Security and the Environment, finds its impacts in the U.S. are more indirect. Water stress may be the main culprit.

To validate this hypothesis and to help differentiate the different mechanisms impacting crop yields at higher temperatures, the research team used a model known as an Agricultural Production Systems Simulator (APSIM). High temperatures had a strong negative effect on corn yield response in the United States, in agreement with the data, but the predominate effect of heat in the model was via increased water stress.

As temperatures increase, plants transpire more water into the atmosphere, just as people sweat more on hotter days. With more hot days, the corn plant finds it harder to maintain growth rates, and at the same time loses more water, which sets up the risk of even more drought stress later in the season.

“APSIM computes daily water stress as the ratio of water supply to demand, and during the critical month of July this ratio is three times more responsive to 2 ºC warming than to a 20 percent precipitation reduction,” writes Lobell and co-authors in a new paper published in Nature Climate Change. “Water stress during July is particularly important for overall biomass growth and final yield, with July being the month with the most total biomass growth.”

Direct heat stress on the plant, such as happens on extremely hot days, played a more minor role in determining final yield. The study suggests that increased CO2 may reduce crop sensitivity to extreme heat by increasing water use efficiency, but gains are likely to be no more than 25 percent.

“The APSIM model has been valuable in its ability to discriminate the importance of these factors,” said Lobell. “Models like these are useful for guiding efforts to develop crops with greater tolerance to increased temperatures, an important component of most adaptation strategies in agriculture, and helping to identify which processes are critical for modeling efforts to consider when projecting climate change impacts.”

The researchers project sensitivity to extreme heat will remain a severe constraint to crop production in the foreseeable future, especially as the region warms. They are now using the models to evaluate different strategies for developing new varieties of corn that can better handle the heat.