High temperatures over several days or more for a crop such as corn (maize) can reduce the crop yield substantially. But new crop research reveals that it is not so much the heat and heat stress that impacts the plants, but water stress caused by increased evapo-transpiration through vapour pressure deficit (VPD) - the plants version of human sweating to cool down.
The new understanding comes from research and modelling by one of Australia's leading crop scientists, Professor Graeme Hammer, from the University of Queensland. Graeme has spent much of the last decade as professor in crop science at UQ's Queensland Alliance for Agriculture and Food Innovation (QAAFI) developing increasingly sophisticated computer models to predict the growth and yield of agricultural crops. Lead author for the study was David Lobell, an Assistant Professor in Environmental Earth System Science and Associate Director of the Center on Food Security and the Environment at Stanford University.
I reported back in 2009 that Climate Change likely to severely damage U.S. crop yields based upon increasing summer temperatures (Schlenker et al 2009). And indeed crop yields do decrease as temperature rises and the number of days above optimum temperature increases. On a global basis a correlation has been observed between reduced yield and higher average seasonal temperatures.
Lead author David Lobell examined global trends in crop production in a 2011 paper - Climate Trends and Global Crop Production Since 1980 (abstract) detailing that:
"...in the cropping regions and growing seasons of most countries, with the important exception of the United States, temperature trends from 1980 to 2008 exceeded one standard deviation of historic year-to-year variability. Models that link yields of the four largest commodity crops to weather indicate that global maize and wheat production declined by 3.8 and 5.5%, respectively, relative to a counterfactual without climate trends. For soybeans and rice, winners and losers largely balanced out. Climate trends were large enough in some countries to offset a significant portion of the increases in average yields that arose from technology, carbon dioxide fertilization, and other factors."
For example, maize has been found to be sensitive to extreme heat with multiple days above 30 degrees C. But the actual mechanism for how temperature affected crop yield was unknown.
High temperatures an indirect driver of plant water stress and crop yield
What Professor Hammer and his co-researchers have shown is that heat is an indirect driver of reduced crop yield through increased plant evapo-transpiration. The evaporative demand for water - causing increased plant water use - that will ultimately cause the decline in crop yield, is much more important, with direct heat stress on reproductive organs a much more minor role.
"These two factors are often related, but until now we were simply attributing projected yield declines to increases in temperature and heat stress - and it's more complex than that," Professor Hammer said.
Here is how the research paper describes what is going on inside the plants:
"....hotter days tend to have higher vapour pressure deficit (VPD) between the saturated leaf interior and the ambient air, and high VPD drives faster transpiration rates. Plants typically respond to higher VPD by reducing stomatal conductance, which effectively saves soil moisture for periods with less evaporative demand, at the cost of reduced carbon assimilation during high-VPD portions of the day."
So what happens when it gets really hot is that the plants increase their transpiration (sweating) to try and keep cool, while closing down their photosynthesis activities. This happens even when soil moisture is maintained through irrigation. The researchers documented this in field research in the US state of Iowa.
"Our computer models are able to separate the mechanisms and explain what is actually going on. Increasing temperatures mean increasing demand for water and so greater plant water use and ultimately more water stress during the crop life cycle." explained Professor Hammer.
The researchers also imply that increased atmospheric CO2 may reduce crop sensitivity to extreme heat by increasing transpiration efficiency. But the sensitivity gains are likely to be no more than 25 per cent. The researchers stress more research is needed in this area.
The crop sensitivity to extreme heat days "will remain a severe constraint to crop production for the foreseeable future. Further increases in T (temperature) will contribute to greater VPD and water stress, and will probably also cause direct heat damage to be increasingly common. Designing appropriate crop development and management strategies for dealing with extreme heat should be an emphasis of future work." the research paper concludes.
To make the point clearer Graeme Hammer used an analogy of the effects upon a person standing in a hot desert: "You would start to sweat more as the temperature increased and more rapidly use up your reserves of water. It's a relatively simple concept, but one that has been overlooked until now." he said.
The abstract for the paper - The critical role of extreme heat for maize production in the United States (abstract) is reproduced below:
Statistical studies of rainfed maize yields in the United States and elsewhere have indicated two clear features: a strong negative yield response to accumulation of temperatures above 30 °C (or extreme degree days (EDD)), and a relatively weak response to seasonal rainfall. Here we show that the process-based Agricultural Production Systems Simulator (APSIM) is able to reproduce both of these relationships in the Midwestern United States and provide insight into underlying mechanisms. The predominant effects of EDD in APSIM are associated with increased vapour pressure deficit, which contributes to water stress in two ways: by increasing demand for soil water to sustain a given rate of carbon assimilation, and by reducing future supply of soil water by raising transpiration rates. 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 °C warming than to a 20% precipitation reduction. The results suggest a relatively minor role for direct heat stress on reproductive organs at present temperatures in this region. Effects of elevated CO2 on transpiration efficiency should reduce yield sensitivity to EDD in the coming decades, but at most by 25%.
- Adapted from University of Queensland media release, 4 March 2013 - Global warming affects crop yields - but it's the water not the heat
- David B. Lobell, Graeme L. Hammer, Greg McLean, Carlos Messina, Michael J. Roberts & Wolfram Schlenker. 3 March 2013, Nature Climate Change (2013) doi:10.1038/nclimate1832 The critical role of extreme heat for maize production in the United States (abstract)
- lead Image - Drought Corn in West Kentucky August 2012, By CraneStation / Flickr, Creative Commons 2.0 Attributuion