Controlled Warming Effects on Wheat Growth and Yield: Field Measurements and Modeling

Climate warming may raise wheat (Triticum aestivum L.) yields in cooler climates and lower them in warmer climates. To understand these contrasting effects, infrared heating lamps were used to warm irrigated spring wheat by 1.5 degrees C (day) and 3.0 degrees C (night) above unheated controls during different times of the year at Maricopa, AZ. Changes in wheat growth with warming were used to test hypotheses for temperature effects on crop growth in the process model ecosys. Infrared heating substantially raised phytomass growth and grain yield under lower air temperature (T-a) following plantings from September through December. The same heating, however, lowered growth and yield under higher T-a following plantings from January through March. Gains in wheat yield of as much as 200 g C m(-2) with heating under lower T-a were attributed in the model to more rapid CO2 fixation and to reduced chilling effects on seed set. These gains were only partially off set by losses from shortened wheat growth periods. Losses in wheat yield of as much as 100 g C m(-2) with heating under higher T-a were attributed in the model to adverse effects of heating on crop water status and on CO2 fixation vs. respiration, to greater heat stress effects on seed set, and to shortened crop growth periods. Model hypotheses thus explained contrasting effects of heating on wheat yields under different T-a found in the field experiment as well as in many earlier studies. Well-constrained tests of these hypotheses are vital for models used to project climate change impacts on agricultural ecosystems.