Impacts of Day Versus Night Temperatures on Spring Wheat Yields: A Comparison of Empirical and CERES Model Predictions in Three Locations

Trends in recent temperature observations and model projections of the future are characterized by greater warming of daily minimum (tmin) relative to maximum (tmax) temperatures. To aid understanding of how tmin and tmax differentially affect crop yields, we analyzed variations of regional spring wheat yields and temperatures for three irrigated sites in western North America that were characterized by low correlations between tmin and tmax. The crop model CERES-Wheat v3.5 was evaluated in each site and used to project future response to temperature changes. Tmin and tmax exhibited distinct historical correlations with yields, with CERES successfully capturing the observed relationships in each region. In the Yaqui Valley of Mexico, historical yields were strongly correlated with tmin but not tmax. However, CERES projections of response to increased tmin or tmax (holding other variables constant) were similar (|6% C), indicating that the apparent historical importance of tmin mainly results from covariation between temperatures and solar radiation and not greater direct effects of tmin on yields. In the San LuisMexicali Valley of Mexico and in the Imperial Valley of California, the opposite was observed: historical yield correlations with tmin and tmax were similar, but projected responses to tmax were roughly three times larger than tmin. The latter is explained by opposing effects of tmin and tmax on grain filling rates in CERES, with higher tmin increasing harvest indices. This model mechanism was not clearly supported by historical data and remains an area of uncertainty for projecting yield responses to climate change. APOTENTIALLY IMPORTANT TRAIT of anthropogenic climate change is asymmetric warming between day and night. On a global average, daily minimum temperature (tmin) has risen more than twice as fast as daily maximum temperature (tmax) over the past century (Easterling et al., 1997), and most models simulate a further reduction in the diurnal temperature range (DTR 5 tmax 2 tmin) in the next century (Dai et al., 2001). For several reasons, the response of crop yields to temperature change may depend on the relative warming of tmin and tmax (Peters et al., 1971; Ziska and Manalo, 1996; Stone, 2001; Peng et al., 2004). Processes such as photosynthesis and transpiration are concentrated in daylight hours and therefore should be more responsive to tmax, whereas processes such as respiration occur throughout day and night. Crop development rates and the duration of critical phases such as grain filling may also be differentially sensitive to tmin and tmax. In addition, impacts of extreme hot or cold temperatures, such as winterkill in wheat or spikelet sterility in rice, depend on changes in daily extremes (Ziska and Manalo, 1996; Porter and Gawith, 1999). Models capable of simulating the crop yield response to temperature and other environmental factors are useful tools to anticipate the impacts of climate change and to develop appropriate responses. Commonly used models of the major cereal crops, such as the models included in the DSSAT software (Jones et al., 2003), consider the effect of temperature on rates of several processes affecting crop yields. These processes include vernalization, crop phasic development, leaf appearance and expansion rates, photosynthesis and respiration, evapotranspiration, and grain filling (Ritchie and NeSmith, 1991; Wilkens and Singh, 2001). Several studies have used crop models to investigate the impacts of asymmetric warming on rainfed crops in the USA. Rosenzweig and Tubiello (1996) compared the effects of tmin and tmax on winter wheat in the central USAusing CERES-Wheat and found that, for identical changes in average temperatures, increasing tmin more than tmax resulted in higher simulated yields than when increasing both by equal amounts. This disparity was attributed to reduced winterkill in experiments with greater tmin increases. Dhakhwa and Campbell (1998), using the EPIC model for maize, soybean, and wheat in the Southeastern USA, similarly report greater simulated yields for scenarios of greater nighttime warming than for scenarios of equal day–night warming. In their case, themain mechanism responsible was reduced water stress in the asymmetric warming scenario, where lower daytime temperatures led to reduced evaporative losses. Whether these models properly represent the separate effects of tmin and tmax has not been specifically addressed. This is primarily because historical interannual variations in growing season tmin are highly correlated with tmax in most regions. Thus, ‘‘validation’’ of the model by comparing yield simulations with historical records does little to elucidate the model’s ability to capture separately the responses to tmin and tmax. However, there are some growing regions in the world for which tmin and tmax are poorly correlated historically. These provide a unique opportunity to develop understanding of tmin and tmax effects. Specifically, there are several regions where interannual correlations beD.B. Lobell, Energy and Environment Directorate, Lawrence Livermore National Lab., Livermore, CA 94550; and J.I. Ortiz-Monasterio, International Maize and Wheat Improvement Center (CIMMYT), Wheat Program, Apdo, Postal 6-641, 06600 Mexico D.F., Mexico. Received 14 July 2006. *Corresponding author ([email protected]). Published in Agron. J. 99:469–477 (2007).