Trends in thermal time during US maize development phases
The sensitivity of US maize yield to temperature varies substantially over the course of the growing season and records of crop development indicate that planting dates have moved substantially earlier, crops spend less time in the field awaiting harvest, and the grain filling period has lengthened. Consequently, these details of how management and climate have altered the temperature experienced by the maize crop over the course of development substantially impact estimates of the environmental influence on yield trends. By using state level phenology data from 1981-2014 we distinguish sensitivity and warming trends within four distinct phases of maize development.
The phase based trend estimates indicate that temperature has had a greater impact on yields, indicating over 1.8 times greater influence of climate and management on estimated yield trends than that implied by fixed trends and sensitivities across the whole growing season. These results generally demonstrate that management has shifted the growing season towards more positive conditions than a fixed sensitivity would have indicated. While the uncertainties are large, the generally positive effect of the greater sensitivity indicated by the phase based trend estimates is highlighted by eleven states with best estimates greater than fixed estimates, while only five states have more negative estimates. These results imply that adaptation to changing conditions has, effectively, been ongoing and underscores it's importance regarding predictions of how climate change will influence maize production.