*2.1. Study Region*

We quantified the Ya, Yp, Yg, and the contribution of suboptimum SDT to Yg of summer maize over the NCP. The study region in this study spans five provincial-level administrative divisions of China (Beijing, Tianjin, Hebei, Shandong, and Henan) and covers 42 prefecture and two provincial-level cities of China (110.98◦ E–122.71◦ E, 32.27◦ N– 41.06◦ N). In the period 2010 to 2015, the accumulated effective temperature (or growing degree days, GDD, with a base temperature of 8 ◦C) during the maize growing season (May to September) and annual precipitation shows a significant spatial gradient with mean values of 1950 ◦C d year<sup>−</sup><sup>1</sup> and 720 mm year<sup>−</sup>1, respectively, for the entire region. The NCP is one of the major cultivation areas for maize in China, and it is approximated to produce 30% of the total maize production of the country [36]. We determined the study region based on administrative boundaries (Figure 1). The region does not correspond to the exact domain of the plain, but it does cover the major maize cultivation areas.
