Spatiotemporal distribution of high temperatures before and after the flowering stage of summer maize and their impacts on yield in the North China Plain

This study aims to clarify the spatiotemporal distribution patterns of the summer maize at the flowering stage before and after high temperatures in the North China Plain (NCP). A systematic investigation was also made to explore their impacts on the yield under climate change. Daily meteorological data were collected from 340 meteorological stations in the NCP (1981-2022). Phenology data of the summer maize were captured from 19 agricultural meteorological stations, and the high-temperature experimental data were from public literature. High-temperature thresholds of 32 ℃, 35 ℃, and 38 ℃ were selected to calculate the high-temperature indicators, including the extreme growing degree days (EDD) and the extreme heat days (EHD). An analysis was then made on their spatiotemporal distribution during the tasseling and grain-filling stages of summer maize in the NCP. Furthermore, an equation was used to assess the impact of the high temperatures before and after the flowering stage on the yield reduction rate of summer maize. The probability of the high-temperature occurrence was also calculated in the different provinces during various growth stages. The results showed that: 1) The high temperatures in the NCP were intensified under the background of climate warming. There was an increasing trend (P<0.05) in the EDD and EHD more than 32 ℃, 35 ℃, and 38 ℃ during the growing season of summer maize. The increasing rates were 13.8 ℃·d/10a and 4.0 d/10a, 4.0 ℃·d/10a and 2.3 d/10a, and 0.4 ℃·d/10a and 0.4 d/10a, respectively. 2) The occurrence of the high temperatures during the tasseling stage followed a pattern of more in the south and less in the north. In the grain-filling stage, the high temperatures shared a spatial distribution of more in the southwest and less in the northeast. High temperatures during the tasseling and grain-filling stages showed an increasing trend over the past four decades. According to the threshold temperature of 38 ℃, the significant warming occurred in the northern Henan province, with the increase rates of 0.4 ℃·d and 0.5℃ d per 10 years, respectively, during the tasseling stage. Significant warming was also observed in the southern Henan province and Renxian County in Hebei province, with the increase rates of 0.4 ℃·d/10a and 0.5 d/10a for the EDD and EHD, respectively. 3) Henan Province was found with the highest probabilities of the high temperatures above 32 ℃, 35 ℃, and 38 ℃ before and after flowering. The impact threshold of the high temperature was approximately 35℃ for the main maize varieties in the NCP. For each 1 ℃·d increase in the accumulated harmful temperature more than 35 ℃, and then the yield reduction rate of the summer maize increased by 0.45%. Furthermore, the impact of the high temperatures before and after flowering was particularly significant in Henan Province in the 2010s. Yield reduction rates ranged between 2% and 5% in the southern part of the province, such as Jiaozuo, and Huaibin in the southeastern. The findings can also provide scientific support to accurately assess the high-temperature risks and disaster reduction strategies at different growth stages of summer maize in the NCP under climate change.