Uncovering the large increases of thermal units during crop phenological phases in China since 1981

Thermal unit (TU) is a critical genetic trait of crops and parameter in crop models. Understanding changes in TU is essential to better comprehend climate adaptation and enhance crop model simulations. However, trends in the field TU at a large scale remain largely unknown and lead to traditional crop modeling framework assuming a fixed TU. Here, using the records for multiple crop (wheat, rice and maize) phenological dates and varieties at 206 agro-meteorological stations across China from 1981 to 2018, we found that (1) the number of varieties on average reached 10 for spring wheat, 14 for winter wheat, 18 for early rice, 17 for single rice, 16 for late rice, 11 for spring maize, 13 for summer maize and 17 for interplanting maize from 1981 to 2018 across China, respectively. (2) TU of these successively replaced varieties during the whole growing period significantly (P < 0.05) increased by 5.68, 2.44, 2.48, 2.38, 1.67, 5.31, 1.16, 3.78 °C d/a for spring wheat, winter wheat, early rice, single rice, late rice, spring maize, summer maize and interplanting maize across China, respectively. (3) The significantly positive correlations (R > 0.40, P < 0.01) between mean temperature and TU during the whole growing period for wheat, rice and maize were found. This observation-based evidence assists breeders in keeping pace with climate change to ensure food security and supports global crop model development.