Spatiotemporal changes in the characteristics of the safe growth period and high temperature damage of ratoon rice in nine southern provinces of South China

In the context of global warming and with increasing cultivated area of ratoon rice, precise control of ratoon rice sowing is of particular importance to ensure safe maturation and for utilizing the optimal temperature and light resources, stabilizing the production of ratoon rice, and ensuring food security. This study aimed to clarify recent spatiotemporal changes in terms of safe sowing date, duration from safe sowing date to safe full heading date (i.e., safe growth period), and high-temperature damage, and provide insights into novel strategies to utilize optimal temperature and light resources, and ensure safe production under the pressure of global warming. After collecting daily average temperature data in the past 39 years (1981–2019) from 601 weather stations in nine southern provinces of China (namely Sichuan, Chongqing, Hubei, Hunan, Jiangxi, Anhui, Jiangsu, Zhejiang, and Fujian), we examined the changes in safe sowing date and safe growth period, as well as high-temperature damage of ratoon rice in these provinces by using methods of climate change trends calculation and abrupt changes analysis. The results showed that the safe sowing date had advanced 3.3 d·(10a)⁻¹, the safe full heading date was delayed by 1.5 d∙(10a)⁻¹, and the safe growth period had extended by 4.8 d∙(10a)⁻¹. The abrupt shift in the safe sowing date occurred in 2001, after then this date occurred 9 days earlier. The greatest change in the safe growth period occurred in 1996, after then this period was 12 days longer than before. The safe sowing date occurred earlier in the south part than in the north part, and the safe growth period was longer in the southeast than in the northwest. High-temperature damage in June to July and August to September tended to increase. Mild, moderate, and severe damage increased by 48.5 times∙(10a)⁻¹, 30.3 times∙(10a)⁻¹, and 37.4 times∙(10a)⁻¹, respectively, in June to July; and by 52.7 times∙(10a)⁻¹, 18.2 times∙(10a)⁻¹, and 34.6 times∙(10a)⁻¹, respectively, in August to September. Moreover, high-temperature damage in June to July was more serious than that in August to September. The annual average numbers of mild, moderate, and severe high-temperature damage in June to July were 83.9, 41.6, and 115.9 times more than those in August to September. The years characterized by abrupt changes in mild, moderate, and severe high-temperature damage were 1998, 1988, and 1986 in June to July, and 1992, 2002, and 2002 in August to September, respectively. In all cases, high-temperature damage increased significantly after the abrupt change years and their occurrence followed the order of mild > severe > moderate in both June to July, and August to September. In conclusion, the advancement of the safe sowing date and the extension of the safe growing period of ratoon rice were advantageous as they benefited full use of temperature and light resources and yield increasing of ratoon rice in nine provinces of South China. However, concurrent increases in high-temperature damage impeded the high yields of high-quality ratoon rice.