Trends in the frost-free period in China from 1951 to 2012

This study explores the spatial distribution of the first frost date, last frost date and the frost-free period in China through an in-depth analysis of the daily minimum temperature.Accumulative anomaly and linear trend estimation are used to describe the trends in the first frost date, last frost date and the length of the frost-free period. Then, the relationship between the first frost date, last frost date and the frost-free period is explored, and the abrupt feature of frost-free period is detected using a Mann-Kendall test. Four major results were obtained. First,the frost-free period decreases with the increase of latitude or altitude, with yearly fluctuation as latitude ascends or altitude descends. Second, over large parts of China(80%), the frost-free period is extended by both a later first frost date and an earlier last frost date. Furthermore, the magnitude of the changes in the three indices(the first frost date, last frost date and frost-free period) are higher in the north and east regions, but lower in the south and west. Third, for the majority of agricultural regions, the delayed first frost date together with an earlier last frost date prolongs the frost-free period. However, in some areas of the southwest agricultural region and the middle and lower Yangtze River agricultural region, the last frost date is later, but is more than offset by the later first frost date, or the first frost date is earlier, but is more than offset by the earlier last frost date, so the frost-free period is still be prolonged. Fourth, the mutation of the frost-free period was significant in Northeast China, Inner Mongolia and Great Wall, Huang-Huai-Hai, Qinghai-Tibet and Gansu-Xinjiang agricultural regions, which constitute half of China. With respect to timing, the mutation of the frost-free period occurred primarily in the eastern monsoon region in the 1980 s and in the western non-monsoon region in the 1990 s. Overall, these changes in the three indices(first and last frost date, and frost-free period length) could affect the suitability of conditions for sowing and harvesting of a variety of crops.