Jointing-Heading Cold Stress and Yield Impact in Winter Wheat of China.

Frequent extreme cold events pose a serious threat to wheat production, but the spatio-temporal variation of cold stress and its impact on wheat yield are poorly known. Based on the analysis of historical climate, phenology and yield data from 161 stations in the main winter wheat-growing region of China during 1981-2009, the spatio-temporal variation of cold stress between jointing and heading and its impact on wheat yield were investigated among four sub-regions in China, including two northern sub-regions, North Sub-region (NS) and Huang-Huai Sub-region (HHS), and two southern sub-regions, Middle-Lower Reaches of Yangzi River Sub-region (MYS) and Southwest Sub-region (SWS). Four cold stress indices were developed to quantify cold intensity, frequency, duration, and drop amplitude and drop rate between jointing and heading based on the observed daily minimum temperature records of the last 29 years at each station. The accumulated cold stress days (ACSD) and accumulated cold degree days (ACCD) indicated that the cold stress frequency and intensity between jointing and heading at low latitude, including Southwest Sub-region (SWS) and Henan province (HA), was more severe than that at high latitude. However, the temperature drop amplitude (TDA) and drop rate (TDR) were more severe in the generally cooler northern wheat-growing regions (NS and HHS) than the generally warmer southern regions of China (MYS and SWS), because cold air normally flows from the northwest. Considering the combined impacts of minimum temperature changes and phenology shift, the general increasing temporal trends of all cold stress indices were observed in most of the study region, especially significant increasing trends for TDA and TDR in HHS, and for TDR in MYS from 1981 to 2009. Phenology shift contributes significantly to the promotion of the uptrend in cold stress during 1981-2009, but not in MYS for ACCD, TDA and TDR. The wheat yield was significantly affected by ACDD and TDR from jointing to heading in HHS and SWS, and was significantly affected by ACDD in MYS and by TDR in NS. During the entire study region, the variation in ACDD and TDR contributed to 28.4% yield variation in wheat. Furthermore, yield losses due to cold stress variation during 1981-2009 were more serious over most areas of HHS, east part of MYS, and small part of SWS and NS. Given the current level and potential increasing risk of jointing-heading cold stress in the main winter wheat growing region of China, targeted adaptation strategies are necessary to ensure food security under ongoing climate change.