Identifying trigger thresholds for heat damage at different growth stages of summer maize: A case study of the Huang-Huai-Hai Plain, China

Climate change has increased uncertainties in maize cultivation, particularly when daily maximum temperatures ($T_{max}^d$) exceed critical thresholds for maize. The thresholds that trigger heat damage (HD) and the severity of its impacts differ across the various growth stages of maize. However, there is a lack of quantitative evidence defining the critical HD thresholds for summer maize at these specific stages. Therefore, determining the HD trigger thresholds for different growth stages of summer maize is crucial for agricultural meteorological services. This study developed a multi-method framework to determine HD thresholds, integrating box plots, kernel density estimation, and probability density functions to establish threshold ranges, followed by ROC curves and the Youden Index to identify critical thresholds. Results revealed critical thresholds of 34.0°C (V0-VE), 34.1°C (VE-V6), 34.6°C (V6-VT), 34.8°C (VT-R1), 33.7°C (R1-R3), and 32.9°C (R3-R6). Validation using county-level independent samples demonstrated an average accuracy exceeding 83 % across all stages. The spatial distribution and evolutionary characteristics of HD were further demonstrated by examining the maximum consecutive HD days from 2021 to 2023 at each growth stage, along with the $T_{max}^d$ from emergence to jointing stage in 2023. Notably, the HDD/HDF indices exhibited negative correlations with relative meteorological yield, particularly highlighting the significant impact of cumulative heat effects during flowering and grain-filling stages on yield reduction. These findings validate the rationality of the identified HD thresholds and provide valuable theoretical insights for monitoring and early warning of heat damage in summer maize cultivation, particularly in the HHH Plain.