Prospective analysis of spatiotemporal variations in chill during winter, heat accumulation for flowering and spring frost in fruit trees in northeast Spain.

Agriculture is highly sensitive to climate variability, and ongoing warming is expected to modify the thermal conditions controlling fruit tree phenology and production. This study analyzes the spatiotemporal variability of winter chilling, spring heat accumulation, and spring frost probability across fruit-growing areas in Aragón (northeast Spain) using a regional, high-resolution agroclimatic approach. Chill portions (CP), growing degree hours (GDH), and spring frost probability occurrence (SFPO) were computed from daily gridded maximum and minimum temperature data at 1 km² spatial resolution. Agroclimatic indicators were derived using statistical, non-experimental methods based on established chilling and forcing models and empirical temperature thresholds. Recent historical variability was characterized using overlapping 30-year historical climate periods while future climate conditions were assessed using daily temperature projections from an ensemble of 18 regional climate models (EURO-CORDEX), dynamically downscaled from global climate models and bias-corrected. Projections were analyzed for a historical reference period (1971-2000) and under the intermediate and high emissions scenario RCP4.5 and RCP8.5 for mid-century (2041-2070) and late-century (2071-2100) periods. Climate projections at ~ 5 km spatial resolution were interpolated to 1 km and bias-corrected using Empirical Quantile Mapping with high-resolution observational data as reference. Changes in CP and GDH distributions were quantified using spatial differences and standardized anomalies relative to historical conditions, and indicators were extracted at the location of existing fruit orchards. Results indicate that effective winter chilling remains within broad ranges compatible with fruit production across all scenarios, with persistent spatial contrasts between western and eastern sectors. In contrast, spring heat accumulation shows a strong and spatially coherent increase, particularly in low-elevation and eastern areas, indicating an increasing influence of spring temperatures on phenological dynamics. Although future scenarios project a substantial reduction in the probability of frost occurrence after early March, increasing heat accumulation may advance phenological development, potentially shifting frost exposure to earlier periods in late winter. Overall, the results indicate that agroclimatic conditions in Aragón are strongly structured by regional climatic gradients, and that climate change is likely to intensify spatial contrasts between colder and warmer production areas rather than producing uniform changes across the region. These results provide a regional agroclimatic framework that can support adaptation planning and the long-term management of fruit production under climate change.