Differential shifts in the phenological timing and duration of Citrus unshiu: Insights from long-term observations and projected warming

This study analyzed temporal trends in the budburst, full bloom, and fruit-enlargement stages of Citrus unshiu, and predicted phenological shifts under increasing temperatures. The end of the fruit-enlargement stage was quantitatively defined using a sigmoid curve fitted to relative fruit weight (%). Over 23 years of phenological observations, all three stages advanced progressively. Differing rates of advancement led to a 3.5-day-per-decade increase in the interval between budburst and full bloom, whereas the fruit-enlargement period remained relatively stable. For each phenological stage, the base temperature and growing degree day threshold were determined based on extrapolation stability and prediction accuracy, with prediction errors (root mean square error) of 4.2, 2.2, and 6.4 days for budburst, full bloom, and end of fruit-enlargement stages, respectively. Simulations with temperature increases up to 7.0°C showed that a 1°C increase in mean temperature advanced budburst by 10.9 days, and full bloom and the end of fruit enlargement by 9.7 days each, whereas the durations between stages responded nonlinearly to warming. These results indicate that even when phenological stages shift linearly with warming, the intervals between them may change in nonlinear ways, underscoring the need for a more adaptive approach to agricultural planning.