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Stress-Deciduous Onset Triggers

In warm climates, onset triggering depends on soil water availability. An accumulated soil water index (SWI_sum) is tracked, which increases daily if the soil water potential (Psi_s,3) is above a -0.6 MPa threshold. Onset is triggered once SWI_sum exceeds 15, and there has been at least 20 mm of rain in the previous 10 days.

In cold climates, onset triggering depends on both accumulated soil temperature summation (FD_sum) and adequate soil moisture. If FD_sum exceeds 15 days below the freezing threshold, a cold-climate onset criterion is used. This requires meeting three conditions: SWI_sum > 15, growing degree-day summation (GDD_sum) exceeding a critical threshold, and daylength greater than 6 hours.

When a new onset growth period is initiated, several variables are reset to zero (SWI_sum, FD_sum, GDD_sum, n_days_active), and the onset counter (t_onset) is set to 30 days. Fluxes from storage into transfer pools occur during this onset period, following established carbon and nitrogen flux equations.