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Seasonal-Deciduous Onset Trigger

The onset trigger for the seasonal-deciduous phenology algorithm is based on an accumulated growing-degree-day approach. The growing-degree-day summation (GDD_sum) is initiated when the phenological state is dormant and the model timestep crosses the winter solstice. GDD_sum is then updated each timestep based on the third soil layer temperature.

The onset period is initiated if GDD_sum exceeds a critical threshold (GDD_sum_crit), which is calculated based on the annual average 2m air temperature. When a new onset growth period is initiated, GDD_sum is reset to 0 and a 30-day onset period counter (t_onset) is started.

During the onset period, carbon and nitrogen fluxes occur from storage pools to transfer pools. A fraction (f_stor,xfer) of 0.5 is used to determine the amount transferred.

If the onset criterion is not met before the summer solstice, GDD_sum is reset to 0 and the growing-degree-day accumulation will not start again until the following winter solstice. This prevents the initiation of very short growing seasons late in the summer in cold climates.

The onset counter (t_onset) is decremented on each timestep after the onset period is initiated, until it reaches zero, signaling the end of the onset period.