Here is a summary of the provided article:

## Onset Periods

The article describes the deciduous phenology algorithms, which specify the occurrence of onset growth periods. During these periods, carbon and nitrogen fluxes are calculated from the transfer pools into the displayed growth.

The key equations are:

- Carbon flux equations (2.20.1 - 2.20.6)
- Nitrogen flux equations (2.20.7 - 2.20.12)

The time-varying rate coefficient controlling the flux out of the transfer pool, `r_xfer_on`, is defined in equation (2.20.13). This produces a flux that declines linearly over the onset growth period, approaching zero in the final timestep.

The article explains that the onset growth period is represented by the variable `t_onset`, which is the number of seconds remaining in the current phenology onset growth period.

Overall, the article outlines the mathematical formulas used to calculate the carbon and nitrogen fluxes during the onset periods of the deciduous phenology algorithms.