Summary:

### Available Carbon

The available carbon (C_avail) for the FUN model is the total canopy photosynthetic uptake (GPP), minus the maintenance respiration fluxes (m_r), and multiplied by the time step (δt). This represents the carbon available for growth after accounting for maintenance respiration.

The key points are:

- C_avail = (GPP - m_r) * δt
- This calculates the carbon available for the FUN module after subtracting maintenance respiration from the total photosynthetic uptake.
- Growth respiration is then calculated on this remaining, available carbon.

The summary captures the main formula and concept of available carbon for the FUN model, as described in the provided text.