How Does Succession Operate?

The exact representation of the succession process is dependent upon the succession extension installed. However, there is some functionality common to all succession extensions. Each succession extension implements the following:

1. Calculate shade. Shade at a site is variable through time and must be calculated. Shade determines which species can establish and is therefore the primary driver of succession. For age-only succession, shade is dependent upon the shade tolerance of the species already established at a site. High shade tolerance creates high shade values. In biomass succession, shade is dependent upon the amount of biomass on the site, regardless of species.
2. Age cohorts. At each time step, a cohort's age must increment by the designated successional time step.
3. Remove old cohorts. If a cohort reaches or exceeds the designated maximum age, succession removes the cohort.
4. Reproduction. Currently, all succession extensions share the same seed dispersal algorithms (none, universal, Ward's). In addition, each succession extension requires the user to provide the probability of establishment for each species in each ecoregion. The probability of establishment is:

P(cohort establishment | sufficient light)

The assumption is that after a cohort establishes, it can persist indefinitely unless removed via disturbance. With age-only succession, the cohort is therefore present. With biomass succession, the cohort may persist at various densities (measured as kg/ha), dependent upon cohort growth rates, age, and competitive pressure.