RECRUITMENT AND ESTABLISHMENT
Recruitment is the process in which seeds establish in an area and grow into new mature individuals. The FORMIND model simulates recruitment using parameters that define the number of seeds that enter the simulated area, and seedling germination thresholds according to light climate conditions.
During recruitment, individuals in the FORMIND model must face 3 distinct processes until reaching a certain size (e.g. 1 cm stem diameter): DISPERSAL, GERMINATION and SPACE LIMITATION.
Each patch in the FORMIND simulated area contains a seedpool, with the available number of seeds for each plant functional type in each time step. Seeds enter the seedpool through seed dispersal, and exit through germination or mortality.
FORMIND includes two different possibilities to model dispersal:
- Using global constant in-growth rates:
Here the number of dispersed seeds is assumed to be introduced to the local community from an intact surrounding forest community. This number is thereby a constant type-specific parameter independent of the density of individuals already existing on the simulated area. For each Plant Functional type a number of seeds per year is defined, and are distributed uniformly through the patches.
- By seed production of mother trees:
During the simulation, each individual is able to produce a predefined type-specific number of seeds as a mother plant, after reaching a certain diameter size (representing the tree’s maturity age). The seeds are then dispersed according to a predefined radius. In this mode, initial densities of plant functional types influence the future composition of the tree community, providing a more complex dynamic.
Seeds in the seed pool can be affected by seed pool mortality, which determines the loss of seeds for the next time step.
A dispersed seed is only able to germinate, when light and seedling density are adequate. For determining the light conditions, the incoming irradiance on the floor is calculated. Incoming irradiance on the floor is possibly reduced due to shading of already existing individuals. Since each seed is already assigned to a Plant Functional Type with a minimal germination threshold, differences in ground light environments can foster changes in whole forest communities.
If light requirements are fulfilled for successful seed germination, it is checked whether enough space is available for their establishment – If the forest floor is already filled with existing trees, none of the germinated seedlings would be able to establish.
Finally, individuals with a diameter smaller than a threshold parameter (Md – diameter-dependent mortality), can receive a differentiated mortality rate.