Trees are removed from the FORMIND model through mortality processes. These can be event-driven, or occur continuously in each time step.
Continuous Mortality in FORMIND consists of a the sum of 3 rates:
M = Mb + Md + Mi
Mb: This is the background mortality rate, specific for each PFT, and applied to all trees.
Md: Stem diameter-dependent mortality rate. Since typically smaller trees face higher mortality rates, this parameter is applied to all trees with a certain diameter, also defined by the user.
Mi: Represents the diameter increment dependent mortality rate. Older trees (age defined by size) also face a higher mortality rate. Therefore Mi is summed to the mortality rate M to reproduce this pattern.
The Mortality rate M can be deterministic, where a removal of a fraction of the individuals from each PFT happens each timestep, or stochastic, where M represents the probability of death of each individual from the PFT group.
- Crowding Mortality caused by Limited Space:
Crowding (self thinning) occurs, if at any height layer the cumulative crown area of all trees on a patch exceeds the size of the patch. In this case, a reduction factor causes the death of individuals, in proportion to the total crown area.
- Mortality caused by Disturbances due to a Falling Tree:
If one tree falls, neighboring trees can be destroyed as well. A dying tree falls down with a predefined probability. The falling target depends on falling direction and on tree height H. The trees in the target patch die according to the a damage rate.
- Mortality due to Fragmentation:
If the simulated area is defined as a fragment, the area around 100 meters from the edge receives an additional mortality factor. More details in Disturbances