NCI mortality

This behavior uses the effects of neighbor competitiveness to influence tree survival ("NCI" stands for neighborhood competition index). A tree's maximum potential probability of survival is reduced due to competitiveness and several other possible factors. You can use certain parameter values to turn these influences on and off to reflect the conditions appropriate for your run.

Trees killed by this behavior will have a mortality reason code of "natural".

Parameters for this behavior

Parameter nameDescription
NCI Crowding Effect Slope (C) The slope of the curve for neighbor effects.
NCI Crowding Effect Steepness (D) The steepness of the curve for neighbor effects.
NCI Max Radius of Crowding Neighbors, in m The maximum distance from a target tree at which neighbors can have a competitive effect.
NCI Max Survival Probability (0-1) The maximum annual probability of survival, as a value between 0 and 1.
NCI Minimum Neighbor DBH, in cm The minimum DBH for trees of that species to compete as neighbors. Used for all neighbor species, not just those using NCI mortality.
NCI Neighbor DBH Effect (alpha) The effect of the DBH of a neighbor tree on its competitiveness for a target species.
NCI Neighbor Distance Effect (beta) The effect of the distance of a neighbor tree on its competitiveness for a target species.
NCI Neighbor Storm Damage (eta) - Complete (0-1) The fraction to which a neighbor's competitive effect is reduced when the neighbor has sustained complete storm damage.
NCI Neighbor Storm Damage (eta) - Medium (0-1) The fraction to which a neighbor's competitive effect is reduced when the neighbor has sustained medium storm damage.
NCI Shading Effect Coefficient (m) The coefficient in the shading effect equation. Set this value to 0 if you do not wish to use shading.
NCI Shading Effect Exponent (n) The exponent in the shading effect equation. If you set the NCI Shading Effect Coefficient (m) parameter to 0, this value is ignored.
NCI Size Effect Mode, in cm The mode of the size effect curve.
NCI Size Effect Variance, in cm The variance of the size effect curve.
NCI Size Sensitivity to NCI (gamma) The sensitivity of a tree's survival probability to its DBH.
NCI Storm Effect - Complete Damage (0-1) The fraction by which a tree's survival probability is reduced when it has sustained complete storm damage.
NCI Storm Effect - Medium Damage (0-1) The fraction by which a tree's survival probability is reduced when it has sustained medium storm damage.
Species i NCI Lambda The scale of the competitive effect of a neighbor tree's species on the target tree's species.
Include Snags in NCI Calculations Whether or not to include snags when finding competitive neighbors for NCI.

How it works

For a tree, the annual probability of survival is calculated as:


Prob. Survival = Max Survival * Size Effect * Shading Effect * Crowding Effect * Storm Effect

Max Survival is the NCI Max Survival Probability (0-1) parameter. Storm Effect, Shading Effect, Size Effect, and Crowding Effect are all optional factors which act to reduce the maximum survival probability and will vary depending on the conditions a tree is in. All values are bounded between 0 and 1.

Size Effect is calculated as:

NCI size effect equation

where:

Shading Effect is calculated as:

NCI shading effect equation

where:

This effect is not required. To omit the Shading Effect, set the NCI Shading Effect Coefficient (m) parameter to 0.

Crowding Effect is calculated as:

NCI Crowding Effect equation

where:

The NCI value sums up the competitive effect of all neighbors with a DBH at least that of the NCI Minimum Neighbor DBH, in cm parameter, out to a maximum distance set in the NCI Max Radius of Crowding Neighbors, in m parameter. The competitiveness of a neighbor increases with the neighbor's size and decreases with distance and storm damage to the neighbor (optional). The neighbor's species also matters; the effect depends on the relationship between the target species and the neighbor species. Seedlings never compete. You set whether or not snags compete in the Include Snags in NCI Calculations parameter.

The crowding effect is optional. You can omit it by setting either the NCI Crowding Effect Slope (C) or NCI Max Radius of Crowding Neighbors, in m parameters to 0.

NCI is calculated as:

NCI equation

where:

The value of Damage Effect is optional. If you elect not to use storms in your run, set all values in the NCI Damage Effect - Medium Storm Damage (0-1) and NCI Damage Effect - Complete Storm Damage (0-1) parameters to 1. If you are using storms, then the value of Damage Effect depends on the tree's damage category. If the tree is undamaged, Damage Effect equals 1. If the tree has medium storm damage, the value is the NCI Damage Effect - Medium Storm Damage (0-1) parameter. If the tree has complete storm damage, the value is the NCI Damage Effect - Complete Storm Damage (0-1) parameter.

The survival probability as calculated above is an annual probability. For multi-year timesteps, the timestep probability is APX, where AP is the annual probability and X is the number of years per timestep. Once a tree's timestep survival probability has been calculated, it is compared to a random number to determine whether the tree lives or dies.

How to apply it

This behavior can be applied to saplings and adults of any species. It cannot be applied to seedlings.

If the Shading Effect term is activated in the growth equation, then the trees to which this behavior is applied must also have a light behavior applied - the Sail light behavior is the one designed to work with the NCI behavior. The use of any other light behavior is at your own risk.

If any storm damage parameters are set to anything other than 1, it is recommended (but not required) that you have the Storm damage applier behavior applied.