SORTIE-ND
Software for spatially-explicit simulation of forest dynamics

# Puerto Rico semi-stochastic - diam only

This behavior combines a deterministic growth function for small trees with completely stochastic growth for larger trees. It's meant to be used when a species uses a height growth behavior as the primary growth method.

### Parameters for this behavior

Parameter nameDescription
PR - "a" Parameter for Deterministic Growth"a" parameter used to calculate deterministic growth when a tree is below the stochastic height threshold.
PR - "b" Parameter for Deterministic Growth"b" parameter used to calculate deterministic growth when a tree is below the stochastic height threshold.
PR - Height Threshold for Stochastic Growth (m)The tree height threshold, in meters, between deterministic and stochastic growth.
PR - Mean DBH (cm) for Stochastic GrowthThe mean for DBH values, in cm, when a tree uses stochastic growth. This is the mean of the DBH value, NOT the amount of growth.
PR - DBH Standard Deviation for Stochastic GrowthStandard deviation for DBH values when a tree uses stochastic growth. This is the standard deviation of the DBH value, NOT the amount of growth.

### How it works

The divide between the two growth functions is defined in the PR - Height Threshold for Stochastic Growth (m) parameter. Trees shorter than this use the following function:

Y = (A * exp(-B * Height)) - Diam

where:

• Y = diameter growth for the timestep, in cm
• a = PR - "a" Parameter for Deterministic Growth parameter
• b = PR - "b" Parameter for Deterministic Growth parameter
• Height = tree height in cm AFTER height growth in the current timestep
• diam = diameter of the tree at which to apply growth (before growth), in cm

Above the height cutoff, trees are assigned random diameters drawn from a normal distribution. The normal distribution is defined by the PR - Mean DBH (cm) for Stochastic Growth and PR - DBH Standard Deviation for Stochastic Growth parameters, and represents the distribution of DBH values, NOT growth values. The amount of growth for a tree is Y = D' - D, where Y is the amount of growth, D' is the new diameter chosen from the normal distribution, and D is the previous diameter. This means that growth can be negative. The effect is to create a tree population with normally-distributed diameters, where any individual tree may jump from place to place within the distribution.

### How to apply it

This function can be applied to seedlings, saplings, or adults of any species. Any tree using this behavior must also use a height-only growth behavior.