# Puerto Rico semi-stochastic - diam onlyThis 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 behaviorParameter name | Description |
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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 Growth | The 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 Growth | Standard 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 worksThe 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 itThis 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. |