SORTIE-NDSoftware for spatially-explicit simulation of forest dynamics
Basal Area Light behavior
This behavior calculates light levels as a function of the basal area of trees in a neighborhood. The light levels are randomized using a lognormal probability distribution. Because of this random element, to ensure some continuity through time, light levels only change when the local neighborhood tree basal area has changed by a certain amount.
Parameters for this behavior
How it works
This behavior uses a grid called Basal Area Light to manage light levels. Light levels are calculated for each cell in the grid; trees to which this behavior are assigned receive the light level of the cell in which they are located.
The behavior begins by calculating the total basal area and angiosperm basal area in the neighborhood of each Basal Area Light grid cell. The neighborhood is a circle with its center on the center of the grid cell, and a radius given in the Basal Area Light - Search Radius for Neighbors (m) parameter. Whether a neighborhood tree counts as angiosperm or conifer depends on what its species is set to in the Basal Area Light - Species Type parameter. Seedlings and snags are never included in these basal area totals. Other trees are only included if their DBH is equal to or greater than the value set in the Basal Area Light - Minimum DBH for Trees parameter. Once the basal areas are calculated, the behavior adds them together to create a total basal area, and compares this value to the previous timestep's total basal area. If the value has not changed by more than the amount set in the Basal Area Light - Minimum BA Change for New GLI (m2) parameter, no further action is taken. The previous timestep's GLI and basal area values are kept.
For each grid cell in which a new GLI is to be calculated, a mean GLI value is calculated as follows:
This mean GLI is translated into a ζ value for the lognormal random draw as follows:
where GLIm is the mean GLI value calculated above, and σ is the Basal Area Light - Lognormal PDF Sigma parameter.
This value is then used to produce a random lognormally distributed number, from the following distribution:
where ζ is the value calculated above, and σ is the Basal Area Light - Lognormal PDF Sigma parameter. This number is the GLI value, between 0 and 100.
Once each Basal Area Light grid cell has gotten a GLI value, trees to which this behavior applies get the value of GLI from the grid cell in which they are located.
How to apply it
This behavior may be applied to seedlings, saplings, and adults of any species.