Software for spatially-explicit simulation of forest dynamics

Basic SORTIE modeling concepts

SORTIE is an individual-based forest simulator designed to study neighborhood processes. This means that the trees in the forest are modeled individually, not as averages or spatial aggregates. Each individual has a location in space. SORTIE specializes in modeling the interactions of trees with their nearest neighbors to study local neighborhood dynamics.

SORTIE state data

The basic SORTIE model state is defined by the plot, trees, and grids. The plot is the underlying location in which the simulation takes place. It has a particular size and shape, and attributes for climate and geographic location. The trees are the individuals making up the forest on the plot. Grids hold additional data that varies from place to place, such as soil chemistry or light level at the forest floor. All of these together define the model state at a particular time.


The processes that act to change the model state are called behaviors. Behaviors often correspond to biological processes. They are individually contained units, but often work together to create a complex, interacting system. For instance, a simulation might consist of three behaviors: a behavior to calculate light levels for trees, one to determine the amount of tree growth as a result of the amount of light, and one to select trees to die if they grow too slowly. Behaviors are placed in a certain order to correctly structure their interactions.

The simulation

Forests tend to operate on annual cycles, and so does SORTIE. The unit of time in SORTIE is the timestep. It represents a set of one or more years. A single timestep consists of each behavior acting once, in their defined order. The process is repeated for the number of timesteps that you set, and that's a single simulation, or run.

The basic structure of the SORTIE system is very simple. Its power lies in its incredible flexibility. Almost every aspect of the model is under direct user control.

The parameter file

When you start the SORTIE software, you are using a tool that helps you to define the state data and behaviors that will make up a simulation. Once you have done this, you have created a parameter file. The parameter file completely defines a run. You can load and run your parameter file any time.