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Stage 1: Modelling the growth of a single tree

Step 1
Add a compartment to the desktop, and rename it size.
Step 2
Draw a flow into the size compartment, and rename it growth.
Step 3
Add a variable to the desktop, above the growth flow, and rename it gr.

The variable gr represents the maximum rate of growth of the tree.

Step 4
Draw an influence arrow from both the size compartment and the gr to the growth flow.

Your model diagram should now look like this:

Step 5
Enter the following expression for the growth flow: gr*(1-size/25)

In this expression, the value 25 is the maximum size the tree will reach. It’s growth rate will fall from the maximum given by gr at the start of the simulation. This form of mathematical expression is a convenient way of achieving this desired behaviour of rapid initial growth reducing as the final maximum size is reached.

Step 6
Set the initial value for the size compartment to 3.
Step 7
Set the value for the variable gr to 0.2.
Step 8
Prepare the model for running.
Step 9
Set up a plotter display helper for the size compartment
Step 10
Run the model