  In this article we build a mathematical model for forest growth and we
compare this model with a computer forest simulator named SORTIE. The main
ingredient taken into account in both models is the competition for light
between trees. The parameters of the mathematical model are estimated by using
SORTIE model, when the parameter values of SORTIE model correspond to the ones
previously evaluated for the Great Mountain Forest in USA. We construct a size
structured population dynamics model with one and two species and with spatial
structure.
