In the first part of this presentation we will present 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 see that the best fit of the parameters of the mathematical model is obtained when the competition for light influences only the growth rate of trees. We construct a size structured population dynamics model with one and two species and with spatial structure. The second part of the talk we investigate the semi flow properties of a class of statedependent delay differential equations which is motivated by some models describing the dynamics of the number of adult trees in forests. We investigate the existence and uniqueness of a semi flow in the space of Lipschitz and C 1 weighted functions. We obtain a blow-up result when the time approaches the maximal time of existence. We will conclude the presentation with an application to a spatially structured forest model.