Modelling the dynamics of forest ecosystems is an urgent task, as the volume of publications in the Russian and world press demonstrates. In the proposed work, a new ecological and physiological model of a mixed forest stand has considered. Basically, it proceeds from the ecological and physiological model of a single-breed forest stand, that had obtained from the analysis of the behavior of an open thermodynamic system, which is a forest ecosystem. Four differential equations are required to describe a two-species stand, with the mutual influence of species being expressed both in interspecific competition for a resource and in mutual ‘support’ in the growth of the trees. The model of mixed stand with two species contains 10 independent parameters that have a clear physical meaning. Six parameters relate to the dynamics of each species, and four ones take into account the interactions of the species during growth. The verification of the model is presented by calculating the biomass dynamics for full two-stage aspen-spruce stands of European part (middle taiga ecoregion) of the first appraisal area. The presented model of the dynamics of forest ecosystems can be used in practical forestry, especially in the transition from an extensive method of forestry to an intensive one.