SPRUCOM - a simulation model of spruce stand dynamics under varying immission exposure. The process model SPRUCOM for forest simulations is presented. It allows the simulation of the dynamic development of spruce stands from planting to maturity. The modelled forest stand is divided into four distinct competition classes, each starting under identical initial conditions, but developing differently in response to the effects of light competition. The competition process leads to reduced photoproduction and successive disappearance of the suppressed classes. This self-thinning is accelerated by the inherent feedback structure. Environment-dependent tree-physiological processes are simulated in different submodels for photoproduction, assimilate allocation, maintenance respiration and biosynthesis of supporting and structural tissue. The model was applied in particular to stands under varying immission exposure. Simulation runs illustrate that even identical pollutant-caused damages for all individuals lead to unexpected effects on stand structure via altered intra-specific competition.