Old-growth forests enable us to explore the natural dynamics and equilibrium of forest ecosystems. They are characterized by a long-term balance of energy, water, carbon, nitrogen and other nutrient fluxes. One option to enhance our understanding of forest ecosystem fluxes is the application of ecosystem models. In this paper we use a species-specific adaptation of the ecosystem model BIOME-BGC to assess the biogeochemical cycle of old-growth forests and test. the reliability of self initialization procedures as they are commonly applied within large-scale forest ecosystem models. We investigated the reliability of such initialization procedures by comparing the modeled equilibrium with data obtained from virgin forests in Central Europe. The results demonstrate that after some modifications the model is able to assess the flux of energy, water, nitrogen and carbon within old-growth forest ecosystems.