A sensitivity analysis, a restricted uncertainty analysis and a validation of a model describing competition between a heather and grass species (HEATHSOL), was carried out. The results of the sensitivity analysis show that the competition process is mostly influenced by parameters determining light and nutrient competition and minimum/maximum nitrogen content of the plant species. The uncertainty analysis was only carried out qualitively. The parameters which contribute most to the uncertainty of the model output are specific leaf area, height coefficients, root length, model initialisation and nitrogen deposition. The validation shows that the model can describe the competition in a Calluna/Deschampsia and Calluna/Molinia vegetation satisfactorily.