This work presents results of investigation of beech forest succession in the Bieszczady Mountains using a FORKOME* model in response to climatic changes. The model was verified according to field observations in 1998-2001 in beech forest in the forest district Procisne in the Bieszczady Mts (Poland). Prediction of tree biomass and tree number was aimed at for the next 600 years. Simulation demonstrated beech domination when mean annual temperature increases by 2°C. However while the decrease in temperature by 2°C fir becomes dominant in research plot. Both one simulation run and Monte Carlo simulation showed comparable results of statistical analysis. 3. scenario (warm and humidity) illustrated an increase in beech biomass. That trend is also characteristic to 4. scenario with parameters set to cold and humidity. In opposite situation - dry climate (6. scenario - cold and dry, 5. scenario - warm and dry) descending number of beech trees resulted in an increase in fir trees number. Change in trees number is noticeable in 7. scenario where air temperature is fluctuating.