The results of forest dynamic modelling via forest gap models in Slovakia are presented in the paper. Our calculations have been carried out by using prof. Smith’s version of Forest gap model. Model requires followed input data for individual forest tree: maximum tree age, maximum diameter, maximum high, maximum yearly seedling establishment scaled to plot. This model contains several response functions which includes light, water balance, and climate responses to the individual trees. We established for a climate change conditions, that reduction of coniferous (spruce, fir) will be larger than deciduous trees. The observed results about increasing production must by interpret with fair sized reserve. The modelling of snow cover changes is closely related to mountain forests presence. The model WaSiM shows noticeable decrease of water supplies in snow in mountain catchments of Nízke Tatry Mts. The negative difference between the year 2075 and the base period will be 32 %.