In this thesis it has been determined how site productivity and tree species composition affect root biomass. Six research plots have been selected, of which three were on productive and three on less productive sites. Pure Norwayspruce, Silver fir and European beech stands have been compared in the mature forest stands on Omphalodo-Fagetum site unit. Basal area, growing stockand stand density per hectare have been determined at each plot and 30 soil cores have been obtained. The roots have been eliminated from the samples, they have been measured and weighed after drying. The results have shown that both, productive as well as the less productive soil, have been deepest in the fir sites. Regarding the length of the roots, productive beech stand achieved the highest value, while maximum root biomass has been found inspruce stand on the less productive site. It has been concluded that the roots penetrate deeper into the more productive sites and that tree composition affect the differences in the amount of root fitomass.