Experimental studies were conducted with forestry mulchers in field protection forest belts and clearings in the North-Eastern region of Bulgaria . Regression models were elaborated to determine the operating performance and fuel consumption of forestry mulchers with a rated power of 70 kW to 245 kW for site preparation of clearings with the amount of wood residues, shoots and bushes from 15 t/ha to 48 t/ha. The mode of influence of the mulcher rotor speed , the concentration of comminuted biomass and the mulching unit power on the performance and fuel consumption were established. When treating clearings with small biomass concentration of 15 t/ha, the productivity of the mulching units with a greater power of 245 kW is 0.392 ha/h, and of those with a smaller power of 70 kW it is 0.086 ha/h. This difference is even greater in clearings with a biomass concentration of 48 t/ha, where the operating productivity of 245 kW mulchers is 0.304 ha/h and that of 70 kW mulchers is 0.021 ha/h. Mulching units with greater power also have a lower relative fuel consumption per unit area. This fact is more pronounced in clearings with a larger amount of wood residues of 48 t/ha, where the fuel consumption of bigger units with a power of 245 kW is 155 l/ha, of units with a medium power of 160 kW it is 217 l /ha, and of units with a smaller power of 70 kW it is 335 l/ha. For treating clearings with relatively less wood residues and shrubs of 15 t/ha, depending on the rated power of mulching units, these fuel costs are respectively 86 l/ha, 122 l/ha and 214 l/ha. The optimal power and rotor revolutions of forestry mulchers when processing cuttings with different concentrations of woody biomass were determined.