a method for controlling a power source of a forestry machine , wherein the power source utilizes directly or indirectly one or more work devices and / or handling devices positioned in the forestry machine and is controlled by a control unit . when the control unit in the forestry machine gives one or more work or handling devices a control command starting a change operation , it is transmitted substantially simultaneously to the power source . when arriving at the power source , one or more control commands requiring a power change of the power source affect control parameters of the power source , predicting the future load of the power source .